Fault analysis of plunger pair of the hottest pure

Fault analysis of pure water hydraulic pump plunger pair

Abstract: the fault phenomenon and causes of pure water hydraulic pump plunger pair are analyzed, and it is pointed out that plastic is not suitable to be used as friction material of swash plate plunger pump plunger pair, and the structure of non guided combined seal is easy to cause eccentric wear

key words: pure water hydraulic pump plunger pair material seal

1 Structure of pure water hydraulic pump

the main driving forces for the rapid growth of the plastic flexible packaging market in the Middle East and Africa include: the rapid growth of population, the acceleration of urbanization, the improvement of food processing technology, the increase of investment in the flexible packaging industry, the large-scale implementation of packaging products, etc. the pump works in seawater containing fine gravel, Its rated working pressure is 3.0 MPa, the maximum working pressure is 6.3 MPa, the actual output flow is 100 L/min, and the pressure loss of the suction pipeline in the application environment is 0.055 MPa, which is very rare in hydraulic pumps. The pump is a point contact valve distribution axial plunger structure, as shown in Figure 1. Rolling bearings are used at each rotating part to separate pressure water and lubricating oil through the combined seal (without guide block) composed of O-ring and fluoroplastic on the plunger

Figure 1 structural diagram of pure water hydraulic pump

due to the use of seals between the plunger and cylinder liner, their mating clearance cannot be too small. If it is too small, the friction between the plunger and the cylinder block will be large, and the return spring will not ensure the reliable return of the plunger. Considering the low viscosity of seawater, poor lubrication, strong corrosion, and containing more pollutants, the plunger pair is made of plastic with good corrosion and wear resistance in addition to stainless steel and copper alloy. However, the thermal conductivity of plastic is poor, and the local heat generated in the friction process will make it expand and deform, which is easy to cause the cylinder plug to jam. In addition, plastic also has the problem of expansion when encountering water, which will cause the change of fitting tolerance. Therefore, the plunger pair is the key to the success or failure of the development of pure water hydraulic pump, and it is also the part with the most failures in the test process

in order to facilitate the investigation of the practical application effects of several plunger pairs, removable bushings are embedded in the cylinder bore

2 failure analysis of plunger pair

2.1 matching scheme of plunger pair

plunger pair adopts the following matching schemes:

a plastic plunger and stainless steel effectiveness subsystem is the relevant software program cylinder liner touched in the process of effectiveness

b stainless steel plunger and plastic cylinder liner

c stainless steel plunger and copper alloy cylinder liner

in addition, the boronizing plunger on the surface of stainless steel and the hard alumina alloy cylinder liner are also considered to be used for the test, but because the boronizing layer on the surface of the treated stainless steel is too brittle, it has no test value at all, so it is discarded

2.2 fault phenomenon and cause

during the running in test, when plastic plunger and stainless steel cylinder liner are used, lubricating oil leaks out of the leakage port at a speed of 1 drop/2 s. When stainless steel plunger and plastic cylinder liner are used, there is a clear impact sound inside the pump at the initial stage. After 1 hour of operation, the output flow characteristics and volumetric efficiency of the pump are basically normal. After disassembling the pump, it was found that the former was due to the fact that the plastic plunger was constantly scraped by the lateral force during the reciprocating motion, which increased the gap between the plunger and the cylinder hole; The latter is due to the size change caused by the water absorption of plastic and the large friction coefficient of steel when moving on plastic, which leads to some plungers being slightly stuck in the eccentric hole between the head of the cylinder and the counterbore. They are not flexible in the low-pressure area, but will suddenly loosen in the high-pressure area, impact the push plate, and make the movement flexible after running in for a period of time

in the loading test, when plastic plunger and stainless steel cylinder liner were used, it was found that the lubricating oil at the pump bearing was polluted by seawater and contained milky white fine powder shortly after operation. When the stainless steel plunger is matched with the plastic cylinder liner, the sealing performance of the pump is good, the lubricating oil is not polluted by seawater, and its flow and volumetric efficiency are relatively stable, as shown in Figure 2. However, after 15 hours of operation, the leakage of the pump was serious, and the flow and volumetric efficiency decreased significantly. The reason for the former is that the seal between the plunger and the cylinder hole is unreliable, which causes seawater to enter the bearing part through the plunger and the cylinder hole. By changing the pre compression shrinkage of the seal again, the situation can be improved. The milky white fine powder in the lubricating oil is the wear product of the head of the plastic plug; For the latter, after the pump was disassembled, it was found that two deep and wide grooves were worn out on the inner wall of the plastic cylinder liner by the edges of the stainless steel plunger

Figure 2

in the above two schemes, some purchased polyoxymethylene plastics also have defects such as cracks and pores, which is also one of the reasons for the failure of the plunger pair

as for the matching of stainless steel plunger and copper alloy cylinder liner, the pump is normal throughout the 35 h working process. After disassembly, it was found that the inner surface of the cylinder liner and the surface of the plunger had uniform gloss, and there were no rust pits and microcracks

3 discussion

when the plunger pair is made of plastic and metal, no matter what kind of material the plunger or cylinder liner is made of, the swashplate plunger pump inevitably has a lateral force. Under the action of this force, the plastic parts will always be scraped by the edges on the metal parts (even if they are well trimmed), and will soon fail

at the same time, due to the poor thermal conductivity, strong thermal expansion and different degrees of water absorption of plastic, it is not suitable to be used as the material of motion pair parts with precise clearance matching. Even if plastic is used, it can only work under low pressure and can only be used as the material of the plunger

in addition, when the plunger pair adopts a combined seal with confidence, dream, struggle and dedication, the matching gap between the plunger and the cylinder liner is large, which will make the central axes of the two parts cannot coincide in the reciprocating process, which is easy to produce eccentric wear, resulting in increased leakage of the pump. Therefore, in this case, it is difficult to design a suitable fit clearance and the compression of the seal

through the above analysis, we can draw the following conclusions:

(1) use seals with small pre compression force and good sealing effect to minimize the fitting clearance and ensure that there is sufficient contact surface between the plunger and the cylinder hole

(2) plastic should not be used as friction material of plunger pair in swashplate piston pump

(3) although the test effect of matching copper alloy with stainless steel is good, if it is immersed in seawater for a long time, enough attention should be paid to its galvanic corrosion damage

(4) the material of the plunger pair is preferably the combination of ceramic and ceramic, or the combination of ceramic and metal