How to Double the Maximum Working Load (MWL) of a Load Arrestor by Including a Pulley
The conventional method of deploying a load arrestor is to connect the cable of the unit directly to the load (see figure 1 in Globestock link below). In this configuration, it is extremely important that the weight of the load must be less than or equal to the maximum working load (MWL) indicated on the label and manual of the unit.
By including an appropriate pulley, it is possible to double the MWL of a load arrestor (see figure 2 in Globestock link below). The obvious advantage is that the MWL is increased without having to invest in another load arrester. This arrangement also reduces the activation speed by half, and that reduces the stopping distance accordingly. The normal activation speed of G-Guards™ is approximately 2 feet per second, so including one pulley would create an activation speed of approximately 1 foot per second.
The reduced activation speed could be a benefit, provided the normal running speed of the application is always considerably less than 1 foot per second. However, for applications where the normal running speed is close to 1 foot per second, this could be a disadvantage, because vibrations and erratic movements may result in the brake engaging when a fall has not even occurred.
Inclusion of a pulley also requires the cable to be doubled over, reducing the distance that the load can travel by half. The pulley must also be as low-friction and low-mass as possible, because the G-Guard’s cable retraction spring has to provide the force to rotate the pulley and keep the cable taught. For maximum performance, more frequent maintenance may be required on the tension of the retraction spring.
Because of the increased MWL that results from including a pulley, it is of utmost importance that the strength of the anchor points must be double the requirement which is stated on the G-Guard’s label and manual. This is likewise for the strength of the anchor point of the pulley itself.
When the G-Guard’s™ brake is activated, there is an axial movement which causes the unit to ‘kick’ outwards a small distance. G-Guards™ kick in direction opposite from where the cable exits the unit. The part of the cable which runs back up to the anchor point should therefore be positioned toward the same side of the G-Guard™ where the cable protrudes, in order to prevent the case from hitting the cable during a kick (see figure 3 in Globestock link below).
In conclusion, we see that including a pulley with a load arrester can have many advantages, provided that all of the important considerations are addressed. Check out our website for a full line of fall safety equipment. Do you have a special fall arrester application? No problem, just give us a call at 813-888-8226. We look forward to hearing from you.