Patent GB-880679-A (GB)
Title: Steering system for track-type tractors
IPC: B62D11/10
Filed: 1960-07-11
Abstract: 880,679. Steering endless track vehicles. CATERPILLAR TRACTOR CO. July 11, 1960 [Aug. 5, 1959], No. 24077/60. Class 79(2). The tracks 10, 11, Fig. 1, of an endless track tractor are driven through the bevel gears 13, 14, combination clutch and brake units 16 and gears 20, 21. The clutch and brake units are operated hydraulically, the power being supplied by a pump 60, Fig. 4, through a flow divider 62 to a valve 39 controlling supply to the brakes and to a valve 38 controlling supply to the clutches. A pilot circuit for operating the valves 38 and 39 from a control lever 25 comprises two variable displacement pumps 26, 27, placed in opposed relationship, and driven by gears 30, 28 and 31, 29 on th driven side of each clutch unit, respectively, so that the output of each is proportional to the corresponding track speed, the output of each being connected to the input of the other by pipes 40 and 41 and their outputs being controlled by a lever 25 through links 32, 33, 34. In the centre position the pumps provide a closed circuit, variation in output of one relative to the other being compensated by the passage 44 and valve 45 attached to lever 25. By moving the lever 25, the valve 45 is closed and the output of one pump is increased and the other decreased resulting in a high pressure in one or other of lines 55 or 56 leading through a reversing valve 36 to the clutch control valve 38 and brake control valve 39. The circuit of pumps 26 and 27 is made up by a pump 46 from a reservoir 47 through check valves 51 and 52, a pressure relief valve 53 being provided. The spool of valve 38 comprises bleed passages 72 through which fluid from the supply line 63 passes to provide a pressure at one end of the spool counter to the applied pilot pressure from either 57 or 58. The clutch-actuating pressure fluid passes to the right or left through one or other of lines 71 and is returned to reservoir through a line 65, the entrance to which is partially blocked by the spool to an extent according to its position as determined by the pressure from the pumps 26 and 27 thus building up a back pressure and since the pressures from 63 and 57 or 58 are equalised by the spool 70, the amount of clutch movement is regulated by the lever 25. Floating pistons 74 and 75 prevent passage of fluid between the clutch-actuating circuit and the pilot circuit. The brake control valve spool 77 is centred by springs 78 of a magnitude such that the spool is not moved by low pressures during which controlled turns by clutch actuation are effected. At a sufficiently high pressure, however, the spool 77 is moved and the right or left brake is operated through a line 80 and booster 23, the pressure being such that the corresponding clutch is completely disengaged. The steering brakes may also be operated by pedals 22. The spool 82 of the reversing valve 36 is connected to a lever 37 (Fig. 1) which may be the reversing lever of the vehicle transmission and operates so that when travelling in reverse, the high pressure in line 55 or 56 is transmitted to the line 57 or 58 opposite to that to which it would be transmitted when travelling ahead. Excessive pressures in the pilot circuit may be relieved through a valve 87 connected to the pressure lines 40 and 41 through check valves 88 and 89.