Background
The invention relates to air-operated hoists in industrial applications that have a load support. The development prevents the system from attempting to lift the load if the total load force exceeds a predetermined amount.
Typically air hoists have motors sized to provide a maximum specific lift capacity (for example, 0.25, 0.5, 1 US ton etc) and controls that control the vertical movement of the lifting mechanism. So, when lifting, the hoist can raise a load up to a predetermined lift capacity.
In industrial situations many applications need a tool or lifting device suspended or otherwise attached to the hoist structure. These are commonly called ‘end-effectors’ or ‘below the hook tooling’. Commonly the weight of this tool combined with that of the load itself would be less than the rated lifting capacity of the hoist, leaving excess capacity. In some situations this can cause unsafe circumstances. For example, when attempting to lift heavier objects than the rated capacity of the end-effector tooling, although less than that of the hoist itself. Whilst the hoist could continue to lift, the tooling would be placed in an unsafe condition. In a related condition the total load may be too much for the rated capacity of the hoist. In this case, if the load is removed from the hoist, the hoist may continue to apply the maximum lift force, causing the end-effector tool to lift abruptly with maximum force resulting in possible damage to equipment and perhaps placing the operator in an unsafe situation.
The tool and/or load is in contact with an obstacle such as an adjacent shelf or machinery component. The hoist may have the capacity to continue upward movement but this would again result in possible damage to equipment or endangering the operator.
Objective
The objective of the invention is to prevent the above situations by stopping further flow of pressurized operating fluid. This may typically be achieved by a pneumatic overload protection circuit comprising a fluid pressure source, a hoist actuated by pressurized fluid and including a load handling device, a fluid conduit between the source of fluid and the hoist, and a sensor to sense the load imposed on the load tooling to control the flow of pressurized fluid through the conduit.
Description
There are various possible embodiments of the invention covered by the patent.
In Figs 1 and 2 a hoist (10) is typically attached to a crane that can be used to rotate the hoist about a longitudinal axis, as well as moving it in a horizontal direction. The hoist assembly includes end-effector tooling (12) to support the load being lifted, and which can be mounted on a bracket on a rotational bracket on a backing plate (16). The end-effector and bracket are guided through pivotal positions by an operating ring (18) that can be held by the operator to facilitate rotation of the end-effector. The bracket position can be fixed by a locking pin and handle (20).
A pneumatic device incorporating two airbags (22) lies between an upper load hook guide plate (24) and a lower hoist hook mounting plate (26) positioned apart. Airbags of 11.25in (286mm) diameter are preferred. A threaded fastener (28) connects the upper ends of the airbags to the upper hoist load hook guide plate. This is, in turn, secured to the backing plate. Threaded fasteners also fasten the lower ends of the airbags to the backing plate, also providing a communication between the source of fluid pressure and the airbags through a regulator (32).
The airbags vary the amount of energy required to move the plates (24 and 26) towards each other by varying the amount of energy required to compress the airbags. The airbags can be manually adjusted to control the pressure maintained in the airbags, and this will determine the lifting capacity of the hoist.
An eyebolt (36) extends through apertures formed in the hoist load hook guide plate (24) and the hoist hook mounting plate (26). This receives an attachment to a raising/lowering device (40). The threaded end of the eyebolt is adapted to extend through an aperture in plate 26 and receives a nut (42) for attachment. The latter contacts a lower surface of the hoist hook mounting plate and limits downward movement of this plate.
A limit-switch assembly (44) is mounted on backing plate 16. It includes an arm on a pivot that is biased to contact the hoist hook mounting plate. Normally the pivoted arm of the limit switch assembly is in contact with the lower surface of the hoist hook mounting plate.
The limit switch is connected to a pilot valve (46 of Fig 3) to permit pressurized fluid to pass through when the pivotal arm of the limit switch is in a first position. If the arm is moved into a second position by upward movement of mounting plate 26, the pilot valve is also moved into a second position to prevent flow of the pressurized fluid. The pilot valve is connected to a control valve for fluid communication and thence connected to the raising/lowering device, allowing it to move upwards. The fluid (air in this case) can flow to atmosphere from the raising/lowering device, causing it to move downwards. However, if the pilot valve is positioned to prevent flow, the fluid pilot signal does not flow to the control valve and is caused to move into a second position. In this second position the control valve prevents flow to the raising/lowering device, preventing it from moving upwards, but as the flow can escape to atmosphere, it allows the raising/lowering device to move downwards.
Backing plate 16 is mounted on a slide on a pair of vertical guide rails and carries the hoist load hook guide plate, the load support bracket and limit switch assembly. The backing plate can be adjusted to any position on these guide rails using the raising/lowering device according to the control panel (50).
Operation
Referring to the circuit shown in Fig 3, pressurized fluid is supplied from source 62 to the pilot valve 46. According to the inventor the fluid pressure using dry, clean air at 90 lbf.in.2 produces satisfactory results. The pressurized fluid flows through the pilot valve to the control valve, putting it in the first position. In this position the control valve allows fluid flow from the main source through the control valve to the raising/lowering device to atmosphere. This permits the raising/lowering device to move upwards and downwards.
When the pilot valve is in the second position, it prevents fluid flow through it, causing the control valve to move to the second position in which the fluid is prevented from flowing from the source to the raising/lowering device, stopping upward movement. However, fluid can flow from the raising/lowering device to atmosphere, allowing downward movement.
Manual control of the pressure regulator (32) sets the pressure maintained in the airbags. Once the required pressure is achieved the regulator prevents further fluid flow. If a reduction of pressure in the airbags is desired, the regulator can be manually operated to release fluid to atmosphere.
In operation the crane positions the hoist in a desired location, and the regulator is manually adjusted to inflate the airbags to a predetermined setting to accommodate the weight of the end-effector tooling and the load weight to be lifted, plus, for example, a margin of 10%. Thereafter the operator uses the control panel to work the raising/lowering device.
An appropriate signal from the control panel causes pressurized fluid to pass from the source through the pilot valve and the control valve to the raising/lowering device, moving it upwards. In turn this causes the eye bolt and hoist hook mounting plate to move upwards as well, with the plate applying a force to the airbags. If this force is less or equal to the force created by the pressure in the airbags, the hoist load hook plate moves upwards with the raising/lowering device, together with the backing plate. The limit switch,
bracket 14, and end-effector tooling also move upwards. This continues until the signal from the control panel is stopped, or the raising/lowering device reaches a maximum allowed height.
On the other hand, if the force applied to the airbags by the hoist hook mounting plate exceeds that caused by the pressure in the airbags, the latter will collapse. This causes the hoist hook mounting plate to move upwards, losing contact with the arm of the limit switch. This forces the pivot arm of the limit switch and pilot valve into the second position, preventing flow through the pilot valve. This causes the control valve also to move to its second position, preventing fluid flow from the source to the raising/lowering device, and thus movement of the raising/lowering device.
To lower the raising/lowering device the correct signal from the control panel causes the raising/lowering device to exhaust fluid to atmosphere, causing downward movement. This causes the eye bolt and hoist hook mounting plate to move downwards also and to apply a force against the airbags. If this force is less than or equal to the force due to pressure in the airbags the hoist hook guide plate moves downwards with the raising/lowering device. The limit switch assembly, bracket and end-effector tooling also moves downwards until the signal from the control panel is stopped, or until the raising/lowering device reaches a minimum allowed height.
If the force from the hoist hook mounting plate on the airbags exceeds that created by the pressure in the airbags, the airbags will collapse. This causes the hoist hook mounting plate to move upwards, losing contact with the limit switch. This causes the pilot valve to move into the second position, preventing flow through the pilot valve. Again, this causes the control valve to move to the second position preventing flow from the source to the raising/lowering device. As flow can escape to atmosphere from the raising/lowering device, this allows the raising/lowering device to move downwards.
Thus the amount of fluid maintained in the airbags controls the lifting capacity of the hoist. If an overload occurs the raising/lowering device is prevented from rising but lowering is permitted. When the overload condition has been removed, the hoist reverts to normal operation.
