Why would anyone consider replacing the nut at the end of their bolt with a three-piece nut? As the standard threaded connection of today originated in the 1700’s, shouldn’t 300 years be long enough to wait for something new? The three-piece nut deserves consideration because it guarantees a reliable connection, doesn’t get stuck and is safer than the standard.
First, here is a description of the standard and the three-piece nut:
– The standard industrial nut is a hexagon shaped metal block with a threaded hole in the middle. The hexagonal shape fits the tightening wrench and the threaded hole mates with the bolt. Tightening with a torque force rotates the nut along the bolt and stretches the bolt to provide the tightening load.
– The three-piece nut consists of three cylindrical parts: an outer sleeve, a washer and an inner sleeve. The inner sleeve threads to the bolt and by turning the outer sleeve with a torque force, the three-piece nut stretches the bolt and provides the tightening load. All relative movements take place within the nut. The three-piece nut does not rotate along the bolt.
The goal of any bolting operation should be to quickly and safely produce a reliable bolting connection. Let’s look at the problems created by the standard industrial nut and how the three-piece alternative resolves them.
Problem Solved #1 – Bolt failures due to inaccurate bolt loads
Standard bolted connections fail because unknown friction forces cause over or under tightening. The main problem with using torque to tighten a standard nut is the variability of tightening load resulting from the applied torque load. Friction forces between the nut, bolt and mounting surfaces significantly affect the tightening load produced by the torque force. Because the three-piece nut has no relative movement between the inner cylinder and the bolt and between the washer and the mounting surface, these frictions are not relevant. All the movement is contained within the three-piece assembly where the friction is tightly controlled.
Problem solved #2 – Removal problems due to thread damage
It is always more difficult to unscrew a standard nut than it was to screw it because thread damage and corrosion increases friction between the mating threads and mounting surfaces. With the three-piece assembly, all relative movements take place within the nut. Bolt thread and the mounting surface friction increases are not relevant to removal. Special design considerations, material selection and lubricant in the three-piece nut guarantee that removal friction is controlled.
Problem Solved #3 – Elimination of pinching dangers on reaction points
People are injured while tightening standard nuts. The equipment that is used to screw and unscrew the standard industrial nut requires a reaction arm to hold the tool in place during operation. As this arm must react against a solid surface, it presents the perfect opportunity to pinch fingers while in use. The three-piece unit eliminates the need for a reaction arm since it reacts internally against the mounting surface thus eliminating the pinch point.
The standard nut is long outdated for industrial applications. Three hundred years is long enough to wait for a new nut design. The three-piece nut eliminates many of the problems created by the standard nut and should be considered for every industrial bolting application.