The latest methods for the use and maintenance of hydraulic plunger pumps 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%, regular maintenance is required. Bearings and aged seals need to be replaced, and friction pairs that exceed the fit clearance should be replaced or repaired to restore their performance.

This article mainly focuses on the straight shaft swash plate piston pump and introduces its usage and maintenance methods.

The oil supply form of the hydraulic pump

Straight shaft swash plate piston pumps are divided into two types: pressure-supplied oil type and self-priming oil type. Most pressure oil supply type hydraulic pumps use oil tanks with air pressure. There are also hydraulic pumps that have their own oil supply sub-pumps to provide pressure oil to the oil inlet of the hydraulic pump. The self-priming hydraulic pump has a strong self-priming capacity and does not require external force for oil supply.

For hydraulic oil tanks that are supplied with oil by air pressure, after each start-up of the machine, the hydraulic oil tank must reach the operating air pressure before the machinery can be operated. If the machine is operated when the air pressure in the hydraulic oil tank is insufficient, it will cause the sliding whip inside the hydraulic pump to be pulled off, and it will also cause abnormal wear of the return plate and pressure plate inside the pump body. For the plunger pump supplied with oil by the oil replenishment pump, after 3,000 hours of use, the operator needs to check the plunger pump 1-2 times a day to see if the sound of the hydraulic pump operation is normal. If it is found that the speed of the hydraulic cylinder has decreased or the machine has become stuck, the make-up oil pump should be disassembled for inspection. Check whether there are any scratches on the edge of the impeller and whether the clearance of the internal gear pump is too large.

For self-priming plunger pumps, the oil level in the hydraulic oil tank must 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 the hydraulic oil, the longer the service life of the hydraulic pump.

Bearings for hydraulic pumps

The most important component of a plunger pump is the bearing. If there is clearance in the bearing, it cannot ensure the normal clearance of the three pairs of friction pairs inside the hydraulic pump. At the same time, it will also destroy the thickness of the hydrostatic support oil film of each friction pair, reducing the service life of the plunger pump bearing. According to the information provided by the hydraulic pump manufacturer, the average service life of the bearings is 10,000 hours. If this value is exceeded, new bearings need to be replaced.

The clearance of the disassembled bearings cannot be detected without professional testing instruments. Only visual inspection can be used. If scratches or discoloration are found on the surface of the rollers, they must be replaced.

When replacing bearings, pay attention to the English letters and model of the original bearings. Most bearings of plunger pumps adopt high-load capacity bearings. It is best to purchase products from the original manufacturer and of the original specification. If another brand is to be replaced, consult experienced personnel with bearings to check the table for replacement. The purpose is to maintain the precision grade and load capacity of the bearings.

Inspection and repair of three pairs of friction pairs

3.1 Plunger rod and cylinder block hole

It can be repaired by the following methods:

(1) If the cylinder block is fitted with a copper sleeve, it can be repaired by replacing the copper sleeve. First, adjust the diameter of a group of plunger rods to a uniform size, and then polish the outer diameter with sandpaper of 1000# or higher.

Three methods for installing copper sleeves on cylinder blocks:

(a) The cylinder block is heated for hot mounting or the copper sleeve is frozen and extruded at low temperature for interference fit. (b) It is assembled with Loctite adhesive. This method requires that the outer diameter surface of the copper jacket has grooves. (c) Tap the cylinder bore, process the threads on the outer diameter of the copper sleeve, apply Loctite glue, and then screw it in for assembly.

(2) For the cylinder block and copper sleeve combined by fusion, the repair method is as follows:

(a) Use grinding rods to grind and repair the cylinder bore by hand or mechanical means; (b) Use a coordinate boring machine to rebore the cylinder block holes; (c) Use reamers to repair the cylinder block holes.

(3) Adopt "surface engineering technology", as follows:

(a) Electroplating technology: A layer of hard chromium is plated on the surface of the plunger; (b) Brush plating technology: Brush plating wear-resistant materials on the surface of the plunger; (c) Thermal spraying or arc spraying or electrical spraying: Spraying high-carbon martensitic wear-resistant materials; (d) Laser deposition: High-hardness wear-resistant alloy powder is deposited on the surface of the plunger.

(4) For cylinder blocks without copper sleeves in the cylinder block holes, the material is mostly ductile iron. Amorphous films or coatings are prepared on the inner wall of the cylinder block. It is because of this special substance on the inner wall of the cylinder block hole that a hard-hard friction pair can be formed. If the cylinder block holes are ground blindly and the surface material on the inner wall of the cylinder block holes is worn off, the structural performance of the friction will also change. If the friction pair with the coating removed is forcibly used, the temperature of the friction surface will rise sharply, and the plunger rod will be glued to the cylinder bore.

In addition, a unique film coating is prepared on the surface of the plunger rod. The coating contains anti-friction, wear-resistant and lubricating functions. This set of friction pairs is actually a hard-soft pair. Once the coating is changed, the friction pair with the best matching material will be damaged. To repair these special plunger pumps, they need to be sent to a professional repair shop.

3.2 Sliding boots and swashplate

The sliding friction between the sliding shoe and the swash plate is the most complex pair among the three pairs of friction in the swash plate plunger pump.

Table 1 lists the clearance between the ball head of the plunger rod and the ball socket of the sliding shoe. If the clearance between the plunger and the sliding shoe exceeds the tolerance, the high-pressure oil in the plunger cavity will leak out from the gap between the ball head of the plunger and the sliding shoe, thinning the oil film on the sliding shoe and the swash plate. In severe cases, it may cause the failure of the static pressure support, metal contact and friction between the sliding shoe and the swash plate, erosion and falling off of the sliding shoe, and scratching of the swash plate by the ball head of the plunger. When the ball end of the plunger rod and the ball socket of the sliding shoe exceed the tolerance by 1.5 times, they must be replaced in groups.

The diameters of the plunger rod and cylinder bore are φ16, φ20, φ25, φ30, φ35, and φ40

Standard clearances: 0.015, 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

The standard clearance with the ball socket of the sliding shoe is 0.010, 0.010, 0.015, 0.015, 0.020, 0.020

Limit clearance: 0.30, 0.30, 0.30, 0.35, 0.35

After the swash plate has been in operation for a period of time, the plane of the swash plate will show an inward concave phenomenon. Before using the platform for grinding, the original dimensions and plane hardness should be measured first. After grinding, measure the grinding amount. If it is within 0.18, there will be no hindrance to the use of the plunger pump. If it exceeds 0.2mm, nitriding should be adopted to maintain the original thickness of the nitrided layer.

When grooves are scraped out on the swash plate plane by the plunger ball head, the method of laser deposition of alloy powder can be adopted for repair. Laser cladding technology can not only ensure the bonding strength of the material, but also guarantee the hardness of the remelted material, without completely reducing the hardness of the surrounding structure.

Manual surfacing welding can also be carried out using chromium phase electrodes. The flat surface of the swash plate that has been repaired needs to be re-heat-treated, and it is best to use a nitriding furnace for heat treatment. No matter which method is adopted to repair the swash plate, the original dimensional accuracy, hardness and surface roughness must be restored.

3.3 Repair of the distribution plate and the distribution surface of the cylinder block

There are two forms of distribution plates: planar distribution and spherical distribution.

For the friction pair of spherical distribution, when the scratches on the distribution surface of the cylinder block are relatively shallow, they are repaired by grinding means. When the groove on the distribution surface of the cylinder block is relatively deep, the "surface engineering technology" should be used to fill the groove first, and then grinding should be carried out. Blind grinding should be avoided 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 platforms with relatively high precision.

Before grinding the cylinder block and the distribution plate, the total thickness dimension and the dimension that should be ground off should be measured first, and then compensated onto the adjustment pad. When the grinding volume of the distribution plate is large, it should be re-heat-treated after grinding.

Hardness standards for plunger pump parts

The recommended hardware for the plunger rod is hs84

The recommended hardness of the plunger rod ball head is greater than hs90

The recommended hardness of the swash plate surface is greater than hs90

The recommended hardness of the distribution plate is greater than hs90

After the cylinder block and the distribution plate are repaired, the leakage of the mating surface can be checked by the following method: Apply vaseline oil to the surface of the distribution plate, block the oil drain channel, place the distribution plate flat on a platform or flat glass plate after applying the oil, then place the cylinder block on the distribution plate, and inject diesel into the cylinder holes at intervals, that is, inject oil into one hole and do not inject oil into the other. Observe for more than 4 hours There is no leakage or cross-flow of diesel in the plunger holes, indicating that the cylinder block and the distribution plate have been ground properly.

4 Conclusion

The service life of a plunger pump is related to regular maintenance and care, the quantity and quality of hydraulic oil, and the cleanliness of the oil. Avoiding wear and tear on the friction pairs of the plunger pump caused by particles in the oil is also an effective way to extend the service life of the plunger pump.

When replacing parts during maintenance, it is advisable to use original factor-produced parts as much as possible. These parts may be more expensive than other counterfeit ones, but they offer better quality and stability. If you purchase counterfeit parts at a lower price, it may seem to save costs in the short term, but this brings hidden dangers and may also cause greater harm to the use of the plunger pump.