Friction in Pulleys or Block and Tackle Systems

The history of the block and tackle is something that is murky at best. We posted an article called how pulley systems changed the world about what is believed to be the origin of the system, but from that point on the components that make up this system have been optimized and improved to where we are today, with what is arguably one of the most efficient machines available to mankind.

As the pulley became more widely used in block and tackle systems, the weight lifted increased and friction became an obstacle. The challenge is the higher the weight and the higher the speed, the more friction is created. The friction comes from the various forces on a pulley when lifting a load.

A weight is attached to the end of the rope, the rope passes through a pulley to a user and, when you pull down on the rope, the weight goes up. The weight exerted on the upper pulley is double the weight being lifted. With one pulley, you have a weight exerting a force down and you have a user pulling down with equal force to make the weight go up.

By having a second pulley, or a two-to-one block and tackle, the weight is shared with the second point, meaning less load on the pulley, but an increase in the amount of rope that needs to travel through the pulley and an increase in the amount of time the pulley is being used. In effect we have less friction over a longer period of time.

Why is it important to overcome friction? there are two primary reasons:

  1. Friction is basically the pulley’s resistance to turning. To make the pulley turn, you need to apply more force, so the more friction in the system, the more force you need to apply.
  2. Friction causes heat; this heat can melt the pulley if it is plastic, or transfer to the rope, causing the fibers to fail. Specific materials are used in bearings to reduce the thermal expansion of the balls, or to reduce the deformation of the ball under load and high heat.

As technology has developed, so have the surfaces being used to reduce friction. Ball bearings and modern low expansion materials with low friction surfaces all help to improve the efficiency of a pulley system. Managing friction and keeping load capacity high have become the biggest challenges in pulley design. Evidence of friction reduction efforts dates way back to ancient Egyptian times when water was used between sandstone surfaces to help them slide. It just goes to show that the reputation of the Egyptians is justified!

In the marine industry there are numerous cases where a pulley is expected to work at high speeds under little to no load and then handle high loads for extended periods of time. Doing this with a ball bearing or a needle design was a challenge and pushed Ronstan® engineers to develop the dual bearing system. This has an outer ball bearing surface for high speed-low load conditions and an inner bushing on polished shaft system for high-load conditions when the ball bearings have deformed, allowing bushing contact.

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