GB2159788A

GB2159788A - Cranes

Info

Publication number: GB2159788A
Application number: GB08514149A
Authority: GB
Inventors: Herbert Halsey
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1984-06-06
Filing date: 1985-06-05
Publication date: 1985-12-11
Also published as: GB8514149D0; GB8414406D0; GB2159788B

Abstract

In a crane having two hoisting ropes (2,3) each rope acts against the piston of a respective hydraulic piston and cylinder unit (15, 16) having the cylinders interconnected to equalise the pressure acting against the pistons, and means are provided for monitoring the movement of the pistons to give an indication of rope failure or differential stretching of the rope. <IMAGE>

Description

SPECIFICATION Cranes This invention relates to cranes.

It is known that for crane applications requiring assurance of a high integrity load support it is common to provide two hoisting ropes or cables (hereinafter referred to simply as ropes). The ropes normally share the load, but must be of adequate strength such that in the event of failure of one of the ropes the remaining rope will carry the load. One means of load sharing is by a process called "double sheaving" whereby each rope is wound onto a common drum, or one of two drums rotatable together, and passes round a pulley carried by the crane hook and returns to the structure housing the drum.

It is an object of the present invention to provide a means of indicating rope failure in double sheaving type cranes and to minimise load movement should this occur.

According to the invention in a crane having two hoisting ropes the tension in each rope acts against the piston of a respective hydraulic piston and cylinder unit the cylinders of which are interconnected to equalise the pressure acting against the pistons, and means are provided for monitoring the movement of the pistons.

Such movement may occur for example by breakage of a rope or by an excessive differential stretch in the ropes. Normally hydraulic fluid will be free to flow between the cylinders to equalise the pressure within them, and any differences in rope length that occur will result in movement of the pistons, means preferably being provided for providing a warning signal, or possibly stopping the further operation of the crane, on movement of the pistons through more than a predetermined distance.

The crane may incorporate, for example, mechanically actuated electrical switches which are operated on excessive movement of the pistons, although photoelectric devices disposed so as to provide an output signal on movement of the pistons through more than a predetermined distance could alternatively be employed.

The warning signals for indicating excessive movement of one or both pistons may be provided, as may be desired, by visual and/or audible devices.

Preferably each rope, after passing around a respective pulley fitted to the hook, passes over a further pulley supported by a piston of the respective piston and cylinder unit, around another pulley fitted to the hook to an attachment point on the drum assembly. Conveniently the hook has four coaxially disposed pulleys, two outer pulleys being associated with one of the ropes, and the two inner pulleys being associated with the other rope, to provide a balanced support in the event of rope failure or excessive differential stretching.

A pressure sensor may be arranged to monitor the pressure of the hydraulic fluid in the piston and cylinder system and arranged to generate a warning signal, for actuating an audible and/or visual device in the event that an attempt is made to raise a load which is heavier than the maximum permitted load for the crane.

Preferably, also, the means linking the two cylinders incorporates a valve which is arranged to be closed on receipt of a signal indicative of a movement of one or other of the pistons, resulting from excessive lowering of the tension in the respective rope, means being provided for generating an alarm signal should this occur.

One embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic drawing.

Referring to the drawing, this illustrates schematically a crane having a single drum 1 formed with two helical grooves (not shown) each containing a rope 2 and 3 respectively.

The rope 2 leaves the drum 1 downwards to a pulley 4 which in turn is fitted to the crane hook 5. After passing under the pulley 4 the rope rises to pass over a further pulley 6 and then downwards under a pulley 7 also fitted to the hook 5 and finally up to anchor point 8. The second rope 3 follows a similar path under a pulley 9 up to and over pulley 10 down and under pulley 11 and upwards to anchor point 1 2 the pulleys 11 and 9 again being fitted to the hook 5. The two hookmounted pulleys that are associated with one of the ropes are disposed outside those associated with the pulleys associated with the other rope so as to give a balanced support should one of the ropes break, although this is not shown in the drawing.The pulleys 6 and 10 are mounted on piston rods 1 3, 1 4 on to which are fitted pistons 15, 1 6 the pistons being movable to a limited extent with respect to the rods. Springs 17, 1 8 each abut onto a collar 19, 20 which is part of a respective piston rod 13, 14, and bear against the tops of the pistons urging them downwards. The pistons are supported by oil contained in cylinders 21, 22. A connecting pipe 23 joins cylinders 21, 22 and contains a valve 24. The piston rods 13, 1 4 also carry arms 25, 26 respectively which contact switches 27, 28 in the pistons raised mode, or switches 29, 30 in the pistons lowered mode. A pressure sensing device 31 is connected to the pipe 23.

In operation the valve 24 is normally open, and oil flows between cylinders 21, 22 to equalise pressure in them. The equal pressure exerts equal forces on pistons 1 5, 1 6 which in turn through the springs 17, 1 8 exert equal forces on pulleys 6, 10 and thus approximately equal forces on ropes 2, 3.

Differences in length due to differences in elasticity of ropes 2, 3 is accommodated by differential movement of pulleys 6, 10 causing flow along the pipe 23 between cylinders 21, 22.

Excessive differential movement will cause operation of either of switches 29, 30 and indicate excessive elasticity which may indicate possible rope failure.

Failure of the rope 2 (or 3) due to a fault in the rope would allow the pulley 6 (or 10) to operate the respective switch 27 (or 28).

Operation of the switch 27 (or 28) is arranged to effect the closure of the valve 24 and also raise an alarm. The closing of the valve 24 prevents oil flow along pipe 23 and conequently prevents piston 1 6 (or 1 5) from lowering which, in turn, limits the lowering of the load supported on hook 5 and prevents operation of switch 30 (or 29).

Loads in excess of the permitted overload will raise the pressure in both cylinders 21, 22 and operate the pressure sensing device 31 which will also raise an alarm. The sensing device 31 will also operate in the event of rope failure if the load carried is greater than half the permitted overload. A second sensing device (not shown) may be connected to the pipe 23 on the opposite side of the valve 24 if necessary.

Loss of oil in the system will permit both pistons 1 5, 1 6 to lower and cause operation of switches 29, 30 which is also arranged to operate an alarm.

Claims

1. A crane of the type having two hoisting ropes supporting a hook comprising two hydraulic piston and cylinder units, the cylinders of which are interconnected to equalise the pressure acting against the pistons, the tension in each of the ropes acting against a respective piston, and means for monitoring the movement of the pistons.

2. A crane according to Claim 1 further comprising means for providing a first signal on movement of at least one of the pistons through more than a predetermined distance.

3. A crane according to Claim 1 or Claim 2 further comprising a pressure sensor arranged to monitor the pressure of the hydraulic fluid in the interconnected piston and cylinder units and to generate a second signal if said pressure exceeds a predetermined value.

4. A crane according to Claim 2 or Claim 3 where in either said first or second signals activates a warning device which may be audible and/or visible.

5. A crane according to Claim 2 or Claim 3 wherein either said first or second signals causes further operation of the crane to be stopped.

6. A crane according to any preceding claim wherein said monitoring means comprise switches which are operated on excessive movement of the pistons.

7. A crane according to Claim 6 wherein said switches are mechanically actuated electrical switches.

8. A crane according to Claim 6 wherein said switches are photoelectric devices which provide an output signal on movement of the pistons through more than a predetermined distance.

9. A crane according to any preceding claim further comprising four pulleys fitted to the hook, two for each rope and a further two pulleys, again one for each rope, the further pulleys being supported by the piston of the respective piston and cylinder unit, each rope passing around a respective hook-mounted pulley, around a respective further pulley and finally around a further respective hookmounted pulley to an anchor point.

10. A crane according to Claim 9 wherein said four hook-mounted pulleys are coaxially disposed, the two outer ones being associated with one of the ropes, and the two inner ones being associated with the other rope.

11. A crane according to any one of Claims 2 to 10 wherein a value is used to interconnect said piston and cylinder unts and is responsive to said first signal so as to close the valve.

1 2. A crane substantially as hereinbefore described with reference to the drawing.