Blocks & Tackles

Author: RC / Antal Research & Developement   Date Posted:22 February 2021 

Blocks & Tackles efficiency

3.1 IDEAL BLOCKS AND TACKLES

 


A tackle is a system of blocks that amplifies the applied force so to lift heavy loads. For example a 4:1 tackle (FIG.1) amplifies the force by 4 times. It means that appllying a force of 50 kg could lift a load of 4 x 50 = 200 kg. The result (4 x 50 = 200 kg) can be easily understood by observing the FIG.2. So far we considered the ideal block (zero friction). Let's see what happens with the real blocks.


3.2 BLOCK EFFICIENCY

Ideal block : friction = 0 (then F = P)
Real block F = P + Friction
Therefore,  with a real block, applying a force F a load lower than F is lifted.
In the previous Technical Info #01, after carrying out load tests on different type of blocks, we found the relationships between the lifted load P and the applied force F, then the ratio P / F which is E : the block efficiency.

The following table shows the efficiency value E of 4 different types of blocks and the corresponding value of the lifted load P assuming an applied force F = 50 kg.
 

 



Applying a force F = 50 kg to the most efficient block (Roller Bearing Block) I can lift a load P = 46,5 kg, I lose only 50 - 46,5 = 3,5 kg for friction (7% off) while, with the Low Friction Ring, I can lift only 33,5 kg, I lose 50 - 33,5 = 16,5 kg for friction (33% off).

This loss becomes greater in complex systems with many blocks with losses that gradually add up with each new block passage. 
Consider again the Roller Bearing Block with and efficiency E = 0,93 that loses 7% with each block passage and see what happens in the 4:1 system already described in paragraph 3.1.

Apply a force F = 50 kg which will be reduced with each passage by 7%, therefore 46,5 after the first, 43,2 after the second then 40,2 and 37,4 after the last. Then the lifted load will be 46,5 + 43,2 + 40,2 + 37,4 = 167 kg and not 50 + 50 + 50 + 50 = 200 kg of the ideal system.

 

3.3 TACKLE EFFICIENCY WITH DIFFERENT TYPES OF BLOCKS

We saw in the previous paragraph that with a 4:1 Roller Bearing Block system and with an applied force F = 50 kg we can lift 167 kg and not the 200 kg of the ideal system, it means a Tackle Efficency = 167 / 200 = 0,84.

Let's repeat the simulation for the other type of blocks and report the results in the following table.
 

 

3.4 CONCLUSIONS

The values of the lifted load in the systems that use the rings are too low and their efficiencies unacceptable. In complex systems it will always be advisable to use solutions with blocks.
Note: above results are derived from tests that depends on the size and type of tested samples and ropes and also on the load range therefore must be considered only indicative.