Patent GB-1104519-A (GB)
Title: Hydrostatic transmission
IPC: B66D1/08
Filed: 1966-03-23
Abstract: 1,104,519. Hydrostatic transmission and control. NEW YORK AIR BRAKE CO. 23 March, 1966 [3 May, 1965], No. 12814/66. Headings F2C and F2D. [Also in Divisions B7 and B8] An hydrostatic transmission, for a marine barge towing system employing a power-operated winch, prevents towing cable overload, maintains the towing cable under substantially constant tension, allows the towing length of the cable to be varied at will, and automatically maintains the selected towing length substantially constant in spite of changes in the towing drag. The hydrostatic transmission comprises swashplate motor/pump units 14, 16 connected in closed circuit by high and low pressure lines 18, 19. The drive-shafts 15, 17 of the rotating barrels of the units 14, 16 are respectively connected to a prime mover 12 and, through a speed-reducing gear-train, a cable winch 13. The unit 14 is of the overcentre type, its swashplate 33 being movable between maximum displacement positions on opposite sides of neutral. The unit 16 has a fixed swashplate. A charge pump 21, driven by the prime mover, maintains the circuit filled through lines 21a, 21b, and high and low pressure relief valves 22, 23 are provided. As described, the swashplate 33 is at the pumping and motoring sides of neutral respectively when its upper end shown is to the right and left of neutral. The swashplate 33 is biased anticlockwise, i.e. towards its motoring side, either by inherent hydraulic pressure unbalance as explained in the Specification, by rotating the usual kidney-ported valve member 29 so that its high pressure port is asymmetrical about the swashplate trunnion axis, or, under high speed, low pressure, conditions, where piston inertia predominates, by auxiliary biasing means such as a spring. The swashplate 33 is controlled by a double-acting fluid-pressure servomotor 36 energized from the charge pump 21 through the line 21a, a pressure responsive valve 38, a line 21c, and a manually set follow-up valve 37. The valve 38 comprises a spool 59 having an intermediate lap position in which a land 63 blocks the line 21c, a rightward supply position connecting the line 21c with the line 21a and a leftward vent position connecting the line 21c to an exhaust 57. The spool 59 is rightwardly urged by a spring 64 reacting on a movable seat 67 positioned by a screw 68 which is adjusted either manually or, through a speedreducing gear train 72, by rotation of the driveshaft 17 of the motor/pump unit 16 when a manually operable clutch 71 is engaged, in which case the seat 67 is moved leftwardly and rightwardly respectively when the winch 13 is reeling-in and paying-out. The spool 59 is leftwardly moved against the spring 64 by pressure in the high-pressure line 18. The valve 37 comprises a spool 51 connected through a hand lever 55 and co-operating with a ported sleeve 39 connected to the swashplate 33 by a follow-up lever connection 41. Spool lands 53, 54 are slightly narrower than their corresponding sleeve ports 43, 45 connected to opposite chambers 36a, 36b of the servomotor 36, so that when the spool 51 is in a null position with respect to the sleeve 39 the ports 43, 45 respectively communicate with both supply ports 44 connected to the line 21c and exhaust ports 42, 46. Operation.-When the towing system is first put into operation, with the clutch 71 disengaged and the valve 38 set for a safe limiting pressure higher than that required by the unit 16 to balance towing drag, the valve 38 supplies charge pump pressure to the follow-up valve 37. If the valve 37 is in its neutral position shown, pressure is vented via the restriction provided by the lands 53, 54 and a resultant back pressure is transmitted to the chambers 36a, 36b of the servomotor 36. Since the servomotor employs a differential area piston, a shifting force is thereby developed to tend to move the swashplate 33 to its pumping side; however, on swashplate movement the follow-up connection 41 eliminates this shifting force. With the valves 38, 37 in their positions shown, the swashplate 33 is maintained in neutral and the hydrostatic transmission is locked. Cable overload prevention.-If a cable overload develops, increased pressure in the line 18 causes the valve 38 to shift and vent the chambers 36a, 36b of the servomotor 36, thus enabling the anti-clockwise bias to move the swashplate 33 to its motoring side to unlock the hydrostatic transmission. When the unit 14 displacement has increased sufficiently to restore desired maximum operating pressure, spring 64 moves the spool 59 to its lap position, and since the connection 41 has shifted the sleeve 59 rightwards; the servomotor 36 is hydraulically locked against further displacement increasing movement. The system then pays-out cable at a rate determined by the unit 14 as long as overload persists, the cable tension being prevented from exceeding the permissible limit determined by the setting of the valve 38. When the cable overload abates, line 18 pressure decreases and spring 64 shifts the spool 59 to its supply position and the servomotor 36 returns the swashplate 33 to neutral. Manual variation of towing cable length.- To lengthen the cable, the lever 55 is shifted towards its limit 55a and the follow-up valve 37 and servomotor 36 act to move the swashplate to its motoring side, the cable load then driving the unit 16 as a pump which in turn drives the unit 14 as a motor. The paying-out speed is determined by the lever 55, and during this paying-out the valve acts to limit system pressure and cable tension as above. When the required amount of cable is paid out, the operator returns the lever 55 to its position shown to return the swashplate to its neutral position. To shorten the cable, the lever 55 is shifted towards its limit 55b to move the swashplate to its pumping side so that the unit 14 acts as a pump which in turn drives the unit 16 as a motor to drive the winch 13 in the reel-in direction, the valve 38 acting to limit pressure, this time by reducing unit 14 displacement. Automatic cable tension control (with variable towing length).-During towing, cable tension is maintained substantially constant by shifting the lever 55 to, and leaving it in, the reel-in position 55b, and by manually setting the valve 38 to a value that enables the unit 16 to just balance the load on the winch 13. Under these conditions, line 18 pressure reaches the val ve 38 setting as soon as the swashplate 33 reaches the position where unit 14 delivery rate equals circuit leakage rate, and the spool 59 shifts to its lap position and prevents further swashplate movement. Since the sleeve 39 is then positioned rightwards of the null position in respect of the spool 51, the servomotor 36 is hydraulically locked against the anti-clockwise bias of the swashplate. If line 18 pressure increases as a result of increasing prime mover speed, valve 38 vents the servomotor chamber 36a and allows the swashplate anti-clockwise bias to reduce unit 14 pumping delivery until normal pressure is restored. If line 18 pressure increases as a result of towing drag change, swashplate 33 must move over to a motoring position to restore normal pressure. If line 18 pressure decreases, the valve 38 supplies the follow-up valve 37, and the servomotor 36 moves the swashplate 33 further towards its maximum displacement position. Automatic towing length control.-To prevent changes in towing length in rough sea operation, the clutch 71 is engaged so that the pressure setting of the valve 38 is automatically increased and decreased respectively as cable is paid-out and reeled-in. If the towing drag increases momentarily, e.g. the towed barge encounters a wave, the valve 38 will vent and allow the swashplate anti-clockwise bias to move the unit 14 to motoring. However, since the screw 68 then shifts the seat 67 rightwards to increase the pressure setting of the valve 38, the swashplate comes to rest at a smaller displacement position to give a higher load balancing pressure. As the wave passes, the towing drag and line 18 pressure decrease to normal, and since the pressure setting of the valve 38 is higher than normal, the spool 59 moves to its supply position and the servomotor 36 moves the swashplate back to its pumping side. The unit 14 then drives the unit 16 as a motor to cause reeling-in, during which the pressure setting of the valve 38, is reduced by rotation of the shaft 17. In consequence, the valve 38 vents the servomotor chamber 36a and allows the swashplate to move towards its neutral position. When the winch 13 is returned to its original position, the swashplate 33 is then in a small displacement pumping position, in which the unit 14 output equals leakage and the spool 59 is in its lap position. If the towing drag decreases momentarily, e.g. the towing tug encounters a wave, the swashplate 33 moves to an increased pumping displacement and the unit 16 reels-in. Since the screw 68 then lowers the pressure setting of the valve 38, the swashplate comes to rest when the line 18 pressure equals the new lower pressure. Since the pressure is less than normal, as soon as the wave passes, the cable drives the unit 16 in the pay-out direction and the valve 38 pressure setting increases. Again, when the winch is returned to its original position, the swashplate 33 is in its leakage-compensating pumping position and the spool in its lap position. The ratio of change of the pressure setting of the valve 38 with respect to shaft 17 movement may be varied by changing the ratio of the gear train 72 or the spring rate of the spring 64, further with the clutch 71 disengaged the normal pressure can be changed by manual actuation of the screw 68. As an alternative control, with the lever 51 in its position 55b and the clutch 71 disengaged, towing length may be changed by manipulating the screw 68 to raise or lower the pressure setting. of the valve 38