GB2037437A - Industrial lift truck load - Google Patents

Abstract

To indicate when a load being handled by the truck imparts an excessive tilting moment to the truck, strain gauges sense the mechanical strain in the anchor pin 5 of the tilt ram 4 for the mast 1 of the truck, the mast being pivotably connected to the ram 4 and to the truck body at 3. The strain gauges are in shear to obtain a reasonable signal strength. Warning lights are illuminated when the load becomes excessive. <IMAGE>

Description

SPECIFICATION Industrial lift truck This invention relates to industrial lift trucks, especially those having an indicator for indicating when the tilting moment applied to the truck becomes excessive.

Several systems to this end have been proposed. In one (British Patent Specification Nos.

1 510 291 and 1 510 292), a ram for tilting the mast relative to the truck is pivotably attached to the truck by means of a pivot pin, and strain gauges associated with the pivot pin sense the bending moment in the pin. However, this arrangement has the disadvantage that unless the bearings for the pin allow it to bend, which is undesirable since the bearing area is then restricted, the available strain gauge signal is relatively weak.

The invention provides an industrial lift truck which includes a mast, a load carrier for displacement up and down the mast, the mast being mounted on a pivot and having at least one ram for rotating the mast about the pivot to vary the tilting of the mast relative to the truck, the ram being pivotably attached to the truck or the mast by way of a pivot pin incorporating strain measuring means, the strain measuring means being arranged to measure the shear strain produced in the pin by a load on the load carrier, the truck also including an indicator arranged to indicate when the signal produced by the strain measuring means reaches a predetermined magnitude, indicating that the tilting moment applied to the truck is excessive.

The measurement of shear strain in the pin overcomes the disadvantage in the prior system referred to above. A stronger signal can be obtained which leads to greater accuracy but without placing any limitation on the bearing area, which in turn permits a stronger coupling to be employed.

Advantageously, the strain measuring means is mounted on a surface substantially parallel to the direction of shear forces acting on the pin. Thus, the pin may have a pair of radial bores extending from positions on the surface of the pin that are diametrically opposite and in a direction substantially perpendicular to the centre line of the ram, and the strain measuring means may be located at the closed end of one of the bores.

Advantageously, the pin couples the ram and the mast or the truck by means of a bearing surrounding the central section of the pin secured to one of the parts and a pair of apertured lugs surrounding the outer section of the pin and secured to the other of the parts, the strain measuring means being located adjacent tothe junction of the bearing and one of the apertured lugs. Preferably, ther are two such strain measuring means being located adjacent to the the junctions.

Advantageously, the mast has a pair of rams for rotating the mast about its pivot, one ram being on each side of the mast, both rams being pivotably attached to the truck or the mast by way of pivot pins incorporating strain measuring means, and the signals produced by the strain measuring means are averaged for feeding to the indicator.

A fork lift truck constructed in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of the truck; Figure 2 is dn enlarged cross-sectional view of a pivot pin; Figure 3 is an enlarged view of the strain measuring means; Figure 4 shows a bridge circuit incorporating strain measuring means; and Figure 5 is a diagram of the circuit in which the bridge circuit is connected.

Referring to Figure 1, the lift truck includes a mast 1, and a pair of forks 2, which are displaceable up and down the mast. At its lower end, the mast has an apertured lug 3 at each side which lugs are pivotably mounted on the chassis of the truck. The truck also has a pair of rams 4 for tilting the mast 1 , the rams being located at each side of the mast. In addition, the truck has a ram (not shown) for lifting and lowering the forks.

Each ram 4 is coupled to the truck chassis by way of a pivot pin 5 which incorporates strain measuring. means in the form of strain gauges. A strain gauge is shown in Figure 3 and the pivot pin in Figure 2. The force exerted on the pivot pins 5 will give a measure of the moment tending to overturn the truck, and can thus be used to indicate when the load on the truck becomes unsafe.

Referring to Figure 2, each pivot pin 5 is secured to the truck chassis by apertured lugs 6 and to the ram 4 by a bearing 7, in which the pin 5 can rotate. Each pin 5 has bores 8 which extend radiaily from positions that are on opposite sides of the surface of the pin, and which are located at the junction of the lugs 6 and sleeve 7. The bores 8 also extend approximately perpendicular to the centre-lines of the rams (which are thus approximately perpendicular to the plane of the paper). Strain gauges 9 as shown in Figure 3 are located at the ends of two of the bores, and are bonded to the surfaces at those ends. The bores 8 are filled with polyurethene. Leads from the strain gauges pass along channel 10 and via protective box 11 to the rest of the circuit.

Since the force on the pin is substantially perpendicular to the plane of the drawing, the strain gauges are subject to shear forces in the plane of the strain gauges. It should be pointed out that the forces exerted by the rams 7 on the pins 5 will change slightly in direction as the rams are extended and retracted, but the change of direction will be relatively small and the resultant force will always be substantially perpendicular to the plane of the drawing.

Referring to Figure 4, the strain gauges 9-are each arranged in a bridge circuit with two reference resistors R in the other two arms. The bridge is supplied with voltage across the lines A, D, and the bridge output is sensed across the diagonal E, F. Resistors Rzb and Rzt are arranged in the strain gauge arms. Ret is a resistance whose value varies with temperature to compensate for variations of resistance with temperature of the rest of the bridge circuit. Rfb is inserted to bring the output across E, F to zero when no strain is applied to the strain gauges.The outputs of the two bridge circuits (only one of which is shown) across E, F are fed in parallel to the rest of the circuit (Figure 5), and the resistors Rs in the supply lines A, D are provided to provide the same output from each bridge circuit for a given loading of the strain gauges.

The indicator circuit to indicate when the load on the fork lift truck is excessive is shown in Figure 5. The bridge diagonal output is fed in at the points E, F and amber and red lights L1 and L2 light up when the output signal reaches two respective predetermined levels corresponding to unsafe overturning moments. The circuit is supplied by a 12 volt battery. R1 and C1 form a nose spike suppression circuit network with resistor R1 limiting the current flowing into capacitor C1. Diode D5 provides protection against a reverse connected supply. ZD1 is a surge suppression zener diode to further suppress line transients and this is followed by inductance L1 and capacitor C2 which provide smoothing of the supply.

VR1 is a voltage regulator integrated circuit with TR1 its series pass transistor to provide additional current. Resistor R3 is the current sensing resistor which sets the internal current limit of the integrated circuit. Capacitor C6 provides frequency compensation for the integrated circuit.

Resistors R6 and R7 form a potential divider to provide a mid-point voltage at C of one half the regulated supply voltage AD. Capacitors C3, C4 and C5 provide smoothing. OA1 is a differential input operational amplifier with variable gain. The gain is varied by resistor RV3 and the associated feedback resistors are R13, R14, R1 5 and R16.

The offset null of the operational amplifier is set by resistor RV2. The input to the amplifier is from the points EF of the bridge circuit.

The zero offset of the system is adjusted by RV1 which is part of a potential divider chain R8, RV1, R9. R10 is included to reduce the sensitivity of the adjustment. The trip points at which the lights L1 and L2 switch on are set by RV4 and RV5 which form part of the potential dividers R1 7, RV4, R1 8 and R26, RV5 and R27 respectively.

This voltage is applied via resistors (R19, R28) to the inverting inputs of the operational amplifiers OA2 and OA3 resepctively. Similarly, the output of OA1 is supplied via resistors R20 and R29 to the non-inverting terminals of OA2 and OA3.

Capacitors C7, C8, C9 and C10 provide some noise decoupling around the operational amplifiers. The large value resistors R2 1 and R30 provide the trips with in-built hysteresis which prevents oscillation and relay chatter when the input signal is very close to the trip point.

The output of each of the trip amplifiers OA2 and OA3 is taken to current amplifiers CA1 and CA2 respectively, which drive the relays RL1 and RL2 respectively. Each set of relays has two sets of change-over contacts, one set of which is arranged to drive a lamp in each case. These lamps are driven via resistors (R35, R36) which allow lower voltage, and therefore more robust lamps to be used. In operation the trip levels are so set that the amber lamp is illuminated before the red.

The circuit includes check resistors R37 and RV6 which can apply a signal which produces a positive output from OA1 to check that the electronics is functioning. The relays RL1 and RL2 may also operate controls to limit operation of the truck at, or near, an overload situation.

Typically it could halve the possible speed of the forks or other operation tending to increase the overtuming moment acting on the truck.

It has been found in tests that the rams 4 have different travels and speeds and tend to twist the mast. The provision of strain gauges on both pins 5 and the connection of the respective bridge circuit outputs in parallel tends to reduce incorrect warning signals.

The fact that the pins 5 are sensed for shear strain, rather than for example for bending moment, has the advantages that a reasonable signal is obtained, a longer bearing can be used (since it is not necessary to provide a shortened bearing to allow clearance for the pin to bend), that the coupling can consequently be made stronger, and that since shear strain remains approximately constant along the length of the pin, the exact positioning of the strain gauges is not important.

An advantage of using a variable gain amplifier OA1 is that the circuit can be used with different trucks whose load is different.

If desired, in addition to the arrangement of lamps described or instead of them, the strain gauges 9 may be coupled directly to a meter to give a continuous indication of the turning moment.

If desired, the safe loading system may be applied to a reach fork life truck.

Claims (6)

1. An industrial lift truck which includes a mast, a load carrier for displacement up and down the mast, the mast being mounted on a pivot and having at least one ram for rotating the mast about the pivot to vary the tilting of the mast relative to the truck or the mast by way of a pivot pin incorporating strain measuring means, the strain measuring means being arranged to measure the shear strain produced in the pin by a load on the load carrier, the truck also including an indicator arranged to indicate when the signal produced by the strain measuring means reaches a predetermined magnitude, indicating that the tilting moment applied to the truck is excessive.

2. An industrial truck as claimed in claim 1, wherein the strain measuring means is mounted in a surface substantially parallel to the direction of shear forces acting on the pin.

3. An industrial truck as claimed in claim 2, wherein the pin has a pair of radial bores extending from positions on the surface of the pin that are diametrically opposite and in a direction substantially perpendicular to the centre line of the ram, and the strain measuring means may be located at the closed end of one of the bores.

4. An industrial truck as claimed in any one of claims 1 to 3, wherein the pin couples the ram and the mast or the truck by means of a bearing surrounding the central section of the pin secured to one of the parts and a pair of apertured lugs surrounding the outer sections of the pin and secured to the other of the parts, the strain measuring means being located adjacent to the junction of the bearing and one of the apertured lugs.

5. An industrial truck as claimed in any one of claims 1 to 4, wherein the mast has a pair of rams for rotating the mast about its pivot, one ram being on each side of the mast, both rams being pivotably attached to the truck or mast by way of pivot pins incorporating strain measuring means, and the signals produced by the strain measuring means are averaged for feeding to the indicator.

6. An industrial lift truck substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.