Results |
 
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Results |
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Although the tower pulley rotates at the same speed in all models, the block takes different times to reach the 14m elevation in the 1-, 2- and 3-pulley models. This is a fundamental principle of a crane system with moveable pulleys.
Block Elevation
For the 1-pulley model, there is no mechanical advantage, and the tower pulley bears the full 10ton weight, resulting in a 98kN tensile load in the cable. When a second pulley is introduced, the tensile load is halved. When a third pulley is added, the cable tension is just 1/3 of its original value.
Cable Tension
The tension time histories appear quite jumpy rather than smooth. This is because the cable is modelled using elements of finite length. Although the element lengths are relatively short (0.34m), the discretisation results in a stepwise elevation process as the guide surface contact model is on a point-by-point basis rather than truly surface based. The problem is exacerbated by the high contact stiffness terms involved. As noted previously, the pulleys have a high contact stiffness to prevent any significant penetration by the cable. The tower pulley in particular, has a very high contact stiffness in order to withstand the compressive radial loads exerted by multiple wraps of cable. Each successive wrap increases the radial force on the pulley as an increasing number of nodes come into contact with it.