In this article we examine the deployment of load arrestors in partnership with pulleys. By doubling the cable under a pulley the user can double the maximum working load of a load arrestor.
Deploying Load Arrestors with Pulleys:
This case study is written with specific reference to our G-Guard™ load arrestors.
A load arrestor is conventionally deployed so that the unit’s cable is directly connected to the load that is being protected (see figure 1). When this is done, it is vital that the weight of the load does not exceed the stated maximum working load (MWL) of the unit.
It is also possible to double the MWL of a load arrestor by running the cable under a suitable pulley (see figure 2). This provides the obvious advantage of being able to increase the MWL of the unit without having to invest in a more expensive device.
It also has the effect of halving the activation speed, thereby stopping the load in a reduced distance, should it fall. The normal activation speed of our G-Guards™ is around 0.6m/sec, so partnering one with a pulley would create an activation speed of approx. 0.3m/sec.
In some circumstances this may be a benefit, but in others it could be a disadvantage. This is because such a system is considerably more sensitive to movement, so vibration of moving loads and erratic movements may result in activation of the brake even when a fall has not occurred.
Furthermore, because the cable between the arrestor and the load would be doubled over, deploying a pulley system would half the distance that the load can be lowered below the arrest device. You would therefore need to calculate whether there is enough cable length beforehand.
The suitability of the anchor points must also be considered if using a pulley system. The strengths of the two overhead anchorages must be the same as is stated on the G-Guard’s label (normally five times the MWL). The protected load attachment point (where the pulley is attached to the load) must be double that which is stated on the G-Guard’s label / manual:
Overhead anchor strength = 1500kg. Pulley anchor strength = 3000kg.
Overhead anchor strength = 2500kg. Pulley anchor strength = 5000kg.
Overhead anchor strength = 5000kg. Pulley anchor strength = 10,000kg.
Similarly, the pulley itself must be designed to work with double the MWL rating of the G-Guard™. For example, a G-Guard™ 500kg would need to be used with a 1000kg pulley, in order to protect loads of up to 1000kg. Correct pulley design is critical for reliable operation and the cable must not be able to come off the pulley during use.
Thought must also be given to the positioning of the load arrest device in relation to the cable. Activation of the G-Guard’s™ brake creates axial movement, causing the unit to ‘kick’ outwards a small distance. G-Guards ™kick to the side, in the opposite direction to where the cable exits the unit. You should therefore ensure that the load arrestor kicks away from where the cable runs back up to its anchorage point (see figure 3).
Because the device’s retraction spring has to rotate the pulley, in order to keep the cable taught, the pulley must be as light and free-running as possible. The additional resistance that the pulley gives will decrease the working life of the G-Guard’s™ internal retraction spring, so additional maintenance may be required in these circumstances. The tension of the retraction spring must be maintained in order to ensure maximum performance.
Please see figure 4 for an example G-Guard™ 300kg pulley system.
So, we can see that deploying a load arrestor with a pulley can be advantageous, but that there are a number of important considerations that need to be addressed when doing so. For more information about our G-Guards™, or for further advice on the safe deployment of load arrestors, please get in touch. We are always happy to assist.
G-Guard™ Load Arrestors:
Globestock offer a range of market-leading, CE-marked load arrestors, with maximum working loads of 300kg, 500kg and 1000kg.
You can view our range of G-Guard™ load arrestors here.