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Today we are looking at comparing the actual mechanical advantage of the SMC 3" Double PMP with the Rock Exotica Mini Machined Double Pulley. We are interested in how difficult tensioning will be with both of these pulleys so we would like to find what is the maximum tension that is obtainable with them in a 5:1 pulley system. using a 3:1 multiplier.
We will be setting up a 5:1 pulley system using Mantra MKII with Alpine WebLock 2.1's as the anchor points. We will use the CMC MPD as our brake for the pulley system. The rope we will use is the PMI 11mm ACCESS PRO, which is connected to the becket of the pulleys with a 5/16" 316-Stainless Dee Shackle. We will also be using a pair of SMC Large Rigging Plate's for the anchor and a Dillon Extreme Dynamometer to measure the force. Our multiplier will be the Elite Multiplier Kit with optional Petzl ASCENSION Handled Ascender, exclusively available in the Shop.
We will setup a ~100 ft. slackline using the above equipment by first using the Rock Exotica Mini Machined Double Pulley's. We will then pull this line as tight as we possibly can and record the peak force that we reach. After a sufficient rest, we will switch out the main pulleys with the SMC 3" Double PMP's and repeat the process, recording the max tension again.
The peak force that we achieved using the Rock Exotica Mini Machined Double Pulley's was 2,770 lbf (12.32 kN).
The peak force that we achieved using the SMC 3" Double PMP's was 3,570 lbf (15.88 kN).
I was quite impressed with the efficiency of the Rock Exotica Mini Machined Double Pulley's. A force of 12.32 kN is quite good for a pulley that weighs just 141 grams. There was a bit of friction at the location where we attached the rope to the becket of the pulleys. This could have decreased our mechanical advantage slightly.
It should be said that the force we achieved with the Rock Exotica Mini Machined Double Pulley's significantly exceeding the working load limit of the pulley (8 kN - 1,798 lbf). This is not safe rigging practice and should never be done on an actual slackline. Never exceed the working load limit of your gear!
What was even more impressive was the force that we achieved using the SMC 3" Double PMP's. It's incredible that a force of 15.88 kN can be achieved by a single person with just 15:1 theoretical mechanical advantage! This is a This would mean that I would have to have exerted a minimum of 238 lbf! We will see below that this number is quite far from the actual force exerted by me.
Now for a little math: In accordance with efficiency ratings of similar pulleys, we can assume that the SMC 3" Double PMP's have an efficiency rating of 95%. We will also assume that the MPD has a 95% efficiency rating. Knowing this, our actual mechanical advantage for our 15:1 pulley system was as follows:
(1 + 0.95 + (0.95)^2) * ( 1 + 0.95 + (0.95)^2 + (0.95)^3 + (0.95)^4 ) = 12.91:1
This means that I had to have exerted a force of 3,570/12.91 = 276.5 lbs to achieve a tension of 3,570 lbf. Assuming that I put the same force on the Rock Exotica pulley system, we can say the that the actual mechanical advantage of the Rock Exotica System was:
2,770 / 276.5 = 10.01:1
Which means that the Rock Exotica Pulleys have an approximate efficiency of
( 1 + 0.95 + (0.95)^2 ) * ( 1 + x + (x)^2 + (x)^3 + (x)^4 ) = 10.01
x + x^2 + x^3 + x^4 = 2.51
By approximation, we get:
x ≈ 0.821 or 82.1%
Based on these assumptions, we can say that the SMC 3" Double PMP are 29.2% more efficient than the Rock Exotica Mini Machined Double Pulley's. Again, this information is based are large number of assumptions.
Now, what does this mean? When should you use the SMC 3" Double PMP's instead of the Rock Exotica Mini Machined Double Pulley's? Well, since the Rock Exotica's only have a working load limit of 8 kN, that should be a good delimiter. Any slacklines that have a force higher than 8 kN should be using the SMC 3's. Also, if you plan on setting up high tension slacklines (7-12 kN) and want to dynamic tricks (jumps, surfs, bounces), then the SMC 3's are a better choice. Finally, if you want to rig longer lines where the tensions will be higher for a longer period of time during tensioning, the SMC's are a better choice as tensioning is significantly easier in the 6-10 kN range compared to the Rock Exotica's.
This test is far from representative of the efficiency ratings of these pulleys. Several other tests involving different scenarios need to be taken into consideration. In the future, I would like to conduct lab tests of the efficiency rating for both of these pulleys in order to better compare them. In order to do this, I will have to have access to a laboratory where the restraints of the tests can be better controlled.
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