Types of washers

[edit] Introduction

A washer is a thin disc-shaped plate, typically with a hole through the centre, that is used in conjunction with a threaded fastener (such as a screw or bolt) to:

• Distribute load.
• Create separation.
• Prevent corrosion and wear.
• Lock fasteners.
• Reduce vibration.

Washers are most commonly made from metal or plastic, although rubber and fibre versions are also available.

Although they can vary widely, in general, washers have an outer diameter that is approximately twice the inner diameter.

There are three main categories of washer:

• Plain washers.
• Spring washers.
• Locking washers.

[edit] Plain washers

Also known as a flat washer, this type of washer is a flat annulus or ring which is used to spread the load of a screwed fastening and prevent damage to the surface being fixed.

Types of plain washer include:

• Penny washer (or Fender washer): A flat washer with a larger outer diameter proportional to the central hole, meaning it can distribute loads more widely.
• Spherical washer: Consists of one radiused surface which, when used with a mating nut, allows for several degrees of misalignment between parts.
• C-washer: Can be slid in and out of position on a bolt or shaft.
• Countersunk washer: When secured, this type of washer creates a flush surface.

[edit] Spring washers

This type of washer has axial flexibility which helps prevent fastening or loosening as a result of vibrations or shock.

Types of spring washer include:

• Belleville washer: Has a slight conical shape which helps maintain tension in assemblies where there is thermal expansion and contraction.
• Curved disc spring: Is only curved in one direction and so is used to support relatively light loads with a wide deflection range.
• Wave washer: Has a ‘wave’ in the axial direction which makes it suitable for use as a cushion spring or spacer.
• Split washer: Has a ring that is split at one point and bent into a helical shape creating more friction and rotation resistance.

[edit] Locking washer

This type of washer is often interchangeable with spring washers and prevents fastening or loosening rotation.

Types of locking washer include:

• Helical spring: Increases the pre-load on the fastener while tightening. This provides protection against loosening.
• Toothed lock washer: Has serrations around its edge that can extend inward or outward to bite into the surface material and provide maximum torsional resistance.
• Tab washer: Has a side tab that can be bent into place against a nut, preventing rotation.

[edit] Related articles on Designing Buildings Wiki

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• Fixing v fastener.
• Furniture, fixtures and equipment.
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