All About Washers and Why They Are Used

Many fastenings involve the use of washers. But people often ask the questions: what are washers for and are they essential? In this post we aim to provide all the information you need on the subject of washers including how they are used, the various types that exist, the materials they are made from and the standards they are required to meet. What are Washers? A washer is a perforated plate or disc, made from a variety of materials, that’s sandwiched between two surfaces subject to rotative friction. For example, between a threaded fastening, such as a bolt or a screw, and the surface into which the fastening is bolted or screwed. We’re all familiar with metal disc washers, such as ‘penny washers’, commonly used in a variety of applications. The origin of the word ‘washer’, in this context, is not known but it has been used since the 14th century when it was spelled ‘whasher’ and defined as “an iron hoope, amongst gunners called a whasher, which seves to keepe the iron pin at the end of the axeltree from wearing the nave”. This reference appears to describe the use of washers in gun or canon axle assemblies where the washer prevented wear. What are Washers Used For? People often question whether washers are essential in fastenings. Here are the main reasons why washers are used and the functions they perform. When a fastening is tightened the washer distributes pressure evenly over the surface ensuring maximum fastening security. Washers provide a smooth, even, level surface for the fastening which reduces the possibility of loosening due to uneven surfaces. Washers are commonly used to protect surfaces to which fastenings are attached. Washers are sometimes used to create seals to prevent leakage or the ingress of water. Washers are sometimes used for spacing, when a fastening is too long. Specialist insulating washers are used in electrical applications. Washers help ensure the integrity of fastenings, enabling them to remain tight and secure, often under extreme conditions involving vibration, stress and pressure. Types of Washers Washers take many forms but can be broadly categorised as either plain washers or spring washers. Plain washers distribute the load imposed by a tightened fastening, preventing surface damage and ensuring the fastening is secure. Plain washers are also used in electrical applications. Spring washers are typically used to prevent fastenings from loosening due to vibration. Locking washers are a form of what are effectively spring washers that are intended to prevent fastenings from becoming loosened. Washer Forms Washers are available in many forms, which can be confusing. Flat washer forms are classified from A to F where each classification identifies the outside diameter of the washer along with its thickness. Here’s a summary of the primary and most popular washer forms and designs along with their typical applications. Plain Washers Round washers with a central perforation are the most common washer form design, but other shapes are also sometimes used. For example, square washers are often used for specific applications such as roofing. Form A flat washers are one of the most ubiquitous washer forms, used in almost every assembly. These are often manufactured to conform with the DIN 125 standard. Form B flat washers are like form A but are thinner. Manufactured to BS4320 standard. Form C flat washers are the same thickness as form A but are of slightly larger diameter. Manufactured to BS4320 standard. Form D flat washers are thin, like form B, and larger diameter like form D. Manufactured to BS4320 standard. Penny washers are also known as fender washers or repair washers. These are amongst the most widely used and recognised form of flat washer. They have a large diameter and are typically used to protect surfaces when tightening fastenings. Spring Washers Spring washers are available in a variety of forms, typically used to help prevent bolted joints from loosening due to vibration. Here are some of the most common forms of spring washer. Square section spring washers are generally used with socket head style screws and bolts. Manufactured to DIN 7980. Rectangular section spring washers are generally used with hexagon head bolted fastenings. Manufactured to DIN 127. Conical spring washers, also known as Belleville washers, disc spring washers, coned-disc spring washers and cupped disc washers. These have a distinctive, conical cup shape which acts as a spring when under pressure in a fastening. Manufactured to DIN 6796. Wave spring washers are another commonly used type of spring washer that helps prevent fastenings from loosening when subject to vibration. Manufactured to DIN 137. Locking or shakeproof washers typically feature serrations around either the internal or external washer edges. They are designed to help prevent the accidental loosening or release of a fastening. Commonly found on electrical connections such as car batteries. Anti-loss or retaining washers typical feature small protrusions from the central hole which prevent the washer from slipping off a matching fastening bolt. Spacer washers are sometimes used to enable fastenings to be effectively tightened, sometimes by stacking multiple spacer washers. Saddle washers are specifically designed for use with round tubing such as that used to manufacture camping chairs and folding furniture. Cup and countersunk washers are primarily used for aesthetic purposes, protecting or concealing the head of a fastening. Cup washers may also involve a cap which will completely conceal the screw or bolt head. Washer Materials Washers are available in a variety materials both metallic and non metallic. Metal Washers Steel is by far the most commonly used metal for washers. Carbon steel, spring steel and various grades of stainless steel are typically used in various applications. Other metals used to manufacture washers include: zinc, iron, copper, brass, aluminium and titanium. Corrosion resistance is an important attribute so many steel washers undergo treatments such as metallic coating (e.g. zinc), electroplating, chemical plating or browning (sometimes called bluing). Non-Metallic Washers Materials used to manufacture not metallic washers include: Nylon Fibre Felt Acetal Rubber (TPR or thermoplastic rubber) Cork Sponge More... Non metallic washers can provide a number of benefits over their metallic counterparts, in some applications. For example: Wear resistance. Plastics like Nylon, PTFE and Acetal provide natural lubricity which increases resistance to wear and can contribute toward extending the part life of bearings, rollers, gears and seals. Corrosion resistance. Non metallic washers simply don’t corrode and are therefore highly resistant to corrosive environments. Excellent sealing properties. Since some non metallic washers compress under pressure they are far more suitable for sealing applications such as tap washers. Impact resistance. Some non-metallic washer materials, such as Nylon and Polyurethane, will absorb impact shock and therefore provide some shock and stress protection. Electrical insulation. Non metallic, insulating washer materials are commonly used in electrical applications. Lightweight. Non metallic washers are commonly used in applications where weight is an important factor, such as aircraft manufacture. Low cost. Non metallic washers tend to cost less than their metal counterparts. Washer Sizes The UK began moving away from the traditional, imperial system of weights and measures, originally established in 1824, to the metric system in 1965. The dimensions and measurements for nuts, bolts washers and fastenings on sale in the UK today are all defined using metric units. UNC and UNF Washers The Unified Thread Standard (UTS) is commonly used in the United States and Canada as the main standard for nuts, bolts and a variety of fasteners. UNC stands for ‘unified coarse’ and UNF refers to ‘unified fine’. UNC and UNF washers are designed to be used with respective nuts and bolts for which they will provide the required clearances. Washer Dimensions When specifying and ordering washers you will typically need to define: The overall dimension (OD) of the washer. This refers to the overall diameter of a circular, flat washer. The internal dimension (ID). This refers to the diameter of the hole in the centre of the washer. The thickness of the washer. In addition you will need to specify the washer form (as previously noted, form A...F) along with the washer material and required finish. Washer Standards High quality washers are manufactured to conform with defined standards. Typically you might see ISO 7089, DIN 125 and BS4320. ISO stands for International Standards Organisation and the ISO 7089 standard defines the requirements for what are called: normal-series, product-grade-A plain washers in the 200 HV and 300 HV hardness classes. DIN stands for "Deutsches Institut für Normung", meaning "German institute for standardisation". The DIN 125 standard aligns with the ISO 7089 standard for washers. BS refers to British Standards. BS4320 is the standards specification for metal washers for general engineering purposes.

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