Another major component in the pulley tensioning system is, you guessed it, PULLEYS! The common standard for creating tensioning system is to use two double pulleys and a mechanical rope brake, thus creating a theoretical 5:1 block and tackle pulley system. You will also need one single pulley to use with your multiplier. Pulleys come in different sheave sizes, WLL/MBS, weight and with either ball bearings or bushings. We’ll go ahead and break those concepts down for you:
- Sheave size - A sheave is the component that the rope comes in contact with inside the pulley. It’s the big round part. Typically, the larger sheave you have the more efficient the pulley is. Furthermore, sheaves usually only accept ropes up to a certain diameter.
- WLL/MBS - You want to make sure your pulley is sufficiently strong for the application. If you’re rigging huge longlines tensioned to 2,000 lbf (9 kN) a MBS of 10,000 lbf (44 kN) should be used to keep with a 5:1 Safety factor.
- Weight - Big pulleys weight more than smaller pulleys. Consider where you’ll be taking your pulleys and determine how much weight you want to carry on your back.
- Ball bearing vs. bushing - Ball bearings are more efficient and smoother to operate but shouldn’t be loaded for long periods of time. When under high loads for long periods of time, the balls can depress into the races of the bearing and damage the race. Bushings are not as efficient but can take higher long term loading. They also squeek under load.
Generally, a large diameter sheave, quality ball bearings and thin rope will yield maximum efficiency in your system. The large sheave size means that the rope does not have to sharply bend and reduces overall friction within the system. Smaller pulleys are great, yet they usually have lower MBS and WLL. So again, selecting the proper pulley for the application is key.