Patent Application
20050226737
Several different types of axial piston hydraulic pumps are known in the prior art. For example, one type of axial piston pump is known as a slipper type axial piston pump. The slipper type axial piston pump comprises a piston body with a substantially spherical ball at one end which engages a bearing of a slipper. Advantages of the slipper type axial piston pump are that the pump has a long life, is robust, and very durable. Additionally, there are low contact stresses within the pump. Though advantageous in some regards, the slipper type axial piston pump has many disadvantages, including that it has many parts including a slipper hold down mechanism that consists of a number of complex parts. Additionally, the hydrostatic balance requires that an orifice be drilled through the piston and slipper, an operation that can be extremely expensive. This orifice causes oil to be lost in the pump thus causing reduced efficiency. Slipper type pistons also have a comparatively higher mass that must reciprocate into and out of a cylinder bore with every revolution of the pump.
A second type of axial piston pump is a bull nose type axial piston pump. A bull nose axial piston pump has a hollow piston body with a spring disposed therein to hold down the piston and the end of the piston engages race elements having a ball thrust bearing therebetween. The bull nose design is very inexpensive because it does not have slippers and thus it is simple to produce. However, the bull nose end of the piston contacts the race of the thrust bearing in a point contact. Because a high load is transferred in this point contact, high contact stresses result. High contact stresses limit the use of bull nose pistons to low power units in applications where comparatively short unit lifetime is acceptable.
Currently there is a problem in the art in that slipper type axial piston pumps suffer from the high manufacturing costs of precision machined slippers, as well as, high manufacturing costs of the piston body. Slipper type axial piston pumps also suffer from lowered efficiency due to the fluid loss that results when establishing hydrostatic oil film on which the slipper runs, as well as, rotational speed limitations that result from the high slipper mass. On the other hand, bull nose type piston pumps are limited in pressure, displacement, and life. These practical limits prevent more wide spread use of bull nose piston pumps. Consequently, regardless of which type of piston is currently used, a manufacturer will incur disadvantages in either pump life or pump expense.
There is a need in the art for an axial piston pump that combines the elements of the slipper type axial piston pump and the bull nosed type axial piston pump to provide an axial piston pump that overcomes the disadvantages found in the prior art. Additionally, there is a need in the art to modify the rotating kit of an axial piston pump that will improve performance and reduce cost of the pump.
Thus, it is a primary object of the present invention to provide an axial piston rotating group which will have performance and life which approaches or exceeds the performance or life of a slipper type axial piston pump, at a low cost which approaches the low cost of a bull nose piston pump.
It is yet another object of the present invention to provide an axial piston pump that displays long life, is robust, is durable, and has low contact stresses without the use of slippers.
Yet another object of the present invention is to provide an axial piston pump that can be produced at a lower cost than a slipper type axial piston pump.
Another object of the present invention is to provide an anti-frictional bearing that engages race members to improve the contact stresses within an axial piston pump.
Still a further object of the present invention is to provide a piston pump that eliminates the need to use pressurized fluid to establish a hydrostatic film on the slipper and thereby improves efficiency.
Another object of the present invention is to provide a pump with slipper like features with a lower reciprocating mass.
These and other objects, features, or advantages of the present invention will become apparent from the specification and the claims.
The present invention is an axial piston pump that combines the technologies of a slipper type axial piston pumps and a bull nose type axial piston pumps. The axial piston pump has at least one elongated hollow piston having a spring disposed within the hollow piston body creating a spring force on a pseudo slipper. Alternatively, a spring may be disposed outside the piston and creating a spring force in the same manner. The pseudo-slipper is in swiveling contact with the piston body and may optionally be in further contact with an anti-frictional thrust bearing that has a first and second race element in parallel relation and a bearing therebetween. If a variable displacement piston unit is desired, an anti-frictional thrust bearing is then operably connected to the swashplate to control the angle of the swashplate.
The figures show an axial piston pump 10 of the present invention. The axial piston pump 10 shows a cylinder block 12 having a piston body 14 disposed within. Additionally, piston body 14 has an elongated bore 16 disposed therethrough that may receive a spring element 18 such that the spring element 18 engages an internal wall of the body 14 at one end and a wall of the cylinder block 12 at the opposite end. At a first end of the piston body 14 is a ball 20 that swivelably connects to a slipper element 22 that can be considered a pseudo slipper. Pseudo slipper 22 engages an anti-frictional thrust bearing 24 having a first race element 26 and a second race element 28 with a bearing element 30 positioned therebetween. Operably connected to the anti-frictional thrust bearing 24 is a swashplate (not shown).
In operation, the spring 18 acts as a hold down force on the piston body 14 thus eliminating additional components. As the piston body 14 moves the slipper element 22 swivels or rotates around the ball portion 20 of the piston body 14 thus causing the a distribution of the piston force on the anti-frictional thrust bearing 24. As the cylinder block 12 continues to rotate the pistons 14 continue to reciprocate into and out of their cylinder bores. In this manner, the hydraulic fluid is displaced and the hydraulic power unit acts as a hydraulic pump or a hydraulic motor.
It should be appreciated that by using a spring 18 within the cylinder bore 16 removes the need for a slipper hold down mechanism thus eliminating unneeded, complex and expensive components. Additionally, the pseudo slipper design decreases contact stresses as compared to a bull nose axial piston and also increases the amount of life of the piston compared to the bull nosed axial piston. Thus, the present invention seeks to approach the high performance of slipper kits while seeking a low cost level which is nearer the cost range of bull nose piston kits. Because the present invention does not require a hydrostatic balance film, as seen in slipper type axial piston kits, oil is not lost through the piston and any efficiency of the piston is not decreased. Additionally, one skilled in the art will understand that spring element 18 can be within bore 16 or optionally outside the bore 16 and still perform a hold down function.
The anti-frictional thrust bearing 24 additionally helps reduce contact stresses as compared to a bull nose type axial piston pump. The anti-frictional thrust bearing can be any one of the group consisting of a ball bearing, a roller bearing, a tapered roller bearing, or another suitable bearing. Additionally, by eliminating the need for a hydrostatic film and thereby eliminating the oil leakage that is used to create such a film, losses are reduced and efficiency is improved. Finally, by replacing a comparatively large and massive slipper of the standard design with a pseudo-slipper of the present invention, reciprocating mass is reduced. Consequently, all of the objects of the present invention have been attained.
It will be appreciated by those skilled in the art that other various modifications could be made to the device without the parting from the spirit in scope of this invention. All such modifications and changes fall within the scope of the claims and are intended to be covered thereby.
