Types of Washers Explained: Flat, DTI, Spring, Belleville, Lock, Nylon and Sealing

Key things to remember

The essentials from this document in plain words. Check the full document before you rely on a figure.

• ✓Standard washers stop the turning nut scouring the surface, and spread the load of the bolt or nut over a larger area.
• ✓Direct tension indicating (DTI or squirter) washers have bumps that flatten to a set level when the required bolt tension is reached, squeezing out silicone as a visible sign. Load indicating washers work the same way without the silicone.
• ✓Split spring washers go under the bolt head with the nut turned to tension. On the nut side, put another hardened flat washer between the nut and the split washer. They do not show over-tightening.
• ✓A Belleville (coned disc) washer acts like a spring; several can be stacked to change the spring rate or deflection. A spherical cone and cup washer pair takes up a 10 to 15 degree misalignment in a joint.
• ✓Tooth lock (star) washers grip through teeth twisted out of plane (Type A) or folded in opposite directions (Type B). Two-piece wedge lock washers use ramped discs to stop vibration loosening.
• ✓Nylon washers, natural or black UV stable, separate dissimilar metals, insulate electrically, protect a bearing surface and absorb shock and vibration.
• ✓Cyclone washers are 25mm OD aluminium and EPDM bonded seals for 14g screws, for crest fixing metal roofing to timber or steel battens and purlins in high-wind (cyclonic) conditions.

Who it helps: Anyone choosing a washer for load spreading, locking, tension checking or sealing a roof fixing.