The safety nut, also known as a catch nut, is a key safety component in screw drives. It runs unloaded parallel to the load-bearing main nut on the screw and is positioned at a defined distance from the main nut (typically 0.5–2 mm) via drive claws.If the main nut fails—for example, due to wear, material fracture, or ball jamming—the safety nut immediately takes over the load. This reliably prevents an uncontrolled drop of the load.
Technical Structure
Assembly
- Positioning behind the main nut
- Adjustable clearance between the main nut and the lock nut
- The clearance also serves to monitor wear on the main nut
Material
- High-strength steel or bronze
- Thread profile precisely matched to the spindle
- For ball screws: Lock nut without balls, purely mechanical interference fit
Advantages of the Catch Nut
Maximum Safety
- Prevents consequential damage and personal injury
- Particularly relevant for vertical applications such as:
- Stage and Event Technology
- Lifting Systems
- Vertical Lifting Axes in Mechanical Engineering
Wear Monitoring
- A change in the distance between the main nut and the lock nut provides an early indication of the main nut’s wear condition
Compliance with Standards
- Required by DIN or other standards in many safety-related applications
Limitations of Ball Screws
Lock nuts are less common with ball screws, as ball screw systems often act as self-locking mechanisms in the event of ball failure. The following alternatives are used:
- Mechanical Limit Switches
Redundant Safety Systems- Double-nut systems with preload
Nevertheless, locknuts are used when the highest safety requirements apply.
Kammerer’s Practice
Kammerer recommends locking nuts specifically for vertical ball screw lifting axes in mechanical engineering—especially for:
- heavy loads
- safety-critical applications
- stricter regulatory requirements