ADJUSTING AND RECOIL MECHANISM

Abstract: A track tension and idler recoil mechanism for crawler tracked vehicles comprises a pair of axially opposed piston and cylinder assemblies (7, 8 and 13, 14) operably connected between a track idler (2) and the vehicle frame (11). A first piston and cylinder assembly (13, 14) is powered by a source of hydraulic fluid under constant pressure and a one-way check valve (21) and a relief valve (22) in the fluid conduit (19) permits displacement of the idler (2) under load at a fluid pressure greater than that required to effect recoil of the idler to a normal position. The second piston and cylinder assembly (7, 8) is filled with an incompressible fluid to displace the idler (2) to effect tension in the track.

Description: ADJUSTING AND RECOIL MECHANISM This invention relates to an improved system for controlling recoil of the track tensioning idler on all crawler track mounted apparatus such as crawler tracked tractors and crawler tracked excavators. All track mounted vehicles utilise a chain type track fitted with various types of grouser shoes, which chain is driven by a sprocket (usually at the rear end of the vehicle) and which is tensioned by an idler (usually at the front end of the vehicle) with a supporting roller or rollers so mounted as to carry the weight of the top of the track and prevent it sagging downwards. The weight of the machine is normally carried on small rollers which run on the side links of the chain for the full distance between the sprocket and the idler. In all instances a tensioning device, usually a screw or an hydraulic ram filled by grease from a grease gun, is required to adjust the tension on the track by forcing the idler further away from the drive sprocket. Also required is a recoil mechanism, commonly a heavy helical compression spring or a plurality of springs, which allows the idler and adjusting mechanism as a unit to move rearwardly or towards the drive sprocket when excess tension is induced in the track. Such conditions may arise when a foreign object becomes lodged in the track chain between the chain and the sprocket inducing considerable tension in the chain leading to damage or breakage. Because of the magnitude of the loads to be resisted by the mechanism and of the limited space available, springs are commonly designed to work at or near the ultimate tensile strength of the steel resulting in limited life. A particular disadvantage of helical spring mechanisms is that displacement, as a function of force applied, is non linear in nature. Another major disadvantage of such systems is that a major shock load is applied to all components whenever a foreign object causing the mechanism to recoil is suddenly dislodged allowing the mechanism, the idler and the track to be rapidly propelled back to. their normal operating position. In an endeavour tσ alleviate the problems associated with mechanical coil spring recoil and track adjusting mechanisms, it has been proposed to utilize fluid pressure mechanisms employing a piston and cylinder assembly in fluid communication with a gas filled accumulation. United States Patent No. 3787097 describes an arrangement wherein the idler roller is mounted on a yoke slidable relative to the tractor frame. The yoke is acted on by a hydraulic actuator which is connected to a gas «- charged hydraulic accumulator by way of a counter balance valve. The counterbalance, valve includes a " differential pressure valve whereby the hydraulic actuator resists recoil of the track idler at a pressure of approximately 1640 psi greater than a biassing pressure exerted by the accumulator to return the idler to its normal operating position. The gas filled accumulator has a biassing pressure of 1900 psi and the system has a relief valve set at 4000 psi to relieve excess pressure on the accumulator. Accordingly the track recoil mechanism is operative only with recoil loads within the range 3450 psi to 4000 psi and any recoil load in excess of psi causes hydraulic fluid to be bled from the system. The main problems with this system are that as the idler is being displaced against the biassing of a compressible gas, the degree of displacement is not directly proportional to pressure applied and thus it is functionally equivalent to a mechanical coil spring insofar as displacement vs. load applied is concerned. Accordingly when the displacing load is suddenly removed the idler recoils with the same force as the displacement load. A further difficulty is associated with the hydraulic fluid bleed off when the idler displacement load exceeds 4000 psi. As the initial track tension is set by pumping a predetermined volume of hydraulic fluid into the piston and cylinder assembly under pressure against the accumulator, any bleed off of hydraulic fluid from the system causes the track tension to slacken. Unless the tractor operator is particularly vigilant and frequently resets track tension there is a likelihood that during turning of the trucked vehicle, the track may become dislodged from the idler roller or the drive sprocket due to insufficient tension. United States Patent No. 4514014 describes an alternative hydraulic fluid operated track adjusting and idler recoil system. The idler roller comprises a generally central axle-like transverse mounting member which is fixedly mounted on the tractor frame. An idler run is rollable mounted on a central support member which is slidably mounted on the transverse mounting member. The support member includes radially opposed cylinders in which is slidably mounted a unitary piston in turn slidably mounted in the transverse mounting member. Track tension is achieved by pumping an incompressible fluid such as grease into the interstice between the piston and cylinder. A recoil mechanism comprising a gas filled accumulator is connected in fluid communication with the fluid located between the piston and cylinder. This system is extremely complicated in construction and thus is expensive to manufacture and to maintain. In terms of its recoil mechanism, it suffers the same disadvantages as that described in the abovementioned United States Patent Specification No. 3787097. It is therefore an object of the invention to provide a control mechanism that is durable and less prone to causing damage due shock loads on the componentry of the track mounted machine by "snapping" forward when a foreign object which has caused the mechanism to recoil, is suddenly dislodged. According to the invention there is provided a track tension adjusting and recoil mechanism for crawler track systems, said mechanism comprising a fluid powered ram operably associated with a tensioning idler in said crawler track system, said ram being in fluid communication with a source of pressurized fluid at a constant pressure, said ram including pressure relief means to prevent fluid pressure within said ram exceeding a predetermined limit under the influence of abnormal operating loads on said system. Preferably the fluid powered ram comprises a pair of opposed fluid powered piston and cylinder assemblies. Suitably one such piston and cylinder assembly comprises a track adjusting mechanism and the other of said piston and cylinder assemblies comprises a recoil mechanism. Preferably said track adjusting mechanism is adjustable between predetermined limits. If required a stop limit for the track adjusting mechanism is movable to modify the position of a predetermined stop limit. The source of pressurized fluid to operate the recoil mechanism may be provided by an hydraulic pump means associated with a crawler tractor hydraulic system or a pump means provided specifically for that purpose. Preferably the source of pressurized fluid is a hydraulic circuit connected to a hydraulic track drive motor. In order that the invention may be better understood, a preferred embodiment will be described with reference to the accompanying drawings. The drawing illustrates schematically the relationship between a crawler track drive sprocket 1, an idler roller 2 and the track adjusting and recoil mechanism 3 according to the invention, when fitted to an excavator having crawler tracks powered by an hydraulic motor. Idler roller 2 is journalled for rotation on a shaft 4 in a slidable carrier 5. Mounted on one end of carrier 5 is a piston 6 located within a cylinder 7. The free end of .piston 6 includes a piston seal 8. Communicating with the interior of cylinder 7 is a passageway- 9 terminating at its outer end with a grease nipple 10 to enable the cylinder 7 to be packed with grease. A track mounting frame 11 is normally associated with the body of the track mounted vehicle (not shown) . Slidably mounted within the mounting frame 11 is a piston and cylinder assembly 12. At the respective remote ends of piston 13 and cylinder 14 are plates 13a and 14a which slidably engage with the mounting frame 11. Affixed to frame 11 are stops «, 15 which limit the extent of relative movement between piston 13 and cylinder 14 by engagement between plates 13a and 14a and respective stops 15. Cylinder 7 is attached to plate 14a by any suitable means such as bolts 16 whereby cylinder 7 moves in concert with plate 14 and attached cylinder 14a. Piston 13 includes piston seals 17. The remote end of each of piston 6 and piston 13 are releasably attached to carrier 5 and plate 13a respectively by a socket and spigot joint. 5. The remote ends of piston 6 and piston 13 are formed as spigots 17 and 18 respectively engageable in corresponding sockets in carrier 5 and plate 13a. Communicating with cylinder 14 is a fluid conduit 19 connected to a source 20 of pressurized fluid at a 0 constant pressure such as an hydraulic pump or the hydraulic circuit associated with an hydraulic track drive motor. A one way check valve 21 is provided in conduit 19 between cylinder 14 and source 20. Between the cylinder 14 and check valve 21 is " connected a further one way check valve 22. Check valve 22 has a preload bias whereby it operates as a relief valve when a predetermined pressure in cylinder 14 or conduit 19 is reached. Pressurized fluid relieved from valve 21 is 0 returned via conduit 22 to a fluid reservoir 23 associated with the source 20 of pressurized fluid. In use the piston and cylinder assembly 12 is initially pressurized at the line pressure in conduit 19 to move plates 13a and 14a against stops 15. Check valve 21 and biassed check valve 22 maintain the piston/cylinder assembly 12 in a pressurized state and thus maintain plates 13a and 14a in their normal operating position against stops 15. The pressure in conduit 19 needs only to be sufficient to overcome friction in the assembly and to support the weight of the track sagging between the idler roller 2 and the drive sprocket 1 and any intermediate support rollers. Any pressure loss in piston/cylinder assembly 12 as a result of fluid leakage around piston seals 17 is compensated by maintaining a pressure in conduit 19 from source 20. With the piston/cylinder assembly 12 in its normal operating position, grease is then pumped into cylinder 7 via grease nipple 10 to move idler roller 2 in a direction away from drive sprocket 1 in order to adjust normal operating track tension. In the event of track elongation due to stretch or wear in the chain links, the position of stops 15 may be adjustable by any suitable means. For example stops 15 may be frictionally engaged on the surface of frame 11 by means of a nut and bolt located in an elongate slot in frame 11. Alternatively, in the configuration as shown in the drawing, the stops 15 provide a finite relative displacement between piston 13 and cylinder 14 and track tensioning is achieved by pumping grease into piston and cylinder assembly 8,7. If in the course of operating the tracked vehicle fitted «with the apparatus according to the invention, a stone or some other obstruction becomes lodged between the track and the drive sprocket, considerable tension will be applied to the track. As the tension in the track increases, the fluid pressure in cylinder 14 increases but one way check valve 21 prevents any pressure loss in the conduit connecting source 20 to the cylinder. At a predetermined level of back pressure in conduit 19, biassed check valve 22 opens against its biassing influence to relieve excess pressure in cylinder 14 and idler roller 2 is allowed to move towards drive sprocket 1 under a constant fluid pressure. When the obstruction between the drive sprocket 1 and the chain is released the back pressure in conduit 19 reduces and valve 21 opens to allow piston/cylinder assembly 14 to be repressurized from source 20 to move plates 13a and 14a to return to their normal operating positions against stops 15. It can be seen therefore that with the system according to the invention, upon release of tensioning inducing loads in the track, the adjustable idler roller mechanism moves smoothly back to a normal operating position under a constant fluid pressure without the shock loads normally associated with a coil spring mechanism. From the foregoing description it will be readily apparent to a skilled addressee that many modifications and variations will be possible without departing from the spirit and scope of the invention. * ' For example the apparatus may employ an adjusting mechanism in the form of a screw threaded nut and shaft in lieu of the grease packed piston and cylinder assembly asociated with the idler roller.

Claims: CLAIMS 1. A track tension adjusting and idler recoil mechanism for crawler track systems, said mechanism comprising a fluid powered ram assembly operably associated with a tensioning idler in said crawler track system, said ram assembly being in fluid communication with a source of pressurized fluid at a constant pressure said ram including pressure relief means to prevent fluid pressure within said ram exceeding a predetermined limit under the influence of abnormal operating loads on said system. 2. A mechanism as claimed in claim 1 wherein said fluid powered ram assembly comprises opposed piston and cylinder assemblies, one of said piston and cylinder assemblies being operable to adjust track tension and an opposed piston and cylinder assembly being operable to effect recoil of a crawler track idler after said idler is displaced under load. 3. A mechanism as claimed in claim 2 wherein said fluid powered ram assembly comprises a first piston and cylinder assembly movable between opposed predetermined stop positions under the influence of hydraulic fluid supplied from a source at constant fluid pressure to provide a *, biassing influence on said track idler only when said track idler is displaced from a normal operating position. 4. A mechanism as claimed in claim 3 wherein a non-return check valve is provided in a fluid conduit between said first piston and cylinder assembly and said source of constant fluid pressure and a relief valve is provided in said fluid conduit between said check valve and said first piston and cylinder assembly to enable displacement of hydraulic fluid from said first piston and cylinder assembly at a predetermined excess pressure greater than the pressure of fluid from said source whereby displacement of said idler under load occurs against a fluid pressure bias greater than the fluid pressure effecting recoil of the idler to said normal operating position. 5. A mechanism as claimed in claim 4 wherein said fluid powered ram assembly includes a second axially opposed piston and cylinder assembly actuable by the introduction of an incompressible fluid to effect a finite relative displacement said second piston and cylinder assembly to adjust the position of said track idler to tension a track assembly to a desired tension. 6. A mechanism as claimed in claim 5 wherein said source of fluid at constant pressure is supplied to said first piston and cylinder assembly only when said crawler track system is in motion. 7. A mechanism as claimed in claim 6 wherein said source of hydraulic fluid at constant pressure comprises a fluid supply connected to an hydraulic crawler track drive motor. 8. A crawler tracked vehicle whenever fitted with a mechanism as claimed in any one of claims 1 to 8. 9. A crawler track tension adjustment and idler recoil mechanism substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: HAMBLIN ALLAN LLOYD, HAMBLIN, ALLAN, LLOYD
Applicants: HAMBLIN ALLAN LLOYD, HAMBLIN, ALLAN, LLOYD
Application year: 1987
Main classification: B62D55
IPC codes: B62D55