Compressor training manual

Thread Content

Chapter 1: Working Principle and Use of Air Compressors Section 1: Working Principle After the drive motor is started, it drives the compressor’s crankshaft to rotate through a V-belt; this rotation is converted into reciprocating motion of the piston within the cylinder via a crank-slider mechanism. As the piston moves from the cover side toward the shaft, the cylinder volume increases; the pressure inside the cylinder is lower than atmospheric pressure, and external air enters the cylinder through the filter and the intake valve ； Upon reaching the bottom dead center, the piston moves from the shaft side to the cover side; the intake valve closes, the volume of the cylinder gradually decreases, and the air inside the cylinder is compressed, causing the pressure to rise. When the pressure reaches a certain level, the exhaust valve is pushed open, and the compressed air flows through the pipes into the air storage tank. In this way, the compressor keeps working in a cycle, continuously supplying compressed air to the tank and thus increasing the pressure inside it, so as to obtain the desired amount of compressed air. Section 2: Installation, Startup, Operation, and Shutdown of Air Compressors (1) Placement of the Machine The air compressor should be installed in a location with good air circulation, sufficient lighting, and a flat surface around it, to facilitate operation and maintenance as well as ensure effective air cooling. (II) Checks and preparations before starting up 1. Check whether all parts of the machine are in normal condition and whether any fasteners are loose. 2. Add lubricating oil: Use compressor oil grade 13 in winter and grade 19 in summer; it is advisable to fill the oil to 2/3 of the sight glass level. Note: In areas with low temperatures, prevent the lubricant from freezing. 3. Turn the air compressor fan by hand 2-3 times to check for any obstruction or abnormal noises. 4. Open the gas supply valve on the gas storage tank and set it to the fully open position. 5. For electric air compressors, the electrician determines the starting method; after wiring, start it in a step-by-step manner to check whether the rotation direction of the crankshaft is as indicated by the arrow on the safety cover ； For diesel-powered air compressors, the diesel engine also needs to be inspected and prepared in accordance with the diesel engine’s manual. (III) Starting (1) Start the motor, and pay attention to whether its rotation direction is correct ； (2) After the motor is running properly, carry out routine maintenance work and gradually open the load-reduction valve to enable the air compressor to operate normally. (IV) Precautions during operation (1) Pay attention to the sounds and vibrations of various components ； (2) Make sure to check whether there is sufficient oil in the oil chamber of the oil filler, whether the oil level in the aircraft’s oil tank is within the range indicated by the oil gauge, and whether the oil supply to all components is functioning properly ； (3) Pay attention to checking the readings of electrical meters and the temperature of the motor ； (4) Every two hours of operation of the air compressor, the oil and water in the intermediate cooler and the aftercooler should be drained once ； The oil and water in the wind pack are discharged once per shift. (5) Pay attention to checking the readings of the temperature and pressure gauges in various sections ； ① The lubricating oil pressure is within the range of (1.47~2.45)×105 N/m2, but not lower than 0.981×105 N/m2; ② The maximum discharge temperature of the cooling water shall not exceed 40°C; ③ The oil temperature inside the engine casing shall not exceed 60°C; ④ The exhaust temperature at each stage shall not exceed 160°C; ⑤ The readings on the primary and secondary pressure gauges are within the specified range. (6) When a shutdown is required, it should be done immediately in cases where there is a disruption in the supply of lubricating oil or cooling water, a sudden increase in exhaust pressure, a malfunction of the safety valve, abnormal noises, or any other unusual conditions occur. (5) Parking (1) Gradually close the unloading valve to bring the air compressor into no-load operation (this step may be omitted in case of an emergency stop) ； (2) Cut off the power supply to stop the machine from operating ； (3) Release the pressure at the exhaust pipe of the final stage ； When the machine is shut down for more than ten days, sufficient lubricating oil should be poured into all friction surfaces. Chapter 2: Repair and Assembly of Air Compressors Section 1: Repair and Assembly of the Crankshaft (1) Common Problems with the Crankshaft The crankshaft is an important moving part in piston-type air compressors; it receives power from the electric motor in the form of torque, and converts this power into reciprocating forces that drive the pistons to compress air and perform work. As a result, it is easy to cause uneven wear on the main journal and crank journal, resulting in the roundness and cylindricity of the crankshaft exceeding the specified values ； Crankshaft scratches and gouges ； In severe cases, cracks, bending, torsional deformation, and keyway damage occur. (II) Repair and assembly of the crankshaft: When repairing the crankshaft, it is necessary to eliminate any deformation of the crank journal, as well as restore the correct position of the crank journal relative to the center of the crankshaft. 1. Inspection of crank journal roundness and cylindricity: Generally, before repairs, it is necessary to first check and determine the degree of deformation and bending of the crankshaft. The roundness and cylindricity of the crankshaft journals can be measured using a dial indicator. When determining roundness (see figure), measurements are taken at two mutually perpendicular positions I-I or II-II on a cross-section; the difference between these two values represents the roundness of the journal ； When determining cylindricity, measurements are taken at points I-I and II-II, 8–10 mm away from the shaft shoulder; the difference between these two values represents the approximate cylindricity of the shaft journal. 2. Measurement of crankshaft radius and repair of crankshaft journal wear: The crankshaft radius can be measured using methods such as marking with a height gauge vernier scale platform and measuring piston stroke. The offset of the crank journal centerline from the crankshaft centerline, caused by uneven wear, can be determined using a dial indicator and the platform marking method. When the roundness and cylindricity of the journal exceed the wear limit by a small amount, or when the depth of scratches, gouges, and corrosion reaches 0.12 mm, it can be ground and shaped manually using tools such as fine files, fine sandpaper, and emery stones. When the roundness and cylindricity of the journal exceed the wear limit by a significant amount ； When the journal has significant scratches, abrasions, or obvious corrosion, it can be turned on a lathe or polished on a grinder. When turning or lapping, it should start with the spindle journal; moreover, to ensure that the journals have the same dimensions after turning or lapping, it is best to start with the journal that is more worn. After turning or lapping, the surface of the journal must be smooth with no tool marks; when sliding bearings are used, the surface roughness should not exceed ； When rolling bearings are used, it should not be greater than . When repairing the crankshaft, care should be taken to plug the inner bore of the journal with a plug. After the crankshaft is repaired, wipe it clean with kerosene and then remove the plug. When turning or lapping journal surfaces, it is essential to strictly maintain the radius of curvature so that it matches the journal; it is absolutely forbidden to machine the end face at the location of the roundness. The fillet radius of the bearing shell can be taken as 1.3 times the fillet radius of the shaft journal. Minor scratches on the rounded edges can be removed by manual trimming or mechanical processing. 3. Repair of crankshaft cracks: Cracks in the crankshaft often occur on the journal surfaces. There are slight axial cracks on the journal, which can be ground at the crack sites. If it can be eliminated, it can still be used. Radial cracks are generally not repaired; a new crankshaft should be replaced. This is because the crankshaft is subjected to stress during use, causing cracks to gradually widen, which can lead to severe breakage accidents. 4. Inspection and repair of crankshaft bending and torsion: The bending and torsional deformation of the crankshaft can be checked using a dial indicator. During inspection, bring the dial indicator probes into contact with the journal and rotate the crankshaft slowly. Minor bending or torsional deformations can be eliminated using turning or grinding methods. Larger bending and torsional deformations can be corrected using the cold pressing method. As shown in Figure 4-11, place the crankshaft on the V-shaped brackets of the platform, and position a dial indicator below the area of the crankshaft journal or crankpin journal where pressure is to be applied; it is advisable to arrange the contacts of the dial indicator beneath the journal that is to be under pressure. Then, gradually increase the pressure; the reverse bending amount during crankshaft correction should be somewhat greater than the original bending amount, with it being appropriate that this value does not exceed 1 to 1.5 times the original bending amount. This results in the straightened crankshaft having a slight reverse bend, in order to improve its resistance to bending during future use. When straightening the crankshaft, it is also necessary to use a hammer or other pneumatic tools to apply \"cold working\" to the crankshaft, depending on the direction and degree of deformation. This helps to convert concentrated plastic deformation into dispersed, minor amounts of plastic deformation, while simultaneously creating compressive stresses on the surface of the crankshaft corners, thereby increasing its fatigue resistance. 5. Repair of crankshaft keyways: When the flat keyway is worn, the original groove may be widened by 5%. The keyway width of the shaft and the hub should be the same. The parallelism between the keyway centerline and the axis line, as well as the fit tolerances, shall all comply with the specifications in the technical documents. The working surfaces of the newly installed keys should fit tightly together with even contact ； Gaps should be left in non-working areas as specified. No padding shall be placed between the key and the keyway. The key must not be convex. 6. Assembly requirements for the crankshaft: The parallelism of the crank pin axis to the common axis line of the main journal axes in the direction of the crank radius shall not exceed the specified values ； The surface roughness of the main journal and crank journals, the fit between shafts and holes, as well as the amplitude of vibration of the shafts in the bearings must all meet the quality standards for maintenance (or refer to Sections 4 and 5 of Chapter 3). The balance weights on the crankshaft must be fixed securely; otherwise, it will cause the crankshaft to lose its balance, leading to cylinder knocking accidents. Section 2: Repair and Assembly of Connecting Rods The connecting rod is a component that transmits the thrust acting on the piston to the crankshaft, and converts the rotational motion of the crankshaft into the reciprocating motion of the piston. The rod structure of the 4L-20/8 type piston air compressor is shown in the figure. (1) Common problems with connecting rods: Deformation of the large end of the connecting rod, wear or damage to the separation surface ； Bending or torsional deformation of the connecting rod, cracks and fractures in the transition area from the connecting rod pin to the rod body ； Damage to the threads of the connecting rod bolt ； The bolt may elongate, break, experience loose fit, develop cracks, or suffer excessive permanent deformation. (II) Repair and assembly of connecting rods 1. Repair of wear on the split surface of the connecting rod head: When the wear or damage on the split surface of the connecting rod head is mild, it can be smoothed out by grinding or polished with sand, or it can also be repaired by appropriate scraping. The trimmed separation surfaces must not be skewed and should remain parallel to each other. It can be checked using the coloring method. The contact angle between the bearing surface of the bearing shell and the shaft journal should be between 90° and 120°, and the contact length must be no less than 80% of the length of the bearing shell. 2. Repair of deformed connecting rod big end: The reason for the deformation of the connecting rod big end is the excessive protrusion of the bearing shell, as shown by δ in the figure. If the protrusion height of the bearing shell exceeds 0.15 mm, or even reaches 0.5 mm, deformation will occur when the connecting rod bolts are tightened. Therefore, during assembly, it should be ensured that the protrusion height of the bearing bush does not exceed 0.05~0.15 mm. During repair, its deformation can first be checked on the platform, followed by machining until the separation surface returns to its original level. 3. Repair of rod bending and torsional deformation: Rod bending and torsional deformation can be checked using a rod straightener. Install the linkage on the corrector (as shown in the figure). The three contact points of the corrector should fit tightly against the flat plate; if there is any gap between any of these contact points and the plate, it needs to be corrected. Correction methods for the bending and torsional deformation of connecting rods include cold correction methods and thermal correction methods. The cold correction method involves correcting the workpiece at room temperature, using vices or special plate clamps to strike it for correction, or employing presses, hydraulic machines, or manual bolt devices. Correct the bent and twisted linkage according to the selected force application points and support points. The thermal correction method involves heating the workpiece to be corrected to 250–300°C and using one of the cold correction methods; after thermal correction, quenching and tempering treatment should be carried out. 1) Correction of bent connecting rods: Place the bent connecting rod with the bent surface facing up on two support blocks spaced at an appropriate distance apart (with the middle part hanging in the air). Place shims under the area under pressure and apply force so that the connecting rod deforms in the opposite direction to its original bend by a sufficient amount. Hold this position for a while, then release the pressure and carry out a correction measurement. Link rod bending can also be corrected using a press or by pressure correction (see figure). 2) Correction of rod torsion. Before performing torsional testing on the component, first secure the rod big end cap to the rod big end using bolts. Place shims under the vise jaws in order to hold the end face of the rod big end in place. Then use special pliers to apply force to the torsional area of the rod body, adjusting it until it is in the correct position (see Figure 4-16). 4. Repair of the connecting rod cap: Cracks or breaks in the area where the connecting rod cap meets the rod body should be repaired by replacing it promptly ； The bushing at the connecting rod pin is worn out; it needs to be replaced. 5. Replacement of connecting rod bolts: Connecting rod bolts are generally not repaired; they should be replaced when any of the following conditions is observed during use ； 1) Damage to the threads of the connecting rod bolt or loose fit of the screw shaft ； 2) Cracks appear on the connecting rod bolts ； 3) Excessive residual deformation in the connecting rod bolts (when greater than 2‰). 6. Assembly requirements for connecting rods: The parallelism of the common plane of the axes of the large and small hole bores in the connecting rod body must meet specified requirements, as well as the cylindricity of these holes ； The roughness of the surfaces of the large and small head holes shall not be greater than ； The clearance of the connecting rod bearings, the fit between the shaft sleeves and the crosshead pins, the contact arc surface between the bearing shells and the shaft journals, the contact length, as well as the fasteners, must all meet the requirements specified for maintenance quality standards. Section 3: Repair and Assembly of the Crosshead. The crosshead is the moving component that connects the piston rod to the connecting rod; it moves back and forth within the crosshead slideway of the engine frame, serving a guiding function. The figure shows a crosshead component diagram. One of the threaded holes at the end of the crosshead is connected to the piston rod, and the depth of the thread connection in the piston rod allows for adjusting the clearance at the dead center between the piston and the cylinder head. Tapered holes are provided on both sides for crosshead pins; these pins are fixed to the crosshead using screw keys and fit together with the rod end bushings. 1. Problems that often occur with the crosshead and crosshead pin: (1) Due to wear, the clearance between the crosshead slide and the housing slideway becomes large, resulting in loud noises during operation ； (2) Delamination, separation, grooving, or nodulation of the babbitt layer cast externally on the crosshead ； (3) Cracks appear in the crosshead body or the plate ； (4) The threads connecting the crosshead to the piston rod are worn or loose, and the flange connecting the crosshead body to the piston rod is damaged ； (5) Wear of the crosshead pin hole and the outer diameter of the crosshead pin, with cracks or scratches on the crosshead pin. 2. Repair and assembly of the crosshead and crosshead pin: Slight wear is allowed on the crosshead slide and the frame slideway, provided that no excessive noise is generated during operation, and the clearance between them does not exceed twice the allowable value. The crosshead body and slider should be replaced if they are cracked. When the cast brass layer on the crosshead exterior peels off or comes loose, the original brass layer must be removed, and it must be poured again and machined. Grooves or nodules formed in babbitt can be restored using the scraping method. When the threads of the crosshead and piston rod are worn or become loose, the crosshead and piston rod should be replaced. When the wear of the crosshead pin hole and the crosshead pin exceeds the specifications given in Table 1-8 of Appendix 1, it shall be repaired as a sliding bearing. The crosshead pin should be replaced if it has cracks or scratches. The repaired or replaced crosshead shall meet the following technical requirements: (1) The perpendicularity of the axis of the crosshead pin hole to the axis of the crosshead’s friction surface, as well as the perpendicularity of the supporting surface on which the piston rod fixing nut is mounted to the axis of the crosshead’s friction surface, must not exceed the values specified in Appendix 1-6 ； (2) The cylindricity of the crosshead friction surface, as well as the coaxiality of the axis of the threaded hole for installing the piston rod with respect to the axis of the crosshead friction surface, must not exceed the specified values ； (3) The surface roughness of the crosshead shall not exceed: 0.8 for the friction surface ； The crosshead pin hole is 0.6 ； (4) Use the coloring method to check the distribution of contact points between the crosshead slider and the frame slideway: the contact points should be evenly distributed, with the contact area accounting for 50%–70%. The fit clearance is specified in Table 1-17 of Appendix 1 ； (5) The cylindricity of the friction surface of the crosshead pin shall not exceed the specified values. Roughness of the friction surface of the crosshead pin: 0.2 when D≤150mm, and 0.4 when D>150mm ； (6) During assembly, the crosshead pin, as well as its connecting bolts and locking devices, must all be tightened and secured properly. Section 4: Repair and Assembly of Pistons The piston is the main component of the compression mechanism in piston-type air compressors. The rotational motion of the crankshaft is converted, through the connecting rod, crosshead, and piston rod, into the reciprocating motion of the piston within the cylinder, thereby compressing the air to generate work. For example, the 4L-20/8 type air compressor uses cast iron conical disc pistons. It is connected to the conical surface of the piston rod, as shown in the figure. Piston ring grooves are machined radially on the outer circumference of the piston, and piston rings made of gray cast iron are installed in these grooves to seal the gap between the cylinder wall and the piston; they also serve to distribute oil and conduct heat. The quality of the contact between the piston rings and the cylinder has a significant impact on both cylinder wear and compression ratio; therefore, strict quality standards must be followed during repair and assembly. 1. Common problems and repair/assembly of pistons, piston rings, and piston rods. Under normal conditions, the piston itself does not come into contact with the inner surface of the cylinder (which is smooth); rather, it is the outer diameter of the piston rings that comes into contact with the inner surface of the cylinder and experiences wear – wear occurs only in the grooves where the piston rings are located. Generally, pistons and piston rings are not repaired; they are replaced instead. It should be replaced immediately in the following situations: 1) The piston has defects such as cracks, fractures, severe scarring, a loose plug, or severely worn piston ring grooves. 2) Piston ring broken or with transitional scuffing ； The piston ring has lost its proper elasticity ； The radial wear of the piston ring thickness is greater than 1–2 mm ； The axial wear of the piston ring width is greater than 0.2–0.3 mm ； The axial clearance of the piston ring in the piston ring groove reaches 0.3 mm or exceeds 1.5 times the designed clearance ； There is a gap between the outer surface of the piston ring and the cylinder wall, and this gap’s total length exceeds 50% of the cylinder’s circumference. 3) The piston rod is bent and severely scratched, with wear exceeding 0.3–0.5 mm. For the replaced piston rings, use spare parts provided by the manufacturer or those manufactured according to the manufacturer’s specifications; they should not be replaced more than three times. For further replacement, piston rings with an increased outer diameter should be used. In the event of an accident, piston cracks or damage ； If the piston groove walls collapse or break, a new piston should be replaced. If cylinder wear requires an increase in the inner diameter of the cylinder (boring), the piston must be remade (with the corresponding increase in the outer diameter of the piston and the size of the piston rings). When replacing the piston, follow these procedures: (1) Remove the cylinder head and turn it upside down on the workbench to facilitate inspection and cleaning ； (2) Remove the piston rod from the crosshead and pull the piston out of the cylinder ； (3) Remove the piston rings from the piston using an expander, inspect them, and clean the areas associated with the piston rings ； (4) Adjust the dimensions, prepare the replacement piston rings, remove burrs and sharp edges, and insert the piston using an expander ； (5) Install the piston with the piston rings into the cylinder and connect it to the crosshead ； (6) Place the cylinder head back on and adjust the top/bottom or front/rear dead center clearances. When removing the piston ring from the piston, follow these steps: (1) Insert the short end of the expander into the cut in the piston ring, and press the long end of the expander by hand to cause the short end to expand inside the cut in the piston ring ； (2) Place the three metal plates between the piston ring and the ring ； (3) Move the middle metal plate along the piston, beneath the piston rings, to the middle of the piston ； The other two metal plates remain beneath the ends of the piston rings ； (4) Move the piston ring along the metal block and remove it from the piston. 2. Quality requirements for the repair and assembly of pistons, piston rings, and piston rods: 1) The perpendicularity of the bearing surface that fits with the piston rod to the axis of the piston’s inner bore shall not exceed the values specified in Table 9 of Appendix 1. 2) Coaxiality of the axis of the outer cylindrical surface of the piston with the axis of the matching inner hole of the piston: 0.025 mm when the piston diameter is ≤ 120 mm ； When the piston diameter is >120–250 mm, it is 0.03 mm ； When the piston diameter is >250–500 mm, it is 0.04 mm. 3) The surface roughness of the cylindrical surface of the piston, the sides of the piston groove, the areas where they meet the outer circumference of the piston rod, and the surface in contact with the piston rod shall all be no greater than 1.6. 4) The newly installed piston rings shall meet the following technical requirements: (1) Surface hardness: When D＞500 mm, HB=89～102 ； When D≤500mm, HB=90～107. Under normal circumstances, the hardness of the piston rings should be 10% to 15% higher than that of the cylinder liner ； (2) The surface of the piston ring must be free from casting defects such as cracks, pores, inclusions, and porosity; there must be no scratches at either end of the ring or on its outer cylindrical surface ； (3) Surface roughness: not more than 1.6 for the outer cylindrical surface, and not more than 0.8 for the end face ； (4) The flatness of the piston ring end face shall not exceed the relevant specifications ； (5) Piston rings processed on a magnetic workbench should be demagnetized after processing. 5) The piston rings are inspected for leaks in the cylinder before assembly. There shall be no more than two light leaks around the entire circumference, and the arc length of each leak shall not exceed 25. The total length must not exceed 45. , and the distance from the piston cut is greater than 30. ； The gap at non-contact areas shall not be greater than 0.04 mm. 6) The side clearance and radial clearance between the piston ring and the ring groove shall meet the design specifications. When pressing the piston ring with the hand, the piston should be able to sink completely into the ring groove, forming a recess. 7) When installing the piston rings in the cylinder, their cut positions should be offset from each other, with an offset angle of not less than 120 degrees. . All cuts should be properly offset from the cylinder’s valve ports, oil filling holes, etc. The gap at the cut should meet the design specifications. 8) Piston rods after major repairs or newly replaced shall meet the following requirements: (1) The coaxiality of the axis of the cylindrical surface that fits with the piston hole and the axis of the friction surface of the piston rod, as well as the roundness of the friction surface of the piston rod, shall comply with relevant regulations ； (2) The perpendicularity of the shoulder surface that interacts with the piston to the axis line of the friction surface on the piston rod shall not exceed the values specified in the attached table ； (3) Surface roughness of the piston rod: not more than 0.2 for the friction surface, and not more than 0.8 at the interface with the piston. Section 5: Repair and Assembly of Cylinders. The cylinder is the main component that constitutes the compression volume in piston-type air compressors. The figure shows the double-walled cast iron cylinder of the 4L-20/8 type piston air compressor, which consists of an inner layer and an outer layer; the space between these two layers forms a cooling water jacket for the flowing cooling water. On both sides of the cylinder are cylinder heads 1, and these cylinder heads have valve chambers 7 for installing the intake valve assembly and the exhaust valve assembly. The cylinder head is connected to the cylinder 2 using studs, while the lower part of the cylinder is connected to the engine block with bolts. During operation, the cylinder is subject to high strain and additional thermal stress, which inevitably leads to wear and damage. Common problems with cylinders include scratches, pitting, and wear on their inner surfaces ； The roundness and cylindricity exceed the specified values ； The cylinder block shows cracks, leaks, etc. (1) Repair of cylinders: 1. When there are minor scratches or abrasions on the inner surface of the cylinder, a semi-circular oil stone can be used to grind along the circumferential direction of the cylinder wall by hand, until no noticeable roughness can be felt to the touch. When there are scratches or wear on the inner surface of the cylinder with a depth greater than 0.5 mm, or when the roundness and cylindricity exceed the specified values. Cylinder boring should be carried out, and the following requirements must be followed during this process: the increase in the cylinder diameter shall not exceed 2% of the original designed diameter; the reduction in wall thickness shall not exceed 10% of the original wall thickness; and the increase in piston force resulting from the increased cylinder diameter shall not exceed 10% of the value specified in the original design. The dimension of the bore that is drilled out in the cylinder is generally not more than 2 mm; if it exceeds 2 mm, pistons and piston rings with a larger outer diameter must be used. When boring cylinders, processing can be carried out using a boring machine or a turnmill, depending on the specific conditions. The surface of the machined cylinder will have tool marks, and polishing should be performed if conditions permit. For small-diameter cylinders, boring and grinding are carried out using a vertical drilling machine, ensuring that the center line of the cylinder coincides with the center line of the vertical axis of the drilling machine. When the scratches on the inner surface of the cylinder are deep and replacement is difficult, copper, silver, or similar materials can be welded at those scratches; after scraping and polishing, they can be used as a temporary solution. 2. Repair of cylinder cracks or leaks 1) Repair of cracks on the cylinder surface: At present, there is no fully effective method for repairing cracks on the surface of cylinders. When necessary, low-pressure (below 1 Mpa) air compressors are sometimes repaired using the manual twisting method with copper wire (see figure). The repair process involves first cleaning the inner surface of the cylinder, and then drilling holes of 1.5–2 mm at both ends of the cracks in the cylinder wall; the depth of these holes should be 1.2 times the depth of the cracks, in order to prevent the cracks from spreading further during operation after the repair. Then, using a **small flat chisel, the crack is carved into a swallow-tail groove along its length; the depth of the groove and the width at its bottom can be equal to the diameter of the drill hole, while the width at the top of the groove can be 0.5 mm less than the width at the bottom. The removed dovetail grooves should be cleaned thoroughly. A copper wire of appropriate diameter is selected, and it is gently hammered and twisted using a small hammer. After twisting, the copper wire should not protrude too much above the surface of the cylinder; only enough excess should remain for scraping and polishing. After scraping, fine oilstones can be used for polishing, so that the surface of the copper material is even with the surface of the cylinder wall, with no unevenities. Cylinders with severe cracks should be replaced. Section 6: Disassembly and Assembly of Air Compressors I. Guidelines for Disassembly 1) Release the remaining compressed air, lubricating oil, and water inside the machine ； 2) The disassembled parts should be stored properly, and must not be damaged or lost ； 3) When removing important parts, attention should be paid to their installation positions, and markings must be made accordingly ； 4) When using lifting equipment to remove large items, attention should be paid to their center of gravity to ensure safety ； 5) If it is necessary to disconnect the machine from its foundation, this should be done last to prevent the machine from toppling over and causing an accident. II. Disassembly sequence: 1) First, remove the intake and exhaust pipes, oil pipes, air filter, oil filler, etc ； 2) Remove the suction and exhaust valves at all levels ； 3) Remove the cylinder heads at each level ； 4) Remove the crosshead nut and take off the piston rod together with the piston ； 5) Hang the cylinder, unscrew the nut that connects the cylinder to the chassis, and remove the cylinder along with its base ； 6) Remove the crosshead pin and take out the crosshead ； 7) Unscrew the bolts and nuts, remove the connecting rod, and make sure to place the bearing shells and shims back in their original positions ； 8) Remove the gear oil pump and the large pulley ； 9) Remove the bearing cover and take out the crankshaft (or, without removing the large pulley, remove the large pulley along with the crankshaft). III. Assembly 1) All parts should be cleaned or wiped clean before assembly. When cleaning with kerosene, it is necessary to wait until the kerosene has evaporated before assembling, and apply machine oil to the mating surfaces ； 2) If any of the mating parts show roughness or burrs on the edges, they should be ground smooth ； 3) During assembly, check the clearance between the various main components as well as the clearance at the inner and outer dead centers of the piston ； 4) Before assembly, the large and small end bearings of the connecting rod should be inspected using the coloring method to check their fit with the corresponding parts; the contact area should generally remain at around 75%. The tightness of the two linkage bolts should be consistent ； 5) The assembly sequence is the reverse of the disassembly sequence. Chapter 3: Use and Maintenance of Piston Air Compressors Section 2: Common Faults, Troubleshooting Methods, and Performance Standards of Air Compressors Section 1: Maintenance 1. The filter elements of the air intake filter and silencer should be cleaned regularly: clean them once a month under normal operating conditions (using compressed air), and replace the filter elements after 500 hours of operation. 2. The compressor oil in the crankcase should be replaced regularly: It should be changed one week after the machine starts operating, and thereafter every 500 hours of operation. When changing the oil, make sure to clean out any sediment present in the crankcase. Be sure to change the compressor oil grade (No. 19 or No. 13) as seasons change. 3. The dirt in the gas tank should be removed regularly: clean it at least once a week. Open the drain plug at the bottom of the air storage tank, and let the air compressor operate at a pressure of 0.2 Mpa for a few minutes to discharge waste air under pressure. 4. Check the reliability of the safety valve on a monthly basis. 5. Regular cleaning of the intake and exhaust valves: at least once every six months, remove the valves, eliminate carbon deposits, and clean them thoroughly. 6. Conduct a comprehensive maintenance check on the main machine once a year, inspecting the clearance between the various key moving parts; those that are excessively worn and affect the machine’s performance should be replaced. 7. Regular tensioning of the V-belt: The V-belt becomes loose after being used for a while; to prevent slippage, it should be tensioned in a timely manner. Apply a force of about 3–4 kg with the hand to the middle of the V-belt; a compression of 10–15 mm is appropriate. If the wear is excessive, replace it with a new belt. Common Faults and Troubleshooting Methods
Fault Phenomenon | Cause | Solution
--- | --- | ---
Difficulty in starting the compressor | 1. Fault in the diesel engine or electric motor 2. The gate valve is not fully open 3. Seizure of the compressor’s moving parts | Repair or replace; open the valve to full position; disassemble and inspect
Insufficient exhaust volume | Abnormal pressure | 1. Leaks in the pipelines 2. Insufficient rotation speed 3. Poor sealing of the intake and exhaust valves 4. Clogged air filter 5. Faulty pressure gauge 6. Excessive wear gap in the cylinder piston rings 7. Faulty pressure regulating valve | Check pipeline connections and tighten them; adjust the tension of the V-belt or increase the diesel engine’s rotation speed; repair, clean, or replace relevant components; inspect or replace components; clean, adjust, or replace them
High oil consumption | 1. Oil leaks 2. Clogged breather 3. Excessive wear, breakage, or misalignment of piston rings 4. Excessively high oil level | Check; clean; check, replace, or adjust; lower the oil level as specified
Abnormal noises and vibrations | 1. Unstable installation of the compressor 2. Loose fasteners 3. Severe wear of moving parts 4. Foreign objects inside the cylinder 5. Collision between the piston top surface and the valve plate | Adjust; tighten; repair or replace; disassemble and troubleshoot; adjust the top dead center clearance (by adding shims)
Excessively high exhaust temperature | 1. Leakage in the exhaust valve 2. Poor cooling effect in the cylinder 3. Severe carbon buildup on the valves | Check and eliminate the issue; check the rotation direction of the flywheel and ensure proper ventilation; clean the valves
Excessively high oil temperature | 1. Severe wear of piston rings 2. Too much or too little lubricating oil | Check and replace; adjust the oil level in the oil reservoir to an appropriate level
Compressor getting stuck | 1. Lack of oil supply 2. Contaminated or degraded lubricating oil 3. Excessively high oil temperature | Add oil to the specified amount; change the oil; check and resolve the issue

Section 2: Standards for Good Condition
The standards for a compressor being in good condition are listed in the table below. Air compressor integrity standards – Inspection items and integrity criteria: Remarks 1: Bolts, nuts, washers, split pins, and guards are all present, intact, and properly tightened. 2: Cylinders and valve chambers: Cylinders have no cracks and there is no air leakage; exhaust temperature ； For single-cylinder engines, the temperature should not exceed 190°C; for double-cylinder engines, it should not exceed 160°C. There should be no deposits or carbonized residues in the valve chamber. The valve discs should be free of cracks, fit tightly with their seats, and have uniform spring pressure. The stroke of the valve discs is generally 2–3.5 mm, or as specified by the manufacturer. The wear level of the piston, cylinder, and piston rings, as well as the clearance in the cylinder, should meet the following standards: (1) The clearance and wear levels for the piston and cylinder are specified in Table 1 below. During the water filling test, the valve seat and valve discs must remain in their normal operating state; if no more than 5 drops of water seep through after 3 minutes of filling, the result is considered acceptable. 2. The inspection records are valid for one year. Continued from Table 1: Inspection Item – Goodness Criteria – Remarks. For cylinders and valve chambers: Unit – mm; Cylinder diameter – Standard clearance – Maximum allowable wear level: 80–120, >120–180, >180–260, >260–360, >360–500, >500–630; Corresponding values for clearance and wear level: 0.12–0.25, 0.15–0.30, 0.18–0.35, 0.21–0.40, 0.25–0.46, 0.30–0.52. For piston rings, the side clearance should be 0.05–0.1 mm, and the gap at the cut edge when installed in the cylinder should be 0.4–0.6% of the cylinder diameter. Piston rings must not have any of the following conditions ； ①Fracture or severe scorching ② The side clearance is 2–2.5 times greater than the specified value ③ The cut clearance is 1.5–2.0 times greater than the specified value. (3) The clearance between the piston and the cylinder should generally not exceed the values specified in the table below (or as specified by the manufacturer of the equipment). For air compressors with a capacity of 10 m3/min or less, 20 m3/min, 40 m3/min, or more: Crankshaft end, in mm: 1.2–2.2, 1.5–2.5, 2.5–4.5. Crankshaft end, in mm: 1.5–2.5, 2.0–3.5, 3.0–5.5. The maximum clearance between the crosshead and the slider should not exceed 0.6 mm, with a contact area of at least 60%. 4. The horizontal deviation of the main shaft bearing should not exceed 0.1‰. The maximum clearance of the bearings should not exceed the values specified in the table below: Axis type – Sliding bearing, Rolling bearing: 30–50, >50–80, >80–120, >120–180, >180–260; 0.1, 0.2, 0.24, 0.3, 0.36, 0.1, 0.17, 0.2, 0.25, 0.30. The temperature of sliding bearings should not exceed 65°C, while that of rolling bearings should not exceed 75°C. The axial contact of sliding bearings should cover at least 3/4 of the length of the bearing shell, with the contact arc at the middle of the bearing shell being between 90° and 120°. Continued table: Serial number, Inspection item, Acceptable standard, Remarks. 5. For the transmission mechanism, the end-face clearance of the elastic coupling should be 2–3 mm less than the maximum axial movement of the shaft; the radial displacement should not exceed 0.15 mm, the end-face inclination should not exceed 1.2‰, and the difference between the outer diameter of the rubber ring and the hole diameter should not exceed 2 mm. The pulleys should be aligned parallel to each other, with an axial offset of no more than 2 mm between them; the end-face deflection should not exceed 2‰ of the pulley diameter, and the belt tension should be appropriate. There should be a gap at the bottom of the V-belt and the pulley groove; the number of belts used must comply with the manufacturer’s specifications. For other types of couplings, the end-face gap shall follow the manufacturer’s guidelines. 6 7 The lubrication system cylinders must use compressor oil of qualified quality; in crankcases with crossheads, the oil temperature must not exceed 60 degrees℃ ； The oil temperature in the crankcase without a crosshead should not exceed 70°C, and there should be no oil leakage. The oil pressure should be within the range of (0.785–2.943)×105 N/m². 1. The flash point of the compressor oil should be at least 215°C. 2. There must be a test report for the compressor oil. 8. Safety devices and instruments such as pressure gauges, thermometers, ammeters, voltmeters, and safety valves must be available and reliable, and they should be calibrated regularly. A safety valve must be installed on the air receiver; a gate valve should be placed between the air receiver and the air compressor, with another safety valve installed in front of that valve. The operating pressure of the safety valve should not exceed 10% of the working pressure. The pressure regulator must function reliably. 1. Records must be kept for one year. 2. The depth to which the thermometer’s probe is inserted into the exhaust duct should be at least 1/3 of the duct’s diameter, or as specified by the manufacturer. 9. The air receiver and the air filter, along with the inlet and outlet pipes of the air compressor, should be cleaned no more often than once a year. The air receiver should have access holes and a drain valve. The air filter should be cleaned regularly, at least every three months. For pipes with shorter operating times, the cleaning interval can be extended appropriately. 10. Electric motors and switchgear must meet the required standards for functionality. The grounding system must be proper. Compressors equipped with a cranking device must have interlocks with the electrical starting system. 11. The exhaust pressure produced by the equipment must meet the values specified on the nameplate, and the volume of exhaust gas must be at least 85% of the value specified on the nameplate. Measurement records must be kept for one year. 12. Cleanliness and documentation: The equipment and the machine room must be kept clean, with tools, materials, and spare parts stored neatly. There should be operation logs as well as records of inspections and maintenance. Section 3: Economic Operation of Air Compressors. The economic efficiency of air compressors is typically evaluated using parameters such as overall efficiency η and specific power Nb. The lower the overall efficiency η and specific power Nb, the better the economic performance. To achieve a higher η and a lower Nb, it is necessary to reduce the shaft power N and increase the displacement QP. Meanwhile, shaft power and displacement are related to the actual operating cycle of the air compressor, mechanical efficiency, etc. Therefore, to ensure the economic operation of air compressors, it is necessary to have a reasonable clearance volume, low suction and discharge resistances, good cooling and lubrication, low inlet temperature and humidity, minimize various types of leaks, and establish a scientific management system for compression equipment. I. Reasonable adjustment of the clearance volume: Although the presence of a clearance volume has no effect on the cycle work required to compress 1 cubic meter of gas, the larger this volume is, the smaller the amount of air drawn in by the air compressor at each cycle, and the higher the temperature at the end of inhalation. As a result, the exhaust volume of the air compressor decreases. However, if the clearance is too small, it may cause the piston to collide with the cylinder, leading to mechanical failures. Therefore, the clearance volume must be adjusted to a reasonable range. Different models of air compressors require different clearance volumes. During adjustment, the clearance volume should meet the relevant regulations or the manufacturer’s requirements. II. Reducing intake and exhaust resistance: Intake and exhaust resistance not only increases power consumption and reduces the volume of exhaust gas, but also raises the exhaust temperature. Therefore, efforts should be made to reduce the suction and exhaust resistance. 1. Clean the air filter regularly. After being in use for a while, dust will inevitably accumulate on the air filter, increasing the air intake resistance and affecting suction. It is generally specified that the resistance of metal mesh air filters should be less than 2453 N/m2. Therefore, it should be cleaned regularly, with the cleaning interval not exceeding three months. 2. Maintain the proper operation of the intake and exhaust valves. To ensure the proper functioning of these valves, it is necessary to follow the points below. 1) Ensure tight contact between the valve seat and the valve disc. Before using the intake and exhaust valves, the valve seat and valve disc must be ground properly, and a water filling test should be conducted; the results must meet the requirements specified in Note 1 of item 2 in Table 15-2. The flow channels of the valve seat and valve cover should be polished smoothly to eliminate any irregularities on their surfaces. 2) The valve springs meet the requirements. If the springs are too soft, they will prevent the valves from closing properly, resulting in air leakage; if they are too hard, it will increase the resistance of the valves. Therefore, the hardness of the springs should be appropriate, and the elasticity of each spring should be consistent. 3) Remove carbon deposits promptly. Due to the high temperature and pressure inside the cylinders, lubricating oil tends to oxidize and form carbon deposits. These carbon deposits, along with the dust that enters the cylinder with the air, can easily block the valve passages and intake ducts, increasing flow resistance and thereby raising the cycle work and exhaust temperature. Therefore, the air valve should be removed promptly and placed in kerosene for cleaning. III. Maintain proper cooling of the air compressor. The efficiency of cooling in an air compressor is closely related to power consumption, exhaust volume, and exhaust temperature. The main way to improve the cooling effect is to place the air compressor in a location with good air circulation, sufficient light, and a flat surroundings, so as to facilitate operation and management and ensure effective air cooling. IV. Maintain proper lubrication of the air compressor. Proper lubrication of the air compressor can improve mechanical efficiency. To this end, qualified lubricating oil should be selected in accordance with the regulations ； The amount of lubricating oil must not be excessive, nor must its supply be interrupted; otherwise, it is both wasteful and increases the risk of explosion ； The oil temperature and pressure should meet the relevant requirements ； Make it a habit to regularly clean the oil tanks, oil pipes, oil filters, and oil injectors to ensure smooth oil flow. As mentioned earlier, under high temperature and pressure, lubricating oil tends to oxidize and form carbon deposits. The presence of carbon deposits not only increases airflow resistance but also poses a risk of spontaneous ignition and explosion under high temperature and pressure, thus becoming a source of safety hazards. To this end, piston rings and seals made of filled polytetrafluoroethylene can be used in place of cast iron, and the oil injector can be removed to convert the cylinder’s oil-lubricated operation to a oil-free one. Oil-free lubrication has the following advantages: 1) It reduces power consumption and increases displacement. PTFE filling is a self-lubricating material with excellent corrosion resistance, tolerance to high and low temperatures, wear resistance, and low friction. After the piston rings made from it have undergone some running-in time in the cylinder, the molecules on the surface layer of these rings will move toward the cylinder wall, gradually forming a continuous and firmly bonded layer of polytetrafluoroethylene that provides self-lubrication; the friction coefficient of this layer is only 0.04, which is much lower than the dynamic friction coefficient of 0.07–0.12 that occurs when there is an oil film between cast iron surfaces. Therefore, with oil-free lubrication, the power consumed by the air compressor to overcome friction is much lower than that with oil lubrication. 2) Improve the safety of compressor operation. With oil-free lubrication, oxidation of oil under high temperature and pressure, which leads to the formation of carbon deposits, is avoided, thereby eliminating potential safety hazards. 3) Saving lubricating oil and improving environmental hygiene: Since polytetrafluoroethylene has good self-lubricating properties, there is no need to inject oil into the cylinder. This not only saves a large amount of lubricating oil but also reduces oil mist in the compressor, thereby decreasing the amount of oil separated in the air receiver and improving environmental hygiene as a result. 4) Prolonging the service life of the cylinder: With oil lubrication, the friction between the cylinder and the piston rings is metal-on-metal friction, whereas without oil lubrication, self-lubrication occurs due to the presence of polytetrafluoroethylene. Therefore, when the latter is used, cylinder wear **decreases, extending its lifespan. In the initial stage of switching from oil-lubricated to oil-free lubrication for cylinders, friction, wear, and leakage all increase because a polytetrafluoroethylene transfer film has not yet formed on the cylinder surface. Therefore, 2-3 drops of lubricant should be injected into the cylinder; once a thin film has formed on the cylinder walls, the supply of lubricant should be stopped. V. Reducing leakage losses 1. Reducing leakage losses in air compressors There are two types of leakage in air compressors: internal leakage that occurs inside the cylinders, and external leakage that occurs at the seals of the machine body; each type will be discussed below. 1) Reduce internal leakage. Internal leakage occurs due to gas leakage between the two chambers of the cylinder. There are two reasons for gas leakage: one is severe or uneven wear of the cylinder and piston rings, which creates gaps at the interface between them ； The second is that the cut positions of the individual piston rings are not offset from each other. To this end, the clearance between the piston ring and the cylinder, as well as the degree of wear, should comply with relevant regulations ； When installing piston rings, the positions of the cuts should be staggered, and all cut positions should also be staggered from the valve ports. Before use, the piston ring should also be inspected for light leakage in the cylinder; it is required that there be no more than two areas of light leakage along the entire circumference, with each such area having an arc length of no more than 25°, and the total arc length of all such areas being no more than 45°. The area where light leakage occurs should be at least 30° away from any cuts. 2) Reduce external leakage. External leakage is mainly caused by poor packing sealing. Therefore, it is necessary to conduct regular inspections and adjustments of the filler to ensure good lubrication. The installation positions of each sealing ring must also be correct. 2. Reducing pipeline network leaks: Due to the long length of underground pipelines and the numerous connections, leaks occur frequently, with the average efficiency of pressure pipelines being only 60-65%. Therefore, for the compression main pipe, welding connections should be used ； For the compressor branch piping network, quick connectors are recommended. VI. Strengthen the management of compression equipment 1) Adhere to a regular rotation and maintenance system for pneumatic tools, in order to reduce their compressed air consumption and improve their efficiency of use. 2) Establish a system for shutting off compressed air supply on time. To this end, it is necessary to understand the pattern of compressor consumption and adjust the number of machines that are turned on or off as needed, so that the total exhaust volume of the air compressors matches the total air demand of the pneumatic tools. This prevents the compressors from frequently operating at idle or with insufficient pressure; moreover, discharging compressed air into the atmosphere should not be used as a method to regulate the compressor’s exhaust volume.