Multi-position measurement of oil film thickness within the slipper bearing in axial piston pumps

Highlights

• A test rig is built for measuring the oil film thickness within the slipper bearing.

• A novel index plate mechanism is first introduced to switch the measured positions.

• Multi-position measurement is available under real working conditions over a cycle.

Abstract

The oil film thickness represents the greatest unknown variable influencing the slipper performance in axial piston pumps. Earlier slipper test rigs suffer from two main drawbacks: inverse kinematics of the slipper and fixed measured positions. However, the real slipper behavior for one cycle should be examined from the standpoint of fluid film lubrication. Therefore, a new slipper test rig is designed to allow the multi-position measurement of oil film thickness within the slipper bearing under real operating conditions. This paper mainly includes a description of the slipper test rig and the developed multi-position measurement method. A group of three displacement sensors can capture the gap height between the slipper and the swash plate in thirty positions through an index plate mechanism. Finally, some sample results of oil film thickness are presented to confirm the capacity of the test rig.

Introduction

Axial piston pumps are widely used in fluid power applications for converting mechanical power into hydraulic power due to their striking attributes such as high efficiency, great power density, available flow regulation, and long service life [1]. Fig. 1 shows the general configuration of a typical axial piston pump. Multiple piston-slipper assemblies are nested in a circular array within the cylinder block at equal intervals about the centerline of the cylinder block. The slipper is connected with the piston head through a spherical joint which allows the slipper to have three rotational degrees of freedom. The retainer helps the slipper to maintain a reasonable contact with the swash plate. The cylinder block is coupled with the shaft through a spline mechanism and held against the valve plate. When the cylinder block rotates together with the shaft, the pistons reciprocate within cylinder bores and the slippers slide on the swash plate. The fluid flows from the intake port into the displacement chamber through the valve plate as the piston is pulled out of the cylinder block. Conversely, the fluid flows out of the displacement chamber as the piston is pushed into the cylinder block. The above motions repeat themself for each revolution of the shaft, accomplishing the basic task of providing high-pressure fluid for the hydraulic circuit.

The efficiency and reliability of axial piston pumps greatly depend on the design of the three lubricating interfaces, i.e., the slipper/swash plate interface, the piston/cylinder block interface, and the cylinder block/valve plate interface. Among the lubricating interfaces within an axial piston pump, the slipper/swash plate interface has received much academic attention. It is a critical design issue for axial piston machines, which is required to fulfill the bearing and sealing functions simultaneously. As shown in Fig. 1, the slipper is characterized by a center pocket in the sliding face which is communicated with the piston chamber through the orifices drilled in the piston and slipper. The pressurized fluid from the displacement chamber is supplied to the slipper pocket and leaks through the gap between the slipper sealing land and the swash plate. The slipper is subjected to external loads from the piston and is lifted by the lubricating film under it. Thus this thin oil film within the slipper bearing helps to reduce the potential metal-to-metal contact between the slipper and the swash plate. The gap height within the slipper/swash plate interface is of importance to the slipper behavior. If it is too large, then the gap leakage increases significantly and the volumetric efficiency drops dramatically. If it is too small, the power losses generated within the interface increase due to viscous friction and the worst mixed friction tends to take place.

During the last forty years, a large amount of research has been conducted on the slipper’s lubrication characteristics. To verify the lubrication model for the slipper bearing, many slipper test rigs have been designed and manufactured. These slipper test rigs are generally divided into three common types in terms of their constructions.

For the first type of slipper test rig, the swash plate is rotated while the piston-slipper assembly remains stationary. This type of slipper test rigs suffers from two main drawbacks. First, the inertia force and viscous friction of the piston-slipper assembly fail to be considered. Second, it is impossible to realize the real flow communication between displacement chambers and pump ports through the valve plate. Hooke et al. [2], [3] built a simple test rig to measure the oil film thickness between the fixed slipper and the rotational swash plate. They mounted three capacitive displacement transducers in the swash plate and thus the oil film thickness could only be measured intermittently. Continuous measurement of the oil film thickness requires sensors to be attached to the test slipper. Bergada et al. [4], [5] constructed a slipper test rig which could measure the oil film thickness and pressure under the slipper simultaneously. Three displacement sensors at 120° intervals and four pressure sensors at 90° intervals were attached to the fixed test slipper. The small tilting angle of the slipper relative to the rotating swash plate was adjusted by four additional positioning screws. Lin and Hu [6] also designed a continuous measurement system of oil film thickness. To avoid damaging the slipper sealing land, the three eddy displacement sensors were not directly mounted in the test slipper; instead, they were distributed uniformly on an auxiliary ring that was attached to the slipper. Additionally, in order to protect the sensor cables, a positioning pin was installed in the auxiliary ring to restrain the spinning motion of the test slipper.

The second type of slipper test rig is superior to the first type regarding the kinematics behavior since it allows the piston-slipper assembly to reciprocate within the cylinder bore. However, the used inverse kinematics still cannot take into account the actual centrifugal effect of the piston-slipper assembly since the test slipper cannot rotate together with the cylinder block like the one in a real pump. Iboshi and Yamaguchi [7] established a test rig to monitor the oil film thickness of the slipper continuously. Three displacement sensors were located in a modified slipper at 120° intervals to obtain the clearance between the slipper and the rotating swash plate. The test slipper was designed larger than the generally used one to improve the measuring accuracy and provide enough installation space. In addition, they limited the slipper spin using an H-formed stopper since the sensors with wires might be damaged by the spinning slipper. Nie et al. [8] built a swash-plate-type single piston equipment to study the tribology characteristics of the slipper bearing for different material combinations under water lubrication. The single piston-slipper assembly was nested within a cylinder bore and accomplished its reciprocating motion by the rotating inclined swash plate. It should be noted that the test rig could not operate under a high pressure since the swash plate was subjected to an eccentric load from the single piston-slipper assembly.

The above two types of slipper test rig benefit from their simplicity in structure and are relatively easy to create. However, the real kinematics behavior of the slipper and instantaneous displacement chamber pressure are suppressed during operation, which fails to consider all physical effects acting on the slipper bearing. Therefore, some researchers have recently attempted to experimentally investigate the slipper in a model pump by modifying commercial pumps. Hooke and Kakoullis [9], [10], [11] installed a cluster of four capacitance displacement transducers in the swash plate of a commercial axial piston pump to measure the oil film thickness underneath the inner land of the slipper. These displacement transducers were positioned in such a way that they could capture the slipper’s tilting motions in the radial and tangential directions. Spencer [12] carried out the measurement of oil film thickness within the slipper bearing in a modified pump using six displacement sensors. These sensors were located at given positions in a specially designed swash plate and only the intermittent measurement of oil film thickness could be obtained. Similarly, Tang et al. [13], [14] measured both oil film thickness and temperature within the slipper bearing using eight displacement sensors and four thermocouples that were embedded in the modified swash plate. Again, the positions of all sensors were fixed in the swash plate, only allowing the intermittent measurements. Zhang et al. [15] first experimentally confirmed the slipper spinning motion by a three-piston pump that was modified based on a standard commercial pump. The sensor was installed in the modified retainer to continuously detect the slipper spinning motion. The sensor signals were transmitted from the rotating retainer to the stationary data collection system via a slip ring unit.

Also, there were some model pumps designed for the experimental investigation on the piston/cylinder block interface and the cylinder block/valve plate interface. Lasaar and Ivantysynova [16], [17] built a three-piston test pump based on a conventional swash-plate-type axial piston pump, which allowed the measurement of the piston friction forces and displacement chamber pressure under normal operating conditions. The sensor data were transmitted from rotating cylinder block to the stationary evaluation unit by telemetry. The telemetry technology was also adopted by Olems [18] when measuring the displacement chamber pressure and thermal distribution within the piston/cylinder block interface in a modified pump. Kim et al. [19], [20] installed a displacement sensor in a hole located at the cylinder block to continuously capture the distance between the cylinder block and the valve plate. Unlike Lasaar and Ivantysynova [16], [17], Kim et al. transmitted the sensor signals from the rotating cylinder block to a static recorder using a slip ring unit. Placing the sensors in the stationary valve plate or housing case was another method for measuring the gap height between the cylinder block and the valve plate [21], [22], [23], [24]. This method required no additional slip ring units, which was more convenient and reliable but at the expense of non-continuous measurements.

In reviewing the literature, it can be seen that slipper test rigs play an important role in understanding the physical effects occurring in the slipper bearing, and much effort has been devoted to them. However, these slipper test rigs suffer from two main drawbacks. Firstly, the commonly used inverse kinematics method in the slipper test rig fails to provide a real operating condition for the measurement. Secondly, the fixed sensors can only capture the slipper behavior at given positions if they are mounted in the swash plate. This means that measurements at more locations will be at the expense of costly sensors and measurement integrity. The purpose of the present work is to solve the problem related to the multi-position measurement of oil film thickness under the slipper in a model pump. A group of three displacement sensors are expected to measure the slipper’s oil film thickness in thirty positions over one cycle by a novel index plate mechanism.