Technical Field This invention relates to systems for tilting truck cabs relative to truck frames to expose truck motors for maintenance. In particular, it relates to such systems havin-g hydraulic lost motion means for permitting hydraulic fluid to move back and forth between the push chamber and the pull chamber of the tilt cylinder(s) while the truck cabs are in their "over-the-road" position, thereby per- mitting the cabs to pivot relative to the frames to acco o- date vertical motion in the suspension system.

Background of the Invention In the design of tiltcab trucks, it is desirable to provide a suspension system between the truck cab and the truck frame to partially isolate the occupants of the truck- cab from vibration and jolts due to unevenness in the roads over which the truck passes. However, if the tilt cylin¬ der(s) are connected directly to the cab frame and the truck frame, as is most convenient, the incompressible fluid in the push and pull chambers of the tilt cylinder(s)

tends to hold the cab rigidly in position relative to the truck frame. This problem is conventionally overcome by providing hydraulic lost motion apparatus which permits hydraulic fluid to move back and forth between the push chambers and the pull chambers of the tilt cylinder(s) while the truck cabs are in their "over-the-road" position, thereby permitting the cabs to pivot relative to the frames to accomodate vertical motion in the suspension system. Examples of such apparatus are shown, for instance, in published United Kingdom patent application No. 2,048,383A and West. German Off. No. 2,458,264.

Brief Summary of the Invention According to the subject invention, the apparatus for permitting hydraulic lost motion between the push chamber and _. he pull chamber of the tilt cylinder(s) comprises a pilot-operated check valve having a through passage (a) which is part of a hydraulic connection between the push chamber and the pull chamber and (b) which is only shut off when hydraulic pressure is applied to the push chamber of the tilt cylinder(s).

Brief Description of the Drawings Figure 1 is a schematic view of a tiltcab truck according to this invention with portions broken away to show the cab-tilting apparatus. Figure 2 is a hydraulic schematic drawing showing the cab-tilting apparatus in the over-the-road position.

Figure 3 is a cross-sectional view on an enlarged scale of the lost motion apparatus shown in Figure 2.

Figure 4 is a hydraulic schematic drawing showing the cab-tilting apparatus in the extension position. Figure 5 is a hydraulic schematic drawing showing the cab-tilting apparatus in the retraction position.

Detailed Description of a Preferred Embodiment of the Invention

Figure 1 shows a tiltcab truck 10 comprising a chassis member 12, a cab member 14 mounted on the chassis member 12 for pivotal movement about an axis 16 from the lowered ^■ first position shown in the drawing to a raised second position. One or more tilt cylinders 18 are operatively connected to the chassis member 12 and the cab member 14 to pivot the cab member 14 back and forth between its first and ^' second positions. A suspension system 20 is opera¬ tively connected between the chassis member 12 and the cab member 14 to partially isolate the occupants of the cab member 14 from vibrations and jolts due to unevenness in the roads over which the truck 10 passes.

Turning to Figures 2, 4, and 5, a pump 22 supplies hydraulic fluid under pressure to the tilt cylinder 18 via hydraulic conduit 24, control valve 26, and hydraulic ^' conduits 28 and 30. The pump 22 obtains its hydraulic fluid from reservoir 32 via hydraulic conduit 34, and hydraulic fluid is returned to the reservoir 32 from the control valve 26 via hydraulic conduit 36.

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A hydraulic conduit 38 containing a bypass valve 40 permits hydraulic fluid from the conduit 24 to return directly to the reservoir 32 when the pressure in the con¬ duit 24 exceeds a predetermined minimum value. The bypass valve 40 is normally biased shut by a spring 42, but pres¬ sure in the conduit acting through a pressure tap 44 moves the bypass valve 40 to the right in Figures 2, 4, and 5 against the spring 42, ultimately allowing the hydraulic fluid to return to the reservoir 32 through the conduit 38. A one-way valve 46 is located in the conduit 28 and oriented so as to be opened by hydraulic fluid under pres¬ sure flowing to the push chamber 48 of the tilt cylinder 18. A pilot piston 50 having a projection 52 sized, shaped, and positioned to open the one-way valve 46 under the influence of pilot pressure is located adjacent to the one-way valve 46.

Preferably, the one-way valve 46 -and the pilot piston 50 are both located in a housing 54, and pilot pressure is supplied to a bore 56 in the housing 54 via a conduit 58 which branches off from the conduit 30. The conduit 58 is threadedly received in an axial port 60 in an end plug 62 which, in turn, is threadedly received in the housing 54.

The pilot piston 50 preferably comprises a spool hav¬ ing two pistons 64 and 66 which slide sealingly in the bore 56 and which are separated by an annular relief 68.

Threaded radial ports 70 and 72 extend through the housing 54 in ^' position to communicate with the relief 68 when the pilot piston 50 is in its central, or over-the-road,

position (shown in Figures 2 and 3) and when it is in its retracting position (shown in Figure 5) . However, the radial ports 70 and 72 are blocked by the piston 64 when the pilot piston 50 is in its retracting position (shown in Figure 4) .

The radial port 70 threadedly receives a conduit 74 which communicates with the interior of the tilt cylinder 18 on the pull chamber side of the normal excursion of the tilt cylinder piston 76 due to unevenness in the road (symbolited by the two broken-line positions of the piston 76 in Figure 2) . The radial port 72 threadedly receives a conduit 78 which communicates with the conduit 28 between the one-way valve 46 and the push chamber 48.

The bore 56 ends in an abutment surface 80 which serves as one limit on the travel of the pilot piston 50. A smaller axial bore 82 extends through the abutment sur¬ face 80 and opens into a chamber 84 which contains the one-way valve 46. The projection 52 is biased to extend through the axial bore 82 and towards the one-way valve 46 by a spring 86 which extends between the piston 66 and the end plug 62, where it is received in a recess 88. The spring 86 can be entirely compressed into the recess 88, allowing the end plug 62 to serve as the other limit on the travel of the pilot piston 50. A threaded radial port 90 extends through the housing 54 in position to communicate with the axial bore 82. The projection 52 is substantially smaller in diameter than the axial bore 82, permitting hydraulic fluid to flow through

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the radial port 90' and the axial bore 82 in both directions even when the projection 52 is in position to extend through the axial bore 82. The radial port 90 threadedly receives one portion of the conduit 28. An end plug 92 is threadedly received in the housing 54 at the end opposite the end plug 62. The end plug 92 contains an axial port 94 which threadedly receives another portion of the conduit 28 and which leads to chamber 84. The one-way valve 46 comprises a ball 96 which valves the opening 98 between the axial bore 82 and the chamber 84 and a spring 100 which extends between the end plug 92 and the ball 96. The spring 100 is stronger than the spring 86 _r so that, in the absence of hydraulic pressure, the spring 100 holds the ball 96 in valving engagement with the opening 98 against the force from the spring 86 transmit¬ ted to the ball 96 via the projection 52.

Operation of the Lost-Motion Apparatus When the truck 10 is travelling .over the road, the tiltcab piston 76 has a central, at rest, position shown in solid lines, from which it moves irregularly in either direction, as suggested by the broken line position of the piston 76 and its rod. As the piston 76 moves down, it forces hydraulic fluid out of the push chamber 48, through a portion of the conduit 28, the conduit 78, the relief 68, and the conduit 74 to the pull chamber 102 above the piston 76. Passage of the hydraulic fluid through the con¬ duit 28 beneath its connection with the conduit 78 is pre¬ vented by the one-way valve 46. As the piston 76 moves

back up, it forces hydraulic fluid out of the pull chamber 102, through the conduit 74, the relief 68, the conduit 78, and the same portion of conduit 28 back to the push chamber 48. Again, passage of the hydraulic fluid through the con- duit 78 is prevented by the one-way valve 46. While a small amount of pressure may be induced in the bore 56 beneath the piston 66, it is normally not enough to unseat the one-way valve 46, particularly since, as previously mentioned, the conduit 58 normally contains a pressure step-down transformer. In the unlikely event that the piston 76 moves upwardly enough to block the conduit 74, further lost-motion movement of the piston 76 is simply abruptly halted, and the piston 76 remains in position until it begins to sink back down towards its rest position. -Hydraulic fluid can flow back from the tilt cylinder 18 to the reservoir 32 during lost motion.. This occurs when too much hydraulic fluid is present in the tilt cylinder 18. In that case, as the piston 76 moves down, a pressure builds up in the push chamber 48 which acts through the conduit 78, the relief 68, the conduit 74, the pull chamber 102, the conduit 30, and the conduit 58 to open the check valve 46. Once the check valve 46 has been opened, hydraulic fluid can flow back to the reservoir 32 through the conduit 28 until the consequent reduction in pressure in the push chamber 48 allows the check valve 46 to close.

Figure 4 shows the cab-tilting apparatus in the exten¬ sion position. In this position, hydraulic fluid from the reservoir 32 is pumped through the conduit 34, pump 22, the

conduit 24, the control valve 26, a first portion of the conduit 28, and the port 90 to the axial bore 82 (where it moves the pilot piston 50 downwardly to its lower stop and where it unseats the one-way valve 46) , through the cham- ber 84, the port 94, and a second portion of the conduit 28 to the push chamber 48, where it causes the tilt cylinder to extend. Outward motion of the tilt piston 76 causes hydraulic fluid to vacate the pull chamber 102 via the conduit 30, the control valve 26, and the conduit 36 to the reservoir 32. Passage of hydraulic fluid through the conduits 74 and 78 is prevented by the piston 64, which blocks the ports 70 and 72.

Figure 5 shows the cab-tilting apparatus in the retraction position. In this position, hydraulic fluid from the reservoir 32 is pumped through the conduit 34, the pump 22, the conduit 24, the control valve 26, and the conduit 30 to the pull chamber 102, where it causes the tilt cylinder 18 to retract. Inward motion of the tilt piston 76 initially causes hydraulic fluid to vacate the push chamber 48 via the conduit 28, the chamber 84, the bore 82 (the one-way valve 46 having been unseated by the projection 52) , the conduit 28 again, the control valve 26, and the conduit 36 to the reservoir 32. The pressure acting on the piston 66 via the conduit 58 is sufficient to move the pilot piston 50 upwards against the force of the spring 100 and the induced pressure in the chamber 84 because (1) it acts first, unseating the one-way valve 46, and, (2) once the one-way valve 46 is unseated, a

hydraulic pathway is open all the way to the reservoir 32, preventing the induced pressure in the push chamber 48 from building up to a value high enough to move the pilot piston 50 downwards, allowing the one-way valve 46 to close under the combined urgings of the spring 100 and the induced pressure. However, the check valve 46 remains open only as long as the pressure remains in the conduit 58. This means that the cab 14 has to be pumped back all the way until the piston 76 passes the opening into the conduit 74. Passage of hydraulic fluid through the conduits 74 and 78 is pre¬ vented during a first stage of the retraction process because both are in communication with the push chamber 48 and during a second state of the retraction process because the conduit 74 is blocked by the piston 76. Once the piston 76 has passed inwardly beyond the . point where the conduit 74 is in communication with the interior of the tilt cylinder 18, however, a new stage is entered which has no analog during the extension of the tilt cylinder 18. In contrast to the situation during extension of the tilt cylinder 18, during which fluid flow through the conduits 74 and 78 is continuously prevented by the piston 64, flow through the conduits 74 and 78 is possible after the piston 76 has cleared the conduit 74 because the conduits 74' and 78 are both in communication with the relief 68. In this situation, the cab 14 falls freely to its rest position, since the weight of the cab 14 induces a pressure in push chamber 48 which acts through conduit 78, relief 68, conduit 74, pull chamber 102, conduit 30, and conduit 58 to open check valve 46. Once the check

valve 46 is open, the hydraulic fluid flows freely back to the reservoir 32 through the conduit 28, the control valve 26, and the conduit 36.

CAVEAT While the present invention has been illustrated by a detailed description of a preferred embodiment thereof, it will be obvious to those skilled in the art that various changes in form and detail can be made therein without departing from the true scope of the invention. For that reason, the invention must be measured by the claims appended hereto and not by the foregoing preferred embodimen .

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