1 the introduction
Because of its structural characteristics, the inclined disc axial piston pump motor can achieve high efficiency and withstand pressure, which can realize many variable functions, so it becomes the ace element in hydraulic technology.But because of its generally have at least four pairs of sliding friction pair: oil distribution plate cylinder block, cylinder piston, piston head - slipper socket, slipper - inclined plate, the lubrication condition is complex, therefore, the durability becomes the key indicators, also is domestic product part of the largest gap with the world advanced level.To cope with strong industrial base strategy, write this article, the main reference Breuer doctoral thesis [1] (Germany aachen university, institute of fluid drive control IFAS 2007) in the literature review part and literature [2 ~ 6].In order to facilitate the retrieval of references, the names of the names are preserved in the Latin alphabet.

2 review
With regard to the friction of ordinary sliding bearings, the German Stribeck summarized the famous Stribeck curve (see figure 1) in 1902 based on a large number of test results.It can be seen that the friction coefficient is the lowest in the mixed friction zone.Because of the least wear of materials in the liquid friction zone, the ideal of sliding bearing is to work at the junction of mixed friction and liquid friction.This curve is of great value to design sliding bearing, so it is the German industrial standard DIN 50281:1977-10 "friction in bearings: concept, type, status, physical quantity".

However, compared with ordinary sliding bearing, the relationship between the load, pressure distribution, geometry and kinematics of axial piston pump is much more complicated.Because of the coupling of each sliding point, the plunger has an uncertain degree of freedom in the socket joint and the cylinder body hole, which makes the calculation of friction contact difficult.Take the plunger, plunger, though there are similar to ordinary sliding bearing of the shaft rotation, axial translation, but also by a slipper to plunger, the lateral force bearing area (block), makes the friction losses from the plunger to become the main part of power loss.Therefore, the accumulated experience on traditional sliding bearing theory can only be applied in a limited way [7-15].

Van Der Kolk (1972) first tried to study the friction between plunger and cylinder block.He designed a tilt table test bench for the experiment.However, since the oblique rotation axis of the experiment overlaps with the plunger axis, the plunger has no axial movement and is only subjected to a rotating lateral force.In experiment and theory, he avoided due to the motion of the plunger axial bearing pressure distribution, the tribology problem is simplified to a tilted, lateral loading of unilateral external edge of sliding bearing pressure increased.He paid particular attention to the plunger's outstretched position.The measurement of pressure distribution in clearance shows that the pressure accumulation mainly occurs in the edge region of the gap.In the theoretical research section, he first used the numerical solution to solve the Reynolds equation [16].

Renius (1974) realized the limitations of the Van Der Kolk test bed, proposed an improved structure and considered the axial movement of the plunger.He USES a fully static bearing measuring set and a compensating plunger to measure the pressure and friction separately.This test rig USES a valve control associated with the corner, which can simulate pumps, motors, or other pressure work, which means that the plunger is under pressure when it is retracted and outstretched.In this way, you can experiment with all the situations that occur in real work.The constant pressure operation that does not occur directly in the actual work is very suitable for understanding the general situation of friction on the cylinder.He conducted a wide range of test parameters, the pressure of 15 ~ 200 bar, Angle 0 ~ 20 °, speed is 2000 ~ 100 r/min.In addition, he conducted a special starting test.He presented his experimental results in the form of classic sliding bearing theory, and discussed the validity and applicability of the similarity number, such as Sommerfeld number, or gumbel-hersey number in his tests.The main results he obtained from the experiment are as follows [6, 17] :

(1) the sliding friction characteristics of the plunger - cylinder body can be described from the driving Angle, demonstrating the effectiveness of the Stribeck curve in the obvious mixed friction region.

(2) shows the similarity criterion Gu = eta/р omega of good availability, here, eta is viscosity, omega is driving speed, p is pressure plunger hole.It is pointed out that, as described by Van Der Kolk, there is no practical significance to the contact of the cylinder block.

(3) the friction of the plunger plays a decisive role in the starting characteristic of the motor, which results in the starting loss of 13% ~ 16% of the torque of the motor theory.At the same time, there is a large leakage in the sliding shoe, which can be explained by a large rotation friction between the ball head and the plunger.

(4) the rotation of the relative driving Angle of the plunger is not consistent with the driving rotation at all working points.Theoretically, the conclusion is that the relative rotation is unfavorable to the friction property.

(5) the linear motion of plunger is established to support the pressure, so that the friction surface is separated, which has a special significance in the motor mode, which is proved by the test of variable parameters.

(6) he was found to have been in the existing oil trap during the test, but did not think the effect was significant.

(7) the gap between plunger and cylinder body shows great influence on the friction process in the experiment, which is less than 1% of the diameter of plunger.The lower end of the gap should be lubricated fully and should not be determined by leakage requirements.

(8) he recommends that the design of the cylinder block fit together: for the pump, the smooth unaccompanied pressure trough, short guide section with short guide section, and long guide section for the motor.

Dowd and Barwell (1974) established a test bed for the friction between plunger and cylinder.The straight-line motion of plunger is realized by a CAM driven, without considering lateral force.The measurement is based on the principle of constant pressure.As an innovation, a metal contact sensor is used to detect whether or not contact is detected by measuring resistance changes between the friction pairs.They studied the effect of the plunger roughness and the material's side, thus determining that the friction will not continue to decrease when the surface roughness is reduced to a certain extent.

Regenbogen (1978) used the same experimental setup with Renius.In addition to the plunger of sliding shoes, he also studied the plunger of the head of the ball and the plunger of the rod support.As a result of the study, he proposed a series of design proposals: such as the maximum deflection Angle, low cost of materials, the clearance of plunger and guide length.For the motor, he recommends that the long guide plunger, but there can be an interrupt to reduce the loss of [19] at high speed.

At the same time, Boinghoff (1977) advanced the study of the sliding shoe of axial plunger.He successfully deduced the slanting force of sliding shoes on the sliding surface of the inclined plate and proved by experiment.The force of the plunger and the ball socket between the plunger and the sliding shoe are included in the calculation.According to his research, the elliptical trajectory of the minimum clearance point between the sliding shoe and the inclined disc does not coincide with the ellipse trajectory of the oblique plane and the plunger axis.Understanding the relative velocity and the clearance changes of sliding shoes, the loss flow of the sliding shoe relative rotation Angle can be calculated by [7].

The experiments of Hooke and Kakoullis (1981) also mainly studied the contact of shoe-plunger.The results of a series of experiments show that the relative rotation of plunger decreases with increasing driving speed, which Renius also found.In addition, the plunger tends to rotate more when the pressure increases, because the friction force resulting from the increase of pressure in the ball socket is higher than that of the plunger side force.

Renvert (1981) presents a variety of test methods for the low speed and starting characteristics of hydraulic motors.As the most commonly used method is to force constant speed rotation, so you can avoid other methods (starting under constant load, hold the motor shaft, constant flow) test results of a lot of discrete [21].The experimental results of his particular systematic approach were adopted by ISO 432-1 and recommended to measure motor starting and low speed characteristics.

Weiler (1982) studied the effect of motor plunger structure on low-speed characteristics by experiment and simulation.He studied the friction and leakage conditions of various contact points and compared the results with the simulation.The simulation model can be used to reconstruct the characteristics of the motor even though some parts have been simplified.Therefore, for the first time, he can not directly test the various plunger, but show the problem of the increased leakage of motor at low speed and start.

Koehler (1984) studied the pressure distribution of the force of the plunger during the start of motor - cylinder block [23, 24].His experiment Settings include a cylinder - driven plunger and a lateral cylinder that allows the lateral load to be applied freely.The simulation model he established can be calculated, taking into account the pressure distribution in the gap after the plunger bending deformation.In order to get the best starting and low speed properties, the best clearance between the plunger and the cylinder block must be about 1 ‰ of the diameter of the plunger.

Ivantysynova (1985) first used Reynolds and energy equation to calculate the non-isothermal flow in the gap, and compared [25] with the test results.The energy equation model adopts the dissipative function of Vogelpohl as the source term.The test kit consists of a double - hole rotating inclined disc pump which can be short-circuited through the control valve.

Ezato and Ikeya (1986) established a test bed for the friction of the plunger - cylinder body.The lateral force and axial force are measured separately through a measuring sleeve supported on a rolling bearing, so only a small lateral force can be applied.The test is conducted in constant pressure mode, with emphasis on starting and low speed characteristics.The influence of the surface roughness, material and hard surface of plunger surface was studied, and the latter showed that [26] was not applicable.

Jacobs (1993) with the method of artificial add particles to test the pump motor, the adoption of an alternative material (by physical vapor deposition (PVD) and the combination of hard surface, can significantly improve the wear characteristics of axial piston pump and sliding characteristics [27].

Fang and Shirakashi (1995) have carried out theoretical and experimental research on [31] for axial plunger machinery.Their simulation model, while solving the Reynolds equation of all positions of plunger stroke, did not consider the dynamic pressure build-up effect caused by the pressure oil discharge.The measured measurements show the beneficial effects of the relative rotation of the plunger, contrary to what Renius and Regenbogen say.

Donders (1998) studied the effects of various kinds of friction pairs in various experiments, and applied the knowledge to the design of [8] for axial plunger of axial plunger for high water base fluid (HFA).He developed a device for measuring the friction and pressure distribution of plunger and sliding shoes.The test bed for measuring plunger friction has a plunger connected to a force sensor enclosure.The plunger has a wedge-shaped gap compensating plunger mounted on the bottom of the plunger.In order to simulate the relative motion between plunger and cylinder body, the cylinder body is driven to and fro by a crank, and the lateral force acting on the ball head of the plunger is produced by an external pressure cylinder.Jang [28], Oberem [29] and Van Bebber [30] also used the same test platform, with some modifications.

 Donders USES a special friction meter to test the friction force.The inclined disk is rotating and the compacting force is similar to the real machine.The tilt of plunger is ignored in the experiment.Experiment, the results showed that the calculated slipper swelled the pressure distribution between the seal can be consistent with the measured data very well, and can be expected, in a relatively high speed, the slipper will float due to fluid dynamics.

Donders tries to derive the loss of the whole machine from the loss of the measured friction pairs, to a certain extent.However, it is proved to be more accurate to simulate the working process of the slanting machine, and it is very important to design the measuring device close to the actual working condition.In particular, the complex interaction between the axial piston mechanical friction components must be taken into account when designing the measuring device.

Manring (1999) USES the same measurements of Ezato and Ikeya on rolling bearings to measure the friction between the plunger and the cylinder block.In this case, the inclined disk does not rotate, and it only does the reciprocating linear motion to generate the plunger's stroke, so it does not simulate the lateral force of the circular motion.According to the test results, a Stribeck curve with exponential function is derived for the mixed friction region.In the model, there is no consideration of the compressive membrane effect caused by plunger movement and rotation.There is no test at low speed region [41].

Tanaka (1999) studied the influence of the macroscopic geometry shape of the plunger on the starting and the friction force [42-43] by experiment.The test bed used a measuring sleeve similar to the Renius test bed to be supported by static pressure.A less rigid plunger leads to lower friction (long guide plunger, measured in mixed friction area).
ZhangYangang (zhang yangang, 2000) studied the measures of improving the low speed and starting characteristics of axial plunger mechanism by [5].He analyzed the friction and leakage in the motor with a method of constant force rotation (see Renvert).In order to deepen the analysis, he used multiple test stations, including the Donders' movable cylinder sleeve, and a single plunger test platform fixed to the lateral force position, equivalent to a minimum speed of 5r/min.He put him in an inclined plate of motor test of friction and leakage loss, as measured by quantitative: actual output torque motor was only 77% of the theory of torque between piston and cylinder friction loss 8.7%, loss of 6.1% between plunger - slipper, cylinder body - 3.8% between the swash plate, slipper - 3.1% between inclined plate, the rest of the loss of 1.0%.

Nevoigt (2000) studied the wear resistance of hydraulic components with hard surface.He tested the friction by using the piston rod of the hydraulic cylinder [32-33].

Liu Ming (Liu Ming, 2001) [13] and Krull (2001) [11] examined the contact of the plunger with oil lubrication on the axial plunger mechanism, with the aim of simulating the machine as a component of the transmission vibration.Liu proposed the analytic description equation based on spatial force, and Krull investigated the required rigid friction value and damping value through extensive test.To this end, he used three different test stations: test table 1, to determine the stiffness and the damping of the cylinder block and the cylinder body;Test bed 2, friction torque in the ball socket;Test bed 3, the stiffness and damping of sliding shoes.Knull does not measure the axial and tangential friction, but is evaluated from the Renius' friction measurement.The data obtained by Knull indicate that in many cases the plunger runs in the mixed friction zone, while the pulsating side force is not enough to get the plunger out of the mixed friction zone.Knull attributed the friction in the sliding ball pit to a good mixing friction;The friction coefficient is very close to the value of the known bronze - steel or brass - steel.Although it is still a problem, that is, through a special test-bed for some of the measurement of the friction coefficient and the approximate formula is enough to accurately reflect the actual machine friction characteristic of the plunger, but Liu's work suggests that use of these data to the axial piston machine is regarded as a rotating oscillation system.Since friction is based on the measurement of Renius, it is difficult to ensure that the range operating at extremely low speeds is effective.

Kleist (2002) developed a simulation program for calculating the friction and leakage of the plunger, and solved the relative motion speed of plunger when the cylinder was rotating.The steady-state and transient components of the average Reynolds equation in the so-called rough lubrication gap determine the force acting on the cylinder.A statistical method of surface roughness based on Partir and Cheng studies [34-35] was adopted for the AFM model (mean flow model).In addition, the solid force part adopts the Greenwood and Williamson contact pressure model to model [36].According to Kleist, it is very important to consider the bearing capacity of surface roughness through rough peak contact, especially at low speed.He also discussed the energy equation considering the dependence of temperature on pressure buildup in the gap of the general solution, but the result is, in his study, don't have to be considered, but said such consideration is useful.Model in order to verify his theory, he established the multiple test rig, is particularly important, one can perform a variety of test of internal support of radial piston pump, friction, temperature, the buildup of pressure in clearance, a such as Donders, movable cylinder body, of the piston lateral load of the test rig can be (37, 39).In addition to the simulation of the friction contact of the cylinder block, he also carried out the calculation of the contact part of the sliding shoe.He pointed out that the contour of the sealing ring and all the chamfering must be considered when modeling, because it has a significant impact on the calculation results.A calculation that considers all sliding contacts because the calculation time is too long to give up.

Based on the results of a series of simulations, he proposed the improvement of design, long cylinder hole and long plunger.
The simulation of piston friction occur in medium speed and small Angle (750 r/min, 15 °), and the modern axial piston motor cannot be compared to poor working conditions.

Sanchen keep Kleist's (2003) and the plunger cavity together with the dynamic calculation of the pressure buildup in combination with PUMA into the pump motor design software, which can output function in the inclined plate adjustment or drive shaft bearing force.The low speed is not considered here (less than 500r/min).The study shows that the accumulation of dynamic pressure in the gaps needs to pay special attention to [40] and [14] if the friction between plunger and cylinder is described.

Wieczorek (2000) proposed a simulation model of the flow of the oblique disk mechanical clearance, CASPAR [15], [44].It can calculate the sliding contact between the slip-disk, plunger - cylinder block and cylinder body - oil plate.Attached can simulate the mechanical (kinematics, dynamics) and hydraulic (pressure building of the cylinder cavity) effect.The effective surface of lubrication is not limited to simple and basic geometrical forms, but can be determined freely within certain limits.Unlike the BHM and PUMA programs developed by Kleist and Sanchen, CASPAR has solved the energy equation in addition to the Reynolds equation and can consider the non-isothermal processes in the gap.The program needs to know the temperature and volume of all parts that define the gap.The contact force in the mixed friction area is 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 shows the principle feasibility of this calculation and gives some examples of calculation.This also indicates that the contact area of the plunger - cylinder body can be considered mixed friction.Due to the very high speed (> 2000r/min) for testing, the simplified calculation of contact force is regarded as reliable.

The work of Olems (2001) focuses on the thermodynamic model of the simulation program CASPAR [45-46].Did he do to the program added that in the heat produced by piston clearance is passed to the cylinder body, and from there to the perimeter leakage of oil in the shell, the contact force is once again with the help of a simplified model description.Tests of temperature sensors in the cylinder of a series of products show that the simulation and measurement results are quite consistent.The measured values are expressed in the relative oblique disc Angle and pressure.The rotating speed and operating mode are given by the "nominal speed" and can be seen by the speed n > 2000r/min.

Oberem (2002) studied the various friction points of axial piston pumps, aiming to develop a axial plunger pump and motor for high water base fluid (HFA).His test bed was further developed from the testbed of Donders' crank drive plunger set.Due to the low viscosity of the medium, almost all frictional processes occur in the mixed friction region.The plunger friction test was carried out at a high speed of 10 ~ 1500r/min at a low speed of 1 ~ 10r/min, all under constant pressure.Only the speed and pressure dependence is tested on a high speed range, the length and clearance of different plunger, and the impact of the extended length and the plunger ring groove.In the low speed range, the repeated test results are discrete, and the reason can be attributed to the failure of the velocity fluctuation and the static bearing of the measuring sleeve.Due to the large proportion of solid friction, the measurement of friction changes, as expected, seems to be a pure coulomb friction rather than relying solely on plunger travel.In order to solve the problem of mixed friction, Oberem puts forward the components and hard surface, or USES the friction material, the preferred ceramic base to replace [29], [47].

Van Bebber (2003) explored the application of gradient carbonation layer to the axial plunger mechanism [30].This process can be used in principle to all the friction parts of the axial plunger machinery, especially the non-ferrous metals commonly used in the cylinder, the oil and the plunger.He considered particularly promising as an alternative with the gradient of hard surface HfCg and ZrCg hafnium and zirconium carbide carbide layer (gradient) features, the thickness of a few microns (intermediate value about 4 microns) layer, the surface is soft, central layer is relatively hard, in the junction and become soft, layer and matrix to obtain a better adhesion effect.In the study, it was found that it was difficult to use the hard surface of the high surface pressure (> 50N/mm2).To improve this, he used various FEM tools and BHM program [38] to study.At the same time, he performed the piston friction test on the existing test bed [47], and the calculation of BHM was only consistent with the high speed.The pressure effect of the plunger edge can be improved by grooving in the bottom of the cylinder hole, but it can not be proved by experiment.Improve the conditions of friction and mechanical hydraulic efficiency is not the main purpose of the study, hard surface layer system of excellent friction characteristics can bring more effect, it can see when each test bed gradient layer test.

Breuer power (2007) adopted a rigid pressure sensor as part of the plunger on the motor at low speed test bench test of the plunger friction [1], through test and calculation, revealing the key mechanism of friction and to improve the design of plunger.The design guide of plunger mechanism is introduced by experiment.

Gels (2011) studied the hard surface and corresponding shape of plunger - cylinder body.In order to achieve better wear resistance, friction pairs can be used to substitute hard - hard combinations for traditional hard - soft combinations: such as the use of tempered steel and carbonated zirconium surface.But in the past, the running and the stage ceased to occur, so it was necessary to process the plunger and cylinder holes in advance to make certain shapes.Through the simulation, find out the proper shape parameter, and considering the processing technology, and then in a single plunger test rig, and a complete is carried out on the piston machine test, the results show that hard - hard friction pair can improve the bearing capacity, and the appearance of fine can improve the efficiency of [2].

Enekes (2012), in addition to study the PVD hard surface friction loss in synthesis of ester without additives, also in the pump shell was investigated through CFD method of the oil energy loss caused by rotating cylinder body agitation, and usually the measures to improve [3].

Scharf (2014) continues to study the friction and wear characteristics of the gradient carbide surface in rapid biodegradation.The test proves that friction can be reduced significantly and durability is improved.It can be used as an auxiliary function by working out the arc of the ball through the plunger and cylinder.By analyzing the lubrication condition in the gap, the different ball arc parameters were investigated and the best shape [4] was found.

3 conclusion
Reviews can be seen from the above, abroad for more than forty years on the research of the axial piston machine, has experienced from simple to complex, from single to comprehensive, continuous advance of the research process, constant, theory combined with the test, the test validation forward on the basis of the theory, based on the more and more close to the actual working condition of the simulation program.This review is only a published paper, and there must be several times more research done in-house.These studies are not, and cannot be fully expressed in drawings and process CARDS.Therefore, it is very naive to think that drawing and process CARDS can catch up with the world's advanced level of ideas.

At present, the working life of the plunger pump in the world's advanced level can reach more than 8000h under the working conditions of the excavator.It can reach up to 15 000h under the working conditions of the crane.Rexroth USES modern design techniques in 2010, completely redesigned plunger variable unit A15VSO;Recently, the work pressure of A4VHO has reached 630bar, which is the result of the industrialization of these long-term studies.

The ancestral training "bao jianfeng from grinding out, plum blossom fragrant from bitter cold", is also applicable here.