GB2038277A - Improvements relating to fork-lift vehicles - Google Patents

Abstract

A fork-lift truck with a multi-stage mast (1), a carriage (10) movable with respect to the mast and a reach mechanism comprising upright members (26, 27) which can support lifting forks, two pairs of links (21, 23) pivoted in a scissor arrangement with one link (21) of each pair being pivoted at its inboard end to the carriage (10) and with the other link (23) engaged at its inboard end by a roller (29) directly with said uppermost stage (2) of the mast in a manner to allow relative movement in a vertical direction, the outboard ends of the links (21, 23) being engaged with the upright members (26, 27), and an hydraulic ram (31) to cause relative pivoting movement of the links (21, 23) of the pairs of links and operation of the reach mechanism. The fork-lift truck can have a counter- weight with control means therefore such that the reach mechanism can only be operated when the counter- weight is in its position of maximum effect. <IMAGE>

Description

SPECIFICATION Improvements relating to fork-lift vehicles The invention relates to fork-lift trucks.

Fork-lift trucks are known having two or three stage lifting masts up which a carriage can be raised.

The carriage can support horizontally projecting forks which can be inserted beneath a palleted load whereby the load can be lifted by the forks, transported to a new location and deposited at the new location which may be at a higher or lower level than the original location.

Forward extension attachments have been proposed having lifting forks thereon and being attachable to the carriage of a fork-lift truck whereby the forks can be moved horizontally with respect to the mast so that, for example, the forks can be used to lift a load from or stack a load on the side of a lorry remote from the side which is accessible to the fork-lift truck.

When lifting loads with fork-lift trucks and particularly fork-lift trucks with forward extension attachments, it is not only the dead weight of the load being lifted which must be considered but also the moment of that dead weight effecting a turning moment on the truck, that is to say the horizontal forward separation of the centre gravity of the load from the mast, multiplied by the dead weight.

Deflection of the upright members of the mast as a result of such moment must also be considered as must the dead weight of the attachment it self and the effect of its moment on the truck.

Previously proposed forward extension attachments have comprised rear upright members securable to the carriage of the mast of the truck, forward upright members which support the forks, a pivoted scissor arrangement connecting the rear upright members and the forward upright members and an hydraulic ram to effect pivoting movement of the scissor linkage to cause separation of the forward upright members from the rearward upright members and thereby forward extension of the forks from the carriage of the mast of the truck.

According to the invention, a fork-lift truck has an upright multi-stage mast, a carriage movable generally vertically with respect to the upper-most stage of the mast and a forward extension arrangement comprising a pair of upright members which can support lifting forks, on each side of the mast a pair of links pivoted one to the other in a scissor arrangement with one link of each pair being pivoted at its inboard end to the carriage and with the other link of each pair engaged at its inboard end with said uppermost stage of the mast in a manner to allow relative movement in a vertical direction, the outboard ends of the links of each pair being engaged with the upright members, one by pivotal engagement and the other in a manner to allow relative movement in a vertical direction, and an hydraulic ram to cause relative pivoting movement of the links of the pairs of links and thereby to cause the upright members to move towards or away from the mast.

Preferably the uppermost stage of the mastcom- prises channel section members joined at their upper ends by a top member and the upright members of the forward extension arrangement are also channel section members, the carriage is movable with respect to the channel section members of the mast by having rollers engaging in the channels of the channel section members and the ends of the links which engage the mast in a manner to allow relative movement in a vertical direction are provided with rollers which engage in the channels of the channel section members.

Preferably the fork-lift truck has a counter-weight at an opposite end thereof to the end at which the mast is provided, the counter-weight is movable longitudinally of the truck by means of an hydraulic ram and the hydraulic ram of the counter-weight and the hydraulic ram of the forward extension arrangement are so coupled to a source of hydraulic pressure fluid that, upon operation of a control to extend the forward extension arrangement, the counterweight is moved to a maximum outboard position with respect to the truck before hydraulic fluid is supplied to the hydraulic ram of the forward extension arrangement to move the upright members away from the mast, Thus the forward extension arrangement can only be extended when the counter-weight is in its maximum outboard position.

The means coupling the rams to the source of pressure fluid preferably comprises a sequence valve which first supplies pressure fluid to the counter-weight movement ram and only pressurises the forward extension ram when the counter-weight ram is fully extended.

Preferably the truck includes level sensing means on the mast such that raising of the carriage up the mast and/or extension of the forward extension arrangement is prevented and/or an alarm is given if the mast is more than a determined angle away from a truly vertical position. Such level sensing means preferably comprise an electrical arrangement including a mercury filled pendulum with an electrical path therethrough such that the electrical resistance to flow of current along said path increases with increasing deviation of the mast from a vertical orientation.A solenoid valve is preferably incorporated in the path of pressure fluid from the source of pressure fluid to a ram to raise the carriage up the mast and/or the ram of the forward extension arrangement such that if insufficient electrical current flows through the mercury filled pendulum due to undue deviation of the mast from a vertical orientation, the solenoid valve remains closed and prevents raising of the carriage up the mast and/or operation of the forward extension arrangement.A trip switch to over-ride the level sensing means is preferably provided, such trip switch being operated when the multi-stage mast is in a lowered position such that some raising of the carriage with respect to the mast and/or operation of the forward extension arrangement is permitted even though the mast is not in a vertical orientation so long as the mast and the carriage thereof are not raised beyond a position at which the trip switch is operated.

The invention is diagrammatically illustrated by way of example in the accompanying drawing, which is a perspective view of an upper portion of a mast of a fork-lift truck according to the invention.

Referring to the drawing, a fork-lift truck has an upright multi-stage mast generally indicated at 1 and comprising an uppermost section 2 having channel section side members 3 and 4joined by a top member 5 and a section 6 below said uppermost section 2 comprising channel section side members 7 and 8 joined at their upper ends by a cross member 9. One or more further sections below said sections 2 and 6 may be provided as desired to give for example a possible lift of the upper end of the section 2 to 30 feet above the ground on which the fork-lift truck is standing.

A carriage 10 is of generally H-shape and comprises a cross member 11 and side members 12 and 13 rigidly secured thereto, the side members 12 and 13 each mounting rollers 14 and 15 which engage in the channels of the channel section members 3 and 4 of the uppermost section 2. Adjacent the lower ends of the side members 3 and 4 there is a transverse member 16 to which one end of each of a pair of chains 17 are connected, the chains passing over pulley wheels 18 on the cross member 9 and extending down to a fixed anchorage on the truck. A ram (not shown) has its piston rod 19 connected to the cross member 9 such that as the ram is extended the section 6 of the mast is raised upwardly. Raising the section 6 upwardly by means of the ram 19 moves the chains 17 over the pulleys 18 and thereby raises the uppermost section 2 of the mast with respect to the section 6.A further pair of chains 20 are provided, each of the chains 20 having one end secured to the transverse member 11 of the carriage 10, passing over a respective pulley wheel (not shown) mounted on the underside of the top member 5 and having the other end secured to the cross member 9 such that as the uppermost section 2 of the mast is raised with respect to the section 6 the carriage 10 will be raised by the chains 20 with respect to the uppermost section 2.

Two pairs of links, each pair in a scissor arrangement are provided with one pair at each side of the mast. Thus first links 21 are pivoted at their lower ends by pivots 22 to the respective side member 12 or 13 ofthe carriage 10, are pivoted to the other link 23 of the respective pair of links by a pivot 24 and at their upper ends mout rollers 25 each engaged in the channel of a respective upright channel section member 26 or 27. A bar 28 extends between the upper ends of the pairs of links 21 on which the rollers 25 are mounted.

The other link 23 of each pair is pivoted at the lower end by a pivot (not shown) to the lower end of the respective upright channel section member 26 or 27 and at its upper end mounts a roller 29 engaged in the channel of the respective channel section member 3 or 4 of the uppermost section 2 of the mast.

Each link 23, intermediate the pivot 24 whereby it is coupled to the link 21 and the position at which the roller 29 is mounted, is secured to a cross bar 30 whereby it is coupled to the other link 23 and an hydrayulic ram is connected between the cross member 11 of the carriage 10 and the cross bar 30.

Supply of pressure fluid to the hydraulic ram 31 to cause it to contract will pull the cross bar 30 down towards the transverse member 11 of the carriage 10 and will thereby move the roller 29 of each link 23 down towards the pivot 22 where the link 21 is connected to the carriage 10 thereby to move the upright members 26 and 27, while maintaining them in a vertical orientation, outwardly from the channel section members 3 and 4 of the upper section 2 of the mast. The upright members 26, 27 are coupled by a pair of transverse members 32,33 on which lifting forks can be secured. The supply of hydraulic fluid to the ram 31 is provided by flexible hydraulic hose (not shown) which passes over a sheave in a housing alongside a lower section of the mast 1 and a counterweight (not shown) is provided to maintain the flexible hose taut.

By providing the inboard ends of the links 21, to be mounted directly on the carriage 10 and in the channel section members 3 and 4 of the mast 1 respectively rather than to a pair of inboard upright channel section members similar to the channel section members 26, 27, the overall weight of the forward extension arrangement can be reduced by approximately one third and the moment of a load carried by forks secured to the transverse members 32, 33 is, when the forward extension arrangement is fully retracted, reduced since the upright members 26,27 can be brought closer to the upright members 3 and 4 of the mast 1 then would be the case if the forward extension arrangement had rearward upright members secured to the carriage 10. The extent of the forward extension possible can be increased, compared with a forward extension arrangement having rearward upright members, without exceeding a maximum moment for a load of determined dead weight.

Preferably the hydraulic ram 31 is connected to a source of pressure fluid by way of a solenoid valve opening and closing of which is controlled by a level sensing device which prevents the solenoid valve opening if the mast is more than a determined angle away from a vertical orientation and is so coupled to an hydraulic ram (not shown) controlling the position of a counter-weight (not shown) of the truck such that the ram 31 can only contract to extend the forward extension arrangement if the counterweight is in a fully outboard position with respect to the truck, that is to say the position in which it has maximum counter-balancing effect.

Claims (9)

1. A fork-lift truck having an upright multi-stage mast, a carriage movable generally vertically with respect to the uppermost stage of the mast and a forward extension arrangement comprising a pair of upright members which can support lifting forks, on each side of the mast a pair of links pivoted one to the other in a scissor arrangement with one link of each pair being pivoted at its inboard end to the carriage and with the other link of each pair engaged at its inboard end with said uppermost stage of the mast in a mannerto allow relative movement in a vertical direction, the outboard ends of the links of each pair being engaged with the up-right members, one by pivotal engagement and the other in a manner to allow relative movement in a vertical direction, and an hydraulic ram to cause relative pivoting movement of the links of the pairs of links and thereby to cause the upright members to move towards or away from the mast.

2. A fork-lift truck according to claim 1, in which the uppermost stage of the mast comprises channel section members joined at their upper ends by a top member and the upright members of the forward extension arrangement are also channel section members, the carriage is movable with respect to the channel section members of the mast by having rollers engaging in the channels of the channel section members and the ends of the links which engage the mast in the manner to allow relative movement in a vertical direction and the upright members to allow relative movement in a vertical direction are provided with rollers which engage in the channels of the channel section members.

3. A fork-lift truck according to claim 1 or claim 2, in which the fork-lift truck has a counter-weight at an opposite end thereof to the end at which the mast is provided, the counter-weight is movable longitudinally of the truck by means of an hydraulic ram and the hydraulic ram of the counter-weight and the hydraulic ram of the forward extension arrangement are so coupled to a source of hydraulic pressure fluid that, upon operation of a control to extend the forward extension arrangement, the counter-weight is moved to a maximum outboard position with respect to the truck before hydraulic fluid is supplied to the hydraulic ram of the forward extension arrangement to move the upright members away from the mast.

4. A fork-lift truck according to claim 3, in whcih the means coupling the rams to the source of pressure fluid comprises a sequence valve which first supplies pressure fluid to the counterweight movement ram and only pressurises the forward extension ram when the counter-weight ram is fully extended.

5. A fork-lift truck according to any one of claims 1 to 4, in which the truck includes level sensing means on the mast such that raising of the carriage up the mast and/or extension of the forward extension arrangement is prevented and/or an alarm is given if the mast is more than a determined angle away from a truly vertical position.

6. A fork-lift truck according to claim 5, in which the level sensing means comprise an electrical arrangement including a mercury filled pendulum with an electrical path therethrough such that the electrical resistance to flow of current along said path increases with increasing deviation of the mast from a vertical orientation.

7. A fork-lift truck according to claim 8, in which a solenoid valve is incorporated in the path of pressure fluid from the source of pressure fluid to a ram to raise the carriage up the mast and/or the ram of the forward extension arrangement such that if insufficient electrical current flows through the mercury filled pendulum, due to undue deviation of the mast from a vertical orientation, the solenoid valve remains closed and prevents raising of the carriage up the mast and/or operation of the forward extension arrangement.

8. A fork-lift truck according to claim 7, including a trip switch to over-ride the level sensing means, the trip switch being operated when the multi-stage mast is in a lowered position such that some raising of the carriage with respect to the mast and/or operation of the forward extension arrangement is permitted even though the mast is not in a vertical orientation so long as the mast and the carriage thereof are not raised beyond a position at which the trip switch is operated.

9. A fork-lift truck substantially as hereinbefore described and illustrated with reference to the accompanying drawing.