Shift rod piston seal arrangement for a vibratory plate compactor

An easily accessible phase adjustment mechanism for a vibratory plate compactor includes an improved seal arrangement to protect against leakage of internal lubricating oil into the hydraulic cylinder providing fluid pressure to the adjustment mechanism. An easily demountable cylinder housing provides ready access to the piston and seal assembly which can then be threadably detached and replaced in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to manually operated vibratory plate compactors and, more particularly, to an improved seal arrangement for the piston of a shift rod used to control movement of the compactor. The seal arrangement is readily accessible and the seals may be individually replaced or a new piston and seal subassembly substituted for the subassembly needing repair or replacement.

Manually operated vibratory plate compactors are well known and commonly used for compacting soil in back-fill, sub-grade and other construction activity compaction applications. In one typical vibratory plate compactor, pairs of parallel shafts carrying eccentric weights are rotated by driving one shaft and transmitting the rotation to the other with a gear arrangement. The eccentric weight arrangement and a drive engine are mounted on a substantially flat compaction plate. An operator's handle with controls is also attached to the plate frame. The operator controls include an actuator which can be used to adjust the rotational position of the eccentric weights on the shafts. Such adjustment alters the phase and vector of the forces generated by the eccentric weights such that the plate compactor may be made to move in a forward direction, a reverse direction, or to remain horizontally stationary, all while imposing vertical compacting forces on the surface beneath the plate.

One common means for adjusting the phase of the eccentric weights is to use a hydraulic actuator including a piston mounted coaxially in or with respect to a bore in the driven input shaft of the apparatus, the piston connected by a shift rod to a carrier head carrying a cross pin that engages a helical groove on the ID of the main input shaft bore. Movement of the shift rod assembly axially in the input shaft bore provides the rotation of the shaft and attached eccentric weights to adjust the phase. Such apparatus is shown, for example, in U.S. Pat. Nos. 4,356,736; 5,010,778; and 5,818,135.

In all of the prior art apparatus of the foregoing general type, the shafts carrying the eccentric weights and drive gears are encased in a housing partially filled with a liquid lubricating oil. The piston on the shift rod is typically connected to a supply of hydraulic fluid which is applied to the free end of the piston, operating either in a bore in the input shaft or in a cylinder housing attached coaxially to the shaft, to move the carrier and cross pin on the opposite end of the shift rod axially to rotate the input shaft for phase adjustment, thereby adjusting the speed and direction of forward and reverse movement of the compactor.

It is known in the prior art to provide the shift rod piston with a seal to prevent hydraulic fluid in the piston cylinder from bypassing the piston and escaping into the main housing. The seal is typically a uni-directional type such as a lip seal or cup seal that expands with increasing hydraulic pressure to inhibit leakage. When actuating hydraulic pressure on the piston is reduced or relieved, the eccentric weights shift in an opposite rotational direction under the influence of rotation of the main input or drive shaft to initially reduce the speed of movement in one direction (typically reverse) to a neutral or horizontally stopped position and then to increase speed in the opposite (forward) direction. Thus, the shift rod piston needs only to be single-acting and, therefore, it has been assumed in the prior art that a unidirectional piston seal to prevent leakage of pressurized hydraulic fluid was adequate.

It has been found, however, that under certain circumstances of operation, lubricating oil in the main housing can become pressurized and escape past the uni-directional seal on the piston where it becomes trapped in the cylinder housing. The lubricating oil in the housing may become pressurized as a result of high temperatures generated during operation. Also, the rapidly rotating shafts in the housing tend to stir up the lubricating oil causing it to atomize and, under pressure, seep past the seal. The accumulation of lubricating oil in the chamber intended to receive pressurized hydraulic fluid interferes with proper movement of the piston and, as a result, eventually interferes with operating movement of the compactor.

The high operating temperatures experienced by these kinds of vibratory plate compactors also create a hostile environment for any type of seal. Thus, the prior art piston seal must be periodically replaced and great care must be taken to avoid contamination of the interior of the housing during seal replacement. In addition, the construction of prior art apparatus has made seal replacement tedious and time consuming, sometimes requiring the removal of the main housing cover and partial disassembly of the eccentric weights from the drive shaft to access the shift rod and piston so the seal may be replaced. Opening the cover plate for the main housing also exposes the entire interior of the mechanism to potential contamination.

SUMMARY OF THE INVENTION

In accordance with the present invention, the shift rod piston is provided with a double seal to protect against leakage of pressurized lubricating oil from the interior of the housing in cooperation with a prior art piston seal to prevent the ingress of hydraulic fluid from the cylinder housing. An improved demountable cylinder housing makes access to the shift rod and piston much easier and the piston is demountably attached to the shift rod so that the entire subassembly of a piston head and new seals may be easily substituted for the old and worn subassembly.

In accordance with the preferred embodiment of the invention, the demountable connection of the piston to the shift rod comprises a threaded connection. The annular piston seals preferably comprise cup seals oriented to face in opposite axial directions. The cylinder housing preferably includes an integral peripheral outer flange that is adapted to engage the outer wall of the main housing. A mounting plate comprising an annular clamping plate holds the cylinder housing flange in engagement with the outer wall and is held in place with a plurality of threaded fasteners. The bore in the cylinder housing preferably comprises a through bore to facilitate machining. A demountable cover plate encloses the outer end of the through bore.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A vibratory plate compactor10includes a horizontal bottom compaction plate11through which vertical compactive forces, generated by an attached rotary eccentric weight mechanism12are transmitted to the soil or other base material underlying the plate11. The compaction plate11, as best seen inFIG. 4, is part of a casting and includes upwardly tapered front and rear portions13to facilitate movement of the compactor in forward and reverse directions. The casting also includes front and rear frame members14that are formed integrally with the compaction plate11and to which are attached an operator's handle (not shown) and a drive engine with supporting brackets (also not shown). Between the front and rear frame members and also forming part of the casting is a generally rectangular main housing15in which the rotary eccentric weight mechanism12is enclosed. The housing is enclosed from above with a removable top plate16.

The rotary eccentric weight mechanism12includes a main rotary input shaft17journaled at its opposite ends in the side walls20of the main housing15with bearings18. A pair of main eccentric weights21are secured to the main input shaft17for rotation therewith. A drive gear22is also mounted on the main input shaft17between the eccentric weights21and rotates with the shaft and weights. One end of the input shaft17extends through the side wall20and has mounted thereon a drive pulley23for operative attachment to the drive engine with a V-belt (not shown). A driven shaft24is also journaled in the side walls20of the main housing15with bearings25. The driven shaft24has a driven gear26centrally mounted thereon and in engagement with the drive gear22on the main input shaft17. A pair of eccentric weights27are also mounted on driven shaft24for rotation therewith. Driving rotation of the main input shaft17transmits a counter-rotation to the driven shaft24via the gears22and26.

As indicated, the eccentric weights21are fixed to the main input shaft17and the eccentric weights27are similarly fixed to the driven shaft24so that they rotate, respectively, therewith. In a manner generally known in the art, the relative rotational positions of the eccentric weights21and27on their respective shafts17and24can be varied to change the phase relationship of the forces generated during operation. The relative rotational positions of the eccentric weights are adjusted by limited rotation of the main input shaft17which transmits a similar but opposite limited counter-rotation to the driven shaft24. This phase adjustment permits the compactor10to be driven in a forward direction at a variably adjustable speed, stopped to operate without horizontal movement, or driven at a variable adjustable speed in a reverse direction.

The adjustment mechanism28for effecting the change in eccentric weight phase is operatively connected to the main input shaft17. This adjustment mechanism includes several features which constitute improvements over the prior art, as will be described hereinafter. The main input shaft17is provided with a long blind bore30and, near the interior end thereof, the shaft wall is provided with a pair of diametrically opposite matched helical slots31. A cylindrical carrier32is slidably mounted in the bore30and is journaled with bearings33on one end of a shift rod34positioned axially in the bore30. On the opposite end of the shift rod34is mounted a piston35by a threaded connection36comprising a threaded OD on the end of the rod34and a threaded ID on a counter-bore in the piston35. The piston35is carried in a cylinder housing37which is provided with a through bore38within which the piston may be reciprocated axially. The cylinder housing37has a lead end provided with a extended sleeve40that extends with the clearance into the bore30of the input shaft17and provides an extended bore for the piston35. Pressurized hydraulic fluid is supplied via a fitting41to the cylinder bore38and acts against the free face of the piston35to move the piston, shift rod34and carrier35in the direction away from the fitting. A cross pin43is mounted in a cross bore42in the carrier32as best shown inFIG. 3. The opposite ends of the cross pin43extend into the helical slots31with a small clearance so that the cross pin may slide in the helical slots. Axial movement of the adjustment mechanism28along the path of the helical slots causes limited rotational movement of the input shaft17and the drive gear22mounted thereon. This limited rotational movement is transferred to the driven gear26mounted on the driven shaft24. The result is relative counter rotational movement of the respective eccentric weights21and27, resulting in the phase adjustment described above and the resultant change in horizontal movement of the compactor10. As indicated, the carrier32is journaled on the end of the shift rod34such that the carrier and the cross pin43rotate with the main input shaft17. Thus, axial movement of the carrier under the influence of hydraulic pressure in the cylinder housing37may be utilized to move the cross pin in the helical slots31to provide on-the-fly phase adjustment while the shafts17and24are rotationally driven.

Referring again toFIG. 1and also toFIG. 4, the bottom of the main housing15provides a reservoir44for a lubricating oil for the various bearings and gears mounted in the housing. Typically, the reservoir44is filled to a fairly low level sufficient to permit the teeth of the gears22and26to pick up lubricating oil during rotation and have it spread throughout the housing by the other rotating parts, such as the bearings and eccentric weights, into which it comes in contact. The rapidly rotating parts tend to break the oil into minute droplets and to even create an oil mist which penetrates and lubricates the bearings and other moving parts. The generation of high operating temperatures inside the housing15results in an increase in internal pressure. Although pressure relief may be provided, it has been found that, in prior art devices, a piston35having only a single seal, will permit the passage of lubricating oil past the piston and into the cylinder housing37. A very small volume of leakage into the cylinder housing where it mixes with pressurized hydraulic fluid, has been found sufficient to interfere with operation of the adjustment mechanism28. As a result, proper control of the compactor is lost. Normal wear of the single piston seal with use and seal degradation at high operating temperatures both add to worsen the leakage problem.

Referring also toFIG. 3, in addition to the single hydraulic pressure seal45typical of prior art constructions, the piston35of the present invention also includes an oppositely acting lubricant seal46at the opposite axial end of the piston. The piston also includes a guide ring49between the two seals45and46, the guide ring being typical of prior art constructions. The lubricant seal45for the piston35of the improved phase adjustment mechanism is preferably a cup seal and may be of the construction and material identical to the oppositely facing hydraulic pressure seal45. Each of the seals is, of course, oriented to enhance sealing engagement in response to increased pressure. A typical seal material for this application would be a polyether-based urethane, but other synthetic rubber materials could also be used. Instead of two separate seals45and46, a single double-acting seal could be used.

Another problem with certain prior art compactor constructions was that, when seal replacement was necessary, access to the piston was difficult and time consuming, and furthermore, often required access to the interior of the main housing and removal of parts of the eccentric weight mechanism. All of this contributed to the potential for contamination. In accordance with the present invention, the cylinder housing37is made to be easily removable from the main housing15, making access to the piston for repair or replacement of the seals possible without direct access to the interior of the main housing15. The side wall20of the main housing15is provided on both sides with large circular openings29, each of which is closed by an end cover19that also provides a housing for the main bearings18. Each end cover19is secured to its respective side wall20with mounting bolts53(seeFIG. 4). The cylinder housing37includes a shoulder39the OD of which provides a pilot surface for centering the cylinder housing in a central opening54in one of the end covers19. The cylinder housing37also includes a peripheral flange47that engages the end cover19when the sleeve40is inserted into the bore30in the input shaft and the pilot shoulder39is received in the central opening54. The housing37is held in place with a clamping plate48which, in turn, is demountably attached to the end cover19with four machine screws50. When access to the piston35and seals45,46is required, the clamping plate48and cylinder housing37are removed to expose the piston. If necessary, the piston may be pulled axially out of the housing so the seals may be removed and replaced. Preferably, however, the entire piston is removed by grasping the shift rod34(e.g. with a pliers) and unthreading the piston at the threaded connection36. Then the entire piston including new seals45and46and guide ring49may be replaced as a unitary subassembly quickly and with a minimum of effort.

It will be noted in the drawings, such as the detail ofFIG. 3, that the throughbore38in the cylinder housing is closed with a cover plate51. The throughbore38itself is utilized simply to make machining more accurate and easy to accomplish (as compared, for example, to blind bores provided in certain prior art constructions). The cover plate51is attached with a number of machine screws52, but the plate does not have to be removed for any repair or maintenance activities. With the improved construction and easy access provided by the subject invention, the piston and seal subassembly may be replaced in about 20 minutes. In the prior art construction without an easy access cylinder housing and requiring access to the piston by removal of the main top plate16, replacement of the piston seals would take three to four hours.