Device for biasing a cylinder drum of a variable-displacement axial piston machine against an associated slide valve member
The present invention refers to a device for biasing a cylinder drum of a variable-displacement axial piston machine against an associated slide valve member and comprising a spring means, the axial piston machine having between the cylinder drum and a drive disk a synchronizing shaft for mutual synchronous rotation of said drum and disk, which shaft at each end has a universal joint of so-called tripoid type for angularly rigid but pivotable connection with the drum and the drive disk, respectively, the synchronizing shaft furthermore being supported at each end approximately axially from the drum and the drive disk, respectively, by a link pin. 
     In axial piston machines, it is essential that the end surface of the cylinder drum is kept biased or pressed against a slide valve surface. For obtaining this, a plurality of different proposals of basically two types have been made during the years, i.e. one type in which the biasing force is obtained by a slotted resilient ring or similar measures acting on the outer peripheral surface of the cylinder drum (see e.g. the Swedish Patent Spfn. 320 269 or its U.S. counterpart), or another type in which the biasing force is applied centrally on the drum. As an example of a device of the latter kind in which the biasing force is centrally applied, see Wahlmark U.S. Pat. No. 3,289,604. The central rotary bearing of the drum by means of a thrust-type roller bearing requires radial space, however, and it has also other drawbacks. The Applicant&#39;s prior Swedish Patent Spfn. 74 06378-5 describes a proposal for biasing a cylinder drum, in which the biasing force is applied to the end surface of the drum opposite to the valve surface thereof by means of a peripheral bearing ring, upon which acts the spring means. Formerly, this last-mentioned structure has been considered to be the most space-saving one obtainable, but during the further development of axial piston machines it has turned out in such machines of variable-displacement type that this biasing device still requires certain valuable space. In connection with ever-increasing hydraulic pressures this bearing ring has also been found to incur noticeable friction losses. 
     There is thus a demand for providing a still more compact structural design of the cylinder drum biasing device, and at the same time such a device has to be free of losses. The main object of the present invention therefore is to suggest a device which makes this possible, and the invention is substantially distinguished in that the spring means is mounted between a link pin bearing cup, axially movably arranged in the center of the drive disk, and an abutment means which is coaxial with and adjustably secured to said disk and the associated machine shaft, such that the biasing force is applied to the drum from said abutment means under the intermediation of said synchronizing shaft. 
     Since in a variable-displacement axial piston machine having a synchronizing shaft of the aforementioned kind there occurs a long series of tolerances of the parts built together, the biasing pressure essentially determining for the operation of the machine cannot be designed into the machine but has to be obtained by adjustments. The major advantage of the design now suggested according to the invention is that this adjustment can be carried out in a very simple and hitherto impossible manner, namely from the outside of the machine through a central bore in the machine shaft, where thus access to the adjustable abutment means can be had. Since the biasing force is transferred only through the synchronizing shaft with its pair of link pins, the most compact design is obtained, since no space-requiring spring arrangements are necessary, either internally or externally of the cylinder drum or in the synchronizing shaft. 
    
    
     By way of example, the invention will be further described with reference to the accompanying drawing, which illustrates a longitudinal section through a variable-displacement axial piston machine provided with the biasing device according to the invention. 
    
    
     In the drawing it is thus illustrated a variable-displacement axial piston machine with a cylinder drum 1 which centrally is rotatably journalled on a pin 2 carried by a valve segment 3. Said segment 3 comprises inlet and outlet conduits for the cylinder drum 1, and for varying the displacement of the machine the segment is reciprocatory moveable in a groove 4 in a machine housing 11 in the plane of the drawing by means of an adjustment device, of which only an associated actuation arm 5 is illustrated. 
     As conventional, in the cylinder drum 1 is arranged a plurality of circumferentially equally spaced cylinder bores 6, such as nine such bores, in each of which a piston 7 runs. Through a piston rod each piston is connected with a part-spherical piston rod end, which is journalled universally pivotable in a corresponding recess in a drive disk 10. In its turn, said disk 10 is rotatably mounted in the surrounding machine casing 11 and rigidly secured to or integral with the machine shaft 9. 
     For securing a permanently synchronous rotational movement of the cylinder drum 1 and the drive disk 10 independent of their mutual angular position for various selected displacements of the machine, they are connected to each other in a conventional way by means of a synchronizing shaft 12. At each end said shaft 12 carries a universal joint 13, preferably of so-called tripoid type, i.e. having three bearing studs 13a extending at right angles from the shaft 12 and equally spaced circumferentially, each stud furthermore carrying a rotatable bearing roller 13b. Said rollers are housed in mating axial bearing grooves 13c formed in the wall of a central bore in the end surface of the cylinder drum, which faces the shaft 12, and of the drive disk 10, respectively. For allowing great adjustment angles of the cylinder drum 1 relative to the drive disk 10 to either side of the central neutral position, namely of an order of magnitude of about 42° or even more, it is well known that both ends of the synchronizing shaft 12 must be free to orbit in a small circular path. For controlling this orbital motion, a link pin 14 is inserted between a bearing cup 14a at the end of the shaft 12 and a central bearing cup 14b in the cylinder drum 1 and the drive disk 10, respectively, so as to support the synchronizing shaft 12 approximately axially therefrom. Preferably, both link pins 14 have a cylindrical shape with spherically rounded ends to rest in the bearing cups 14a, 14b. 
     According to the present invention the bearing cup 14b is now formed in a support means 15, which is mounted in the drive disk 10 coaxially therewith and which is axially movable in a central bore 16 extending through the drive disk and the machine shaft 9 rigidly secured thereto. In said bore 16 is inserted an abutment means 17, and between said means 17 and the support means 15 is mounted a compression spring 18. The axial position of the abutment means 17 in the bore 16 and hence the value of the force applied by the compression spring 18 to the cylinder drum 1 under the intermediation of the synchronizing shaft 12 is defined by a set screw 19, threaded into a threaded portion of the bore 16. When needed, said set screw may be lockable by means of a locking screw 20 axially outside the same. It is to be particularly remarked that said screws 20 and 19 thus are accessible from the free end of the machine shaft 9, which is located outside the machine housing 11. In order to restrict the possibilities of the cylinder drum 1 to raise axially from its engagement with the valve segment 3, it is furthermore preferred that the spring travel between the support means 15 and the abutment means 17 be restricted by a spacing member 21. In the embodiment illustrated in the drawing, in which the the helically-coiled compression spring 18 is long and thin, it is thus suitable that said spacing member 21 has the form of a rod, located inside the spring and having a length which provides for an axial play or clearance between the abutment means 17 and the support means 15 of the order of 0.1 mm. 
     In the device according to the invention it has thus turned out that the necessary biasing force on the cylinder drum 1 does not need to be applied directly on said drum but the synchronizing shaft 12 might advantageously be utilized for the transmission of said biasing force in spite of the angular position between the shaft 12 and the cylinder drum 1 and the drive disk 10, respectively. In this way it primarily achieved that the space requirements for the cylinder drum 1 and its biasing mechanism are reduced and moreover, an adjustment of the biasing force on the cylinder drum 1 can be carried out very easily from the outside of the machine without any need for dismantling the same in any way. 
     Although in the embodiment described above, the support means 15 is made axially movable separately in the bore 16 in the drive disk 10, said support means 15 might also be rigidly secured to or integral with a cup-shaped bearing means (not illustrated), in which are formed the bearing grooves 13c and which is non-rotatably but slightly axially movably connected with the drive disk 10. Also other modifications of the above-described embodiment of the inventive subject matter can be made within the scope of the accompanying claims, as will be obvious to the artisan.