What If We Have A Hoist That Has An Identity Crisis?

What If We Have A Hoist That Has An Identity Crisis?

CX 92 Staging What If

In July last year we touched on the topic of adding a second brake to a generic industrial grade chain hoist (D8 Compliant or equivalent) to make it “safer”, and this topic has reared its ugly head again here in the world-of-explaining-chain-hoist-compliance.

It started with a fairly innocuous conversation with a potential client who was looking to fly a 1,000kg load and didn’t want to fit safety tethers.

Nothing fancy required, no special group stops or multi point lifts, no performers hanging off trusses, no moving loads over the audience - just lift a speaker cluster for a show without needing to send someone “up there” to tether the load off before the punters or crew can walk under the cluster.

We quoted said client for basic, no-frills, fixed speed, 1,000kg D8+ hoists, compatible with existing control gear, and got ready for the normal discussions regarding price and “special” discounts.

It was a bit of a surprise then when we were informed that “other” brands of “identical” chain hoists are of the order of half the cost of the machines that we had suggested. This was a little more than someone being enthusiastic about margins or optimistic about exchange rates so we dug a little deeper.

It turns out that the “identical” hoists that our humble 1,000kg D8+ machines were being compared to were in fact 1,000kg D8 machines that are set up for a second (optional) brake, and were being presented as “Dual Compliance” machines that can be used as D8+ (with a load of 500kg and the optional second brake fitted).

The perception was that because the documentation presented for these “other” hoists stated “D8+” that the hoist could be used to lift 1,000kg and not require a tether before allowing people under the load.

We explained (tried to, at least) that the machines were genuine D8+ machines only when the overload clutch was set to operate for a load of 500kg and a second brake was fitted (with the second brake circuit arranged for staggered application of the brakes). If the overload clutch was left set to operate for a load of 1,000kg, then no matter how many extra brakes were fitted, that the hoist would only ever be a D8 machine.

The jury is still out as to whether this explanation fell on deaf ears or not, but we tried.

The article we published in June last year examined the dangers of adding a second brake without a control circuit that ensures that the 2 brakes can never apply simultaneously, and this has significant implications when we look at the concept of “Dual Compliance” hoists.

A hoist which is designed to arrest and hold a load of 1,000kg will have a brake of double the torque (braking capacity) of a hoist designed to arrest and hold a load of 500kg.

When we add a second brake to this 1,000kg hoist, without the appropriate brake timing circuit, then we have a (set of) brake(s) with 4 times the torque required to arrest and hold a 500kg load.

D8+ hoists have a chain Factor of Safety that is double that of a D8 hoist, but this dramatically increased braking torque effectively negates this higher Factor of Safety.

Doubling the brake capacity of a hoist does not necessarily improve the safety of the hoist.

A D8 hoist with a second brake fitted is not a D8+ hoist - it is a D8 hoist with an extra brake, and that extra brake may well be compromising the safety of the hoist.



Jands Staging
provide articles to the "Staging What If" section in CX Magazine. If you have any questions regarding this article then please comment below or email info@jands.com.au.

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