This invention relates to clamps for the lifting of steel plate and, more particularly, to a plate lifting clamp without a neutral position which can be released from gripping the plate and locked open from a position remote from the clamp.
Hoists are used to lift heavy steel plate in steel warehouses or similar type operations. A lifting clamp, connected to the hoist, is used to grip the edge of the steel plate to permit the steel plate to be lifted by the hoist for transporting the plate in the warehouse. See, for example, U.S. Pat. Nos. 4,491,358 and 4,702,508.
Many prior art clamps are locked closed and open by lever operation. All lever operated clamps have a neutral position between the locking and unlocking positions of the operating lever. Because such prior art lever operated clamps have a neutral position, the operator is able to place the clamp into the neutral position while the clamp is suspending a steel plate from its lifting shackle. Whenever the locking mechanisms on these prior art clamps are in the neutral position, clamping engagement of the work piece is dependent on the gravitational force on the work piece. A sharp blow or bump to the steel plate can cause a temporary loss of tension on the lifting shackle thereby disengaging the gripping cam from the plate and causing the clamp to release and drop the plate.
The locking mechanism on most of these clamps is a single device that is directly connected to the linkage operating the gripping cam. It is this device that permits the locking mechanism to be shifted into the neutral position while the clamp is being suspended from its lifting shackle and carrying a steel plate. Under normal operating conditions, such clamps are relatively safe if the operator diligently adheres to the safety instructions that are provided by the manufacturer of the clamp. Unfortunately, serious accidents occur because the operators ignore these safety instructions and shift the locking mechanism to the neutral position while the clamp is under load. One of the principal reasons that these safety instructions are ignored is that the locking mechanism of the clamp cannot be unlocked and locked open from a remote location. When the clamp is used to lift a plate to a high location, often the operator will place the locking mechanism in the neutral position after lifting the plate and before raising the plate to the high location, thereby avoiding having to climb up to the clamp to unlock its locking mechanism. Further, in some cases, after the worker has climbed up to the clamp's position to release the clamp, the worker must also lift the clamp off the steel plate.
Thus, the lifting clamps of the locking and unlocking type have serious deficiencies. The prior art clamps cannot be locked open or locked closed from a remote position below the clamp. The locking and unlocking mechanisms of the clamp are directly connected to each other and, in some cases, also connected to the linkage operating the gripping cam. Movement of the cam linkage, such as by a sharp blow, to the open position while the locking mechanism is in the neutral position, can cause the locking mechanism to lock the cam in the open position. The locking mechanisms of the prior art clamps have a neutral position which creates an unsafe condition.
It is an object of the present invention to overcome these deficiencies of the prior art. In particular, the present invention eliminates the neutral position of the locking mechanism of the clamp, the locking and unlocking mechanisms of the present invention being completely independent. The clamp of the present invention may be remotely actuated to release the clamp thereby eliminating the need for operators to climb up to the clamp. Further, the clamp of the present invention cannot be unlocked or opened while there is tension on the lifting shackle.
Other objects and advantages of the present invention will appear from the following description.