differential pulley block的用法和样例：
- The lower part of a pulley block.
- A pulley block is needed to lift that heavy machine .
- The individual strands of each cable were hung in pulley blocks.
- To demonstrate the proposed framework, the design of the compensation of pulley block in derricking mechanism was presented.
- The mentioned method is applied to design of pulley block compensation in luffing mechanism and the results show that the method is feasible and effective.