Introduction

Theoretically, the multiplier pulley is responsible for two-thirds of your mechanical advantage (turns your 5:1 into a 15:1, or your 9:1 into a 27:1). For this reason, the multiplier pulley is a critical component to how efficiency your pulley system is. I wanted to test just how important it is by comparing 4 different pulleys with varying efficiencies.

Methods

I setup an 80 foot line with Mantra MKII using the Alpine WebLock 2.2 as my webbing anchor. My pulley system was a 5:1 consisting of a pair of SMC 3" Double PMP's as the main pulleys, the CMC MPD as the brake, a pair ofSMC Large Rigging Plates for the connection point, and 100 feet of PMI 11mm Access PRO Static Rope.

The four pulleys I compared are the SMC CR Pulley, the Rock Exotica Mini Machined Single Pulley, the Rock Exotica Machined Rescue Pulley, and the SMC 3" Single Pulley.

My method was to use each of the pulleys listed above as the multiplier on the system listed above and see what sort of tensions I could reach. I started with the SMC CR Pulley and worked up to the SMC 3" Single Pulley.

Results

With the SMC CR Pulley, I was able to reach 3,050 lbf (13.56 kN).

With the Rock Exotica Mini Machined Single Pulley, I was able to reach 3,414 lbf (15.19 kN).

With the Rock Exotica Machined Rescue Pulley, I was able to reach 3,618 lbf (16.09 kN)

With the SMC 3" Single Pulley, I was able to reach 4,048 lbf (18.01 kN).

Discussion

Impressive results! As suspected, the larger sheaved pulleys achieved higher tensions compared to the smaller sheaved pulleys. This is mainly due to the torque over the friction at the axle. For instance, the SMC 3" Single Pulley has 67% more torque than the Rock Exotica Machined Rescue Pulley, which gives it a big advantage when pulling tension. This combined with the fact that the bearings in the SMC 3" Single Pulley are phenomenal, makes it one of the best pulleys on the market for your multiplier.

As a lightweight, mid-range multiplier pulley that is fully capable of achieving really high tensions, the Rock Exotica Machined Rescue Pulley performed very well. I would use this pulley if weight were a big issue.

The main reason why the SMC CR Pulley did much worse than the rest of the pulleys is because of the lack of ball-bearings. The sheave on this pulley is made from nylon and it just spins about the axle. There are not even bushings on this pulley! That means that there is a ton of friction which is robbing you of your mechanical advantage when you are pulling your line tight. This pulley is okay if you are on a really tight budget and you don't need high tensions, but remember, with a main pulley system that consists of non-ball-bearing pulleys and a camming-style brake, you will notice an even bigger difference between the SMC CR Pulley and the SMC 3" Single Pulley. 

Future Research

In the future I would like to compare a few other pulleys to the SMC 3" Single Pulley. I'd also like to see what sort of advantage having a compound multiplier does for the efficiency of the system.