Generally, a bent axis type hydraulic pump is a drive disc on a rotational shaft, coupled with a cylinder block through pistons which are reciprocably received in the cylinder block. Therefore, when the bent axis type hydraulic machine is a hydraulic pump, the hydraulic reaction forces acting on the pistons on the high pressure side in a discharge stroke are supported by the rotational shaft through the drive disc. Similarly, when a hydraulic motor, the hydraulic reaction forces which act on the pistons on the high pressure side in the suction (feeding) stroke are supported by the rotational shaft through the drive disc.
Accordingly, in a bent axis type hydraulic machine of this type, the rotational shaft is subject to radial and thrust loads of the hydraulic reaction forces, and therefore it is necessary to mount the rotational shaft in a suitable condition for supporting these loads.
In this connection, it has been the general practice in the prior art to result to the so called mechanical type support which mechanically supports the rotational shaft rotatably by ball or roller bearings capable of supporting the radial and thrust loads, the partial hydrostatic support which mechanically supports either the radial or thrust loads by a roller or ball bearing while supporting the other load by a hydrostatic bearing, or the total hydrostatic type support which supports the entire loads hydraulically by means of hydrostatic bearings.
Of these various types of shaft support, a hydraulic machine employing a shaft support of the partial hydrostatic type bearing is described, for example, in Japanese Laid-Open Patent Application 60-224981, wherein a rotational shaft is supported by a hydrostatic thrust bearing composed of a stationary bearing and a movable bearing, with the movable bearing being provided with springs in an outer ring to counteract the thrust load which acts on the rotational shaft, along with pistons which are located on the side of the outer ring to generate a pressure in the same direction as the springs and to which oil pressure is applied from the high pressure area in the cylinder block.
On the other hand, a hydraulic machine supporting a shaft totally with hydrostatic bearings is described in Japanese Laid-Open Patent Application 59-131776, which is provided with a radial load bearing sleeve and a thrust load bearing plate within a casing, in combination with a drive flange which is provided movably between the bearing sleeve and the bearing plate to serve also as a drive disc. The drive flange has one end face thereof securely connected to a rotational shaft and the other end face coupled with pistons. Further, a plural number of pressure chambers constituting a hydrostatic radial bearing, are defined between the outer peripheral surface of the drive flange and the bearing sleeve, and a plurality of drive shoes, constituting a hydrostatic thrust bearing, are provided on one end face of the drive flange. The pistons and drive flanges have oil passages bored therein for supplying high pressure oil independently to the hydrostatic radial and thrust bearings from the corresponding cylinders, to thereby hydrostatically support the radial and thrust loads.
When the bent axis type variable displacement hydraulic machine is a hydraulic pump, the hydraulic reaction forces acting on the pistons in the high pressure area in the discharge stroke are supported by the rotational shaft through the drive disc. In such a case, the point of exertion or the acting point of the resultant force of such hydraulic reaction forces is located in an eccentric position with respect to the axis of the rotational shaft. Besides, the positions and the number of pistons in the high pressure area changes with the rotation of the cylinder block, so that the location of the acting point of the resultant force of hydraulic reaction forces also changes with the rotation of the cylinder block, generally drawing a locus of the shape of ".infin.". Therefore, the center of the locus of the resultant force of hydraulic reaction forces is usually referred to as an acting point of mean hydraulic reaction force. When the bent axis type hydraulic machine is a hydraulic motor, the acting point of the resultant location off the axis of the rotational shaft.
As a result of the off-axis location of the acting point of the resultant force of hydraulic reaction force relative to the axis of the rotational shaft, the drive disc is subject not only to the radial and thrust load components but also to moment components which are induced by the radial and thrust load components.
However, in the prior art bent axis type hydraulic machine, especially in case of the hydraulic machine of aforementioned Japanese Laid-Open Patent Application 59-131776, a hydrostatic radial bearing is constituted by supplying high pressure oil from pistons to corresponding pressure chambers which are formed in 90.degree. shifted positions between the drive flange and bearing sleeve according to the total number of the pistons, and a hydrostatic thrust bearing is constituted by supplying high pressure oil from pistons to corresponding drive shoes with pressure chambers provided on the drive flange according to the total number of the pistons, thereby supporting the radial and thrust loads and the moment load about the rotational shaft. Namely, the conventional hydrostatic radial and thrust bearings have a load supporting capacity only in the areas on the side of the acting point of the resultant force of hydraulic reaction force where high pressure oil is supplied from the pistons, and not in the areas away from the acting point or in the areas where high oil pressure is not supplied.
Nevertheless, the above-mentioned prior art hydraulic machine is required to support the imposed radial and thrust loads and the moment load about the rotational shaft not only by the hydrostatic bearing in the areas on the side of the acting point but also by the hydrostatic bearing in the areas away from the acting point.
Consequently, there occurs an imbalance between the hydrostatic bearing with a load supporting capacity, which is located on the side of the acting point, and the hydrostatic bearing without a load supporting capacity, which is located on the side away from the acting point, resulting in non-uniform thickness of oil films which are formed on the sliding surfaces of the drive flange and drive shoes. This will induce metal contact of the sliding surfaces in the areas away from the acting point, accelerating localized abrasive wear of the sliding surfaces and increasing the leak flows.
Especially in a case where the sliding surfaces of the hydrostatic radial bearing, such as the outer peripheral surfaces of the drive flange serving as the drive disc or the inner peripheral surfaces of the bearing sleeve, are abraded during use over a long time period, the gaps between the surfaces are broadened to promote unstable radial vibrations of the drive flange as well as the vibrations of contact between the piston and the cylinder block. As a result, a problem arises in that the increased fletching abrasion at the contacting portion brings about an increased degree of contacting stress of the pistons.
The present invention contemplates to solve the above-mentioned problems of the prior art, and has as an object the provision of a bent axis type variable displacement hydraulic machine employing a partial and/or total hydrostatic bearing support which can ensure operations of high stability and reliability by suppressing abrasive wear in the radial direction and leak flow rate during use over a long time period.