168 An Improved Grab Crane. 



winding round their respective barrels. During this 

 operation of closing and digging, the counterbalance barrels, 

 s s, take in the slack of the two hoisting chains, and assist, 

 instead of retard, the engine while performing the operation. 

 The engines by continuing to pull the lifting chains, after 

 the jaws are closed, raise the grab with its load, the counter- 

 balance barrel still taking in the slack of the lifting chains, 

 and assisting the operation of hoisting. When the grab has 

 attained the required height the brake is applied to stop the 

 lifting chain pulleys on the crane, the engines being at the 

 same time disengaged. The engines are next engaged to 

 pull the lowering chain, v, which being wound round the 

 larger chain barrel, o, on the grab, causes it to revolve, thus 

 unwinding the warping chains from their respective pulleys, 

 and dragging up and opening the grab jaws. When the 

 grab is opened the lowering chain is held by its brake, and 

 and the grab remains open, partly suspended by the lower- 

 ing chain and partly by the lifting chains. The grab can 

 now be lowered by the pressure being slightly taken off the 

 lowering chain brake. 



Having gone through these operations, it will be observed 

 that, unlike other machines for effecting a similar purpose, 

 the digging of the grab in any material is not interfered 

 with by the counterbalance, which assists, instead of retards, 

 the engines in performing this operation; while the whole 

 weight of the grab bears on the points, and enters the 

 material to be lifted. In other machines the grab cannot 

 be raised open, but in the machine just described the 

 grab jaws can be opened by the opening chain when the 

 grab is in any position whatever, and the grab can be 

 raised while open to any required height. Again, in other 

 machines when opening the grab the load is suddenly trans- 

 ferred from the lifting to the lowering chain, which throws 

 a severe strain on the chains and other parts of the crane ; 

 but in this machine the load is discharged while all the 

 chains are taking part of the strain, and there is no sudden 

 strain put at any time on them. Another advantage is 

 gained by having two lifting chains, either of which will 

 continue the work should the other break, and when they 

 are both at work they prevent the grab from swinging and 

 twisting the chains, which goes on to some extent in other 

 machines. The weight of the barrel shaft of the grab tends 

 to close the jaws. 



