Castellated nut
A castellated nut (also called a castle nut or a slotted nut) is a nut with slots (notches) cut into one end. The name comes from the nut’s resemblance to the crenellated parapet of a medieval castle. The bolt has a single hole drilled through its threaded end. The nut is tightened on the bolt so that the slots are aligned with the hole. The nut is then secured with a cotter pin or with locking wire. Castellated nuts are used in low-torque applications, such as holding a wheel bearing in place.
Fastener materials
Nuts and bolts are used to fasten two surfaces together. This involves tightening and locking the nut. In applications subject to vibration, it is important to reduce the chance of failure by torquing the nut to a tension that exceeds the expected maximum load on the bolt. As the load changes, the clamping force exerted by the bolt changes, but the tension on the bolt stays the same.[1] Additional protection against failure is provided by various locking devices.
The most common bolt materials are carbon steel, stainless steel and alloy steels. The tensile strength[2] of carbon steel is typically 375 Mpa (55 ksi); stainless steel 1,400 Mpa (200 ksi); heat treated and alloy steels up to 2,000 Mpa (300 ksi). The strength of a nut/bolt combination depends primarily on the strength of the bolt. The first six threads of the nut take up essentially the whole load.[3]
Locking devices
Split-beam locknuts
Split-beam locknuts have slots cut into the top. They bear a superficial resemblance to castellated nuts. In split-beam nuts, however, the slotted areas are pressed together, reducing the thread diameter. When a bolt is threaded through the slotted area, the compressed segments are forced apart, producing friction forces that prevent subsequent movement of the nut.
Lockwires
Lockwires are used to connect nuts securely.[4] They are threaded through holes in the nut, twisted together, and connected to adjacent nuts. They are configured so that tension on the wire tightens the nut. Any tendency of a nut to loosen is counteracted by the tightening force it exerts on adjacent nuts. Lockwiring is used in aerospace and other critical applications. Castellated nuts can be lockwired.
Jam nuts
Two nuts are tightened over the bolt and then torqued against each other. The smaller (jam) nut is tightened first. This puts stress on the bottom of the bolt threads (the side closest to the mating surface). The larger nut is then tightened against the jam nut. This puts stress on the top of the threads. At the junction of the two nuts, the thread is clamped by forces in opposite directions. The two nuts are locked to the bolt at this point.[5]
Lockwashers
Helical washers are compressed (like a spring) when the nut is tightened. When fully compressed, however, they are functionally equivalent to a flat washer and provide no additional locking ability.[6]
Toothed (star) washers have teeth that bite into the nut (or bolt head) and the mating surface.
Other locking devices
Serrated-face nuts have serrations on the bottom that dig into the mating surface. Some nuts (e.g. Nylok) are manufactured with nylon inserts that apply friction forces to the bolt. Locking adhesives (e.g. Loctite) are epoxy adhesives that bond the nut and bolt together.
References
- ^ see Bolted joint
- ^ Tensile strength refers to the force (stress) required to break a fastener apart. Tensile strength is measured in units of pressure (newtons per square meter in the metric (SI) system; pounds-force per square inch in the U.S. customary system). The conversion factor is 1 psi = 6894.757 Pa, or 1 ksi ≈ 6.89 Mpa. In engineering practice, “ksi” are called “kips.”
- ^ Johns WE, Notes on Nuts and Bolts, p. 1. http://www.gizmology.net/nutsbolts.htm
- ^ FAA advisory circular AC 43.13-1B Chapter 7, “Safetying.” Builder’s Guide to Aircraft Materials, Rev.1 (January 15, 2006). http://www.auf.asn.au/scratchbuilder/safetying.html
- ^ Calvert JB. The Jam Nut. http://www.du.edu/~jcalvert/tech/jamnut.htm
- ^ Barrett, RT. Fastener Design Manual. NASA Reference Publication 1228, 1990, p.9. http://www.tcnj.edu/~rgraham/barrett/manual1-A.html
Bibiography
Barrett, RT. Fastener Design Manual. NASA Reference Publication 1228, 1990. http://www.tcnj.edu/~rgraham/barrett/manual1-A.html
