Pumps – Three or more cylinders arranged in parallel – radial – or...
Reexamination Certificate
1999-05-06
2002-06-18
Tyler, Cheryl J. (Department: 3746)
Pumps
Three or more cylinders arranged in parallel, radial, or...
C091S499000
Reexamination Certificate
active
06406271
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to swashplate type axial-piston hydraulic pumps, and in particular to innovations which increase the efficiency, adjustment range and speed capability and reduce the noise, size, weight and cost of such pumps.
Swashplate type axial-piston hydraulic pumps are well known in the art and typically include a generally cylindrical cylinder barrel rotatably mounted within a pump housing. One or more pump piston bores, having pump pistons reciprocably mounted therein, are disposed around the rotational axis of the cylinder barrel in parallel, or almost parallel alignment therewith. The ends of the pistons project beyond the end of the cylinder barrel so as to engage the surface of an angled swashplate stationarily mounted adjacent the end of the cylinder barrel within the pump housing. When the cylinder barrel is rotated within the housing, shoes, mounted to the piston ends, follow the surface of the angled swashplate with the result that the pistons are reciprocated within their respective piston bores. A valve plate, disposed adjacent the end of the cylinder barrel furthest from the swashplate, controls the ingress and egress of hydraulic fluid from the piston bores such, that a pumping effect is produced in response to rotation of the cylinder barrel within the pump housing.
Although highly advantageous in various applications, swashplate type axial-piston pumps are presently somewhat inefficient and their operational adjustment and speed range is too narrow when used e.g. as a vehicle transmission. (The adjustment range being the ratio of maximum to minimum swashplate angle which can be used efficiently). In addition, hydraulic pumps are generally too large, heavy and noisy at high power throughput and costly.
The inefficiencies are caused by friction due to high mechanical contact forces and leakage. These forces are representing mechanical loads like the side forces between piston and piston bore, retainer plate and shoe and retainer plate and retainer ring, or they are rest forces of loads which are hydrostatically balanced like the forces between shoe and swashplate, in the joint of the shoe and piston and cylinder barrel and valve plate. Furthermore, the friction force components of the mechanical contact forces produce tilting or cocking especially between the shoe and swashplate and the cylinder barrel and valve plate. Components of the piston side forces will increase the tilting between the cylinder barrel and the valve plate. These movements result in noticeable leakage and wear.
Attempts have been made to reduce the tilting or cocking of the cylinder barrel by applying counter forces which balance or nearly balance the tilting moment. Henry-Biabaud (U.S. Pat. No. 3,444,690) tries to balance axial forces and side forces of the floating spherical distributor/valve plate with radial forces of a reduction gear, located at the outer periphery of the cylinder barrel.
Further attempts have been made to reduce the tilting by supporting the cylinder barrel through a bearing located in the plane of the piston side forces on the shaft. The pulsating, resulting piston side force results in radial vibration of the shaft and therefore high frequency, small amplitude tilting of the cylinder barrel at the valve plate, creating additional leakage and wear.
Several prior attempts have been made to overcome the tilting and cocking of the shoe in relation to the face of the swashplate resulting in leakage and wear. The friction in the joint prevents the shoe from adapting fully to the face of the swashplate. A slight tilting is required, resulting in an excentricity of the mechanical force at the face of the shoe which overcomes the moment of friction in the joint These attempts to reduce the required moment and therefore the degree of tilting are directed toward the reduction of friction forces in the piston joint and increased countermoments through excentric hydraulic and mechanical forces at the face of the shoe and mechanical forces at the backface of the shoe due to a spring loaded or form locked retaining mechanism.
Previous attempts to reduce the moment of friction at the joint have lead to the increase of the pressure field within the joint to reduce the mechanical contact force by increasing the hydrostatic force, or to minimize the ball diameter with high friction forces on a small lever arm. Both solutions have had only limited success. The moment of friction on a small ball, limited by a sufficient encirclement of the socket around the ball, typically 20 or more past the geometric center of the ball, and the neck diameter between ball and piston, remains high, and a ball, noticeably larger than the piston diameter, providing space for a sufficient pressure field to reduce the mechanical contact forces, cannot be received deeper into the piston bore, therefore creating noticeable higher piston side forces or a reduced swashplate angle due to a longer lever arm between piston joint and piston bore.
Several prior art attempts have been made to create a sufficient excentric force at the face of the shoe to overcome the moment of friction of the piston joint to avoid or reduce the tilting of the shoe. This has been achieved through a not fully hydrostatically balanced axial shoe force creating a mechanical contact force, acting on the face diameter of a slightly tilted shoe, or through hydrostatic forces created through several pressure fields larger than needed which are partially depressurized due to the tilting of the shoe, creating an off-center hydraulic force. (Pat. SU 1421-894-A1). Various previous attempts have been made to provide these fields with a sufficient amount of fluid and pressure without producing an excessive amount of leakage, instability of the shoe movement, difficulties in fabricating the throttle arrangements and high sensitivity to wear or contamination. (Thoma, UK Pat. 983.310; Pat. SU 1463-951-A).
Both methods to create a counter moment to the friction moment at the joint do not produce the changing counter forces, needed at various swashplate angles and rotational positions during each revolution. Therefore, the remaining rest force or hydraulic forces are oversized at smaller swashplate angles and result in additional losses. The effects of the retaining mechanism are stated later.
Several attempts have been made to reduce the leakage and wear sensitivity at the shoe. Deflecting ends at the face of the shoe have been utilized to provide a hydrodynamic pressure field, thus reducing the size and leakage of the required hydrostatic pressure field. (Espig et al, U.S. Pat. No. 3,521,532). High strength materials for the shoe, such as steel, have been utilized with enclosed bearing material at the face of the shoe to reduce deterioration during service. (Alexanderson et al, U.S. Pat. No. 3,263,623). In both attempts, the shoe socket end is fitted over the ball at the piston by 230 or more.
Prior attempts to reduce the frictional losses between piston and piston bore have been directed toward establishing hydrodynamic or hydrostatic pressure fields at the contact areas. The variants have typically been the clearance between piston and piston bore and the elasticity and/or shape of the ends of the piston bores to improve the conditions for a hydrodynamic pressure field or to establish a hydrostatic pressure field. (Thoma, U.S. Pat. No. 3,216,333).
Additional losses occur due to mechanical preloads (spring force) between the cylinder and the valve plate and the retaining mechanism, shoe and swashplate. The spring load is needed to hold the parts in position at no or very low pressure rates and to provide additional forces at the back face of the shoe to reduce its tilting. The pre-load forces are generally constant and result in high percentile losses at low pressure rates and small swashplate angles.
Several attempts have been made to use form-locked retaining mechanisms for the shoes to eliminate the effects of the pre-load forces in several sections of the mechanism, between shoe and swashplate, s
Reinhart Boerner Van Deuren s.c.
Tyler Cheryl J.
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