Lock Washers

Lock washers are used to resist loosening of bolted joints. There are essentially two types: spring action and tooth.

Spring action lock washers include split (helical spring), which are also called spring washers. These washers, placed under the nut or screw head, compress as the fastener is tightened and the spring-back tension deters loosening. Also, because of their spring-like take-up action, split lock washers can compensate for slight material wear. But because spring types flatten when tightened, their "locking" action doesn't come into play until the fastener has slightly loosened. Use regular for Grade 2 hex cap screws and high strength alloy steel for Grade 5 and 8 hex cap screws.

Tooth lock washers—internal, external, internal-external and countersunk external—create a ratchet action by biting into the nut or screw head and the surface it contacts. Despite their widespread usage, tooth lock washers can scratch mating surfaces.

A final option is the lock set pair. Also called vibration-resistant lock washers and Nord-Lock washer assemblies, the set consists of two pieces: a top and a bottom washer. These reusable washers have wedges on one side, which should be installed interlocked, and radial teeth on the opposite side. When the screw or nut is tightened, the teeth bite into the screw head or nut and the mating material to prevent slippage, while the wedges increase tension to resist loosening.

Aside from the additional time required, it can be difficult to install lock washers in tight places or when the bolt or screw is pointing down.

The different types of lock washers are described below…

**Table 1. Lock Washers.** A list of common lock washer styles and types.
Lock Washers
Hi-Collar Lock Washers
These lock washers fit in space-limited areas due to their small outside diameter (OD). Designed for use with smaller head screws like socket head cap screws, hi-collar lock washers have increased thickness to preserve their locking ability. They are also called high collar lock washers, high collar spring lock washers and spring washers.
Lock Set Pair Lock Washers
Lock Set Pairs are supplied as an assembly: one pair includes a top and a bottom washer. Install these reusable washers with the wedges interlocked and radial teeth on the opposite side. Also known as Nord-Lock washer assembly and vibration-resistant lock washers.
Split Lock Washers
These helical spring lock washers are also referred to as spring washers and split ring lock washers and are available in regular, heavy pattern and high strength alloy steel. Heavy pattern offers a larger outside diameter (OD). High strength alloy steel washers are used with Grade 5 and 8 fasteners.
Countersunk External Tooth Lock Washers
Countersunk external tooth lock washers are used with standard 82° flat and oval head screws in countersunk holes. Sometimes called countersunk external lock washers and countersunk external tooth washers.
External Tooth Lock Washers
When used under a screw head, external tooth lock washers require a large head—like binding, pan or round—because the washer teeth must make contact with the head. Since the teeth are on the largest radius, which results in greater twisting resistance, this type is preferred when the greatest loosening resistance is desired. Also known as external lock washers and external tooth washers.
Internal Tooth Lock Washers
Internal tooth lock washers work best with small screw heads, such as fillister, and when the teeth—and the surface marks they create—need to be hidden for appearance reasons. Because the teeth must make contact with the mating surface, do not use with oversized holes. Also referred to as internal lock washers and internal tooth washers.
Internal-External Tooth Lock Washers
Having a larger outside diameter (OD) than "external" and "internal" types, internal-external tooth lock washers are used when a larger bearing surface is needed to optimize an electrical connection or because a hole is oversized or irregular. They are also used as an insert between two adjustable pieces to maintain mechanical alignment and are also called internal-external lock washers and internal-external tooth washers.

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