Work Safe. On Rope. 09 274 8090

Treetools Blog

Pulley ratings: it's like comparing apples with pears

Richard Tregoweth - Tuesday, August 30, 2011

Equipment ratings, in general, can be confusing at the best of times but the information laser etched on the plates of the DMM Pinto and ISC Fixed Micro are the most graphic in their differences.

At first glance many customers are puzzled by the figures presented - the ISC Fixed Micro appears to be stronger than the Pinto? Yet one look at the construction of the DMM Pinto pulley and you know it must be stronger than its ISC alternative.

As is common with numbers, you can make the figures tell whatever story you want.

The ISC Fixed Micro has the Minimum Break Strength (MBS) stated in kilo Newtons: 24kN at the carabiner connection point and 12 kN on each leg running from the sheave, with 12 kN on the bottom becket.

The becket figure on the ISC Fixed Micro is a little unusual. In theory, if 24 kN can be loaded at the top anchor point(s) then it should be capable of 24 kN at the bottom.

But the bottom becket on the ISC Fixed Micro pulley has a stated MBS of only 12 kN, exactly half that of the top two holes. Our assumption is the top anchor holes are supported by two plates (one on each side of the sheave) while the bottom becket has only one plate, connecting the two side plates, therefore the load rating is halved?

In contrast to the ISC pulley, the DMM Pinto has two sets of figures stated. The MBS and the WLL (Working Load Limit). The WLL is calculated by applying a predetermined Safety Factor of 5:1. That means the 50 kN MBS of the DMM Pinto is reduced to 10, 5, 5 and 10 kN respectively when the Safety Factor is applied.

To recap, the ISC Fixed Micro displays Minimum Break Strength and assumes you will apply a Safety Factor of your own to calculate a safe Working Load Limit for the pulley.

The DMM Pinto applies a 5:1 Safety Factor to the MBS beforehand and displays the safe Working Load Limit for you, by dividing the 50 kN MBS exactly five times. And… the bottom becket of the DMM Pinto will take the same load as the top connection point.

This makes comparing the face value ratings of these two pulleys a bit like comparing apples to… well, pears!

For the record, while these two pulleys appear similar in size, the DMM Pinto is twice as strong as the ISC Fixed Micro - see graphic below.

The numbers displayed above each pulley depict MBS (Minimum Break Strength) relative to the parts of the pulley. In the case of the DMM Pinto the Safety Factor of 5:1 has NOT been applied. Now you are comparing apples with apples.

Treetools New Zealand ©. All Rights Reserved.