One concern when designing a lift platform based on screw jacks is how to know if the travel nut (which bears the load) is worn out or about to fail — especially when the lift operates in a place where personnel might get hurt if the mechanism fails.
In these machines, a follower nut assembly (sometimes called a wear-indicator nut) can mount vertically with the load in a downward direction to leverage Newton’s law of gravity. In other words, wear-indicator nuts work where the load is above the nut assembly.
As the primary (load-bearing) travel nut traverses the load, it wears … and its threads get thinner. Eventually, the thread won’t be able to bear the machine load and will fail.
To prevent this from happening without warning, wear-indicator nuts travel with the primary nut. As this nut’s internal threads thin, it will exhibit an increasing amount of play and the gap between it and the follower wear-indicator nut becomes smaller. This narrowing gap indicates the amount of wear on the thread. To be clear, the follower wear-indicator nut usually bears no load; it’s merely twinned to the primary nut to follow along.
In some variations, the gap between the nuts is set to ¼ of the pitch. This gap can be measured with a filler gage. When accessibility to the travel nut is restricted, in some applications we have incorporated a proximity sensor to indicate closure of the gap.
Follower wear-indicator nuts for screws are either steel or bronze. On assemblies with the latter, a closing gap actually lets the wear-indicator nut bear a small portion of the load — which in turn makes an audible noise to alert maintenance personnel of detrimental screw-nut and assembly wear.
Below is a video from Nook Industries on their wear-indicator nut offering to help end users know when wear is approaching the point of catastrophic failure.