2008 Edition: Lifting Techniques and Quick Reference of Common Data, as well as Design, Selection, Calculation of Machinery and Equipment, and Safe Operation Techniques

Common Safety Techniques and Requirements for Lifting, Hoisting, and Transporting I. Common Lifting Ropes and Hoisting Equipment (1) Ropes Ropes are used in lifting operations to tie down, transport, and lift objects. Common types of ropes include hemp ropes, chemical fiber ropes, and steel wire ropes. 1. Types and uses of hemp ropes. Rope can be divided into three types: white-brown rope, mixed hemp rope, and thread hemp rope. Among them, the white-brown rope has higher strength and is widely used. Rope is a type of cord commonly used in lifting operations, offering advantages such as light weight, flexibility, and ease of tying. It is generally used for bundling lighter objects, and can also be used as a lifting rope, cable, or balancing rope. Breaking tension and safety factor of hemp rope. To ensure the safety of lifting operations, the tensile force exerted on the hemp rope in actual use should be less than its breaking strength during testing, thereby providing a certain safety margin during operation. The allowable stress of hemp rope is only 1/3 to 1/10 of its breaking tensile force. The safety factor for white-brown ropes is shown in the table below. When using other types of hemp ropes, the safety factor should be higher than that of white-brown ropes. The allowable tensile force for white-brown ropes is shown in the table below. The allowable tensile force of old hemp ropes is 40% to 60% of that of new hemp ropes. If the breaking tension data for hemp rope is unavailable, the allowable tension value can be estimated using the following formula, which already takes a safety factor of 5 into account. S = 5d2, where S is the allowable tensile force, in Newtons (N) ； d———Diameter of the hemp rope, in millimeters (mm). Safety factor of white-brown rope, Applications: Manual handling (lifting, pulling), Bundling, Hanging or using pulley systems for lifting. Cable ropes; used in critical applications or mechanical operations. Safety factors: 3, 4, 5, 6, 10. Allowable tensile force of domestic Qiyu brand white-brown rope: Hemp rope diameter d (mm), Weight per roll (kg), Breaking tensile force (N), Allowable tensile force (N). Minimum value of pulleys to be used: D > 10d (mm); K = 5, K = 3. Values: 6, 8, 11, 13, 14, 16, 19, 20, 22, 25, 29, 33, 38, 41, 44, 51, 57, 63. 6.5, 10.5, 17.0, 23.5, 32.0, 41.0, 52.5, 60.0, 70.0, 90.0, 120.0, 165.0, 200.0, 250.0, 290.0, 330.0, 450.0, 500.0. 2000, 3250, 5750, 8000, 9500, 11500, 13000, 16000, 18500, 24000, 26000, 29000, 35000, 37500, 45000, 60000, 65000, 70000. 400, 650, 1150, 1600, 1900, 2300, 2600, 3200, 3700, 4800, 5200, 5800, 7000, 7500, 9000, 12000, 13000, 14000. 660, 1080, 1910, 2660, 3160, 3830, 4330, 5330, 6160. 8000, 8660, 9660, 11660, 12500, 15000, 20000, 21660, 23330. 100, 100, 150, 150, 150, 200, 200, 200, 220, 250, 290, 330, 380, 410, 440, 510, 510, 510. Note: Each roll of hemp rope is approximately 200 meters long. (3) Precautions for using hemp ropes: ① Hemp ropes should only be used to tie together lighter objects; they must not be used with mobile lifting equipment or in situations where high forces are involved. ②For rope used in pulley systems, the diameter of the pulley must be no less than 10 times the diameter of the rope. ③Rope knots should be avoided in narrow areas such as pulley grooves. ④When bundling components, padding should be used at the corners of the components. ⑤Do not expose hemp rope to corrosive chemicals; store it on dry wooden boards, keeping it away from moisture and high temperatures. ⑥When the uniform wear on the surface of the rope exceeds 30% of the rope’s diameter, or when the depth of local damage exceeds 10% of the rope’s diameter, it should be used at a reduced capacity ； Use is prohibited in case of broken strands. 2. Chemical fiber ropes (1) Characteristics of chemical fiber ropes. Chemical fiber ropes mainly include nylon ropes and polyester ropes, and they feature being lightweight, soft, corrosion-resistant, and having good elasticity. It is typically used to suspend components and equipment with a smooth surface or those whose surfaces must not be worn. Both nylon ropes and polyester ropes have considerable elasticity; when a load is first lifted, the ropes stretch significantly. When the allowable stress is reached, their maximum elongation can be around 40%. Therefore, chemical fiber ropes can provide cushioning for the load, but they increase the instability during lifting. (2) Precautions for using chemical fiber ropes: ① Chemical fiber ropes melt easily when exposed to high temperatures; therefore, they should be kept away from direct sunlight and flames. ②Chemical fiber ropes have high elasticity and are unstable during lifting; care should be taken to prevent the load from swinging and causing injury. Furthermore, once the rope breaks, it rebounds significantly, so measures should be taken to prevent injury from such rebound. ③Chemical fiber ropes have low friction; when they are released from the mooring post under load, it is necessary to prevent the rope from slipping completely and causing injury. 3. Steel wire ropes (1) Uses of steel wire ropes. Wire ropes are widely used in the lifting, luffing, and towing mechanisms of cranes. It can also be used as tension ropes for mast cranes, support ropes for cable cranes and aerial cables, etc. In lifting operations, it is often used to tie components and materials together as well as as rigging. (2) Types of steel wire ropes. When manufacturing steel wire ropes, the steel wires are first twisted into strands, and then these strands (usually 6 strands) are twisted around a core to form the rope. There are many types of steel cables. In lifting operations, the wire rope commonly used is 6×17＋1 ； 6×19＋1 ； 6×37+1. That is, a 6-strand steel wire rope is used, with each strand consisting of 17, 19, or 37 steel wires, and the rope contains an organic core. The steel wires of 6×17+1 wire ropes have a larger diameter and are relatively hard; they are not easy to bend, and are generally suitable for use as cables ； 6×19+1 is generally used for slings and in winches ； The steel wires of the 6×37+1 wire rope have a small diameter and are relatively soft; they are often used in lifting machinery that operates at high speeds as well as in pulley systems. Steel ropes can be classified into types such as right-hand twist, left-hand twist, and mixed twist, depending on the twisting method used. Cross-twisting is widely used in lifting machinery; in such steel ropes, the direction in which the wires are wound is opposite to the direction in which the strands are wound, which prevents twisting and loosening under load. Rope cores are divided into three types: hemp cores (cotton cores), asbestos cores, and metal cores. The selection of wire ropes should take into account the pulley diameter, the safety factor and tensile strength of the wire rope, as well as the slinging factors. (3) Breaking tension and safety factor of the steel wire rope. The breaking tension of steel wire ropes can be found in relevant manuals. During use, the appropriate specification of wire rope should be selected based on the maximum static tensile force encountered in actual operations and the safety factor. It must meet the following strength requirement: S_break ≥ KS, where S_break is the breaking tension of the wire rope, in Newtons (N); S_break = φ∑S_wire, and φ is the conversion coefficient for the breaking tension of the wire rope, as shown in the table below ； ∑S wire——total breaking tension of the steel wire, in Newtons (N). K———Safety factor for steel wire rope, to be selected as per the table below ； S———The safe lifting capacity of the steel wire rope, in Newtons (N). At the construction site, if it is difficult to obtain relevant information and the breaking tension of the wire rope is unknown, it can be calculated using the following approximate formula: S_break ≈ 1/2d2. Here, S_break represents the estimated breaking tension of the wire rope, in kilonewtons (KN) ； d———Diameter of the steel wire rope, in millimeters (mm). Conversion factor for the breaking tension of steel ropes. Structure of steel ropes: Fiber core – 1×7, 1×19, 1X(19): 0.88; 6×7, 6×12, 7×7: 0.88; 1×37, 6×19, 7×19, 6×24, 6×30, 6X19, 6W(19), 6T(25), 6X(24), 6W(24), 6X(31), 8×19, 8X(19), 8W(19), 8T(25), 18×7: 0.85; 6×37, 8×37, 18×19, 6W(35), 6W(36), 6X(37): 0.82. Safety factor K of steel ropes. Type of crane: Characteristics and scope of use. Minimum safety factor for steel ropes: Manual cranes: 4.5; Motorized cranes – light type: 5, medium type: 5.5, heavy type: 6. Winches with a capacity of less than one ton: 4. Cable cranes: Steel ropes used to carry loads: 3.5. Steel ropes for various purposes: Transporting hot metals, flammable materials, and explosives: 6; Binding objects: 6. Cables: 3.5. Ropes: 8–10. (4) Precautions for selecting and using steel ropes: ① The specification of the steel rope should be chosen based on different applications. For lifting heavy loads or using with pulleys, wire ropes of the 6×37 or 6×61 specification can be chosen ； For use as cables or traction ropes, a 6×19 specification steel wire rope can be chosen. ②The diameter of the steel wire rope should be selected based on the magnitude of the load it is intended to bear and the allowable tensile force of the rope. ③The length of the steel wire rope should be such that, when the hook is in its lowest working position, there are still 2–3 turns of slack on the drum, thereby preventing the rope end plate from bearing the pulling force directly. ④Before using new steel wire ropes, their certificates of conformity should be carefully checked to ensure that their performance and specifications meet the required standards. ⑤When the steel wire rope passes through the pulley, the diameter of the pulley groove should be slightly larger than the diameter of the rope. If the diameter of the pulley groove is too large, the steel wire rope is prone to being flattened ； The diameter of the groove is too small, causing the steel wire rope to wear out easily. To extend the service life of the steel wire rope, the ratio of the diameter of the drum and pulley to the diameter of the steel wire rope can be selected as referred to in the table below. Crane types and usage scenarios: Pulley-type drum and wire rope diameter ratio. Tower, truck, crawler, and general cranes. Manual operation: D≥16d; Mechanical operation – Light type: D≥16d, Medium type: D≥18d, Heavy type: D≥20d. Cranes other than those mentioned above: Manual operation: D≥18d; Mechanical operation – Light type: D≥20d, Medium type: D≥25d, Heavy type: D≥30d. Winches with a capacity of less than one ton: D≥12d. Cranes equipped with grabs: Category 1 light type: D≥20d, Category 2 light type: D≥30d. (II) Lifting attachments: Various types of lifting attachments are required for lifting operations. Commonly used ones include slings (jacks), shackles, hooks and eye hooks, balance frames, and pulleys. The lifting device should be simple in design, easy to use, and straightforward to disassemble, in order to save labor and time while ensuring the safety and reliability of lifting operations. 1. A sling is a lifting device made of steel wire rope; therefore, the allowable tensile force of the steel wire rope is also the allowable tensile force of the sling. However, it is also necessary to know the magnitude of the internal forces within the sling. The internal force of the sling is the actual pulling force generated by the sling when lifting an object; in operation, this internal force should not exceed its allowable pulling strength. That is, the internal force in the sling ≤ allowable tensile force = breaking tensile force / safety factor. The internal force in the sling is related not only to the weight of the object but also to the angle between the sling and the horizontal plane; the larger the angle, the smaller the internal force ； Conversely, the smaller the angle, the greater the internal force in the sling, and its horizontal force also exerts considerable pressure on the object being lifted. The ideal position of the sling is vertical, but this is not always achievable; it is generally advisable to keep the angle at least 30°, with the optimal range being between 45° and 60°. The figure below shows the effect of different angles β on the load-carrying capacity of a two-branch sling. The tensile force acting on the wire rope slings, as well as the diameter of the wire rope, can be determined using the following formula: N = Q/n·1/sina = Q/n·L/a ≤ P/K. Here, N represents the tensile force acting on each wire in the sling ； Q———Weight of the object being lifted ； n———Number of strands in the slinging wire rope ； a———Horizontal angle between the slinging wire rope and the load ； a、L———as shown in the figure below ； P———Breaking tension of the entire wire rope ； K———Safety factor. Example (figures omitted): Hooks and shackles used on slings to lift heavy objects; a safety factor of not less than 5 is sufficient ； If heavy objects are to be tied directly, proper protective measures should be taken at the points where the rope comes into contact with the sharp edges of the objects, with a safety factor of 5 to 8 ； Without taking any protective measures, use 8–10 ； For heavy loads, precision items, or oversized objects, to ensure absolute safety, in addition to taking proper protective measures, a safety factor of 10 is applied. 2. Shackle: Shackles are used to connect lifting pulleys and secure slings; they are one of the important tools in lifting operations. The common types are pin-type and screw-type. The spiral type is more commonly used; it is generally forged from carbon steel and consists of two main parts: a horseshoe-shaped ring and a cross pin. For the convenience of disengagement, the regular shackles have been modified into semi-automatic shackles, whereby the horizontal shaft has been changed to one that can be pulled with a rope from the ground; pulling this shaft out enables disengagement ； The pull rope is relaxed, and the horizontal axis returns to its original position thanks to the elastic force of the spring. If the shackle lacks a certificate of conformity, it shall be subjected to a tensile test at 1.5 times its rated capacity before use. The wear of the bend rings and cross pins should be checked regularly; if severe wear, deformation, or fatigue cracks are detected, they should be replaced promptly. When using shackles, the specified load limit must not be exceeded, and they should only bear tensile forces. It is strictly prohibited to lift with the steel wire rope on either side of the shackle. At construction sites, if an immediate assessment of the allowable lifting capacity of a shackle is needed, an approximate calculation can be used: Q = 4d12, where Q represents the allowable lifting capacity of the shackle in kilograms ； d1———Diameter of the shackle pin (mm). 3. Hooks and eye hooks are the most widely used lifting devices. Hooks come in single-hook and double-hook types. A hook is a component with a circular, closed shape, commonly used in cranes capable of lifting very heavy loads. For ease of use, swivel shackles are generally used in lifting heavy equipment and specialized cranes. Hooks and eye bolts are generally forged, and their surfaces should be smooth, free of any scratches, notches, sharp edges, cracks, etc. They should be inspected every 1 to 3 years; if cracks are detected, use should be stopped immediately. If the wear depth on the critical fracture surface exceeds 10%, calculations based on the actual cross-sectional dimensions should be carried out to determine whether use should be discontinued or the load should be reduced. It is not allowed to repair hooks with defects such as cracks by welding. 4. Balance beam: When lifting items such as roof trusses and large motor rotors, it is necessary to ensure both the balance of the items and protection against damage caused by ropes; generally, a balance beam (commonly known as an iron pole) is used for such lifting operations. This lifting method is simple, safe, and reliable. It can withstand the horizontal forces generated by inclined lifting, reduce the stress on the object being lifted, and improve the loading conditions on the lifting lugs; as a result, the object does not suffer from dangerous deformation. Additionally, it allows for a shorter length of lifting ropes and a lower lifting height. The balance beam can be designed in crossbeam, triangular, H-shaped, and other forms. 5. Pulleys and pulley blocks are among the important tools in lifting operations. A pulley is actually a modified version of a lever; the axis of a fixed pulley corresponds to the fulcrum O of the lever, OA is the lever arm, and OB is the load arm, when the fixed pulley is in equilibrium. N×OA = Q×OB; since OA = OB, it follows that N = Q. As shown in the diagram below for the case of equilibrium with a movable pulley: (1) Calculation of the pulley system. The amount of effort saved by a pulley system depends on the number of working ropes that share the load of the object. Due to the friction at the pulley bearings, when the pulley system is in operation, the tension exerted on each rope is not the same; moreover, the tension in the driving rope is not simply the weight of the object divided by the number of working ropes. II. Safety Techniques for Common Lifting Equipment (I) Jacks A jack is a simple lifting tool that allows heavier objects to be lifted or moved using relatively little force; it can also be used to correct any installation errors in equipment and components that have already been placed in position. Place it under the object while working; no additional auxiliary tools such as pulley systems or steel cables are required ； Based on their structure, they can be divided into rack type, screw type, hydraulic type, and others. When using a jack, it must be installed vertically on a solid and reliable base; the bottom should be padded with sleepers, and wooden planks should also be placed at the top to prevent objects from sliding and causing accidents. When lifting large and heavy horizontal objects, one end should be lifted first; the slope should not exceed 3° (1∶20), and shims should be placed in the gap between the side of the object and the ground. When using two or more jacks to work simultaneously, they should be of the same specification, and the load carried by each jack should be less than 50% of the jack’s rated load. When using a hydraulic jack, be sure to remove air from it. Both screw-type and rack-type jacks should be equipped with limiting devices to prevent the screw or rack from coming completely out. When in use, they should be operated at the maximum lifting height specified; if no such specification exists, the lifting height shall not exceed 75% of the total length of the screw or rack. When the teeth of the screw or nut and the rack are worn, the operating conditions should be reduced; if the wear exceeds 20%, they should be discarded. (II) Manual hoists and electric hoists: A manual hoist, also known as a chain block or monkey hook, is a lightweight and labor-saving lifting tool. When used in conjunction with a tripod, it can be used to lift medium and small-sized equipment or to move equipment over short distances; it can be used in both vertical and horizontal positions. When in use, care should be taken to prevent the chain from jumping out of the groove on the wheel, and to avoid the hook from flipping over when it is horizontal. Pulling the chain does not allow arbitrary increase in the number of people; forceful pulling is not permitted. During operation, first attach the hook, pull the chain in the opposite direction to loosen the lifting chain, thereby allowing the manual hoist to achieve its maximum lifting distance. After the lifting chain is under load, it is necessary to check whether the engagement between the chain and the sprocket is proper and whether the self-locking function is effective. If the weight of the heavy object is unknown, as long as one person can pull it, work can proceed ； If one person cannot pull it, the reason must be investigated; two or more people should not force it. Electric hoists use wire ropes as lifting rigging, while manual hoists use chains as lifting rigging. Electric hoists generally have a lifting capacity of 0.5 to 5 tons and a lifting height of 8 meters. Its range of application is broader than that of a manual hoist. Electric hoists must be equipped with upper and lower limit safety devices to prevent the hook from being too high or too low. (III) Winch A winch, also known as a capstan, can be used to lift and pull equipment; it can be used independently or installed on other lifting machinery as a power source, and is therefore widely applied. Based on the driving method, winches are mainly divided into two types: manually driven and electrically driven. 1. Manual winches: Manual winches usually include capstans and hand-cranked winches. Due to its low lifting capacity and high labor intensity, it is often used in areas without power supply. A winch operates on the principle of levers, and its structural principle is shown in the figure below. When in use, first wind the steel wire rope 4 to 6 times around the drum. A dedicated person should tighten the pull rope, and the steel wire rope should be kept tidy at all times to prevent it from getting stuck. After the operation is complete, use an iron rod to secure the push rod to prevent accidental rotation that could cause injury. A hand-cranked winch should be equipped with a ratchet stop and a friction brake to prevent it from reversing due to being pulled by heavy loads. 2. Electric winch: Electric winches have greater pulling force than manual winches, offer higher speeds, and are safe and easy to operate. They are widely used for lifting, loading/unloading, or other towing tasks. They consist of a base, drum, reducer, electromagnetic brake, motor, and other components. The lifting capacity is generally 1 to 10 tons. An electric winch, when used in combination with a mast and pulley block, can lift heavy and large equipment. The brake of the electric winch is a weight brake, with an electromagnetic release used to release the brake. Cheek brakes are generally used, as they can handle large rotational torques, making them particularly suitable for various types of cranes. This type of brake uses weight to apply the brake force, an electromagnet or electric motor-driven release mechanism to release it, and an adjustment screw to precisely set the brake clearance. To allow adjustment even after the brake pads wear out, the linkage connecting the two brake pads can also be adjusted, as shown in the figure below (omitted). The fixation of the brake wheel to the shaft should be checked every 3 to 6 months to prevent accidents caused by the separation of the brake wheel from the shaft. The fixation of the winch is crucial for ensuring the safety and reliability of the lifting operation; it should be secured using specialized anchor piles, with wire ropes used on each side to prevent lateral movement or toppling. The winch should be installed outside the lifting area, in a flat location where it is easy to observe the movement of the items being lifted or moved. The steel wire rope should be horizontal, wound in from below the drum, and as perpendicular to the drum’s axis as possible. Only in this way can the steel wires be arranged neatly, without overlapping or pressing against each other. If necessary, a guide pulley can be placed right in front of the winch; this pulley should be kept at a certain distance L from the drum, so that the maximum deviation angle of the steel wires does not exceed 1.5°–2°. This prevents excessive wear on the edges of the guide pulley’s groove, thereby extending its service life. Let the length of the reel be a meters. From triangle ABC, we have: L/0.5a = cot a; a ≥ 1.5° for reels without grooves, and a ≥ 2° for reels with grooves. If a = 2°, then L/0.5a = cot ≈ 29, so L = 15a meters. Assuming the length a of the drum is 0.8 to 1.2 meters, the distance from the guide pulley to the drum should be 12 to 18 meters. When the winch is used in conjunction with a mast, the distance between its installation location and the mast should be twice the height of the mast. The following precautions should be observed when using an electric winch: (1) Regularly inspect the electrical wiring; in particular, the brakes must be safe and reliable, and the enclosure should show no signs of electrical leakage. (2) Regularly apply lubricant; gearboxes are generally filled with No. 30 engine oil, while sliding bearings are lubricated with yellow glycerin. (3) The sound when the gears are meshing should be normal; if there is any noise, the machine should be stopped for inspection and repair. (4) When the wire rope on the drum reaches the maximum lifting height, at least 3 safety turns should remain. (5) When multiple winches are operating simultaneously, unified command and synchronized operation are required. (6) The winch should be covered with a rain shelter to protect it from rain, and its base should be raised using wooden planks to prevent moisture. (7) When selecting a winch, it is necessary to ensure that the operational requirements are met by the winch’s basic parameters, and overloading must be avoided. Selection of winches: When selecting a winch, it is necessary to calculate the following operational parameters, primarily traction force, the speed of the steel wire rope, and the rope capacity of the drum. (1) Wire rope speed: refers to the lifting speed of the wire rope, calculated using the following formula: V = π × D′n × i / minute, or V = π × D′n × revolutions. Here, V represents the wire rope speed (meters/minute) ； D′———Calculated diameter of the drum wrapped with steel wire rope (meters) ； D′＝D＋（2m－1）d ； D———Diameter (meters) ； d———Diameter of the steel wire rope (meters) ； m———Number of layers in which the wire rope is wound around the drum ； n_elec——motor speed (revolutions per minute) ； n rev——roll drum speed (rev/min) ； i———Mechanization. (2) Cable capacity of the drum: refers to the length of steel wire rope that can be wound on the drum. L = 2π(D1 + D2 + D3 + …… + Dm), because: D1 = D + d, D2 = D + 3d, and Dm = D + (2m – 1)·d. Therefore, L = Zπ{mD + d[1 + 3 + 5 + …… + (2m – 1)]}. (3) Winch pulling force: This refers to the actual pulling force exerted by the steel wire rope when it is wound around the drum a certain number of times. The actual pulling force is sometimes the same as the rated pulling force; when there are fewer layers of wire rope, it is slightly greater than the rated pulling force. (IV) Masts: Due to their simple structure, low manufacturing costs, and ease of installation and removal, when equipped with electric winches, masts can be used for lifting construction components and equipment during installation work. Therefore, various lifting masts are widely used at construction sites. Its disadvantage is that a cable needs to be installed. Masts can be made of wooden and metallic materials (steel pipes, shaped steel, steel plates). Common types include single-leg, A-frame, rocker arm, and cantilever masts. 1. Wooden mast. Wooden masts are made from straight and sturdy logs, and they are generally used for light lifting tasks with a lifting capacity of 3 to 5 tons and a lifting height of 8 to 12 meters. When using independent wooden masts, it is important to note the following: ① Do not use rotten or damaged logs. ②The steel wire rope holding the pulley must be wrapped twice around the upper part of the mast before falling onto the crossbeam support. ③The number of cables depends on the height of the mast and the amount of load; generally, it should not be less than 5 to 6 cables. ④For the connection between the cable and the mast, as well as between the pulley rope and the mast, it is better if the distance between these two points is as small as possible. If there is a distance, a bending moment will be generated. The image below shows the correct binding method. The diagram below shows the incorrect method; the three forces do not converge at a single point. In addition to the pressure N, a bending moment M is also generated, which reduces the lifting capacity of the mast. ⑤A certain tilt angle (not exceeding 10°) should be maintained so that the object does not collide with the mast during lifting. (2) Steel pipe independent mast. Masts are made of steel pipes, and to enhance their lifting capacity, angle steel is sometimes welded around the exterior of the steel pipes. Angle steel is also used to extend the mast, with its length generally being twice the diameter of the mast. It is generally used for lifting tasks up to 30 meters in height, with a lifting capacity of up to 30 tons and a lifting height of 30 meters. When a mast is used, a guide pulley is tied to the base of the mast, through which the lifting steel wire rope is guided toward the winch. Ropes are also tied to the base of the mast to secure its position. When tying the cables and the lifting pulley block, efforts should be made to ensure that the force acting on the cables and the force acting on the lifting pulley block intersect at a point on the center line of the steel pipe. In this way, the mast does not have to bear, or only has to bear, a small bending moment. (3) Truss-type independent mast (also known as trapezoidal-type independent mast). This type of mast is welded from angle steel or steel plates, with a square cross-section in most cases. Four diagonal struts of smaller size at the four corners form a single unit; it is constructed as a series of sections, the length of each section being determined by transportation considerations. The sections are connected to one another using connectors and bolts. Usually, except for the first and last sections, all the intermediate sections have equal lengths. The number of intermediate sections used determines the height of the mast, while the cross-sections of the first and last sections taper off to form conical shapes. A steel plate is welded to the top of the first section for securing cables ； The tail section base is sometimes designed as a ball-joint type to facilitate changes in the tilt angle of the mast during operation. The maximum lifting capacity of such masts can exceed 100 tons, and the lifting height can reach over 50 meters. (4) Herringbone mast. Generally, two logs are tied together with steel wire ropes, which is why it is called \"two logs tied together\". The angle between the two rods is usually 30°. It can also be made of steel pipes. The bottom of one of the rods is equipped with a guide pulley, through which the lifting rope is led to the winch. Another steel wire rope is used to secure the two feet and connect them to the ground anchor, so as to ensure stability at the bottom of the A-frame mast during lifting. At the intersection of the masts, hemp ropes or steel cables are used for binding depending on the lifting load; 20 to 40 loops are tied, in two layers, with each loop pressed tightly together. Wooden wedges are inserted into any gaps, and spikes or fixing blocks are nailed above and below the knots to prevent them from coming loose. The mast is secured at both the front and back with two ropes that form an angle of 45° to 60° with each other. (5) Rocking arm mast. It involves adding a boom that can be raised or swung from side to side to an independent mast; this boom is mounted at the middle or 2/3 height of the independent mast. It enables not only the vertical lifting of heavy objects but also their horizontal movement within the range of motion of the boom. It is also known as a swivel mast, swinging boom, or tilting frame. The rocker arm mast evolved further into masts or cranes; both the main and auxiliary masts are of lattice structure. The lifting height reaches 50 meters, the boom length is 40 meters, and the lifting capacity ranges from 10 to 40 tons. The swing angle of the boom of the rocker mast as it moves from side to side ranges from 120° to 270°. If a cap with an axis is installed at the top of the mast, and a steel plate with holes around its edges is fitted onto this cap to secure the cables, with the length of the suspension rod being 0.6 to 0.8 times that of the main mast, then the suspension rod can rotate 360° freely beneath the cables, or it can rotate along with the mast as it turns. 2. Erection of the mast: After the mast has been assembled on the ground, with cables tied to its top and passed through pulley systems, and guide pulleys secured at its base, the mast can be erected. Generally, there are the following methods: (1) Shifting method. The mast is placed on the ground with its center of gravity at the installation site, and the pulley of the auxiliary mast is connected 1 m to 1.5 m above the mast’s center of gravity. The length of the auxiliary mast is about 2/3 of the mast length. The mast is lifted; since the center of gravity is below the lifting point, the top rises while the lower end slides along the ground to its installation location. The mast has been raised to 60°–70° to position and secure its base, after which the cables are tightened to pull the mast into place. (2) Rotation method. The auxiliary mast lifts the main mast, allowing it to rotate around the lower pivot. The mast must be held in place with guy ropes. Once the mast is raised to an angle of 60°–70° with the ground, it can be verticalized by tightening or loosening the cables; the auxiliary mast’s height is 1/3–1/4 of the main mast’s height. (3) Right-angle method. Also known as the toppling method. The auxiliary mast is placed at the base of the main mast and perpendicular to it; by knocking over the auxiliary mast, the main mast is lifted. During the tilting process, it is necessary to constantly hold the ropes on both sides of the mast to maintain its stability; the base of the mast should be secured with a guide rope to prevent it from sliding. The height of the auxiliary mast should be 1/3 to 1/4 of the mast’s own height. Masts that are not very tall can usually be installed manually by hand, without the need for additional auxiliary masts. When moving the mast, it is important to ensure that the angle of inclination of the mast in the direction of movement does not exceed 15°–20°. When moving the cables, this should be done one by one, under the unified command of a designated person. 3. Checking methods for wooden masts and steel tube masts: Since lattice masts are manufactured after being designed, no checking is required. Below, only the checking methods for wooden masts and steel pipe masts are described. (5) Ground anchors and cables 1. Ground anchors Ground anchors are used to secure cables, winches, guide pulleys, etc., and should be installed in accordance with the design requirements. Generally, for ground anchors, a pit is dug, a crossbeam is buried, and the soil backfill must be compacted in layers; the cable of the ground anchor is led out of the ground in a straight line ； Buried beams, if made of wood, must be free from insect damage, cracks, and decay. Earth anchors shall only be subjected to forces in the specified direction to ensure safe use. Horizontal ground anchors and pile-type ground anchors are two commonly used types of ground anchors at construction sites. (1) Horizontal ground anchor. It involves laying the crossbeam on the bottom of a pre-dug pit; one end of the rope is pulled out through a groove at the front of the pit, buried, and then the area is filled in with soil and compacted to complete the structure. The dimensions and burial depth of the crossbeam should be determined based on the stress experienced by the ground anchor and the properties of the soil; generally, the burial depth ranges from 1.5 m to 3.5 m. Beams can be made of logs, square timbers, steel tube bundles, channel steel, or I-beams. The rope is tied to one or two points on the crossbeam. The tensile force exerted on a horizontal anchor can be broken down into a vertical upward component and a horizontal component, resulting in an upward pulling force. The force is used to keep the pressure on the soil from exceeding its allowable lateral pressure; therefore, vertical baffles can be used to increase the compressed area and reduce the lateral pressure on the soil. The backfill soil and stones must be compacted; the diameter d of the piles refers to their smallest diameter. (2) Pile-type ground anchor. It involves driving angle steel, round steel, steel pipes, section steel, or logs vertically or at an angle (tilted in the direction opposite to the applied force) into the soil; the soil’s grip and stabilizing effect on the piles enables them to withstand certain tensile forces. Although its load-bearing capacity is low, it is easy to use and saves effort and time, which is why it is widely used. The length of the piles is usually 1.5m to 2m, with a penetration depth of 1.2m to 1.5m; the steel wire for root formation is attached at a height of about 30mm above the ground surface. To enhance the anchoring strength of the pile, a long retaining beam is buried close to the pile at a depth of 0.4m to 0.9m above the ground level in front of it. Depending on the soil conditions and load, dual, triple, or multiple units are used. III. Safety Technology for Common Lifting Equipment Any lifting machinery consists of two main parts: the working mechanism and the metal structure. The working mechanism is used to carry out a certain predetermined action, while the metal structure serves as the supporting framework. The working mechanisms include four main types: lifting, rotating, luffing, and traveling. To meet various different operating conditions, different types of cranes have been developed. The simplest cranes are equipped only with a lifting mechanism; to expand the working area and increase mobility, other mechanisms such as rotation, luffing, and traveling are added. (1) Main components of a crane A crane is primarily composed of the following basic components: (1) Power unit: prime movers such as internal combustion engines or electric motors. (2) Braking devices: electromagnetic brakes, hydraulic push-rod brakes, electro-hydraulic brakes, mechanical belt brakes, etc. (3) Transmission devices: gear transmission (straight, helical, herringbone gears), worm gear transmission, chain transmission, screw transmission, and hydraulic transmission, etc. (4) Working device: The working devices of the lifting mechanism include the hook sleeve, lifting rope, pulley block, and lifting drum. Rotating machinery are rotating support devices. The speed change mechanism is part of the boom system (rope-driven luffing, which includes the luffing rope pulley block and the luffing drum). The moving mechanism consists of traveling wheels, the frame, track plates, and roller wheels, etc. (5) Operating systems: mechanical, electrical, hydraulic, and pneumatic operating systems, etc. In addition to the main components mentioned above, there are various safety devices such as overload limiters, amplitude indicators, travel limiters, torque overload limiters, protective devices, as well as various interlocked safety switches and light and sound alarms. (II) Several common types of cranes 1. Bridge crane: This type of crane is mainly used in workshops and warehouses; it can also be used in outdoor warehouses, but an elevated structure is required to lay the tracks. Its advantage is that it does not take up floor space in the workplace and does not hinder traffic on the work floor. Its working range is a rectangle, with the width being the crane’s width and the length being the maximum traveling distance. It consists of a main beam, a trolley that moves on the main beam, and other auxiliary components such as driving mechanisms. Since it operates in the air, it is closely related to equipment and personnel on the ground; especially when lifting molten steel, improper operation or sudden damage to the winch components can lead to serious accidents. Bridge cranes shall undergo a static load test with a load exceeding 25% of their rated load capacity; the load should be lifted by about 100 mm and held in that position for 10 minutes. The deflection of the main beam during this test should be checked, and if any permanent deformation is detected, it indicates that the strength of the main beam is insufficient. The deformation of the main beam must be less than 1/700 of the span. The ratio of the main beam’s height H to its span L should meet the following requirements: for truss structures, H/L ≥ 1/4–1/16; for plate-frame structures, H/L ≥ 1/15–1/17; for box-shaped structures, H/L ≥ 1/20. The ratio of the span to the distance between the large wheels should be less than 7, otherwise it may lead to the wheels getting stuck in the track or the crane derailing. Safety devices that a bridge crane should have: (1) Car stop frames at both ends of the tracks: Buffer blocks are installed at each end of the main beam to reduce the impact when the crane moves to its extreme positions. (2) Automatic switch to limit the car’s travel distance: This can be omitted in normal operations and is used only in demanding tasks such as cranes in steel plants; the phenomenon of the car exceeding its limit position is controlled by buffers. Ordinary buffers are made of hardwood. (3) A hook limiter is installed in the winch mechanism: it prevents the hook from reaching the maximum allowable height; if the hook exceeds this limit, the current is interrupted and the load stops automatically. 2. Crawler cranes are widely used. Their advantages include high flexibility and ease of use; they can rotate 360° in any direction while carrying a load, and they can also move forward while carrying a load on flat and solid surfaces ； It moves at a slow speed and generally operates within a limited range, making it suitable for use in areas where there is a high concentration of work tasks; it is also the main method used for lifting. Crawler cranes have less stability because the distance between their two tracks is small, which limits the range of movement of the crane’s center of gravity. When using them, it is necessary to follow the operating procedures and avoid lifting loads beyond the specified capacity. Crawler cranes have a low traveling speed, and the tracks exert high pressure on the ground; therefore, they require good roads to move on. A crawler crane is composed of components such as a chassis, a slewing platform, an engine, a winch, a pulley block, a boom, a counterweight, and tracks. The stability of crawler cranes is a key condition for ensuring their safe operation; over 60% of crane accidents are caused by stability issues. The pressure exerted by its tracks on the ground is 0.8–1 Mpa when the vehicle is stationary with no load, 1–1.9 Mpa when it is moving without a load, and 1.7–3 Mpa when lifting a load. Therefore, certain requirements should be met for on-site roads. Usually, the roads are in poor condition during lifting operations, so measures such as sleepers or walkways are used to ensure the safety of crane operations. Common crawler cranes can be selected by referring to the table below. Common numbers of crawler cranes, names and model specifications: W—50 (formerly W—501～505), 10t; West German 952; 30t. W—100 (W—100～1004), 15t; East German VB—162—1, 30t. W—200 (W—2002, electric crane), 50t; Polish KV—1206B, 15t; O—1252, 20t; Japanese 1055BEC, 91t. 3. Tire and truck-mounted cranes: The working mechanism of tire cranes is basically the same as that of crawler cranes; they use special chassis and special tires, while their traveling mechanism is similar to that of truck-mounted cranes. Its advantage is that it operates faster than crawler cranes, is easy to relocate, and can travel on roads. This type of crane is not suitable for working on soft or muddy ground. Its lifting height and lifting capacity are lower than those of crawler cranes, making it suitable for light-duty factories or loading/unloading operations. The domestically produced QL-40 type tire crane currently has a maximum boom length of 42 meters. Its traveling speed is slower than that of road-based cranes but faster than that of crawler cranes, generally ranging from 4.5 to 9 kilometers per hour. A truck crane is a type of crane in which the lifting mechanism is mounted on a vehicle chassis or a special chassis; its driving cabin is separate from the crane operation cabin, and it has the same advantages as tire-mounted cranes. During lifting, braces must be used to ensure stability, thereby increasing stability and reducing the load on the tires; it is generally not advisable to drive with a load. In the vehicle type, when the rotating rod is under full load, an overturning accident can easily occur if the ground is tilted or the subgrade is poor, causing the supports to sink. Furthermore, when lifting at an angle in violation of regulations, a horizontal pulling force is generated as a result, which is a factor that undermines the stability of the crane. Imported Japanese Tannoda ； Gaten 40 tons ； For Japanese Sute 70-ton and 127-ton hydraulic truck cranes, when loaded, the swing range of the turntable from side to side should not exceed 90°. When under full load, it is necessary to avoid lifting heavy objects to high positions while swinging, and driving with the load suspended is not permitted. 4. Gantry cranes are generally used for stacking and loading/unloading operations in outdoor precast component yards; they can lift heavy objects and transport them over short distances. It consists of a main beam, columns, a winch mechanism, a crane mechanism, and other components. The span varies depending on factors such as the lifting capacity, and is generally between 5m and 30m; the door height ranges from 5m to 10m. To reduce the span, extended arms can be added at one or both ends. When the span is very small while the overhang is quite long, its stability conditions are particularly poor and it is prone to toppling; therefore, counterweights must be added to the base frame of the pillar (the wheel frame). The influence of wind force must be taken into account when calculating stability. For stability calculations, in a stationary state, the test load should be 1.5 times the weight. During operation, a walking safety device that prevents overturning accidents even under a 25% overload and wind pressures of 0.05 Mpa (0.025 Mpa at the coast) must be installed; track clamps are required to hold the lifting frame firmly on the tracks when not in use. 5. Tower cranes: Tower cranes are a type of crane with a vertical tower and a rotating boom; depending on their structure, they can be divided into lower-swinging and upper-swinging types. Tower crane with a tall tower body. For ease of transportation and installation, the tower body can be designed as a self-extending type; during use, its sections are attached to the building to enhance stability. The working characteristic of this crane is its relatively tall tower ； Operations such as walking, lifting, and rotating can be carried out simultaneously. The most prominent major accidents involving this type of crane are tower collapse, arm breakage, and accidents that occur during installation and dismantling. (III) Safety techniques for using cranes: Crane operations can sometimes pose a risk to the safety of the items being lifted as well as to human life due to errors in command, operator performance, or bundling. Therefore, it is necessary to strengthen safety management and follow procedures strictly. (1) The luffing of the boom in crawler cranes is achieved through worm gear reduction, and the main cause of boom collapse accidents is operational errors. First, when operating the boom lift lever (gear shift), the pinion on the worm is moved, which in turn shakes the worm shaft gear; this results in no gear being engaged and thus the boom falling. Second, the boom was removed before it had come to a complete stop (by changing the boom position from the lifting mode to another mode). Therefore, it is important to note that when a crane is lifting heavy loads, efforts should be made to avoid raising or lowering the boom. If such movement is necessary, the boom’s transmission mechanism and brakes must be checked carefully. During routine maintenance, pay attention to the brakes of the boom winch to ensure they are in good working condition; actions should only be taken after confirming this. The load must not exceed the allowable weight limit, and it is strictly prohibited to change the control lever before the boom has settled properly. (2) Crawler cranes should avoid moving while lifting loads as much as possible. If movement is necessary, the boom should be turned so that the tracks are parallel to the ground, and the crane should move slowly, keeping the lifted load no more than 50 cm off the ground. The weight lifted should be kept at 2/3 of the crane’s maximum lifting capacity. (3) When lifting heavy objects, the slings should remain vertical, and the lifting and lowering movements should be smooth; avoid emergency braking or shocks as much as possible. Do not operate the equipment beyond its capacity; if one or two items being lifted are about 15% over their weight limit, special technical measures must be taken in such cases to enable lifting despite the overload, such as adding counterweights to the back of the crane or using cables behind it. When the crane is fully loaded or nearly fully loaded, it is prohibited to perform two actions simultaneously. (4) Cranes must not operate under overhead power lines. When working on one side of an overhead power line, the minimum distance between the crane arm, steel wire rope, or heavy objects and the overhead power line shall be in accordance with Article 9 of the \"Safety Management Regulations for Crane Machinery\" issued by the Ministry of Labor. Transmission line voltage: Below 1 kV, 1–20 kV, 35–110 kV, 154 kV, 220 kV. Minimum allowed distance from the transmission line: 1.5 m, 2 m, 4 m, 5 m, 6 m. Step voltage shock: When a wire breaks and falls to the ground, current flows from the point of impact into the ground, creating a step voltage. Its voltage value gradually decreases as one moves away from the point of impact; within a range of about 20 meters, anyone or any animal that approaches this area will get an electric shock, the severity of which depends on the distance between the feet. Therefore, in the event that a wire breaks or the crane arm comes into contact with high-voltage lines, the driver should first disconnect the crane arm from the power source. If this is not possible, the driver must not leave the crane; those present on site should enclose the dangerous area and have the power supply cut off by the relevant authorities before anyone can leave. If there is no one on site, the driver should calmly close all control levers, then jump down with both feet together (ensuring that the body does not touch the crane body), and continue to move out of the dangerous area by jumping with both feet or one foot at a time (without stepping across). (5) When the crane is operating under full load, the range of rotation of the turntable from side to side should not exceed 90°; lifting horizontally is generally avoided to prevent overturning. When using a truck crane, make sure to secure the legs properly. (6) When operating in ditches or slopes, cranes should maintain a necessary safety distance from them (usually 1.1 to 1.2 times the depth of the ditch or slope) to prevent collapses that could lead to overturning. (7) Self-propelled cranes should operate in a horizontal position; once the crane is parked, the allowable slope shall not exceed 3°. IV. Common Safety Instructions for Lifting, Hoisting, and Transporting Operations (1) Technical Instructions for Vertical Transportation Frames and Hoist Platforms Vertical transportation frames are used to transport construction workers, tools, and materials vertically within scaffolds. Those currently in use include derricks, gantries, and single-pole lifting frames, etc. The hoisting platforms of derricks and gantries should be equipped with reliable safety devices to prevent serious accidents during operation as well as when loading and unloading. The safety devices for hoisting platforms include: 1. The platform stop safety device is a mechanism designed to prevent the platform from falling in case the braking system of the winch fails during loading or unloading; it comes in two forms: a safety strut and a safety hook. The commonly used device at present is a safety bar mechanism, which consists of a safety bar and a safety clip. The safety card also helps to ensure smooth and easy loading and unloading. To ensure safe operation, the brake of the winch and the safety strut for stopping the hook block should be used together, while also paying attention to unified command for lifting and lowering. 2. The safety device for the sling wire rope consists of a steel telescopic “tongue” placed inside a seamless steel tube. Its function is to eject the \"tongue\" from inside the tube the moment the wire rope of the sling basket breaks, allowing it to rest on the crossbar of the derrick or gantry, thereby preventing the sling basket from falling downward. (II) Safety technical instructions for winches 1. In construction, winches are usually installed temporarily, either by using the pre-existing holes in the base or by wrapping steel cables around the base to secure it to an anchor. A pressure plate is added to the rear of the base to ensure that the winch does not slide, shift, or tip over during operation. 2. The end of the wire rope should be led out from below; the center of the drum must be perpendicular to the centerline of the first guide pulley ahead. An open-type pulley shall not be used for the first guide pulley, and the pulleys must be secured with ground anchors – they must not be tied to the vertical transportation frame. 3. The distance between the pulley and the winch should be at least 8–12 meters; for winches over 3 tons, this distance should be greater than 15 meters. The steel wire rope should be wound around both ends of the drum, with an inclination angle not exceeding 1.5 to 2 degrees. 4. To ensure safety, when the lifted load is at its lowest position, the steel wire rope should not be fully unwound from the drum; at least 3 turns of safety coil should remain, in addition to the turns fixed by the clamps. 5. When installing the winch, choose a location that is slightly elevated, has good visibility, and has a solid foundation. Winches installed outdoors should have measures to protect them from rain and impact. Generally, a simple shed is set up; once the shed is in place, it must ensure that the operator can see the lifting and lowering of the lifted objects as well as their location. 6. The electrical control system of the winch should be located next to the operator. Ensure that the settings are reliable and effective to prevent electric shock. 7. The operator of the winch must be trained, pass the assessment, hold a valid certificate to work, and be assigned to a specific machine. A test run should be conducted before operation; it is necessary to check whether the braking equipment is sensitive and reliable, whether the connecting fasteners are tight, and whether the operating conditions and safety devices meet the requirements. Only after confirming everything is in order may the vehicle be started. 8. The winch must not be operated under overload, and the steel wire rope must not drag on the ground during operation. Protective devices should be installed when passing through the channel; it is not allowed for people to step on it or vehicles to drive over it, and it is strictly prohibited for anyone to cross the operating steel wire rope. 9. Precautions for installing ground anchors: (1) Ground anchors should be installed after conducting necessary calculations based on the tension of the cables, taking into account an appropriate safety factor to ensure sufficient anchoring strength. Select the specifications and type of ground anchor based on the calculations and installation conditions. (2) The ground anchor may only be subjected to forces in the specified direction; the direction of the anchoring wire rope should preferably be consistent with the direction of the force acting on the ground anchor. (3) The ground anchor should be buried in a dry area to prevent it from being soaked by rainwater. (4) It is strictly prohibited to use wood that is infested with insects, decayed, or cracked as ground anchors. (5) It is strictly prohibited to use unstable objects on site, utility poles, equipment in operation, pipelines, or structures of unknown tonnage as substitutes for ground anchors. (6) During use, a dedicated person should be assigned to oversee the ground anchor, and regular inspections should be carried out, especially after rain. If any problems are detected, measures should be taken promptly. (III) Safety technical instructions for various types of masts: In addition to strictly complying with the relevant safety production regulations set out in these procedures, various types of masts must also adhere to the following requirements: 1. General requirements (1) Various types of masts should be used within the scope approved by their design. (2) After various masts are assembled, they must undergo static and dynamic load tests; they can only be used after passing these tests. (3) When handling mast materials, lift and place them gently; avoid throwing or dropping them, and store them properly. When moving together in a group of two or more people, the shoulders on both sides and the pace should be synchronized to avoid pinching hands or stepping on feet. (4) When assembling the mast, use thorns to drill holes; it is strictly prohibited to use fingers or screws as substitutes. When tightening and removing screws at high heights, it is advisable to use a wrench, applying appropriate force. (5) When the cable stay crosses a road, its installation height should be no less than 7 meters. A certain safety distance should be maintained between the guy wire and the high-voltage lines. (6) The guy wires should be arranged reasonably to ensure even tension. For the connection of stays to the top of the mast, shackles should be used, while couplers should be used for connections to the ground anchors; the number of stays is determined according to the design specifications. (7) Earth anchors shall be calculated. The use of trees and utility poles is strictly prohibited. If it is indeed necessary to use columns or similar elements, calculations must be carried out and approval from the relevant authorities obtained. After rain, after thawing, and during trial lifts, personnel should be sent to conduct inspections, and measures should be taken promptly if any safety issues are detected. (8) The walkway on the mast must be firm and solid, free of water accumulation on the surface, without any holes underground, and have sufficient bearing capacity. When used on a floor slab, the slab must be calculated and reinforced if necessary. (9) When the mast is moved, it should be tilted slightly forward; adjacent shrouds must be shifted alternately, and it is strictly prohibited to release those within a 180° range at the same time. When walking on the overhead walkway of the mast, the walkway should be securely fixed. (10) Before using various masts, their bases must be properly secured. (11) It is strictly prohibited for operators to tie cables to the boom. For boom pulley blocks that can turn left and right during lifting and lowering, bidirectional connectors should be used at both ends. 2. Single-mast (1) The allowable inclination angle of a single-mast should generally not exceed 5:1 of the ratio of the mast’s length to its horizontal projection. (2) For a single-mast pole used in a fixed location, at least six wind braces are required; for a mast used mobilely, at least eight wind braces are necessary. 3. Cantilever mast (1) For a cantilever mast, its main shaft must be vertical. A cantilevered mast with a back arch; the back arch of the main mast must be installed as required, and its use without this arch is strictly prohibited. (2) When turning the mast, it should be done slowly at first, then faster, and again slowly, with gentle acceleration and braking. (3) When the mast is near full load, it is generally advisable to first turn it before lowering the cantilever, in order to avoid twisting of the main mast. 4. Cross-mast (1) The two poles of the cross-mast should be of equal length and strength; the angle between them should not exceed 30°, and the span should remain relatively fixed. (2) Two guy wires should be installed at the front and three at the back. When lifting sideways beside the mast, additional side guy wires must be installed accordingly. (3) When moving the diagonal mast, the speeds of the two legs should be consistent to avoid jamming or twisting.