Technicals: Engineering Washers

 What is an engineering washer, what are they used for and how do they work?

A washer has multiple purposes depending on it’s’ shape, form and material. Generally speaking, a washer is used to:

• protect the surface of fastened materials,
• spread the load/ pressure at the hold points of fastened materials,
• assist with tightening and loosening of a fastener,
• control the spacing of connecting components.

Types of Washers and Links

• Flat Washers - Form A, Form B, Form C
• Imperial Table 3 Washers.
• Repair, Penny Mudgaurd Washers.
• Shakeproof Lock Washers.
• Spring Lock Washers.
• Taper Washers

Flat Washers

Washers are manufactured to international standard specifications in various sizes, materials and shapes. Flat washers thin, formed in discs and most commonly have an internal and external round shape. Some washers might have a square internal and/ or external shape. The inner hole and/ or outer shape can be asymmetric, depending on the washer's function. The specified size will correspond to the fastener's (usually a hex bolt/ set) diameter. e.g. an M10 thread with a 10mm shank diameter will need a 10mm internal washer diameter. Washers are usually metal or plastic. High quality bolted joints require hardened steel washers to prevent the loss of pre-load due to Brinelling (permanent indentation) after the torque is applied.The grade steel can either match the grade of the fastener or be 1 grade lower. e.g an 8.8 high tensile bolt commonly goes with 4.6 grade washers (mild steel).

Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Washers usually have an outer diameter (OD) about twice the width of their inner diameter (ID).

Washers are important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces and phosphor washers are used for high abrasion and corrosion resistance.

A table to show: Metric, Form A Flat Washer Dimensions

Metric DIN Standard

Form A, Din Standard DIN 125A flat washers are most common in Europe 

Metric British and American Standards

Metric flat washers, manufactured to British Standard BS4320 specications range from form 'A' through to form 'G' and offer more size variants than options the German Din standards. BS4320 Form A flat washers are virtually identical to Din125A. 'Type' is not to be confused with 'form' (but often is). The British Standard for Metric Series Metal Washers (BS4320) written in 1968 coined the term 'form'. The forms go from A to D for Bright Metal and denote outside diameter and thickness. They can be summarised as -

• Form A: Normal diameter, normal thickness (thick)
• Form B: Normal diameter, light thickness
• Form C: Large diameter (wide), normal thickness (thick)
• Form D: Large diameter (wide), light thickness (thin)
• Forms E to G relate to black metal washers.

USA Standard Metric

Metric Fasteners used in USA are specified to ANSI B18.22M standards and includes the full range of size variations as British Metric.

Imperial Inch British and American Standards

British Imperial flat washers are to BS4310 and American Imperial are to ASME B18.21.1.