Research into the Design of Grabs by Tests on Models

Abstract

Various lines of research into grab design have been tried since investigations started in 1953. It was finally decided to concentrate on the technique of using models. Laboratory apparatus was constructed consisting of one-twelfth scale clamshell and wide-span grabs which could be made to dig into suitably sized samples of iron ore, the ‘pick-up’ being measured and recorded.

The main conclusions are that existing grabs are proportioned in about the best way possible, and that extra weight increases the efficiency only with very lumpy materials, where the efficiency is, in any case, low. Also, weight for weight, the wide-span grab is more efficient than the clamshell.

Following analytical treatment of the grab mechanisms, results of further tests with model grabs suggest that for maximum pick-up the jaws should exert as high a torque as possible, especially towards the end of the grabbing cycle.