SWASHPLATE HOLDDOWN MECHANISM

Abstract: A swashplate holddown mechanism for a variable displacement hydraulic unit comprises means (41, 41) mounting a displacement control (32) and associated linkage (36.38) on one side of an axial centreline (28), and a swashplate centring mechanism (50) on the diametrically opposite side of the axial centreline. The mounting means (40, 41) and centring mechanism (50) provide first and second axial biasing forces respectively, holding the swashplate (24) firmly under bias against its bearing means (26). The mounting means (40, 41) requires a degree of axial movement of the linkage (36, 38) to permit the first axial biasing force to take effect.

Description: Field of the invention In variable displacement hydraulic units, especially pumps of either the single flow direction or the reversible flow type, it is desirable to have means which positively locate the swashplate in a zero displacement position when there is no control input to move the swashplate to a stroking position, and to hold the swashplate firmly down on its bearings. The invention of our copending European Application No. 86900488.7 (EP 204837A), from which this Application is divided, provides a simple and compact means for centring or levelling the swashplate, that is holding it in a zero displacement position. In addition, there is a need for the mechanism of the present invention which is used as a holddown device for the swashplate to help retain the swashplate in its bearing seat. Background of the Invention Many hydraulic units of the variable displacement type have a rotating cylinder block with pistons axially movable therein. The displacement of the hydraulic unit is proportional to the stroke of the pistons within the cylinder block. Where the hydraulic unit is of the axial piston type, the pistons or piston slippers engage a tiltable swashplate to vary the stroke of the pistons. When the swashplate is perpendicular to the axis of the cylinder block, the swashplate is in the neutral or a zero displacement position and the hydraulic unit has no output. In order to maintain the swashplate in its zero displacement position when no control forces are applied thereto, various swashplate levelling or centring mechanisms have been utilized. Generally such centring mechanisms are a plurality of springs which apply opposite biasing forces on the swashplate at points spaced from the tilt axis of the swashplate. U.S. Patent No. 3359727 (Hann) shows the centring springs to be placed within hydraulic servo mechanisms which are utilized to control the tilt of the swashplate. Such springs may be of a short unstressed length or have a length limiting means to prevent engagement of the spring with the servo piston until the swashplate tilts toward the servo cylinder containing the spring. This, however, requires very accurate spring lengths or adjustment thereof to minimize backlash and insure that the centring force of a given spring does not start until the swashplate is tilted toward that spring but still assures that the spring starts to act on the swashplate exactly when the swashplate is in the zero displacement position. Another prior proposal is in US Patent No. 4283962 (Forster) which discloses a swashplate centring mechanism for a variable displacement hydraulic unit comprising a housing, a cylinder block rotatable in the housing about an axial centreline with pistons axially movable therein and a swashplate tiltable about a tranverse axis perpendicular to the centreline and having a cam surface engageable by the pistons to control the stroke of the pistons within the cylinder block. The centring mechanism of Forster comprises a yoke member having a pair of spaced apart swashplate contact points, one disposed on each side of a plane containing the axial centreline and the transverse axis. Biasing means bias the yoke member towards the swashplate whereby both of the cam contact points are intended to contact the swashplate when the swashplate is centred, or at its zero displacement position. In Forster, at the zero displacement position of the swashplate the yoke member is intended to come into firm abutment with a pair of eccentrically mounted stop members. Adjustment of the individual stop members is necessary both to set the precise angle of zero displacement and to eliminate any backlash, or slack, between the stop members and the yoke member. This dual adjustment is difficult because either of the stop members can lift away from the yoke, allowing backlash therebetween, without affecting the zero displacement setting. Another version of a swashplate levelling and holddown device is taught in U.S. Patent No. 4142452 (Forster + Heyl) teaching a cradle type swashplate resting in a roller bearing pocket and having four swashplate positioning devices located in the corners of the hydraulic unit housing. In one embodiment of Forster + Heyl, all four mechanisms are servo pistons with prestressed springs such as mentioned above. In another embodiment of Forster + Heyl two of the locating mechanisms, located on one side of the tilt axis of the swashplate, are servo units while the two locating mechanisms located on the opposite side of the tilt axis are spring units. Since the spring units are only on one side of the tilt axis, the spring units cannot be used as a levelling device but can only counterbalance the axial biasing force of the servo cylinders on the opposite side of the tilt axis. Even in the first embodiment where the four spring servos apply an axial holddown force on the cradle swashplate, that is to hold the cradle swashplate against its roller bearings, the four springs must be critically dimensioned and adjusted during assembly to provide a spring centring function on the swashplate. In the prior art structures such as Forster + Heyl, in order to have counter-balancing spring centring, it is quite critical that the holddown springs have precisely matched axial force characteristics which requires adjustment and the associated extra parts and assembly steps. Such adjustment must compensate for levelling and backlash. Without complete backlash adjustment, accurate levelling cannot be achieved. Furthermore, use causes a spring to lose its spring rate or take a set and this characteristic alters any previous adjustment. Even though the spring rate loss characteristic may only be a few percent of the total force supplied by the spring, any difference in spring rate loss has a major effect upon the centring forces of the spring and thus prevents swashplate from centring at its zero displacement position. Summary of the Invention The present invention is directed to a swashplate holddown mechanism for the swashplate which does not require the accurate matching of the holddown force of different springs, and which results in a centring mechanism that is easier and more accurate to adjust than that of Forster. The invention provides a swashplate holddown mechanism for a variable displacement hydraulic unit comprising a housing, a cylinder block rotatable in the housing about an axial centreline and having pistons axially movable therein, a swashplate tiltable about a transverse axis perpendicular to the centreline and having a cam surface engageable by the pistons to control the stroke of the pistons within the cylinder block, bearing means on the housing supporting the swashplate for tilting movement about the said transverse axis, and a displacement control means operatively connected to the swashplate by linkage means to vary the tilt of the swashplate and thereby to control the axial position of the pistons in the cylinder block, CHARACTERISED IN THAT the holddown mechanism comprises mounting means locating the linkage means on one side of the cylinder block and permitting axial movement of the linkage means parallel to the axial centreline spring means axially biasing the linkage means towards the swashplate bearing means to apply a first axial biasing force on the swashplate at a position on one side of the axial centreline and a swashplate centring mechanism located on the opposite side of the axial centreline applying a second axial biasing force on the swashplate parallel to the first biasing force but on the opposite side of the axial centreline, whereby the swashplate is firmly held under bias against its bearing means. Brief Description of the Drawings Fig. 1 is a sectional view of a hydraulic unit having a positive swashplate centring and holddown mechanism according to the present invention. Fig. 2 is a sectional view taken along lines 2-2 of Fig. 1 and showing the positive centring and holddown mechanism and its cooperation with the cradle swashplate. Fig. 2A is a partial sectional view taken along lines 2A-2A of Fig. 2 showing an eccentric adjustment mechanism which may be used. Fig. 3 is a schematic view showing the cooperation of the centring mechanism with the swashplate as the swashplate moves from a centred position. Fig. 4 is a sectional view taken along line 4-4 of Fig. 1 showing the mounting of the centring mechanism relative to the side cover. Fig. 5 is a side view taken along line 5-5 of Fig. 1 showing a rotatable side cover which may be used to mount and adjust the centring mechanism as an alternative to the adjustment mechanism of Figures 2 and 2A. Fig. 5A is a sectional view taken along line 5A-5A of Fig. 5. Brief Description of the Preferred Embodiments Fig. 1 shows an axial piston hydraulic unit 10 having a cylinder block housing 12 and an end cap 14. Located within the housing 12 is a rotable cylinder block 16 having plurality of axially sliding pistons 18 located therein. Each piston has a slipper 20 which engages a planar front cam surface 22 of a cradle type swashplate 24. The swashplate 24 is mounted on a pair of semi-circular roller bearings 26 for tiltable movement about a transverse swashplate axis 27, which is perpendicular to a cylinder block axis or centerline 28. Such axial piston hydraulic units using a cradle swashplate are well known and the particular structure of the parts heretofor described are not material to the present invention. Located in the upper portion of Fig. 1 is a displacement control input 30 having a pair of servo cylinders 32 (only one shown) acting on a pin 34 to move a control lever 36 having a central pin 38. A bolt 40 wedges the lever 36 into a tapered groove 41 on the side of the swashplate 24. With the lever arm 36 secured to the side of the swashplate 24, the control lever 36 must follow the same tilting or pivotal movement of the swashplate 24 within its bearings 26. The swashplate is actually a portion of a cylinder wherein the center of pivotal movement of the swashplate 24 is the swashplate axis 27 which is located forward of the front face of the swashplate forming the cam surface 22 for the piston slippers 20. Thus pivotal movement of the swashplate 24 also results in identical pivotal movement of the control lever 36 about the swashplate pivot axis 27. The central pin 38 is located as close to the pivot axis 27 as possible although, as seen in Fig. 1, it is spaced slightly forward of the axis 27 to prevent interference with other parts of the hydraulic unit such as the piston slippers 20 or the slipper holddown structure. Thus, pin 38, being substantially on axis 27, has very little movement induced by pivotal movement of the control arm 36 when a control input is applied on servo pin 34. The particular control input is not of particular importance, and the input could also be manual or electrical in place of the hydraulic input provided by the servo cylinders 32. Central pin 38 is secured to an angled bracket 42 which is axially biased by a spring 44 seated in a pocket 46 of the end cap 14. The axial biasing force is applied through bracket 42, pin 38, lever 36 and bolt 40 to the upper side of swashplate 24 as shown in Fig. 1. This provides a holddown force on the swashplate 24 biasing the swashplate against the upper of the two roller bearings 26. It is envisioned that the levelling features of the present invention can be used on not only the cradle type swashplate 24 as shown on the drawings, but is also equally applicable to a trunnion mounted or other mounted swashplate. However. where a cradle type swashplate is used the centring mechanism of the present invention applies a holddown force on the opposite side of the swashplate 24 which cooperates with the holddown force of spring 44 as just described to keep the cradle type swashplate 24 seated in the bearings 26. Now referring to Figs. 1, 2 and 3, the preferred form of centring mechanism will now be described. Located at the lower side of the housing as viewed through Fig. 1 is a side cover 48 which amounts the centring mechanism. The centring mechanism comprises a cam member 50 which is actually movable along a cam axis 52 parallel to the cylinder block axial centerline 28. The cam 50 includes a leg portion 54 having a pair of mounting slots 56 and 58 positioned about mounting pins 60 and 62 respectively. The cam 50 is furthermore provided with a transverse member or crossbar 64 having a pair of wings which extend perpendicular to the cam axis 52. At the outer ends of the crossbar 64 is a pair of rounded contact points 66 and 68 designed to engage the front Surface of the cam 24. The two contact points 66 and 68 are in a plane perpendicular to cam axis 52. While the contact points 66 and 68 engage two of the four corners of a rectangular faced swashplate, the cradle swashplate may also be provided with two bosses 70 and 72, the former of which is shown in both Figs. 1 and 2, which extend outwardly from the body of the swashplate 24 to form a planar surface which is engaged by contact points 66 and 68. This permits a narrower swashplate body to provide clearance for other elements. On the crossbar 64 and opposite the contact points 66 and 68 are angled portions 74 and 76 which have riveted thereto spring seats 78 and 80. Each of the spring seats provides a mounting for an outer spring 82 and an optional inner spring 84. Springs 82 and 84 may abut flat against the face of the end cap 14 as in Fig. 1 or can sit in pockets 86 and 88 formed in the end cap 14 as in Fig. 2. In the preferred form of practising in the invention, one of the pockets such as 88 is deeper than the other pocket 86 for reasons to be explained later. The springs 82, and also the optional springs 84 when utilized, provide an axial biasing force to the right as seen in Figures 1, 2 and 3, on the cam member 50, to bring at least one of the contact points 66 or 68 into engagement with the swashplate 24. Since the axis 52 of the cam member is parallel to the axis 28 of the cylinder block, the cam 50 can move to the right until both contact points 66 and 68 engage the swashplate 24, at which time the planar cam surface 22 of the swashplate 24 upon which the piston slippers 20 ride is perpendicular to the cylinder block 16. Under such conditions herein referred as a zero displacement condition. rotation of the cylinder block does not generate flow if the hydraulic unit 10 is a pump and produces zero torque output if the hydraulic unit 10 is a motor. In Fig. 3. the swashplate 24 and the cam 50 are shown in solid lines when in the zero displacement position. However, when the swashplate 24 is tilted counterclockwise about axis 27 due to the servo 32 or other input, the upper portion of the front face of the cam 24, which is engagement with the contact point 66, forces the cam 50 to move to the left against the bias of both the upper and lower springs 82 and 84. This left position is represented by the contact point 66′. Since the whole cam 50 moves to the left, the lower contact point, now 68′, is no longer in engagement with the lower portion of the swashplate 24 which has tilted to the right. Clockwise rotation of the swashplate 24, such as a reverse mode of operation, causes the lower portion of the swashplate 24 to move the cam 50 again to the left, but with the lower contact point 68′ now in engagement with the swashplate 24. When the swashplate 24 is in either the clockwise or counterclockwise position as described above, the cam 50 is still biased towards the right by the springs 82 and 84 so as to bias the swashplate 24 toward a centring position, that is a position with the piston slipper riding cam surface 22 to be perpendicular to the axis 28 of the cylinder block 16 when no input control forces are applied to the swashplate 24. In such centred or neutral position, both contact points 66 and 68 engage the front surface of the swashplate 24 to positively retain the swashplate 24 in the zero displacement position. Since a line joining the contact points 66 and 68 is perpendicular to the cam axis 52 and the centreline 28, and since they are both part of the cam 50 which can only move along the cam axis 52, there is no possible relative movement between the contact points 66 and 68. Thus, the swashplate 24 is positively centred to the zero displacement position. If, for some reason, one set of the springs has a different biasing force than the other set of springs, this cannot cause tilt of the cam 50 about cam axis 52 (once established). Since the cam member 50 moves only along cam axis 52 and is not subject to tilt, several embodiments are envisioned to provide adjustment of the cam axis 52 to take up manufacturing tolerances and assure, that the cam axis 52 is parallel to the centreline 28 of hydraulic unit 10. In the embodiment taught in Figures 1, 2 and 3, the pins 60 and 62 are of a diameter substantially equal to the width of the slots 56 and 58 so that the edges of the slots 56 and 58 engage both sides of the pins. The pin 60 and 62 have enlarged 10 heads 60′ and 62′ respectively which trap the axial member 54 against the inside face of the side cover 48 when nuts 89 are tightened on threaded portions of the pins 60 and 62. However, as best seen in Fig. 2A, a central portion 60″ of one of the pins 60 is eccentric to the pin 60 so that rotation of the pin 60 can move the cam leg portion 54 vertically as seen in Fig. 2, since the eccentric portion 60′, engages the slot 56. Thus, even if the pins 60 and 62 are not in perfect parallel alignment with the centreline 28, rotation of the pin 60 adjusts the cam axis 52 until a parallel relationship is achieved between the cam axis 52 and the centreline 28. Once such parallel relationship is established, it is assured that the contact points 66 and 68 of the cam 50 positively position the cam 24 at zero displacement condition when there are no outside control forces applied to the swashplate 24. For the adjustment of the eccentric 60″, the pin 60 is provided with a slot 90 which can be used to rotate the pin 60 when a securing nut 89 is loosened. The outer end of the pin 60 is intended to be flush or recessed relative to the outer surface of the sideplate 48 as shown in Fig. 2A, The adjustment mechanism shown in Fig. 1 extends beyond the outer face solely for clarity purposes. While it is only necessary for one of the pins 60 or 62 to have the eccentric 60″, for adjustment of the cam line 52, it is also contemplated that both pins 60 and 62 may be provided with eccentric portions to aid in adjustment of the cam axis 52. Figs. 5 and 5A show alternative means for adjusting the cam axis 52. While the side cover 48 is shown as circular, other shapes may be utilized. However, the circular form has a particular advantage when the side cover mounting bolts 92 pass through arcuate slots 94 in the circular side plate 48. By loosening the side cover bolts 92, the side cover 48 may be rotated slightly clockwise or counterclockwise relative to the housing 12. The side cover 48 may be provided with internal edges 96 which form slots that trap the cam leg portion 54. Thus, as the side cover 48 is rotated, the cam axis 52 is adjusted until the parallel with the centreline 28. With such side cover adjustment mechanism, the pins 60 do not need the eccentric 60″ since such a second adjustment mechanism would be redundant. Thus, the threaded pins 60 with nuts 89 could be replaced with rivets. Since the edges 96 form slots which trap the cam leg 54, the pin slots 56 and 58 are slightly wider than the diameter of the pins 60 and 62 to prevent any interference fit. Fig. 4, taken as a cross section through the hydraulic unit, shows the compact space saving relationship of the cam 50 relative to a rectangular internal cavity 12′ of the housing 12 circumscribing the rotating cylinder block 16. As stated earlier, the cam 50 is held snug against the side cover 48 by the pins 60 and 62 and their enlarged heads 60 and 62. In Fig. 4, in which the adjustment means is the alternative version of Fig. 5 utilizing the edges 96 to trap the cam leg portion 54, the cam 50 is mounted with its leg portion 54 recessed into the slots formed by edges 96 of the side cover 48. In the version contemplated in Figs. 1, 2 and 3, the cam leg 54 could be mounted flush with the inside surface of the side cover 48, and the cover 48 without the slots could be of less thickness. Utilizing either embodiment, the cam leg 54 is located along a transverse centreline 98 of the housing 12 where there is little clearance between the rotating cylinder block 16 and the side cover 48. However, since the leg portion 54 of the cam 50 is flat, it occupies very little space in this transverse dimension. The wings of the crossbar 64 are bent inwardly as the wings extend outwardly from the housing transverse centreline 98. However, the clearance between the rotating cylinder block 16 and the corners of the housing cavity 12′ is considerably greater than the radial clearance along transverse centreline 98. This permits the springs 82 and 84, whose diameter is considerably greater than the width of the cam leg 54, to be located in the corners where there is greater clearance. While this is most convenient from a clearance standpoint, other designs have been tested using two springs located closer to the transverse centreline 98, and it is also possible to use a single spring on the cam axis 52, although this necessitates a greater width to the housing 12. Not only do the springs 82 and 84 provide the biasing force for the cam 50 to generate the centring force to the swashplate 24, the same spring forces can also be used for swashplate holddown biasing the swashplate 24 against the lower bearing 26 as seen in Fig. 1. AS stated above when describing the holddown function of the upper spring 44, this is particularly important when a cradle type swashplate is used. With the embodiment taught in the drawings, that is with the centring or levelling cam 50 located on one side of the cylinder block 16 and the control mechanism 30 located on the opposite side of the cylinder block, the centring springs 82 and 84, along with the control spring 44, provide axial biasing forces on both sides of the cradle swashplate 24 to keep securely seated against both bearings 26. It is also contemplated that the springs 82 and 84 on one side of the cam 50 are of substantially the same length as the springs 82 and 84 on the other side of the cam 50, but are seated in a pocket 86 of a depth D₁ different from the depth D₂ of pocket 88 so as to provide a different prestress on the springs on one side of the cam as compared to the opposite side. This different prestress of the springs provides a slight rotational canting bias on the cam 50 at the neutral position so that the sides of the slots 56 and 58 positively engage opposite sides of the pins 60 and 62 (in the Fig. 2 embodiment) or that the cam leg 54 engages diagonally opposite edges 96 of the slots formed in the rotational side cover 48 (in the Fig. 5 embodiment). This assures that any manufacturing clearance, between the slots and the pins in Fig. 2 or the cam leg 54 and the edges 96 in Fig. 5, is taken up when the cam 50 is in its neutral position. Thus, once the adjustment is made to bring the cam axis 52 into parallel relationship with the centreline 28, further adjustment is not necessary, and all backlash, or freeplay, is removed from the centring or levelling mechanism when the swashplate is in its zero displacement position. It is furthermore noted that since springs 82 and 84 do not directly engage the swashplate 24, but only the cam contacts 66 and 68 positively centre the swashplate 24, there are no problems with backlash as with the spring systems of previous designs. Furthermore, with the present invention, change in spring characteristics during use, or improper adjustment of the spring at time of manufacture, does not cause tilting of the swashplate from its zero displacement position. In fact no spring adjustments are necessary with the present design even during later repair or spring replacement. Another advantage of the present design is that the swashplate centring mechanism is located on the side cover of the housing to facilitate assembly separate from the assembly of the rotating block and swashplate within the housing 12 and from only one side of the housing. Thus multiple side covers or a complicated spring/servo assembly are avoided. It can be seen that the present invention, as described above, meets the objectives of providing a compact, inexpensive, and easy assembly of a swashplate centring mechanism that has the particular advantage of swashplate holddown.

Claims: 1. A swashplate holddown mechanism for a variable displacement hydraulic unit comprising a housing (12), a cylinder block (16) rotatable in the housing about an axial centreline (28) and having pistons (18) axially movable therein, a swashplate (24) tiltable about a transverse axis (27) perpendicular to the centreline (28) and having a cam surface (22) engageable by the pistons to control the stroke of the pistons within the cylinder block, bearing means (26) on the housing supporting the swashplate for tilting movement about the said transverse axis, and a displacement control means (32) operatively connected to the swashplate by linkage means (36, 38) to vary the tilt of the swashplate and thereby to control the axial position of the pistons in the cylinder block. CHARACTERISED IN THAT the holddown mechanism comprises mounting means (40, 41) locating the linkage means (36, 38) on one side of the cylinder block (16) and permitting axial movement of the linkage means (36, 38) parallel to the axial centreline (28), spring means (44) axially biasing the linkage means (36, 38) towards the swashplate bearing means (26) to apply a first axial biasing force on the swashplate (24) at a position on one side of the axial centreline (28), and a swashplate centring mechanism (50) located on the opposite side of the axial centreline (28) applying a second axial biasing force on the swashplate (24), parallel to the first biasing force but on the opposite side of the axial centreline (28), whereby the swashplate (24) is firmly held under bias against its bearing means (26). 2. A swashplate holddown mechanism according to claim 1, wherein the swashplate (24) is a cradle swashplate and the bearing means (26) comprise a pair of mutually spaced arcuate roller bearings mounted on the housing (12) and engaging the swashplate (24) on arcuate surfaces formed on the swashplate on a face opposite the piston cam surface (22), the pair of bearings permitting tilting movement of the cradle swashplate about the transverse axis (27). 3. A swashplate holddown mechanism according to either preceding claim, wherein the linkage means (36, 38) comprises a lever arm (36) attached to the swashplate (24) and pivot means (38) located on the lever arm, and the displacement control means (32) comprises piston means (32) for applying control forces to the lever arm (36) to induce a tilting motion of the swashplate (24) and the spring means (44) applies the first axial biasing force to the pivot means (38). 4. A swashplate holddown mechanism according to claim 3, wherein the lever arm (36) is secured to the swashplate (24) for tilting movement therewith about the transverse axis (27), the pivot means (38) being located substantially on or near to the transverse axis (27) whereby the pivot means (38) is subjected to at most a limited movement due to control inputs to the lever arm (36). 5. A swashplate holddown mechanism according to any preceding claim, wherein the centring mechanism (50) comprises a cam member (50) axially movable along a cam axis (52) parallel to the axial centreline (28), the cam (50) having a pair of spaced apart swashplate contact points (66, 68) disposed one on each side of the transverse axis (27), and spring means (82, 84) biasing the cam member (50) towards the swashplate (24) whereby at least one of the contact points (66, 68) is always in engagement with the swashplate (24). 6. A swashplate holddown mechanism according to claim 5, wherein the cam member (50) comprises a cam leg (54) mounted for axial movement on the housing (12) parallel to the centreline (28) and a cross member (64) perpendicular to the cam leg (54) and including the spaced apart pair of contact points (66, 68), the spring means (82, 84) comprising a pair (82, 82 or 84, 84) of springs applying a biasing force to the cross member and located one on each side of the cam leg wherein the springs bias the cam member (50) towards the cradle swashplate (24) so that both the cam contact points (62, 64) contact the swashplate (24) when the swashplate is in its zero displacement position.

Inventors: BECK, RICHARD JR., LOUIS, JOSEPH E.
Applicants: SAUER, INC.
Application year: 1985
Main classification: F01B3
IPC codes: F04B1, F01B3, F01B13