Abstract
The article describes the influence of displacement of the swash plate rotation axis on the efficiency of axial piston pumps. The dead space volume was defined as the volume of the working chamber in the extreme position of the piston at the end of the pumping phase and its variability was determined as a function of the position of the axis of rotation of the swash plate and the swing angle of swash plate. The influence of swash plate rotation axis displacement on leaks between piston and cylinder was determined. It has been proven that displacement of the swash plate rotation axis reduces the dead space volume and leaks, thus improving the volumetric efficiency visible at small swash plate swing angles. The results of a comparative study of the volumetric and total efficiency of a modified design of a pump that allows the rotation axis to be repositioned are presented. The efficiency curves for the swash plate rotation axis crossing the shaft rotation axis and for the displaced swash plate axis of rotation are compared. The article also presents the author’s construction of an axial piston pump with displaced swash plate rotation axis with a follow-up mechanism of capacity change controlled by a stepper motor. The diagram and view of the test stand are shown, as well as the results of tests of volumetric and hydromechanical pump efficiency.
Access provided by Autonomous University of Puebla. Download conference paper PDF
Similar content being viewed by others
Keywords
1 Introduction
Piston pumps with swivel swash plate are often used in modern hydraulic systems. The design of such pump is schematically shown in Fig. 1. Shaft (1) is connected to drum (2) by means of a spline. In the drum there are pistons (4) in cylindrical chambers, finished with slippers (5). Pistons, during the rotation of the drum, due to the contact of slippers with the swash plate (6), perform the reciprocating movement in the cylindrical chambers. The face of the rotating drum contacts a fixed timing plate (3) with two kidney holes. One of these openings is connected to the suction port and the other to the pump delivery port. Thus, the cylinder chambers from which the pistons slide out are connected to the suction channel by means of a timing plate and the liquid is sucked in. As the shaft continues to rotate and the pistons pass through the dead-end, they begin to approach the timing plate - then the chambers are connected to the delivery channel and liquids are pressed out of the chambers [1, 3]. Figure 1 also indicates the volume of dead space, i.e. the volume occupied by the fluid in the cut-off cylinder chamber when the piston is in its extreme position at the end of the pressing phase [4, 5].
2 Materials and Methods
2.1 Displacement of Swash Plate Rotation Axis
The standard axial piston pumps have one thing in common: the swash plate axis of rotation crosses the shaft axis of rotation. The volume of the dead space increases as the swing angle of swash plate decreases (see Fig. 2a). If the axis of rotation of the swash plate is displaced as shown in Fig. 2b, the volume of this space will be constant regardless of the swing angle of swash plate. The displacement of the pump Vs is a function of swash plate swing angle γ, piston spacing D and piston diameter dt [1,2,3]:
If the axis of rotation of swash plate intersects the axis of rotation of the shaft, as in Fig. 2a, then for the angle γmax the volume of dead space is minimal and increases with the reduction of the swing angle of swash plate γ according to the following relation [8]:
VDmin is the minimum dead space volume for maximum swing angle of swash plate γmax. This space includes a part of the cylinder, the window of the cylinder drum and the empty space inside the piston.
2.2 Influence of Displacement of Swash Plate Rotation Axis on the Dead Space Volume
Figure 3 shows the situation when the axis of rotation of the swash plate is shifted in relation to its original position by ax and ay. The boundary condition is that the swash plate is positioned at its maximum swing angle to obtain a minimum volume of dead space VDmin. Thereby, regardless of the position of the axis of rotation of the swash plate, its position at the maximum swing angle will be the same. Consider the situation of the rotation of the swash plate relative to the displaced center of rotation from the angle γmax to the angle γ in a system of coordinates with the center in the place where the center of rotation of the swash plate was originally located.
The volume of dead space as a function of the position of the center of rotation and the swing angle of the swash plate may be described as [8]:
The coefficient C depending on ax and ay will be [8]:
Example results of simulation of a change in the dead space volume as a function of the position of the center of rotation of the swash plate is presented in Fig. 4. The greatest influence on changes in the dead space volume has the movement of the axis of rotation in the direction ay.
2.3 Influence of the Dead Space Volume on the Volumetric Flow Losses
At the end of pumping phase, the residual oil under high pressure is located in the dead space. Thus, when the suction manifold opens, the medium cannot be sucked at the beginning of cycle, because the oil contained in the dead space expands, causing back-flow to the suction duct. Therefore, the volume of actually sucked oil is reduced, thus decreasing the volumetric efficiency. The higher delivery pressure and the larger dead space, there is the higher influence on the phenomenon mentioned above.
The change in the volume of the expanded oil at a pressure drop Δp will be [5]:
Since the phenomenon of liquid expansion in the pump is adiabatic, the secant isentropic bulk modulus KI should be used to describe it. The resulting flow rate losses for a pump driven at speed n with the number of pistons z can be determined as:
These losses directly affect the reduction of the flow generated by the pump and thus the volumetric efficiency.
2.4 Influence of Displacement of Swash Plate Rotation Axis on the Length of Piston - Cylinder Gap
Figure 2 shows that the displacement of the swash plate rotation axis is carried out under the condition that the position of the swash plate for the maximum angle is the same as for a pump with a non-translocated rotation axis, causing the pistons to slide more into the cylinder drum, hence for the disc rotation angle γ2 the distance of the piston face from the bottom of the cylinder chamber changes from L0 to Ld. The difference is:
The length of the piston-cylinder gap depends on the angle of rotation of the shaft [6], so greater insertion of pistons into the cylinder drum will result in an increase in the average length of the piston-cylinder gap and will reduce the leakage occurring in this node.
2.5 Influence of Displacement of Swash Plate Rotation Axis on the Volumetric Efficiency
Comparative tests were carried out in the Laboratory of Hydraulics of the Gdansk University of Technology on a pump with the possibility of moving the swash plate axis of rotation. The basic parameters of the pump were as follows:
-
piston spacing D = 58 mm
-
piston diameter dt = 14,15 mm
-
maximum swing angle of swash plate γmax = 15,65°
-
minimal volume of dead space VDmin = 4,27 cm3
The results of the tests shown in Fig. 5 showed a noticeable increase in volumetric efficiency for small swing angles of swash plate. These tests confirmed the influence of the position of the swash plate axis of rotation on the volumetric efficiency and were an impulse to design a variable displacement pump with a displaced axis of rotation.
3 Results
3.1 Prototype Pump Construction
The pump shown in Fig. 6 is a prototype construction where the axis of rotation of the swash plate is displaced so that the volume of the dead space remains constant when the swash plate swing angle is changed. The swash plate (2) is fixed in the body by two bolts (3), which form its axis of rotation. The capacity is changed by using a piston (7) ended with a roller. The torque that loads the swash plate has the same turn every time, so there is no need for additional spring support of the plate. The change of the pump capacity is done by moving the control spool (10) by means of the stepper motor (11). This spool controls the flow of liquid through the moving cylinder (12) to the chambers of the actuator (7). Feedback is realized mechanically. Pusher (8) through lever (9) acts on cylinder (12) by moving it in the direction of the control slider movement. The movement of the cylinder continues until it reaches the appropriate position in relation to the control spool. The kinematic ratio of the mechanism, i.e. the ratio of piston stroke (7) to control spool stroke (10), depends on the length and position of the center of rotation of lever (9). A spring (13) acts on cylinder (12), which eliminates clearance in the mechanism and presses cylinder (12) against lever (9). This pump can have two directions of pressing and can be used for hydraulic closed systems. In such a construction, the pump has a symmetrical design, has two displacement pistons and two follow-up mechanisms. When changing the direction of flow, it is necessary to move the axis of rotation by means of hydraulically controlled bolts (3). Main parameters of tested pump were as follows:
-
geometric displacement q = 40 cm3/rev
-
pistons spacing D = 67 mm
-
piston diameter dt = 15,17 mm
-
number of pistons z = 9
-
dead space volume VD = 3,244 cm3
-
max swash plate swing angle γmax = 20°
-
nominal pressure pnom = 35 MPa
Figure 7 shows the operation of the mechanism of changing capacity when increasing the swash plate swing angle. The follow-up mechanism is delivered straight from the pump pressure collector.
3.2 Test Stand
A scheme of the test stand is shown in Fig. 8. A view of the test stand is shown in Fig. 9. The tested pump was supplied with a supply pump. The suction pressure is regulated by a relief valve (5) and the delivery pressure by a valve (6). During the tests, pressures in front of and behind the pump, oil temperature in the suction connection, flow rate, torque and speed were recorded. Used working fluid was mineral oil of viscosity class VG 46.
A list of used test apparatus is presented in Table 1.
3.3 Efficiency Characteristic
Volumetric and hydromechanical efficiency as a function of differential pressure and swash plate swing angle for viscosity 20 and 40cSt and rotational speed 2000 rpm are shown in Fig. 10 and 11.
4 Results
The prototype of an axial piston pump with a displaced axis of rotation of the swash plate, based on a commercial pump working mechanism (pistons with slippers, cylinder barrel, timing plate), has achieved a satisfactory volumetric efficiency, comparable to that of commercial pumps. Due to the fixed dead space volume, independent of the swash plate swing angle, the volumetric efficiency was improved for small swash plate swing angles and high pressures. The reason for the reduced hydromechanical efficiency is probably due to a shaft that is too long and can vibrate during pump operation. Tests have also shown problems with the follow-up mechanism, such as too much leakage through the directional valve and too high friction forces on the follow-up cylinder seals, causing it to lock. The results of the tests will be used to create a second, upgraded prototype.
The article is a result of project funded by The National Centre for Research and Development within the framework of program LIDER.
Project no.: LIDER/22/0130/L-8/16/NCBR/2017 Project title: Hydro-mechanical automatic gearbox for agricultural vehicles and heavy machinery. Value of funding: 1 197 500,00 PLN.
References
Ivantysynova, M., Ivantysyn, J.: Hydrostatische Pumpen und Motoren. Konstruktion und Berechnung. Vogel, Wurzburg (1993)
Manring, N.D., Johnson, R.E.: Modeling and designing a variable-displacement open-loop pump. J. Dyn. Syst. Meas. Control 118, 267–271 (1996)
Osiecki, A.: Hydrostatyczny Napęd Maszyn. WNT, Warszawa (2004)
Osiecki, L.: Mechanizmy rozrządu hydraulicznych maszyn wielotłoczkowych osiowych. Wydawnictwo Politechniki Gdańskiej, Gdańsk (2006)
Osiecki, L.: Wpływ przestrzeni martwej na straty energetyczne w pompach wielotłoczkowych. Hydraulika i Pneumatyka 3/2007 (2007)
Scharf, S., Murrenhoff, H.: Measurement of friction forces between piston and busching of an axial piston displacement unit. Int. J. Fluid Power 6(1), 7–17 (2005)
Załuski, P.: Influence of the position of the swash plate rotation axis on the volumetric efficiency of the axial piston pumps. Machines, Technologies, Materials issue 11/2014, Sofia (2014)
Załuski, P.: Wpływ położenia osi obrotu wychylnej tarczy na sprawność objętościową pomp wielotłoczkowych osiowych. Ph.D. dissertation, Gdansk University of Technology (2017)
Załuski, P.: Prototype pump testing, Research report in project LIDER/22/0130/L-8/16/NCBR/2017 Gdańsk (2019)
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 2021 The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG
About this paper
Cite this paper
Załuski, P. (2021). Experimental Research of an Axial Piston Pump with Displaced Swash Plate Axis of Rotation. In: Stryczek, J., Warzyńska, U. (eds) Advances in Hydraulic and Pneumatic Drives and Control 2020. NSHP 2020. Lecture Notes in Mechanical Engineering. Springer, Cham. https://doi.org/10.1007/978-3-030-59509-8_12
Download citation
DOI: https://doi.org/10.1007/978-3-030-59509-8_12
Published:
Publisher Name: Springer, Cham
Print ISBN: 978-3-030-59508-1
Online ISBN: 978-3-030-59509-8
eBook Packages: EngineeringEngineering (R0)