Safety nuts for fall protection

1. How a safety nut works

The nut is load-free during normal operation. It is mounted at a defined distance from the load-bearing nut or load-bearing worm gear via driver claws, and therefore, it's not subject to wear in the thread. Safety nuts can only perform their function if they can 'catch' the worn load-bearing nut or spindle. If the safety nut is mounted in the wrong load direction, it won't catch the load, and the load-bearing nut or spindle will fall towards the ground.

2. Structure of a safety nut

For trapezoidal threaded spindles, there are several ways to use a safety nut:

The connection between the worm wheel and the safety nut is made through integrated claws. The nut combination is always produced as a set. Replacing only one nut will result in a change to the original distance measurement.

3. Wear measurement via distance dimension

Since the worm wheel and the safety nut are always delivered at a precisely defined distance from each other, wear can be measured by observing changes in this distance. The distance is typically one-quarter of the spindle profile. This distance can also be monitored electronically using an inductive limit switch. The switch activates when the defined measurement is undershot. This type of electronic monitoring is required for lift platforms used for transporting people and with lifting heights exceeding 400 mm.

Note: If the distance falls below one-quarter of its original value, the nut combination should be replaced.

4. Types of safety nuts

1. Safety nuts have the same length as the main nut and are capable of fully supporting the adjustable load. Even with a worn load-bearing nut, only one lifting movement should be carried out to shut down the system.

Reference nuts are shorter and resemble a thread ring. They are used solely to measure wear—specifically, a change in the gap between the worm wheel and the reference nut. This measurement can be monitored either visually/tactilely or automatically via sensors.

5. What a safety nut cannot do

The use of a safety lock nut does not create a self-locking gearbox. Self-locking is generated exclusively by the friction elements under load, such as the worm wheel in a trapezoidal screw drive.

A safety lock nut cannot secure against both compressive and tensile forces. If both directions need to be secured, it's worthwhile to use an additional safety nut. Although this configuration is considered unusual, there are no technical limitations preventing it.

6. Safety nuts in ball screw drives

Additional lock nuts in the area of ball screws are not equipped with balls but instead form the counterpart to the ball screw profile. Safety nuts in this context are relatively uncommon because they provide little additional benefit (worn balls block the system and prevent collapse) and do not allow for wear monitoring (the nut and safety nut are manufactured as a single piece without a variable visible gap).

7. When is a safety nut used?

This question is primarily answered by the risk assessment during the planning phase. Depending on the maintenance cycle, potential consequential damage if a failure occurs, or risk to life, a safety nut can reduce risk. Certain applications, such as stage construction, explicitly require a safety nut according to DIN standards. Consider such a solution early on. Retrofitting is possible with traveling nut variants, but extending the nut will then reduce the lifting height.