Patent GB-1501955-A (GB)
Title: Industrial trucks
IPC: B62D5/07
Filed: 1975-10-15
Abstract: 1501955 Steering systems RANSOMES SIMS & JEFFERIES Ltd 15 Oct 1975 42360/75 Heading B7H An industrial truck is provided with at least one hydraulic auxiliary service mechanism 51, 52 and a power or power assisted steering mechanism 9, a valve 5 or 6 for controlling flow to and from the auxiliary mechanism which when in its neutral position allows fluid to flow directly from its inlet to its outlet and then to the power steering mechanism. In one arrangement, Fig. 4, the valves 4, 5, 6, which respectively control the lift 52, tilt 51 and some other service, are connected in series and with the valves in the neutral position fluid passes directly to the steering mechanism. Displacement of valves 5, 6, establishes paths of flow both to and from their respective services, whereas displacement of valve 4 permits fluid to flow to the lift cylinder 52 but the valve provides no return path. A solenoid actuated valve 23, which may be omitted (Fig. 1, not shown), bypasses the valves 4, 5, 6 to allow the steering mechanism to be operated when the valves are displaced from the neutral position. In other embodiments (Figs. 6 and 7, not shown) valves 4, 5, 6 are connected in parallel so that flow to the steering is still possible when one of the valves is actuated. The solenoid 40 of valve 23 may be connected to switches 32, 35, 36 operated by the valves so as to be actuated when the valves are displaced. The series wound pump motor 2 is included in a circuit comprising a main switch 13 which controls a contactor 14 connecting the battery 15 to the motor 2, and a pulse controller 16. A speed feedback system 18 is provided to monitor the load on the hydraulic services to maintain the pump speed at a consistent level. In another arrangement (Fig. 3, not shown) the valves operate switches (17a-17c) which select a particular pulsing frequency of the controller to cause the appropriate flow to the selected auxiliary service. The control circuit for the embodiment of Fig. 4 is shown in Fig. 5. A main key switch 21 effects energization of a contactor 22 which connects the battery 25 to motor 26 in series with a pulse controller 27 and further connects the battery to a switch 32 coupled to the lift valve 4 and in series with contactor 33. When valve 4 is operated contactor 33 is energized and shorts out part of the field winding of motor 26 to allow the motor to give maximum speed for example 2500- 3200 r.p.m. Contactor 33 also controls closure of switch 34 connected in parallel to switches 35 and 36 respectively coupled to valves 5 and 6. The switches 35, 36 are in circuit with a foot switch 37 which is closed when the vehicle is operating and the coil of valve 23, so that if valve 5 or 6 is actuated when the vehicle is operating valve 23 is actuated. Controller 27 serves to maintain the pump speed at one of two substantially constant levels, e.g. 600 to 900 r.p.m. It comprises a control circuit pulse generator 42 which controls a power circuit 43 in the form of a thyristor chopper. The circuit 42 has a first input connected via contactor 22 to the battery 25, a second input connected to the motor armature at its junction with the field windings and a third input connected to a current probe 44. By means of these inputs the motor speed may be ascertained and the pulse frequency adjusted to provide one of the preselected speeds. The speed selected is determined by a fourth input which when supplied with battery voltage causes the control circuit to provide the lower of the speeds and when not so supplied the higher speed. The fourth input includes a change over switch 31 which operates to disconnect the battery when switches 35 or 36 are operated.