Abstract:
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.

Description:
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&#39;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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a vertical sectional view taken laterally through a vibratory plate compactor incorporating the apparatus of the subject invention. 
       FIG. 2  is a horizontal section taken on line  2 — 2  of  FIG. 1 . 
       FIG. 3  is an enlarged sectional detail taken on line  3 — 3  of  FIG. 2 . 
       FIG. 4  is a side elevation of the apparatus shown in  FIG. 2   
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A vibratory plate compactor  10  includes a horizontal bottom compaction plate  11  through which vertical compactive forces, generated by an attached rotary eccentric weight mechanism  12  are transmitted to the soil or other base material underlying the plate  11 . The compaction plate  11 , as best seen in  FIG. 4 , is part of a casting and includes upwardly tapered front and rear portions  13  to facilitate movement of the compactor in forward and reverse directions. The casting also includes front and rear frame members  14  that are formed integrally with the compaction plate  11  and to which are attached an operator&#39;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 housing  15  in which the rotary eccentric weight mechanism  12  is enclosed. The housing is enclosed from above with a removable top plate  16 . 
   The rotary eccentric weight mechanism  12  includes a main rotary input shaft  17  journaled at its opposite ends in the side walls  20  of the main housing  15  with bearings  18 . A pair of main eccentric weights  21  are secured to the main input shaft  17  for rotation therewith. A drive gear  22  is also mounted on the main input shaft  17  between the eccentric weights  21  and rotates with the shaft and weights. One end of the input shaft  17  extends through the side wall  20  and has mounted thereon a drive pulley  23  for operative attachment to the drive engine with a V-belt (not shown). A driven shaft  24  is also journaled in the side walls  20  of the main housing  15  with bearings  25 . The driven shaft  24  has a driven gear  26  centrally mounted thereon and in engagement with the drive gear  22  on the main input shaft  17 . A pair of eccentric weights  27  are also mounted on driven shaft  24  for rotation therewith. Driving rotation of the main input shaft  17  transmits a counter-rotation to the driven shaft  24  via the gears  22  and  26 . 
   As indicated, the eccentric weights  21  are fixed to the main input shaft  17  and the eccentric weights  27  are similarly fixed to the driven shaft  24  so that they rotate, respectively, therewith. In a manner generally known in the art, the relative rotational positions of the eccentric weights  21  and  27  on their respective shafts  17  and  24  can 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 shaft  17  which transmits a similar but opposite limited counter-rotation to the driven shaft  24 . This phase adjustment permits the compactor  10  to 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 mechanism  28  for effecting the change in eccentric weight phase is operatively connected to the main input shaft  17 . This adjustment mechanism includes several features which constitute improvements over the prior art, as will be described hereinafter. The main input shaft  17  is provided with a long blind bore  30  and, near the interior end thereof, the shaft wall is provided with a pair of diametrically opposite matched helical slots  31 . A cylindrical carrier  32  is slidably mounted in the bore  30  and is journaled with bearings  33  on one end of a shift rod  34  positioned axially in the bore  30 . On the opposite end of the shift rod  34  is mounted a piston  35  by a threaded connection  36  comprising a threaded OD on the end of the rod  34  and a threaded ID on a counter-bore in the piston  35 . The piston  35  is carried in a cylinder housing  37  which is provided with a through bore  38  within which the piston may be reciprocated axially. The cylinder housing  37  has a lead end provided with a extended sleeve  40  that extends with the clearance into the bore  30  of the input shaft  17  and provides an extended bore for the piston  35 . Pressurized hydraulic fluid is supplied via a fitting  41  to the cylinder bore  38  and acts against the free face of the piston  35  to move the piston, shift rod  34  and carrier  35  in the direction away from the fitting. A cross pin  43  is mounted in a cross bore  42  in the carrier  32  as best shown in  FIG. 3 . The opposite ends of the cross pin  43  extend into the helical slots  31  with a small clearance so that the cross pin may slide in the helical slots. Axial movement of the adjustment mechanism  28  along the path of the helical slots causes limited rotational movement of the input shaft  17  and the drive gear  22  mounted thereon. This limited rotational movement is transferred to the driven gear  26  mounted on the driven shaft  24 . The result is relative counter rotational movement of the respective eccentric weights  21  and  27 , resulting in the phase adjustment described above and the resultant change in horizontal movement of the compactor  10 . As indicated, the carrier  32  is journaled on the end of the shift rod  34  such that the carrier and the cross pin  43  rotate with the main input shaft  17 . Thus, axial movement of the carrier under the influence of hydraulic pressure in the cylinder housing  37  may be utilized to move the cross pin in the helical slots  31  to provide on-the-fly phase adjustment while the shafts  17  and  24  are rotationally driven. 
   Referring again to  FIG. 1  and also to  FIG. 4 , the bottom of the main housing  15  provides a reservoir  44  for a lubricating oil for the various bearings and gears mounted in the housing. Typically, the reservoir  44  is filled to a fairly low level sufficient to permit the teeth of the gears  22  and  26  to 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 housing  15  results in an increase in internal pressure. Although pressure relief may be provided, it has been found that, in prior art devices, a piston  35  having only a single seal, will permit the passage of lubricating oil past the piston and into the cylinder housing  37 . 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 mechanism  28 . 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 to  FIG. 3 , in addition to the single hydraulic pressure seal  45  typical of prior art constructions, the piston  35  of the present invention also includes an oppositely acting lubricant seal  46  at the opposite axial end of the piston. The piston also includes a guide ring  49  between the two seals  45  and  46 , the guide ring being typical of prior art constructions. The lubricant seal  45  for the piston  35  of 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 seal  45 . 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 seals  45  and  46 , 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 housing  37  is made to be easily removable from the main housing  15 , making access to the piston for repair or replacement of the seals possible without direct access to the interior of the main housing  15 . The side wall  20  of the main housing  15  is provided on both sides with large circular openings  29 , each of which is closed by an end cover  19  that also provides a housing for the main bearings  18 . Each end cover  19  is secured to its respective side wall  20  with mounting bolts  53  (see  FIG. 4 ). The cylinder housing  37  includes a shoulder  39  the OD of which provides a pilot surface for centering the cylinder housing in a central opening  54  in one of the end covers  19 . The cylinder housing  37  also includes a peripheral flange  47  that engages the end cover  19  when the sleeve  40  is inserted into the bore  30  in the input shaft and the pilot shoulder  39  is received in the central opening  54 . The housing  37  is held in place with a clamping plate  48  which, in turn, is demountably attached to the end cover  19  with four machine screws  50 . When access to the piston  35  and seals  45 ,  46  is required, the clamping plate  48  and cylinder housing  37  are 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 rod  34  (e.g. with a pliers) and unthreading the piston at the threaded connection  36 . Then the entire piston including new seals  45  and  46  and guide ring  49  may be replaced as a unitary subassembly quickly and with a minimum of effort. 
   It will be noted in the drawings, such as the detail of  FIG. 3 , that the throughbore  38  in the cylinder housing is closed with a cover plate  51 . The throughbore  38  itself 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 plate  51  is attached with a number of machine screws  52 , 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 plate  16 , replacement of the piston seals would take three to four hours.