Review of Research on Axial Piston Pump Motors in the Last 40 Years

The swash plate axial piston pump motor can achieve high efficiency and pressure resistance due to its structural characteristics, so it has become the trump card component in hydraulic technology. However, because it generally has at least four pairs of sliding friction pairs: oil distribution plate-cylinder body, cylinder body-plunger, plunger head-sliding shoe ball socket, sliding shoe-swash plate, the lubrication situation is complicated, so durability becomes Its key indicators are also the biggest gap between domestic products and the world's advanced level.

Compared with ordinary sliding bearings, the relationship between load, pressure distribution, geometry and kinematics acting on the friction pair of axial piston pumps is much more complicated. Due to the coupling of individual sliding points, the plunger has uncertain degrees of freedom in the ball joint and cylinder bore, making the calculation of the frictional contact rather difficult. Take the plunger as an example, although the plunger also has the rotation of the shaft similar to the ordinary sliding bearing, and has axial translation, but it is also affected by a sliding shoe acting on the plunger, which is outside the bearing area (cylinder body). The lateral force makes the friction loss caused by the plunger become the main part of the power loss.

Therefore, experience accumulated on the basis of traditional sliding bearing theory can only be applied to a limited extent. The earliest attempt by van der Kolk (1972) to study the problem of friction between the plunger and the cylinder. He designed and built a swash plate test bench for experiments. However, because the axis of rotation of the swash plate in the experiment coincides with the axis of the plunger, the plunger has no axial movement and is only subjected to a rotational lateral force. Experimentally and theoretically, he avoided the bearing pressure distribution due to the axial movement of the plunger, and simplified the tribological problem to an inclined, external laterally loaded sliding bearing with increased unilateral edge pressure. He paid particular attention to the point where the plunger extends the most (bottom dead center). Measurements of the pressure distribution in the gap showed that pressure buildup mainly occurred in the edge region of the gap. In the part of theoretical research, he used the numerical solution method to solve the Reynolds equation for the first time.

Recognizing the limitations of the van der Kolk test rig, Renius (1974) proposed an improved structure that takes into account the axial movement of the plunger. He used a fully statically loaded measuring sleeve and a compensating plunger to measure pressure and friction separately. This test rig uses an angle-dependent valve control to simulate pumps, motors, or isobaric operation, where the plunger is under pressure both as it retracts and as it extends. In this way, it is possible to experimentally simulate all situations that arise in actual work. Isobaric operation, which does not occur directly in practice, is very suitable for understanding the approximate conditions of the friction occurring on the plunger. He carried out tests with a wide range of parameters, pressures from 15 to 200bar, inclination angles from 0 to 20°, and speeds from 2000 to 100r/min. In addition, he performed special starting trials. He presented his experimental results in the form of classical plain bearing theory, discussing in detail the validity and applicability of similarity quasi-numbers, such as the Sommerfeld number, or the Gümbel-Hersey number, in his tests. The main results he obtained from the experiment are as follows.

1) The sliding friction characteristics of the plunger-cylinder can be described from the driving angle, which proves the validity of the Stribeck curve in the obvious mixed friction region.

2) Demonstrates good availability of the similar quasi-number Gü = ηω / р, where η is the viscosity, ω is the driving speed, and p is the pressure in the plunger bore. Point out that the effect of increased rim pressure, as described by van der Kolk, is moot for plunger-cylinder contact.

3) The friction of the plunger plays a decisive role in the starting characteristics of the motor, which causes the starting loss to reach 13∼16% of the theoretical torque of the motor. Also, large leaks often occur at the shoe, which can be explained by the high rotational friction between the ball head and the plunger.

4) The rotation of the plunger relative to the drive angle does not coincide with the drive rotation at all operating points. From theoretical considerations, it was concluded that relative rotation is detrimental to the frictional properties.

5) The linear movement of the plunger is particularly important in the motor mode for the establishment of the supporting pressure, thereby separating the surfaces of the friction pair, which was confirmed by the test of variable parameters.

6) He found oil trapped in the test, but thought the effect was not significant.

7) The gap between the plunger and the cylinder has been shown to have a great influence on the friction process in the experiment, and it is recommended to be less than 1% of the diameter of the plunger. The lower limit of clearance should be determined by adequate lubrication and not by leakage requirements.

8) He made suggestions on the design of the plunger-cylinder fit: for the pump, use a smooth short plunger without a pressure equalizing groove and with a short guide section, and use a long guide section for the motor. Dowd and Barwell (1974) set up a test rig to study the friction between the plunger and cylinder. The linear motion of the plunger is driven by a cam, regardless of lateral forces. Measurements are based on the constant pressure principle. As an innovation, a metal contact sensor is used: contact is detected by measuring the change in resistance between the friction pairs. They studied the effects of plunger roughness and material pairing to determine that friction does not continue to decrease beyond a certain level of surface roughness.

Regenbogen (1978) used essentially the same experimental setup as Renius. In addition to plungers with sliding shoes, he also studied plungers with ball heads and plungers supported by connecting rods (inclined axis pumps). As a result of the study, he made a series of design suggestions: such as maximum deflection angle, low-cost material pair, plunger clearance and guide length. For the motor, he suggested, a long guide plunger, but could have a break to reduce losses at high speeds. Almost at the same time, Böinghoff (1977) advanced the study of sliding shoes for axial piston machines. He succeeded in theoretically deriving the tilting force of the sliding shoe on the sliding surface of the swash plate, and confirmed it through experiments. The forces acting on the plunger and the ball joint between the plunger and shoe are included in the calculation. According to his research, the elliptical locus of the minimum clearance point between the sliding shoe and the swash plate does not coincide with the elliptical locus of the intersection point of the swash plate plane and the plunger axis. Knowing the relative speed and the change of the clearance under the shoe, the loss flow of the shoe relative to the rotation angle can be calculated.

The experiments of Hooke and Kakoullis (1981) also focused on the shoe-plunger contact. The results of a series of experiments showed that the relative rotation of the plungers decreases with increasing rotational speed of the drive, which was also found by Renius. In addition, the plunger is more inclined to rotate when the pressure is increased because the increase in friction at the ball joint due to the increase in pressure is higher than the increase in the lateral force of the plunger.

Renvert (1981) proposed a variety of test methods to study the low speed and starting characteristics of hydraulic motors. The most commonly used method is forced constant speed rotation, because it can avoid the large dispersion of test results of other methods (starting at constant load, fixed motor shaft, constant flow). The results of his particularly systematic tests were adopted by ISO 4392-1 as the recommended method for measuring motor starting and low speed characteristics. Weiler (1982) studied the influence of the motor plunger structure on the low-speed characteristics by means of experiments and simulations. He conducted detailed studies of friction and leakage at various contact points, comparing the results with simulations. The simulation model reproduces the behavior of the motor fairly well despite some significant simplifications in some parts when it was built. Thus, for the first time, he was able, without testing directly at each plunger, to demonstrate the problem of increased leakage at the shoe at low speeds of the motor and at start-up.

Koehler (1984) studied the pressure distribution in the plunger-cylinder gap due to friction during motor start-up. His experimental setup consisted of a plunger driven by a cylinder and a side force cylinder through which side loads could be freely applied. He created a simulation model that calculates the pressure distribution in the gap, taking into account the bending deformation of the plunger. He proposed that in order to get the best starting and low-speed characteristics, the optimum gap between the plunger and the cylinder must be about 1‰ of the plunger diameter.

Ivantysynova (1985) used the Reynolds and energy equations for the first time to numerically calculate the non-isothermal flow in the gap and compare it with the test results. The energy equation model uses Vogelpohl's dissipation function as the source term. The test setup consisted of a two-bore swash-plate pump whose discharge chambers could be short-circuited by a control valve. Ezato and Ikeya (1986) developed a test rig for the study of plunger-cylinder friction. The lateral force is measured separately from the axial force via a measuring sleeve supported on a rolling bearing, so that only small lateral forces can be applied. The test was carried out in constant voltage mode, focusing on starting and low speed characteristics. The influence of the plunger surface roughness, material and hard surface was studied, the latter was not shown to be applicable at the time of the test. Jacobs (1993) experimented with pump motors by artificially adding pollutant particles and suggested that the combination of an alternative material and a hard surface layer (by physical vapor deposition PVD) can significantly improve the wear characteristics of axial piston pumps and sliding properties. Fang and Shirakashi (1995) conducted a theoretical and experimental study of the axial piston mechanism. Their simulation model, although solving the Reynolds equation for all positions of the plunger stroke, did not consider the effect of dynamic pressure build-up due to pressure discharge. The measurements performed showed a beneficial effect of the relative rotation of the plungers, contrary to what Renius and Regenbogen said.

Donders (1998) used various experiments to study the effect of various friction pairs and applied the obtained knowledge to the design of axial piston mechanism for high water base fluid (HFA). He developed devices for measuring the friction and pressure distribution of plungers and sliding shoes. The test rig for measuring plunger friction consists of a plunger attached to the force transducer housing. The plunger has a wedge-shaped clearance compensating plunger mounted on the bottom of the plunger. In order to simulate the relative movement between the plunger and the cylinder, the cylinder is reciprocated by a crank, and the lateral force acting on the plunger ball head is generated by an external pressure cylinder. Jang, Oberem and vanBebber also used the same test rig, with some minor modifications.

Donders used a special tribometer for the sliding shoe friction test. The swash plate is rotating, and the pressing force is similar to the real machine. The tilt of the plunger was ignored in the test. Tests have shown that the calculated pressure distribution between the sealing protrusions of the shoe can agree very well with the measured data, and it can be expected that the shoe will float due to hydrodynamic forces at high relative speeds.

Donders attempted with some success to derive the losses of the entire machine from the measured losses of the individual friction pairs. However, the facts have proved that in order to accurately simulate the working process of the swash plate machine, it is very important to design a measuring device close to the actual working condition. Especially the complex interaction between the mechanical friction parts of the axial plunger must be taken into account when designing the measuring device.

Manring (1999) used the same measuring sleeve installed on the rolling bearing as Ezato and Ikeya to measure the friction force between the plunger and the cylinder. Here, the swash plate does not rotate, only reciprocating linear motion to generate the stroke of the plunger, so there is no side force to simulate circular motion. Based on the test results, a Stribeck curve approximated by an exponential function is derived for the mixed friction region. The extrusion film effect produced by the concomitant motion and rotation of the plunger is not considered in the model. The low speed region was not tested.

Tanaka (1999) experimentally studied the effect of the stiffness of the plunger and the macroscopic geometry at the end face of the plunger on the starting and friction forces. The test rig uses a hydrostatically supported measuring sleeve similar to the Renius test rig. A less rigid plunger results in lower friction (long guide plunger, measured in the mixed friction zone).

Zhang Yangang (2000) studied measures to improve the low speed and starting characteristics of axial piston machines. He analyzed friction and leakage in motors by means of constant forced rotation. In order to deepen the analysis, he used several test rigs, including Donders' single plunger test rig with movable cylinder liner and fixed lateral force part, and the equivalent minimum speed is equivalent to 5r/min. He quantified the friction and leakage losses he measured in a swash plate motor test: the actual output torque of the motor is only 77% of the theoretical torque, the friction loss between the plunger and the cylinder is 8.7%, and the loss between the plunger and the sliding shoes is 8.7%. 6.1%, 3.8% between the cylinder block and the swash plate, 3.1% between the sliding shoe and the swash plate, and 1.0% for the rest.

Nevoigt (2000) studied the use of hard surfaces to improve the wear resistance of friction pairs of hydraulic components. He used the hydraulic cylinder piston rod to conduct a friction test to investigate the wear and tear.

Liu Ming (2001) and Krull (2001) investigated the plunger with oil-lubricated contacts on an axial plunger machine with the aim of simulating this machine as a vibration-transmitting element. Liu proposed analytical equations describing the individual elements acting on the basis of forces acting in space, while Krull investigated the required values of rigid friction and damping through extensive experiments. For this, he used three different test rigs: Test rig 1, to determine the stiffness of the plunger and cylinder and the damping in between; Test rig 2, the frictional torque in the shoe ball socket; Test rig 3, the stiffness and damping. Knull did not measure axial and tangential friction, but estimated it from Renius' friction measurements. The data obtained by Knull showed that in many cases the plunger was operating in the mixed friction zone and that the pulsating lateral force was not sufficient to dislodge the plunger from the mixed friction zone. Knull attributes the friction at the shoe socket to well-lubricated mixed friction; the coefficient of friction is very close to the known bronze-steel or brass-steel values. Although it remains questionable whether the coefficients of friction and approximate formulas obtained from some measurements on a special test rig are sufficient to accurately reflect the friction characteristics of plungers in real machines, Liu's work shows that using these data is sufficient to An axial piston machine is viewed as a rotary oscillating system. Because friction is based on Renius measurements, it is difficult to guarantee effectiveness in the very low speed range.

Kleist (2002) developed a simulation program to calculate the friction and leakage of the plunger, and solved the relative movement speed of the plunger when the cylinder rotated. The forces acting on the plunger were determined from the steady-state and transient components of the average Reynolds equation for the so-called rough lubrication gap. The AFM model (Average Flow Model) used employs a statistical approach to surface roughness based on the study by Partir and Cheng. In addition, the solid force part is modeled using the contact pressure model of Greenwood and Williamson. Kleist showed that it is very important to consider the load capacity of the surface roughness on the contact of the gap through the asperity peak, especially at low speeds it cannot be neglected. He also discusses a general solution of the energy equation that takes into account the dependence of the temperature in the gap on the pressure buildup, but obtains results that are not necessarily considered in the case of his study, but says that such considerations are useful. In order to test his theoretical model, he built several test rigs, notably an internally supported radial piston pump capable of various tests - friction, temperature, pressure build-up in the gap, a Donders It is a test bench with a movable cylinder and a lateral load on the plunger. In addition to simulating the frictional contact of the plunger-cylinder, he also performed calculations for the shoe-swashplate contact. He points out that the profile of the seal ring surface and any chamfers must be modeled, as this has a significant impact on the calculation results. A calculation that considers all sliding contacts, aborted because it takes too long to calculate. Based on the results of a series of simulations, he proposed an improved design, a long cylinder bore with a long plunger. The above simulation of plunger friction occurs at moderate speed and small inclination angle (750r/min, 15°), which cannot be compared with the harsh working conditions of modern axial piston motors.

Sanchen (2003) continued the work of Kleist by incorporating the dynamic calculation of the pressure buildup in the plunger chamber into the pump motor design software PUMA, so that the forces acting on the swash plate adjustment mechanism or the drive shaft bearing can be output. Low speed (<500r/min) is not considered here. Studies have shown that the process of dynamic pressure build-up in the gap requires special attention if the friction that occurs between the plunger and the cylinder is to be described.

Wieczorek (2000) proposed a simulation model CASPAR to describe the mechanical gap flow of the swash plate. It can calculate the sliding contact between shoe-swash plate, plunger-cylinder and cylinder-distribution plate. The mechanical (kinematics, dynamics) and hydraulic (pressure build-up in the cylinder cavity) effects can be simulated in addition. The lubricating effective surface is not limited to simple basic geometric forms, but can be freely determined within certain limits. Unlike the BHM and PUMA programs developed by Kleist and Sanchen, CASPAR solves the energy equation in addition to the Reynolds equation, allowing non-isothermal processes in the gap to be considered. The program requires knowledge of the temperature and volume of all components that define the gap. The contact forces occurring in the mixed friction zone are described by a simplified model. The result of the calculation is the distribution of pressure and temperature and the leakage of the gap. This work demonstrates the principle feasibility of such calculations and gives some computational examples. This also shows that mixed friction can be considered in the plunger-cylinder contact region. Since only very high rotational speeds (>2000 r/min) were used for the test, the simplified calculation of the contact force was considered reliable.

The work of Olems (2001) focuses on the thermodynamic model of the simulation program CASPAR. He added to this procedure that the heat generated in the plunger gap was transferred to the cylinder block and from there to the leakage oil in the surrounding housing, and the contact force was again described by means of a simplified model. Experiments with temperature sensors installed on the cylinder block of a series of products show that the simulated and measured results are in good agreement. Measurements are expressed relative to swash plate inclination and pressure. The speed and mode of operation are given by the "nominal speed", from the figure it can be seen that the speed n>2000r/min.

Oberem (2002) investigated various frictional parts of axial piston pumps with the goal of developing an axial piston pump and motor for high water base fluid (HFA). His test rig was a further development of Donders' test rig for crank-driven plunger sleeves. Due to the low viscosity of the medium, almost all friction processes take place in the mixed friction region. For the plunger friction test, the high speed is 10-1500r/min, and the low speed is 1-10r/min, all under constant pressure. The dependence of speed and pressure, different plunger lengths and clearances, and the influence of protrusion length and plunger ring groove were tested only in the high speed range. In the low speed range, repeated test results were scattered, which can be attributed to speed fluctuations and failure of the hydrostatic bearing of the measuring sleeve. Since solid friction accounts for a large proportion, the measured friction change, as expected, is a pure Coulomb friction rather than solely dependent on plunger travel. In order to solve the problem of mixed friction, Oberem proposed hardening the surface layer of the part, or replacing it with a friction reducing material, preferably a ceramic base. van Bebber (2003) explored the application of graded carbide layers to axial piston machines. In principle, this process can be used for all friction parts of axial piston machinery, especially it can replace the non-ferrous metals usually used in cylinder block-oil distribution plate and plunger-cylinder block. The gradient hard surface layers HfCg and ZrCg (gradient hafnium carbide and zirconium carbide layers), which he considers particularly promising as alternatives, are characterized by softer surfaces and softer layers in the middle of the layer in thicknesses of a few μm (median value about 4 μm). It is hard and becomes softer at the junction of the layer and the substrate for better adhesion. Difficulties were found in the study to use a hard surface where the plunger-cylinder contact normally has high surface pressures (>50N/mm²). To improve this, he used various FEM tools and BHM programs for his research. At the same time, he carried out the plunger friction test on the existing test rig, and the calculation using BHM only agrees at higher speeds. The plunger edge pressure effect can theoretically be improved by slotting the bottom of the cylinder bore, but it cannot be proven experimentally. Improving frictional conditions and mechanical-hydraulic efficiency is not the main purpose of this study, and the excellent frictional properties of the hard surface system can bring more effects, which can be seen in the gradient layer tests performed on various test benches.

Breuer (2007) used a rigid piezoelectric force sensor as a part of the plunger, and tested the friction force of the plunger on a low-speed motor test bench. Through tests and calculations, the key mechanism of friction generation was revealed, and it was used to improve the plunger. plug design. The design guideline of the plunger mechanism is drawn out through the experiment.

Gels (2011) studied the hard surface of the plunger-cylinder and the corresponding shape. In order to achieve better wear resistance, the friction pair can use a hard-hard combination to replace the traditional hard-soft combination: such as using quenched and tempered steel plus zirconium carbide surface. But the previous running and stage will no longer take place, therefore, it is necessary to process the plunger and the cylinder bore to a certain shape in advance. Through simulation, find out the appropriate shape parameters, and consider the processing technology, and then test on a single plunger test bench, and a complete plunger machine, the results show that the hard-hard friction pair can improve the bearing capacity capability, while the fine form factor increases efficiency.

In addition to studying the friction loss of PVD hard surface in synthetic ester without additives, Enekes (2012) also studied the energy loss of the oil in the pump housing due to the rotation of the cylinder through the CFD method, and usually Improvement measures.

Scharf (2014) continued to study the friction and wear characteristics of the gradient zirconium carbide surface in rapid biodegradation fluid. Tests have proven to significantly reduce friction and improve durability. It can play an auxiliary role by machining the ball arc in the plunger and cylinder hole in advance. By analyzing the lubricating condition in the gap, different ball arc parameters are investigated, and the best shape is found.

It can be seen from the above that, for decades, the working conditions of axial plunger machinery have gone through a continuous research process from simple to complex, from single to comprehensive, and what remains unchanged is that the combination of theory Test, promote the theory on the basis of test verification, and establish a simulation program that is more and more comprehensive and close to the actual working conditions on this basis. At present, the working life of the plunger pump at the world's advanced level can reach more than 8,000 hours under frequent shocks, such as excavators; it can reach more than 15,000 hours under infrequent shocks, such as cranes; Rexroth Using modern design technology in 2010, the plunger variable unit A15VSO was completely redesigned; the working pressure of Rexroth's A4VHO that appeared recently can reach 630bar, all of which are the industrialization results of these long-term continuous research.

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