GB2133772A - Hoists having load support in the event of hoist failure - Google Patents

Abstract

A hoist (10) having lifting means (18, 19) for a very heavy load (20) also has rotatable threaded rod load supporters (21) depending from the hoist to which the load is coupled by nuts (22) and arms (23). The supporters are rotated by a slipping drive so that the nuts try to move in advance of an rise or fall of the load. The advance drive applies a raising or lowering force to the supporters which are free to move axially within small tolerances. At the limit of the tolerances brakes are applied to the rotation of the supporters. In the event of the hoist releasing the load, the load becomes supported by the arms and nuts on the supporters (21) with the supporters firmly braked. <IMAGE>

Description

SPECIFICATION Hoists having load support in the event of hoist failure This invention relates to hoists having load support in the event of hoist failure.

In the operation of nuclear reactors and ancillary plant very heavy loads have to be raised and lowered. Typically, a shielded transport flask for irradiated fuel elements may weight 100 tonnes. A stand-by support is required for such loads to safeguard against damage to the load or objects below the load should the load inadvertently fall whilst being hoisted or transferred in position. Preferably, the stand-by support should have effect as soon as possible after the hoist failure occurs so that large forces are not built up. The present invention is directed to this consideration and has one application to the hoisting of a transport flask being moved above sodium carrying pipes servicing a sodium filled body such as a "fast" nuclear reactor.

A hoist according to the present invention having stand-by load support comprises: (a) a hoist body including a raisable and lowerable lifting grab hook or ties; (b) rotatable threaded rod load supporters depending from the hoist body; (c) nuts engaged with the supporters and carried by members attached to the load; (d) means including a slippable drive for rotating the supporters in advance of the rise and fall of the load; (e) means permitting the supporters to have a rise and fall capacity within small tolerances; and (f) brakes on the supporters for braking their rotation at the limit of the tolerances.

A hoist according to the invention will now be described with reference to the accompanying drawings in which: Figure 1 presents a diagrammatic elevation; Figure 2 shows a sectional elevation at the region marked 11 in Fig. 1; and Figure 3 is an elevation in the direction Ill of Fig. 2 with additional structural members.

In Fig. 1 a hoist 10 is shown having a body 11 movable at flanged wheels 1 2 on a track 1 3 supported on a floor 14. The body has a pair of lifting drums 1 5 and a lifting cable anchor 16. A lifting cable 17 extends from the drums 1 5 and passes to a pulley block 1 8 to which can be attached a load 20 by lifting ties 1 9. The load has nuts 22 attached to it by way of arms 23 and the nuts engage rotatable threaded rod load supporters 21.

The supporters 21 are carried at their upper ends in carriers 24 having a gimbal connection 25 with a slewing ring 26 mounted at the bottom of a transition tube 27 extending downwardly from the body 11. Control arms 28 are shown which adjust the scoops of fluid drive couplings in the carriers 24.

In Fig. 2 a load supporter 21 in connected with a worm gear 29 via a key 29a. The gear 29 is driven by a worm 30. The worm 30 is driven via pulleys, belts, a gear box, a fluid coupling and a drive motor, these details not being shown.

The supporter 21 has a head 31 which carries a ring-shaped brake shoe 32. The shoe, and hence the supporter, is carried on springs 33 above a ring 34 on one race 35 of a bearing 36. The other race 37 of the bearing is on a support 38 which also carries an annular brake housing 39 cooperating with the brake shoe 32. The bearing 36 has freedom to slide on head 31 when the supporter 21 moves downwards and on housing 39 when the supporter moves upwards.

The brake shoe has upper and lower braking surfaces 40, 41 which run with a clearance of 2.5mm relative to brake surfaces 42, 43 on the housing 39. A limit switch 44 is located at 5mm above the top of the head 31.

The springs 33 are precompressed to a load of (say) 2 tonnes and accept the loading (about 1 tonne) of the supporter 21 without further compression.

In normal operation for raising the load, the worm 30/gear 29 are poered in a direction to rotate the supporters so that the supporters apply a lift to the load at nuts 22. Until the load is actually caused by the cable 1 7 to raise, this lift will react on the supporters 21 to pull them down. When this occurs the springs 33 compress from the position shown in Fig. 2 and brake surfaces 41, 43 come into contact which causes slip to take place in the drive motors. In general, the supporters 21 drive the nuts 22 at about 20% faster than the hoist moves the load and hence there is substantially continuous self-braking with the supporters rotating slightly in advance of the rise and fall of the load.

As the raising of the load at cable 1 7 takes place, the supporters 21 are lifted and the brake surfaces become free and slip ceases or reduces at the drive motors. However, as the drive motors always have a capability of driving the supporters 21 at a rate such that the nuts can potentially rise and fall faster than the nuts can be moved by the hoist 10, brake surfaces re-engage and slip at the drive motor recurs.

On lowering of the load, a similar situation is set up except that brake surfaces 40 and 42 cooperate once the precompression in springs 33 is overcome.

In the event that the hoist 10 should fail in any way so that the load is freed from the hoist then the supporters 21 will retain the load in the position it occupied at the point of failure with brake surfaces 41, 43 preventing any tendency for the supporters to rotate. At this point the drive motors will also trip out of service but they should not trip out of service in the normal course of operation even though they frequently experience slip when the supporters 21 are not precisely following the movement of the load.

In the event that the drive to the supporters 21 fails whilst the hoist continues to raise the load then the supporters could be subjected to a compressive load such as to cause them to bow. This is avoided in the construction shown in the drawings in that, before a bow causing load is applied to the supporters the head 31 rises (with the brake shoe 32 sliding on the head) so that the head 31 contacts the limit switch 44 and trips the hoist drive.

In Fig. 3 consideration is given to the arrangement in which the hoist exists on a crab movable on a bridge (that is the floor 14 of Fig. 1 is a part of a bridge). The stress conditions in the load supporters 21 arising in the event of hoist failure are then governed to some extent by the location of the crab on the bridge. Failure of the hoist with the crab at mid span of the bridge creates a lower stress (as there is a degree of resilience from the bridge) than failure at the end of the bridge and directly over a building stanchion (where there is very little resilience from the bridge).

To accommodate the greater stress situation in the load supporters the hoist is carried on compressible pads below the hoist body. As shown in Fig. 3 pads 50 are mounted below a housing 51 for the driven worm gear (29 in Fig. 2) to which load supporters 21 are keyed. The pads are in the form of a short aluminium cylinder 52 with a recess 53 split to provide a pair of spaced pads 50 which can be withdrawn for inspection and replacement without dismantling other parts of the hoist.

With hoist failure the pads 50 compress and thereby allows a reduction in maximum stress in the load supporters.

Claims (7)

1. A hoist having stand-by load support comprising (a) a hoist body including a raisable and lowerable lifting grab hook or ties; (b) rotatable threaded rod load supporters depending from the hoist body; (c) nuts engaged with the supporters and carried by members attached to the load; (d) means including a slippable drive for rotating the supporters in advance of the rise and fall of the load; (e) means permitting the supporters to have a rise and fall capacity within small tolerances; and (f) brakes on the supporters for braking their rotation at the limit of the tolerances.

2. A hoist as claimed in claim 1 in which the brakes are of annular form with annular brake shoes having upper and lower braking surfaces and annular brake housings having upper and lower braking surfaces, and clearances between said braking surfaces which define the small tolerances within which the supporters have a rise and fall capacity.

3. A hoist as claimed in claim 2 in which the supporters rotate on bearings between the supporters and the brake housings and the bearings have an axial freedom of movement under applied forces in an upward direction on both the supporters and the brake housings.

4. A hoist as claimed in claim 3 in which the brake shoes lie between heads on the load supporters and are subjected to the loading of axially acting springs precompressed to a greater loading than the weight of the load supporters.

5. A hoist as claimed in any preceding claim in which the load supporters can rise by a distance greater than the limit of the rise tolerances imposed by the brakes to contact a limit switch to avoid the hoist applying a bowing load to the load supporters.

6. A hoist as claimed in any preceding claim in which the threaded rod load supporters are carried on pads compressible on hoist failure.

7. A hoist substantially as hereinbefore described with reference to the drawings.