Question

History of single purchase crab

Answer 1

In use the crab is generally bolted down to a rough timber framework which extends some distance behind it. The framework is then loaded with bricks or cast-iron fire-bars, or some such weighty material, in order to give the crab the necessary stability. The rope (wire or manilla) from the pulley blocks is then passed two or three times round the drum, and on the winch being turned is either allowed to coil up on the drum or the free end past the drum is held by hand and coiled on the ground as it comes in. These crabs are made to lift (with the aid of three sheave pulley blocks) weights from 2 to 20 tons, the load up to which they may be safely used being marked on the side frame by the manufacturer.