Application and Maintenance of Hydraulic Plunger Pump in Construction Machinery

The efficiency of the hydraulic system of construction machinery mainly depends on the volumetric efficiency of the hydraulic pump. When the volumetric efficiency drops to 72%, routine maintenance is required, such as replacing bearings and aging seals, and replacing or repairing friction pairs that exceed the fit gap to restore their performance.
This article mainly introduces the use and maintenance methods of the straight axis swashplate plunger pump.
1. Oil supply form of hydraulic pump
The straight axis swashplate plunger pump is divided into two types: pressure oil supply type and self priming type. Pressure oil supply hydraulic pumps mostly use oil tanks with air pressure, and there are also hydraulic pumps with their own makeup pumps that provide pressure oil to the inlet of the hydraulic pump. The self priming hydraulic pump has a strong self priming ability and does not require external oil supply.
The hydraulic oil tank that relies on air pressure for oil supply must wait for the hydraulic oil tank to reach the operating air pressure before operating the machinery after each start of the machine. If the air pressure in the hydraulic oil tank is insufficient and the machine is used, it will cause pulling off of the sliding whip inside the hydraulic pump, resulting in abnormal wear of the return plate and pressure plate inside the pump. After 3000 hours of use, the operator needs to inspect the plunger pump 1-2 times a day to check if the hydraulic pump operates normally with a supplementary oil pump for oil supply. If it is found that the hydraulic cylinder speed drops or the vehicle is stuck, the makeup oil pump should be disassembled and inspected to see if there are scratches on the edge of the impeller and if the clearance between the internal mim powder metallurgy alloy steel gear pump is too large.
For self priming plunger pumps, the oil in the hydraulic oil tank should not be lower than the lower limit of the oil gauge, and a sufficient amount of hydraulic oil should be maintained. The higher the cleanliness of hydraulic oil, the longer the service life of the hydraulic pump.
2 Bearings for hydraulic pumps
The most important component of a plunger pump is the bearing. If the bearing has clearance, it cannot guarantee the normal clearance of the three pairs of friction pairs inside the hydraulic pump, and it will also damage the hydrostatic support oil film thickness of each friction pair, reducing the service life of the plunger pump bearings. According to the information provided by the hydraulic pump manufacturer, the average service life of the bearing is 10000 hours, and if it exceeds this value, it needs to be replaced with a new one.
The clearance of the dismantled bearing cannot be detected without professional testing instruments, and can only be visually inspected. If scratches or discoloration are found on the surface of the roller, it must be replaced.
When replacing bearings, attention should be paid to the English letters and models of the original bearings. Most plunger pump bearings use high load capacity bearings, and it is best to purchase products from the original manufacturer and specifications. If replacing with another brand, experienced personnel should be consulted to check the table and exchange the temperature resistant zirconia ceramic bearings, in order to maintain the accuracy level and load capacity of the bearings.
Inspection and repair of three pairs of friction pairs
3.1 Plunger rod and cylinder body hole
Table 1 shows the replacement standards for plunger pump parts (see Figure 1). When the various clearances listed in the table exceed the tolerance, the following methods can be used to repair them:
(1) If the cylinder body is equipped with a copper sleeve, it can be repaired by replacing the copper sleeve. Firstly, trim the diameter of a set of plunger rods to a uniform size, and then polish the outer diameter with sandpaper of 1000 # or more.
Three methods for installing copper sleeves on the cylinder block:

(a) Cylinder body heating and hot fitting or copper sleeve low-temperature freezing extrusion, interference assembly; (b) Adopting Loctite adhesive assembly, this method requires a groove on the outer diameter surface of the copper jacket; (c) Tapping the cylinder hole, processing the outer diameter of the copper sleeve thread, applying Loctite, and then screwing it into the assembly.
(2) The repair method for the cylinder body and copper sleeve combined with melting and burning is as follows:
(a) Using a grinding rod, manually or mechanically grind and repair the cylinder hole; (b) Use a coordinate boring machine to re bore the cylinder bore; (c) Use a reamer to repair the cylinder bore.
(3) Using “surface engineering technology”, the method is as follows:
(a) Electroplating technology: plating a layer of hard chromium on the surface of the plunger; (b) Brush plating technology: Brush plating wear-resistant materials on the surface of the plunger; (c) Thermal spraying, arc spraying or electric spraying: spraying high carbon martensitic wear-resistant materials; (d) Laser deposition: Depositing high hardness wear-resistant alloy powder on the surface of the plunger.
(4) The material of the cylinder body without copper sleeve is mostly ductile iron, and amorphous thin films or coatings are prepared on the inner wall of the cylinder body. Due to the presence of this special substance on the inner wall of the cylinder block hole, a hard hard friction pair can be formed. If the cylinder block holes are blindly ground and the surface material on the inner wall of the cylinder block holes is removed, the structural performance of the friction will be improved. If the friction pair with the coating removed is forcibly used, the temperature of the friction surface will sharply increase, and the plunger rod and cylinder hole will bond.
In addition, a unique thin film coating is prepared on the surface of the plunger rod, which contains wear reduction, wear resistance, and lubrication functions. This set of friction pairs is actually a hard soft pairing, and once the coating is changed by humans, it will destroy the best matching material of the friction pair. To repair these special plunger pumps, it is necessary to send them to a professional repair shop.
3.2 Sliding shoes and swashplate
The sliding friction between the sliding shoe and the swashplate is the most complex pair among the three pairs of friction pairs in the swashplate plunger pump.
Table 1 lists the clearance between the plunger rod ball head and the sliding shoe ball socket (see Figure 2). If the clearance between the plunger and the sliding shoe exceeds the tolerance, high-pressure oil in the plunger cavity will leak out from the clearance between the plunger ball head and the sliding shoe, causing the oil film between the sliding shoe and the swashplate to become thinner. In severe cases, it can cause the static pressure support to fail, causing metal contact friction between the sliding shoe and the swashplate, causing the sliding shoe to burn and fall off, and the plunger ball head to scratch the swashplate. When the plunger rod ball head and sliding shoe ball socket exceed the tolerance by 1.5 times, they must be replaced in groups.
Diameter of plunger rod and cylinder hole plunger rod φ sixteen φ twenty φ twenty-five φ thirty φ thirty-five φ forty
Standard clearance 0.015 0.025 0.025 0.030 0.035 0.040
Limit clearance 0.040 0.050 0.060 0.070 0.080 0.090
Plunger rod ball head
Standard clearance with sliding shoe ball socket 0.010 0.010 0.015 0.015 0.020 0.020
Limit clearance 0.30 0.30 0.30 0.35 0.35 0.35
After a period of action of the swashplate, the plane of the swashplate will appear concave. Before using platform grinding, the original size and plane hardness should be measured first. After grinding, measure the grinding amount again, if it is within 0.18, it will not hinder the use of the plunger pump; If it exceeds 0.2mm or more, nitriding should be used to maintain the original nitriding layer thickness.
When the plane of the swashplate is scraped out of grooves by the plunger ball head, laser cladding of alloy powder can be used for repair. Laser cladding technology can ensure both the bonding strength of the material and the hardness of the remelted material, without completely reducing the hardness of the surrounding tissue.
It is also advisable to use chromium phase welding rods for manual surfacing. The repaired inclined plate surface needs to be re heat treated, preferably using a nitriding furnace for heat treatment. Regardless of the method used to repair the swashplate, it is necessary to restore the original dimensional accuracy, hardness, and surface roughness.
3.3 Repair of the flow plate and cylinder block flow surface
The port plate has two forms: flat port and spherical port.
The friction pair of spherical flow distribution is repaired by grinding when the scratches on the flow distribution surface of the cylinder block are relatively shallow; When the groove on the cylinder block’s flow surface is deep, “surface engineering technology” should be used to fill the groove before grinding. Blindly grinding is not allowed to prevent the copper layer from thinning or oil leakage from the steel base.
The friction pair in the form of planar flow distribution can be ground on a platform with relatively high accuracy.
Before grinding the cylinder block and port plate, the total thickness and the size that should be ground off should be measured first, and then compensated to the adjustment pad. When the grinding amount of the valve plate is large, it should be reheated after grinding to ensure the hardness of the hardened layer (see Table 2).
Table 2 Hardness Standards for Plunger Pump Parts
Recommended Hardware for Plunger Rod HS84
Recommended hardness of plunger rod ball joint>HS90
Recommended hardness on the surface of the swashplate>HS90
Recommended hardness of valve plate>HS90
After repairing the cylinder block and valve plate, the following method can be used to check the leakage of the mating surface: apply petroleum jelly oil to the valve plate surface, block the oil drain passage, apply the oil, and place the valve plate flat on the platform or flat glass. Then, place the cylinder block on the valve plate, and inject diesel into the cylinder hole. Inject oil at intervals, that is, one hole is filled with oil, and one hole is not filled with oil. Observe for more than 4 hours, and there is no leakage or communication of diesel in the plug hole, The grinding of the cylinder body and the valve plate is qualified.
4 Conclusion
The service life of a plunger pump is related to regular maintenance, the quantity and quality of hydraulic oil, and the cleanliness of the oil. Avoiding particles in the oil from causing wear on the friction pair of the plunger pump is also an effective way to extend the service life of the plunger pump.
When replacing parts during maintenance, it is recommended to use original factory produced parts as much as possible. These parts are sometimes more expensive than other imitation parts, but their quality and stability are better. If you purchase cheap imitation parts, it seems to save costs in the short term, but it may also bring hidden dangers and greater harm to the use of plunger pumps