The present invention relates to fluid-pressure apparatus such as a pump or motor.
In particular, the present invention relates to a radial type of fluid-pressure apparatus wherein pistons are circumferentially distributed about the axis of a crankshaft projecting substantially radially therefrom in the manner of spokes of a wheel.
The present invention is particularly applicable to hydraulic pumps or motors utilizing a liquid such as oil.
Although the following description relates to a radial type of hydraulic apparatus wherein the pistons have a star arrangement, the invention is also applicable to an in line arrangement of the pistons.
In conventional fluid-pressure apparatus of the above type the circumferentially distributed pistons are connected to a crankpin of the crankshaft by way of a spherical coupling structure. Each piston is tubular and extends into the interior of a sleeve which has an inner surface slidably engaging the outer surface of the piston. The sleeves are coupled by way of ball joints to the housing and the oil under pressure which circulates therein flows into the interior of the sleeves as well as into the interior of the pistons which are formed with axial bores for this purpose.
Conventional structure of the above type suffers from a number of drawbacks. Thus such conventional structures have undesirably high manufacturing costs particularly because of the precise fit required between the pistons and the sleeves in which they slide as well as because of the ball joints which are required to connect the outer ends of the sleeves to the housing. These conventional structures result in a high degree of friction during operation with a resulting undesirable reduction in efficiency. When operating at pressures on the order of 200-250 atmospheres, there is a considerable deformation of the components as well as an undesirably high degree of wear, leakage of oil, and seizing is not uncommon. In addition, because of the large mass of the pistons and sleeves and the relatively great distance between the crankpin and the part of the casing to which the outer ends of the sleeves are connected by the ball joints, it is not possible to achieve speeds greater than 400-500 r.p.m.