GB2132972A - Crane hoist protection system - Google Patents

Abstract

The reversible drive for the whip hoist of a crane comprises a pair of hydraulic motors 11,12 coupled to the whip hoist winch and a reversible hydraulic pump 13 for supplying hydraulic fluid to the motors via main raise and lower fluid lines 14,15, the arrangement being such as to form a closed loop hydraulic circuit. A normally set winch brake comprising an automatic brake and clutch arrangement 22 is controlled by an hydraulic brake cylinder 23. The brake cylinder 23 is spring biassed to the position in which the brake is applied and it is actuated to release the brake by fluid pressure in the lower fluid line 15, thereby to allow the winch drum to pay-out cable. A protection system for the whip hoist comprises a bypass closed loop circuit connected between the raise and lower fluid lines 14,15 and including relief and shut-off valves 24,25. These valves are arranged to limit the fluid pressure in the raise line and, hence, the winch cable tension. In this connection, a load cell positioned on the crane boom is responsive to an overload tension in the winch cable to produce an hydraulic signal which is transmitted, via a line 61, to actuate the protection system. Over-ride and control valves 41 and 50 are responsive to this hydraulic signal for, respectively, overriding the normal operation of the winch brake and opening the shut-off valves 25, thereby to release the winch brake and cause the bypass circuit to function. <IMAGE>

Description

SPECIFICATION Crane hoist protection system The present invention relates to a system for protecting a crane hoist overloading and, more particularly, although not exclusively, to such a system for protecting the high speed or whip hoist of a marine crane.

Marine cranes, particularly, marine cranes used in offshore operations for loading and unloading cargo from supply ships, are frequently required to operate in rough seas and can be subjected to unusually large shock loads as a result of the motion of a ship.

Hence, a crane operator may snatch a load from a ship as it falls into the trough of a wave, the load or hook may become snagged in a heaving ship or to a ship which is drifting away from the crane, or an operator may try to lift which is heavier than the safe working load. All these conditions can cause severe overloading to the crane structure, resulting in a risk of damage to the crane and injury to operating personnel.

Typically, a marine crane used in offshore operations has a rigging arrangement for hoisting and lowering comprising a main hoist and a high speed or whip hoist, which is generally used for off loading because of its higher speed capability. The whip winch drive is by a variable displacement hydraulic pump through at least one fixed or variable displacement hydraulic motor operating in a closed loop circuit with the pump. A counterbalance valve, disposed in the closed loop circuit, meters the flow of hydraulic fluid from the motor, when lowering is selected, and throttles the fluid flow so that the motor be driven to lower a load and full control of the load is maintained. A disc-shaped automatic brake associated with the whip winch drum is spring set, thereby preventing any downward drift of a suspended load when the winch drive is in neutral.

During lowering, this brake is released by the hydraulic pressure in the lowering circuit and, if the pressure is for any reason cut off, the brake will set, automatically preventing the load from falling. A one-way clutch connecting the brake to the winch drum allows a load to be raised while the brake remains set, affording positive control. The displacement of the hydraulic motor may be automatically adjusted during operation to give high hoist speeds for light loads or maximum lift capability for heavier loads at slower hoist speeds. During normal lifting operations, loads are monitored by a safe load indicator which gives a warning if the safe working load is exceeded.

It is an object of the present invention to provide a hoist protection system for protecting a crane from the risk of accident due to severe overloading of the hoist, such as might be caused when the crane hook, or a load being manipulated by the crane, becomes snagged in a heaving ship or a ship or other vehicle which is drifting or moving away from the crane. it is another object of the invention to provide such a protection system which functions even if the crane engine stalls or is otherwise stopped.

The invention consists in a hoist protection system for a crane having at least one hydraulic winch motor, an hydraulic pump device selectiveiy operable to drive the motor in opposite directions through raise and lower fluid pressure lines, and a winch brake and associated release means for controlling rotation of the winch drum, said system comprising relief valve means disposed in a bypass closed loop circuit connected between the raise and lower fluid lines, whereby to limit the fluid pressure in the raise line and, hence, the winch cable tension, means for detecting an overload tension in the winch cable, and over-riding means responsive to said detecting means for over-riding the normal winch brake release operation to release the winch brake or maintain it released.

Preferably, a shut-off valve is connected in the bypass circuit in series with the relief valve means, this shut-off valve being opened in response to the detecting means detecting an overload tension in the winch cable so as to cause the bypass circuit to function.

The bypass circuit may incorporate an auxiliary hydraulic motor/pump set, at least part of the fluid flow through the bypass circuit being used to drive the auxiliary hydraulic motor, and the auxiliary pump having its output connected to the bypass circuit downstream of the auxiliary motor output for supplying hydraulic fluid to the bypass circuit. A second relief valve may be connected to the bypass circuit upstream of the auxiliary pump connection thereto for exhausting hot hydraulic fluid from the bypass circuit to drain. Hence, the auxiliary motor pump set has two purposes. Firstly, to provide charge pressure to maintain the integrity of the winch motor and other components when the protection system is operated with the crane engine stopped. Secondly, to supply cool fluid to the bypass circuit which effectively ejects hot fluid from the circuit via the second relief valve.It will be appreciated that considerable energy is produced in pulling cable off the winch drum at a tension of, perhaps, several tons. This energy is dissipated in the form of heat at the bypass relief valve means and, thus, requires dispersal away from the bypass closed loop circuit.

The over-riding means, conveniently, comprises a control valve arranged normally to connect the lower fluid line to the brake release means, so as to actuate the latter and release the brake when lowering is selected, and responsive to the detecting means to couple a separate source of fluid pressure to the brake release means. This separate fluid pressure source, preferably, comprises a fluid pressure accumulator arranged to be charged with hydraulic pressure during normal operation of the crane.

Such a pressure accumulator enables the protection system to function even if the crane engine stalls or is stopped. In one embodiment, the bypass relief valve means in preset to operate at a pressure corresponding to 130% of the safe working load. The detecting means is also arranged to detect such an overload and in response thereto supplies a pilot signal to the over-ride control valve to actuate the latter and permit stored pressure in the accumulator to operate the brake release means, open the shut-off valve in the bypass circuit and operate a warning signal in the operator's cab. The accumulator is pressurised by the hydraulic system whilst the engine is running. When the crane engine is shut down, the stored pressure in the accumulator may, for example, leak away in approximately 30 minutes and fall below the pressure required to actuate the protection system.This means, threfore, that the crane is not protected by the system if an overload occurs 30 minutes or more after the engine has stopped. Restarting the engine recharges the accumulator pressure.

So that the protection system is maintained actuated by sustained overloads, but automatically reverts backto normal drive following a transient shock load of short duration, the system preferably includes means for locking the over-riding means in its actuated state in response to the detection of an overload for more than a predetermined duration.

For example, a predetermined time delay of 0.5 second may be built into the detection circuit and sustained overloads or more than this duration may be arranged to lock the protection system in its actuated or set state. Once set, the system may enable the cable to unwind from the winch drum until the overload falls below a predetermined valve, and it can be manually deactivated. If the overload falls below the predetermined valve before all the cable has unwound and if the hazzard has then be averted, return to normal controlled operation may only be achieved by manually operating a valve in the driver's cab. To this end, the protection system may include manually-operated valve means for both activating and deactivating the over-riding means.The activating valve means may be arranged to supply pilot hydraulic pressure derived from the accumulator to the over-riding means to actuate the latter, whilst the deactivating valve means may be arranged to release the locking means upon manual actuation.

An empty hook, for example, of the whip hoist of a marine crane could feasibly foul a ship as it falls into a wave trough, inducing severe overload on the crane so quickly that the protection system of the invention may not be able to respond. Accordingly, particular in marine crane applications, it is preferred that the present invention be used in combination with a motion compensation system of the type described in our European specification No.

0041345. This allows the winch to reverse its direction and payout line under low tension should a crane hook be fouled in such circumstances.

If, through any unforeseen circumstances, manual activation of the protection system according to the invention is required, then this can be achieved by operating the manual activating valve means. If, however, the engine stalls or otherwise stops whilst a load is suspended from the crane, it can only be lowered or raised by restarting the engine.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which Figure 1 is a schematic representation of a marine crane embodying the invention and a ship being off-loaded by the crane, and Figure 2 is a schematic hydraulic circuit diagram illustrating one embodiment of the invention applied to the ship hoist winch of the crane of Figure 1 and adapted for use with a motion compensation system as described in our European patent specification No.0041345.

Referring to Figure 1 of the drawings, the marine crane 1 illustrated has a deck 2 and a machinery housing 3 rotatably mounted on a fixed pedestal 4 which may be part of an offshore platform anchored at sea, such as, an oil drilling platform. An operator's cab 5 projects forward from the housing 3 and a boom 6 is suitably footed on the front end of the deck 2. The boom 6 is conventionally supported by means of an A-frame assembly 7 and stays 8. The crane 1 also has a conventional rigging arrangement for hoisting and lowering which includes a main hoist hook 9 and a high speed or whip hoist hook 10.

As is conventional, the whip hook 10 is generally used for off-loading because of its higher speed capability, and the protection system hereinafter described is incorporated in the whip hoist hydraulic control circuit.

Referring to Figure 2, the drive for the whip hoist comprises a pair of bi-rotational variable displacement hydraulic motors 11 and 12 coupled to a whip hoist (not shown) and a reversible variable displacement hydraulic pump 13 for supplying hydraulic fluid to the motors 11,12 through main raise and lower fluid lines 14,15. A closed loop hydraulic circuit is formed between the pump 13 and the motors 11,12so that the hydraulic fluid delivered by the pump 13 drives the motors which, in turn, drive the whip hoist winch in either direction. Discharge of hydraulic fluid from the pump 13 into the raise line 14 drives the motors 11,12 to turn the whip hoist winch in a direction to draw in the whip hoist cable and raise a load attached to the hook 10.Conversely, discharge of oil by the pump into the lower fluid line 15 rotates the motors in the opposite direction to drive the whip hoist winch to payout rope and lower the load. A pair of conventional counterbalance valves 16,17 are disposed in the raise line 14 on the raise sides of the motors 11,12. When the pump 13 is stroked to raise a load, the flow in the raise line passes through the check valves 18,19 associated with the valves 16,17. When the pump is stroked to lower a load, the counterbalance valves are piloted to their open positions under the control of pilot lines 20 connected to lower line 15 so that fluid may pass through the motors 11,12 and rise line 14to return to the pump 13. In thins way, full control of the load is maintained during lowering and the motors are prevented from overrunning the pump and causing cavitation in the system.

The hydraulic winch drive and its controls are described in more detail in our aforementioned European specification and a fuller expianation is believed unnecessary herein for the understanding of the present invention. Suffice it to say that the pump 13 is a conventional servo-controlled axial piston pump, control pressure being applied to the servo-mechanism under the control of a manually operated hoist control valve which is disposed in the operator's cab 5 and which is movable between three positions, that is, two positions in which the whip hoist hook 10 is raised and lowered or a third neutral position in which the whip hoist winch is stationary. The hydraulic winch drive pump 13 may also be arranged to drive the main hoist winch motor. Motor and pump leakage and cooling flows are returned to the crane circuits through the case drain line 21.

In accordance with standard practice, a normally set winch brake is provided comprising an automatic brake and clutch arrangement 22 which is controlled by hydraulic release means including an hydraulic brake cylinder 23. The brake is a spring set, hydraulically released brake which in normal operation prevents the winch drum from rotating until the operator moves the hoist control valve from its neutral position. The brake operates through a one-way over-running clutch so that the brake effectively operates on only one direction, that is, to prevent lowering of a load. The brake release cylinder 23 is spring biassed to the position in which the winch brake is applied, and it is actuated to release the brake in response to fluid pressure in the lower fluid line 15 to allow the winch drum to payout cable, as is more fully described below.

Connected across each pump 11,12 is a bypass closed loop circuit including relief and shut-off valves 24,25. The relief valves are connected to the raise sides of their associated motors and their low pressure ports are connected to the shut-off valves 25. The latter comprise seated valves normally shutting-off the flow but arranged to be piloted open by venting of a pilot line 26, as described below. The low pressure ports of the shut-off valves 25 are connected to the lower fluid line 15 of the winch motors via a return line 27. However, part of the flow from one of the shut-off valves is bled-off through a flow regulator 28 to drive an auxiliary hydraulic motor pump set 29 which injects hydraulic fluid into the bypass closed loop circuits.To this end, the low pressure port of the flow regulator 28 is connected through a line 30 to the input of the motor 29M which has its output connected to the return line 27 via a filter 31 and a check valve 32. It is also connected to the junction between the filter 31 and the valve 32, together with the low pressure port of the other shut-off valve 25, via a line 33. The pressure in the lines 30 and 33 are controlled by two relief valves 34,35 having their low pressure ports connected to the case drain line 21. The valve 34 is set at a higher pressure than the valve 35. The pump 29P is connected to draw hydraulic fluid from the hydraulic fluid tank 36 and injects this fluid into the line 27 of the bypass circuits via a filter 37 and a check valve 38. This injected fluid has two purposes.

Firstly, to provide a charge pressure to maintain the integrity of the winch motors and other components when the protection system is operated with the crane engine stopped and, secondly, to inject cool fluid into the bypass circuits so as to eject hot fluid from the latter at the relief valve 35 which may, for example, be set at a pressure of 130 p.s.i.. It will be appreciated that considerable energy is inputted in pulling cable from the winch at a tension of, for example 6.5 tons. This energy is dissipated in heat form in the bypass relief valves 24 and, thus, requires dispersal away from the bypass circuits.

Provision is also made for supplying charge pressure to the hydraulic circuitry of the motion compensating system via a valve 39 and under the control of a pressure relief valve 40 connected to the case drain line.

A hydraulically operated brake over-ride valve 41 permits normal operation of the brake release cylinder 23 whilst providing a means for over-riding the normal operation and supplying pressure to the brake release cylinder from a fluid pressure accumulator 42. The valve 41 normally connects the lower fluid line 15, via a line 43, its supports 41A and 41 P and a line 44 to the brake release cylinder 23. The accumulator 42 is connected to the normally closed port 41 B of the over-ride valve via a line 45 and is charged by an auxiliary hydraulic pump 46 coupled to the drive of the pump 13 and connected to the accumulator via a pressure relief valve 47 and a check valve 48. The accumulator may be charged to a pressure of, for example, 800 p.s.i. and is sized to provide effective protection for a miniumum period of, say, 30 minutes should the crane engine stop.A gauge 49 located in the operator's cab 5 indicates the charge pressure of the accumulator. The latter also supplies fluid pressure to control valves 50-53 as explained in more detail below.

The shut-off valves 25 are controlled by the hydraulically operated control valve 50. Port 50P of the valve 50 is connected to the accumulator line 45, port 50T is connected to the port 41 T of the valve 41, port 50B is connected via a flow regulator 54 and line 55 to locking means comprising a delay unit 56 and a fluid operated control valve 57, and port 50A is connected to the pilot line 26 of the shut-off valves 25. The ports 41T and 50T are connected to the case drain line. The valve 50 is normally in the position in which the ports 50T and 50B are interconnected and the line 26 is blocked so that the shut-off valves 25 are maintained closed.The valves 51 and 50 are actuated by pilot pressure supplied to their pilot ports through a pilot line 58 controlled by the hydraulically operated control valve 59. The valve 59 is arranged to receive a pilot signal from the motion compensation system, via the line 60, indicating when the latter system is activated. When activated, the motion compensation system provides an automatic low tension overload system which makes the protection system redundant and this valve 59 blocks the hydraulic signal applied, via its ports 59P,59A, to the pilot ports of the valves 41,50 in these circumstances.

For automatically activating the protection system, a load cell (not shown) is positioned on the crane boom and measures the tension in the whip hoist cable and converts this to a hydraulic signal which is transmitted via a line 61 to a load indicating gauge in the crane operator's cab. A pressure switch 62 is also disposed in this line and detects an overload in excess of a predetermined amount, say, 130%, and provides an electrical output signal to the protection system, this signal being connected to operate the solenoid-operated 3-port hydraulic control valve 53 having its port 53P connected to the pressure accumulator by the line 45. Its port 53A is connected via double-check valves 63,64 to the port 59P of the blocking valve 59.The port 53T of the valve 53 is connected to the case drain line 21 and the valve 53 normally connects its port 53A to the case drain.

A signal as to the state of readiness of the protection system is provided in the operator's cab 5 by cab light (not shown) controlled by a pressure switch 65 responsive to an hydraulic positive feedback signal in the line 55. The port 57A of the valve 57 is connected to the double-check valve 64 and the pressure switch 65 is connected to the junction between the port 57A and this check valve.

Two manually operated valves 51,52 mounted in the operator's cab are provided for deactivating and activating the protection system. The port 52A of the valve 52 is connected to the double-check valve 63, the two ports 51 P,52P are connected to the accumulator line 45, the two ports 51T,52T are connected to the case drain line, and the ports 51A is connected to the pilot port of the valves 57. The valves 51,52 are normally in the positions in which the ports 51A and 52A are vented to the drain line.

The protection system operates as follows. When an overload signal is applied to the solenoid of the valve 53, the latter is actuated to connect the accumulator pressure at port 53P to the outiet port 53A. The resulting hydraulic output signal is transmitted, via the double-check valves 63,64 and the valve 59 to the pilot ports of the valves 41 and 50.

The valve 41 thereupon over-rides the normal brake release operation and admits accumulator pressure present at port 41 B to port 41 P and, hence, to the brake release cylinder 23. Simultaneously, the action of the pilot signal on the valve 50 firstly vents the pilot line 26 of the shut-off valves 25, via its ports 50A and 50T, to the case drain line, thereby causing the shut-off valves to open and allow the bypass release valves 24 to relieve the motor pressure and, hence, limit the tension in the whip hoist cable. The second function of the valve 50, when actuated, is to admit accumulator pressure at its port 50P to port 50B. This signal is applied to the port 57P of the valves 57 after a time delay of approximately 0.5 second which is imposed on the signal by the regulator 54 in conjunction with the delay unit 56.The latter limits the build-up of pressure until the delay unit piston has been displaced against the action of a spring.

After the predetermined time delay, the signal at 57P is supplied via the port 57A to the double-check valves 64 where it replaces the signal originally applied to port 59P of the valve 59 from the valves 53, thereby to retain the valves 41,50 in their actuated positions. Hence, if the original input signal is sustained for 0.5 second or more, a positive feed-back signal in the line 55 ensures that the protection system control valves 41,50 remain actuated, maintaining the system in a state of readiness, as indicated by the warning light in the operator's cab actuated by the pressure switch 65. If the signal on the line 55 is not sustained for this predetermined period, as will be the case where a load is snatchedoff a ship with consequent transient overloads, then the protection system will not remain in its state of readiness.

Actuation of the manual activate valve 52 admits accumulator pressure to the double-check valve 63 and operates the protection system, as described above. This provides a means for a crane operator manuallyto activate the system should the need arise. It also provides a test faciiity whereby manual operation may be used to confirm the satisfactory operation of the control valves 41,50 by energising the cab mounted light controlled by the pressure switch 65. The manual deactivate valve 51 enables switching off of the protection system once the positive feed-back signals has locked the valves 41,50 in their actuated positions. The hydraulic pilot signal from the deactivate valve 51 actuates the valve 57, thereby interrupting the feed-back signal in the line 55.However, the manual deactivate valve 51 cannot be used to interrupt the function of the protection system whilst there is an overload signal present on the line 61.

The protection system described above may be used in conjunction with a rope pull-out device. If, after the protection system has been activated, the overload persists and all the cable is unwound from the winch drum, such a device permits the rope to release from the drum. The rope anchor fitted to the winch drum allows the rope to pull free from the drum at a load less than the safe-working load.

Whilst a particular embodiment has been described, it will be understood that various modifications can be made without departing from the scope of the invention. For example, the protection system described above may be fitted to a winch which is not provided with a motion compensating system, in which event, the part of the circuit comprising the components 39 and 40 and the valve 59 will be omitted.

CLAIMS (filed on 5.1.84) 1. A hoist protection system for a crane having at least one hydraulic winch motor coupled to a winch, an hydraulic pump device selectivey operable to drive the motor in opposite directions through raise the lower fluid pressure lines, and a winch brake for controlling rotation of the winch drum, said system comprising relief valve means disposed in a bypass closed loop circuit connected between the raise and lower fluid lines, whereby to limit the fluid pressure in the raise line and, hence, the winch cable tension, means for detecting an overload tension in the winch cable, and over-riding means responsive to said detecting means for over-riding the normal winch brake operation to release the winch brake or maintain it released.

2. A system as claimed in claim 1, including a shut-off valve connected in the bypass circuit in series with the relief valve means, said shut-off valve being opened in response to the detecting means detecting an overload tension in the winch cable so as to cause the bypass circuit to function.

3. A system as claimed in claim 1 or 2, in which the bypass circuit incorporates an auxiliary hydraulic motor and pump set, at least part of the fluid flow through the bypass circuit being directed to drive the auxiliary hydraulic motor, and in which the auxiliary

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. accumulator by the line 45. Its port 53A is connected via double-check valves 63,64 to the port 59P of the blocking valve 59. The port 53T of the valve 53 is connected to the case drain line 21 and the valve 53 normally connects its port 53A to the case drain. A signal as to the state of readiness of the protection system is provided in the operator's cab 5 by cab light (not shown) controlled by a pressure switch 65 responsive to an hydraulic positive feedback signal in the line 55. The port 57A of the valve 57 is connected to the double-check valve 64 and the pressure switch 65 is connected to the junction between the port 57A and this check valve. Two manually operated valves 51,52 mounted in the operator's cab are provided for deactivating and activating the protection system. The port 52A of the valve 52 is connected to the double-check valve 63, the two ports 51 P,52P are connected to the accumulator line 45, the two ports 51T,52T are connected to the case drain line, and the ports 51A is connected to the pilot port of the valves 57. The valves 51,52 are normally in the positions in which the ports 51A and 52A are vented to the drain line. The protection system operates as follows. When an overload signal is applied to the solenoid of the valve 53, the latter is actuated to connect the accumulator pressure at port 53P to the outiet port 53A. The resulting hydraulic output signal is transmitted, via the double-check valves 63,64 and the valve 59 to the pilot ports of the valves 41 and 50. The valve 41 thereupon over-rides the normal brake release operation and admits accumulator pressure present at port 41 B to port 41 P and, hence, to the brake release cylinder 23. Simultaneously, the action of the pilot signal on the valve 50 firstly vents the pilot line 26 of the shut-off valves 25, via its ports 50A and 50T, to the case drain line, thereby causing the shut-off valves to open and allow the bypass release valves 24 to relieve the motor pressure and, hence, limit the tension in the whip hoist cable. The second function of the valve 50, when actuated, is to admit accumulator pressure at its port 50P to port 50B. This signal is applied to the port 57P of the valves 57 after a time delay of approximately 0.5 second which is imposed on the signal by the regulator 54 in conjunction with the delay unit 56.The latter limits the build-up of pressure until the delay unit piston has been displaced against the action of a spring. After the predetermined time delay, the signal at 57P is supplied via the port 57A to the double-check valves 64 where it replaces the signal originally applied to port 59P of the valve 59 from the valves 53, thereby to retain the valves 41,50 in their actuated positions. Hence, if the original input signal is sustained for 0.5 second or more, a positive feed-back signal in the line 55 ensures that the protection system control valves 41,50 remain actuated, maintaining the system in a state of readiness, as indicated by the warning light in the operator's cab actuated by the pressure switch 65. If the signal on the line 55 is not sustained for this predetermined period, as will be the case where a load is snatchedoff a ship with consequent transient overloads, then the protection system will not remain in its state of readiness. Actuation of the manual activate valve 52 admits accumulator pressure to the double-check valve 63 and operates the protection system, as described above. This provides a means for a crane operator manuallyto activate the system should the need arise. It also provides a test faciiity whereby manual operation may be used to confirm the satisfactory operation of the control valves 41,50 by energising the cab mounted light controlled by the pressure switch 65. The manual deactivate valve 51 enables switching off of the protection system once the positive feed-back signals has locked the valves 41,50 in their actuated positions. The hydraulic pilot signal from the deactivate valve 51 actuates the valve 57, thereby interrupting the feed-back signal in the line 55.However, the manual deactivate valve 51 cannot be used to interrupt the function of the protection system whilst there is an overload signal present on the line 61. The protection system described above may be used in conjunction with a rope pull-out device. If, after the protection system has been activated, the overload persists and all the cable is unwound from the winch drum, such a device permits the rope to release from the drum. The rope anchor fitted to the winch drum allows the rope to pull free from the drum at a load less than the safe-working load. Whilst a particular embodiment has been described, it will be understood that various modifications can be made without departing from the scope of the invention. For example, the protection system described above may be fitted to a winch which is not provided with a motion compensating system, in which event, the part of the circuit comprising the components 39 and 40 and the valve 59 will be omitted. CLAIMS (filed on 5.1.84)

1. A hoist protection system for a crane having at least one hydraulic winch motor coupled to a winch, an hydraulic pump device selectivey operable to drive the motor in opposite directions through raise the lower fluid pressure lines, and a winch brake for controlling rotation of the winch drum, said system comprising relief valve means disposed in a bypass closed loop circuit connected between the raise and lower fluid lines, whereby to limit the fluid pressure in the raise line and, hence, the winch cable tension, means for detecting an overload tension in the winch cable, and over-riding means responsive to said detecting means for over-riding the normal winch brake operation to release the winch brake or maintain it released.

2. A system as claimed in claim 1, including a shut-off valve connected in the bypass circuit in series with the relief valve means, said shut-off valve being opened in response to the detecting means detecting an overload tension in the winch cable so as to cause the bypass circuit to function.

3. A system as claimed in claim 1 or 2, in which the bypass circuit incorporates an auxiliary hydraulic motor and pump set, at least part of the fluid flow through the bypass circuit being directed to drive the auxiliary hydraulic motor, and in which the auxiliary

pump has its output connected to the bypass circuit downstream of the auxiliary motor output for supplying hydraulic fluid to the bypass circuit.

4. A system as claimed in claim 3, including a second relief valve connected to the bypass circuit upstream of the auxiliary pump output connection thereto for exhausting hot hydraulic fluid from the bypass circuit.

5. Asystem as claimed in any one of the preceding claims, in which the over-riding means comprises a control valve arranged normally to connect the lower fluid line to the brake release means so as to actuate the latter and release the brake when lowering operation of the winch motor is selected, said control valve being responsive to the detecting means to couple a separate source of fluid pressure to the brake release means.

6. A system as claimed in claim 5, in which the separate fluid pressure source comprises a fluid pressure accumulator arranged to be charged with hydraulic pressure upon operation of the hydraulic pump device.

7. A system as claimed in any one of the preceding claims, including means for locking the over-riding means in its actuated state in response to detection of an overload for more than a predetermined duration.

8. A system as claimed in claim 7, including manually operated valve means for selectively activating and deactivating the over-riding means, said activating valve means being arranged to supply pilot hydraulic pressure derived from the accumultor to the over-riding means to actuate the latter, and said deactivating valve means being arranged to release the locking means upon manual actuation.

9. A hoist protection system for a crane, constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.