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1 Overview: A truck crane is a widely used construction machine. It can move at relatively fast speeds, boasts good mobility and adaptability, does not require an external power source since it has its own power system, can operate in outdoor environments, and is easy to operate with great flexibility. For these reasons, it is extensively utilized in fields such as transportation, urban construction, firefighting, large-scale material handling sites, infrastructure development, and emergency services. The use of hydraulic lifting technology in truck cranes provides high load-bearing capacity, allowing them to operate under conditions of impact, vibration, and harsh environments. Since the actions that the system’s actuators need to perform are relatively simple and the requirements for positional accuracy are low, the system relies mainly on manual operation. For the hydraulic system of Nachi-Fujikoshi piston pumps used in lifting machinery, ensuring reliability and safety in operation is of utmost importance in the design process. A truck crane is based on a suitable truck, to which appropriate lifting components are added to form a complete truck crane; the truck’s own power source is used to power the hydraulic system of the Nachi-Fujikoshi piston pump in this crane ； When the crane is in operation, the car’s tires are not under any stress; instead, the entire car is lifted using four hydraulic support legs, and the various components of the crane are deployed to carry out the lifting task ； When it is necessary to relocate the crane operation site, the various components of the crane must be retracted onto the vehicle, so that the vehicle can return to its normal transportation capacity and be used for relocation. General truck cranes have the following functional requirements: 1) The entire machine must be able to be moved easily alongside the truck, meeting the needs for mobility and flexibility in field operations without the need for a power source ； 2) During lifting operations, the leg mechanism can lift the entire vehicle, raising all of its tires off the ground and protecting them from direct stress caused by the lifting load. Moreover, the supporting position of the hydraulic legs can be maintained for an extended period of time, preventing the issue of weak legs when lifting heavy objects ； 3) It allows for arbitrary adjustment and balancing of the crane arm length and tilt angle within a certain range, to meet various lifting requirements ； 4) Allow the boom to rotate and lock freely within 3600 degrees ； 5) Enable the lifted load to be raised and lowered within a certain speed range, and allow it to stop under load at any position; no slippage occurs when starting movement with a load. 1) Leg mechanism: It lifts the vehicle’s tires off the ground during lifting operations, raising the entire vehicle so that the load does not press on the tires, and it also allows for adjusting the levelness of the vehicle; it usually has a four-leg structure. 2) Boom rotation mechanism: Enables the boom to rotate 360 degrees and can be locked in place at any position. 3) Boom extension mechanism: Allows the boom to be adjusted within a certain range and positioned, thereby changing its working length. It is generally a 3-section or 4-section telescopic sleeve structure. 4) Boom luffing mechanism: allows the boom to be adjusted to any angle between 150 and 800 degrees, thereby changing the tilt angle of the boom. 5) Hook lifting mechanism: Allows the load to be raised and lowered within a specified range, and to come to a stop under load at any position. The lifting and lowering speeds can be adjusted continuously within a certain range. WeChat/QQ official account: Cycloidal hydraulic motor engineer – worth following. 2 Working Principle: The Q2-8 type truck crane is a medium and small-sized crane (with a maximum lifting capacity of 8 tons). Its hydraulic system, which uses Nachi-Fujikoshi piston pumps, is shown in Figures 8-10 as well as in the set of product photos. The operation of this crane is primarily achieved through manual control to enable the independent operation of each cylinder. During lifting operations, it is generally a single action; in a few cases, there is a combined action involving two cylinders. To simplify the structure, the system uses one hydraulic pump to supply fluid to all the actuating elements in series. Under light load conditions, the various series-connected actuating elements can be combined in any way to enable several of them to operate simultaneously, such as extension and rotation, or extension and luffing occurring at the same time. WeChat/QQ official account: Cycloidal hydraulic motor engineer – worth following. In the hydraulic system of car cranes equipped with Nachi-Fujikoshi piston pumps, the power for the hydraulic pumps is provided by the car engine, via a power take-off unit mounted on the chassis gearbox. The hydraulic pump is a high-pressure fixed-displacement gear pump. Since the engine’s speed can be manually adjusted via the throttle, even though it is a fixed-displacement pump, the flow rate it delivers can be controlled within a certain range by adjusting the degree to which the vehicle’s throttle is opened, thereby enabling stepless speed control ； The rated pressure of this pump is 21 MPa, the displacement is 40 min/r, and the rated speed is 1500 r/min ； The hydraulic pump draws oil from the fuel tank through the central swivel joint 9, the switch 10, and the filter 11 ； The pressure oil output is sent in series to each actuator through the swivel joint 9, as well as through the operation of the manual valve assemblies 1 and 2 of the multi-way directional control valve. When the crane is not in use, the hydraulic system of the Nachi-Nisshin plunger pump is in a unloaded state. The specific operations of various components in the hydraulic system of the Nachi Fujikoshi plunger pump are as follows: 1) Leg retraction and extension circuit – This truck crane is equipped with two legs at the front and rear of its chassis, and mechanical mechanisms are used to retract and extend each leg. A hydraulic cylinder is installed on each leg, and the movement of the legs is driven by these hydraulic cylinders. The two front legs and the two rear legs have their extension or retraction controlled respectively by the three-position four-way manual directional control valves A and B in the multi-way directional valve 1. All directional control valves feature an M-type neutral function, and the oil circuits are connected in series. It is crucial to ensure the reliability of each leg when it is extended; therefore, each hydraulic cylinder is equipped with a two-way locking circuit to guarantee that the legs are securely locked, preventing the occurrence of a \"soft leg\" phenomenon during lifting operations or the legs from sliding down on their own while the crane is in motion. At this time, the flow of oil in the system is as follows: from the fuel inlet path of the front legs, through the power take-off box → hydraulic pump → valve A in multi-way directional control valve 1 → into the fuel inlet chambers of the two front leg cylinders ； Oil return path: Oil return chambers of the two front leg cylinders → Valve A in multi-way directional control valve 1 → Neutral position of Valve B → Rotary joint 9 → Neutral positions of Valves C, D, E, and F in multi-way directional control valve 2 → Rotary joint 9 → Oil tank. Oil inlet path for the rear legs: power take-off box → hydraulic pump → neutral position of valve A in multi-way directional control valve 1 → valve B → oil inlet chambers of the two rear leg cylinders ； Oil return path: Oil return chambers of the two rear leg cylinders → neutral position of valve A in multi-way directional control valve 1 → valve B → swivel joint 9 → neutral positions of valves C, D, E, and F in multi-way directional control valve 2 → swivel joint 9 → oil tank. 2) Boom rotation circuit: The boom rotation mechanism uses a hydraulic motor as the actuating element. The hydraulic motor drives the turntable to rotate through a worm gear reducer and a pair of internally meshing gears. Since the speed of the turntable is low, at only 1–3 revolutions per minute, the speed of the hydraulic motor is also low; therefore, there is no need to install a braking circuit for the hydraulic motor. In the system, a three-position four-way manual directional control valve C among the multiple directional control valves 2 is used to control the three operating modes of the turntable: forward rotation, reverse rotation, and locked position. At this time, the flow of oil in the system is as follows: intake path → power take-off box → hydraulic pump → valves A and B in multi-way directional control valve 1 in their neutral position → rotary joint 9 → valve C in multi-way directional control valve 2 → intake chamber of the swing hydraulic motor ； Oil return path: Return chamber of the rotary hydraulic motor → Valve C in multi-way directional control valve 2 → Neutral positions of valves D, E, and F in multi-way directional control valve 2 → Rotary joint 9 → Oil tank. 3) Telescoping circuit: The boom of a crane consists of a basic arm and a telescoping arm; the telescoping arm is fitted inside the basic arm, and a telescoping hydraulic cylinder controlled by a three-way four-position manual directional control valve D is used to drive the extension and retraction of the boom. To prevent the boom from dropping due to its own weight, a balance circuit is provided in the hydraulic system. At this time, the flow of oil in the system is as follows: intake path → power take-off box → hydraulic pump → valves A and B in multi-way directional control valve 1 in their neutral position → rotary joint 9 → valve C in multi-way directional control valve 2 in its neutral position → directional control valve D → intake chamber of the telescopic cylinder ； Oil return path: Return chamber of the telescopic cylinder → Valve D in multi-way directional control valve 2 → Neutral positions of valves E and F in multi-way directional control valve 2 → Rotary joint 9 → Oil tank. 4) Luffing circuit: The luffing of the boom is achieved using a hydraulic cylinder to change the angle of inclination of the boom. The variable amplitude hydraulic cylinder is controlled by the three-position four-way manual directional control valve E. Similarly, to prevent the boom from dropping due to its own weight during luffing operations, a balance circuit is provided in the hydraulic system. At this time, the flow of oil in the system is as follows: intake path → power take-off box → hydraulic pump → valve A (neutral position) → valve B (neutral position) → rotary joint 9 → valve C (neutral position) → valve D (neutral position) → valve E → intake chamber of the luffing cylinder ； Oil return path: Boom cylinder oil return chamber → Valve E → Valve F neutral position → Rotary joint 9 → Oil tank. 5) Lifting and lowering circuit: The lifting mechanism is the main working component of an automobile starter; it consists of a low-speed, high-torque variable-displacement hydraulic motor that drives the winch to function. The forward and reverse rotation of the hydraulic motor are controlled by the three-position four-way manual directional control valve F. The adjustment of the crane’s lifting speed is achieved by changing the speed of the engine, which in turn alters the output flow rate of the hydraulic pump and the input flow rate of the hydraulic motor. A balance circuit is provided in the return oil path of the hydraulic motor to prevent heavy objects from falling freely ； The hydraulic motor is also equipped with a balance circuit that features a one-way throttle valve, as well as a braking circuit made up of single-acting cylinder rods. When the system is not in use, the spring force within these cylinder rods is used to brake the winch, preventing the lifted load from falling ； When the crane lifts a load, taking advantage of the brake’s characteristic of opening after a delay, it is possible to prevent the load from sliding down when the winch is used for lifting. At this time, the flow of oil in the system is as follows: intake path → power take-off box → hydraulic pump → valve A (neutral position) → valve B (neutral position) → swivel joint 9 → valve C (neutral position) → valve D (neutral position) → valve E (neutral position) → valve F → intake chamber of the winch motor ； Oil return path: winch motor oil return chamber → Valve F → Rotating joint 9 → Oil tank. 3 Performance Analysis As can be seen from Figures 8-10, the hydraulic system of this Nachi-Fujikoshi plunger pump consists of basic circuits such as pressure regulation, speed control, direction reversal, locking, balancing, braking, and multi-cylinder unloading. Its performance characteristics are as follows: 1) In the pressure regulation circuit, a safety valve is used to limit the maximum operating pressure of the system, preventing overload and providing safety protection for the crane during lifting operations with excessive loads. 2) In the speed control circuit, the opening degree of the manual control valve is adjusted to regulate the speed of the workpiece mechanisms (except for the lifting mechanism), which is convenient and flexible, fully reflecting the people-oriented approach of allowing humans to directly operate the equipment. 3) In the locking circuit, a two-way hydraulic lock composed of liquid-controlled check valves is used to lock the front and rear legs in place, ensuring reliable and safe operation. This prevents any leg from becoming weak during the entire lifting process. Even in cases where the engine stalls or the hydraulic pipes rupture, the two-way hydraulic lock continues to function properly for an extended period of time. 4) In the balance circuit, an improved one-way hydraulic control sequence valve is used as the balance valve to prevent the structure from descending due to the weight of the load during lifting, boom extension/retraction, and luffing operations; it ensures stable and reliable operation. However, the presence of back pressure in one direction results in certain power losses for the system. 5) In the multi-cylinder unloading circuit, a multi-way directional control valve structure is employed; each of the three-position four-way manual directional control valves in this structure has an M-type neutral position function. By connecting these valves in series within the oil circuit, it is possible to make any one of the working mechanisms operate independently ； This series connection structure also allows the mechanisms to operate simultaneously in any combination under light load ； However, using 6 reversing valves connected in series increases the unloading pressure of the hydraulic pump and reduces system efficiency; but since cranes are not machines that operate frequently, these losses have little impact on the system. 6) In the braking circuit, a brake composed of a one-way throttle valve and a single-acting brake cylinder is used; braking is achieved through a properly adjusted spring force. The braking action is fast. Since a hydraulic cylinder is required to compress the spring in order to release the brakes, the release process is slow, which helps prevent the load from sliding when lifting heavy objects, thus ensuring safe lifting. Additionally, in the event of the vehicle engine stalling or a malfunction in the Nachi-Nisshin plunger pump hydraulic system, braking can be initiated quickly to prevent the lifted load from falling.