Locking Devices

Vibration may cause nuts, bolts or screwed fasteners to become loose or even fall out; this is inconvenient and may even be dangerous. There is a number of methods by which loosening due to vibration can be resisted and some of these methods provide positive locking, which means the bolt or screw cannot become undone.

Adhesives Epoxy resin-based adhesives can be used to lock parts together; however, they do provide permanent locking and removal of a part is unlikely to be achieved without damage. To deal with this, you can buy adhesives which will resist the loosening effect of vibration on screws and bolts, but which will not prevent subsequent removal of the screw.


Besides fitting locking devices you should also check bolts and screws in equipment which is subject to vibration in use are, wherever possible, fitted with their heads uppermost so they cannot fall out under the effect of gravity, even if they do loosen. You should also make regular checks on the tightness of any screwed fasteners which are frequently subjected to vibration.

Types of locking device

Locking devices available include nuts, clips, keys and washers. Plain washers under bolt and screw heads are placed there to spread the load on the parts being clamped and to protect the surfaces of those parts; they are not resistant to vibration. There are, however, other types of washer which can serve as locking devices.

Spring lock washers


Also known as split washers, these are the washers most commonly used to resist loosening. This type is a small spring which fits between the bolt head and the work, with the ends of the spring shaped so they will dig into the work and the nut when the nut (or screw) ‘is tightened. These washers are available in sizes to fit bolts and screws from 3mm up to 50mm (or 2in) in diameter; the usual pattern is a single coil spring, but there are double coil spring lock washers available.

Tooth lock washers


Also known as shakeproof washers, these are circular washers with teeth or prongs on the outer circumference (external tooth) or on the inner circumference (internal tooth) or on both inner and outer circumferences. When the screw or bolt is tightened, the teeth deform and dig into the work and the fastener; this prevents loosening. These washers are often used under self-tapping screws fastening sheet metal panels. It is not usual to use plain washers as well as tooth lock washers, although the teeth of the latter are likely to scratch the surface of the work on which they are used.

Lock washers

Lock washers

Otherwise called tab washers, these are used with hexagonal bolts or nuts. As supplied, the flat washer is placed between the work and the bolt head or nut; when the bolt has been tightened, the tabs on the washer are hammered over against the edge of the work and against the flats on the bolt or nut.

Another type of lock washer is round and has a projection on the inner circumference which fits into a slot on the screw or bolt; the edge of the washer is bent up against a flat on the nut and this prevents the nut turning relative to the screw as opposed to the workpiece.

Lock nuts Sometimes called jam nuts, these hexagonal nuts are thinner than a standard nut; to use one you should tighten the lock nut down on the work and tighten a standard nut down on top of the lock nut. Then, holding the upper nut with one spanner, use another spanner with slim jaws to slacken the lock nut so it jams against the upper nut. It can be difficult to follow the correct procedure for fitting these nuts because many spanners have jaws which are insufficiently slim; consequently lock nuts are often fitted above the standard nut which is slackened to jam against the lock nut while the lock nut is held.

Elastic stop nuts


With this type of device there is a slot inside the nut above the thread which holds a fibre or nylon ring. As the nut is tightened down, this insert is deformed by the bolt thread and the very high degree of friction between the thread and the insert prevents the nut loosening. Elastic stop nuts are frequently used to secure electrical equipment such as transformers or loudspeakers.

Castle nuts


Sometimes known as castellated nuts, these are hexagonal nuts with a circular shape above the hexagon; they are intended for use with bolts or studs which have been drilled across their diameter. There are three pairs of slots diametrically across the top of a castle nut. To use a nut, tighten it down onto the work until it is as tight as required and a pair of slots are in line with the drilled hole in the bolt; place a cotter pin through the slots and hole and bend it over to provide a positive locking device.

Castle nuts are commonly used to secure the front wheel bearings on cars. When fitting a bearing there must be no shake on the bearing (it must not be able to move backwards and forwards on its shaft); at the same time the securing nut must not be so tight it prevents the bearing rotating. A castle nut is particularly suitable in this and similar situations; it allows the degree of tightness to be finely adjusted since there are six possible locking positions for each turn of the nut.

Slotted hexagonal nuts

Slotted hexagonal nuts

These are used in the same way as castle nuts, but they are cheaper. The slots for the cotter pins are cut across the flats of the hexagon and these nuts do not have a circular portion above the hexagon.

Spring nuts


Made in thin spring steel in a variety of shapes, these are mainly used with self-tapping screws — although they can be used with ordinary screws. When a screw is inserted, it passes through two metal prongs; as it is tightened, the nut tends to flatten from its original curved shape and the prongs press very firmly into the screw thread and prevent it loosening. Because the nuts are very thin they can be used in restricted spaces and are extensively used on domestic appliances and cars for fixing panels. One type of spring nut called a push nut can be used on rods or pins which are unthreaded; the rod or pin is placed in the hole in which it is to be fitted and the nut is simply pushed on from the back.

Stamped nuts


These are very thin hexagonal nuts stamped out of spring steel. There are prongs in the hole of the nut which, as the nut is tightened down on the screw, press very firmly against the thread of the screw. Used as a locking device, the nuts are fitted to a bolt or screw above the normal nut. They do not require much tightening effort, but they do provide a very effective locking device. Where a bolt or screw does not carry a heavy load, a stamped nut can be used in place of an ordinary nut.



Specially designed to fit in grooves on shafts to prevent the shaft or parts fitted on the shaft moving in a longitudinal direction, these clips provide a very positive locking device. A slot has to be machined into the shaft, however, to accept them and they are not easily fitted or removed without special pliers (circlip pliers).



For very light work where small diameter shafts have to be held in position, rubber 0-rings can be used. They are simply rubber rings which are slid onto the shaft until they rest in a groove on the shaft at the required position.

Cotter pins


Also known as split pins, these have a number of uses. They can be fitted to a castle or slotted nut to prevent the nut turning. They can also be used on pins and shafts to prevent the pin or shaft coming out of a hole; when using them in this way, it is worth placing a flat washer on the pin or shaft before fitting the cotter pin.

Keys and pins

It is often necessary to fit a wheel or pulley to a shaft in such a way that the wheel cannot turn on the shaft. To do this, a slot can be milled into the shaft and the wheel and a key inserted to prevent relative movement between the two parts. The main types of key available are parallel keys, tapered or gib keys and Woodruff keys which fit into a radiused slot on the shaft. The slot or keyway has to be accurately machined; since this requires a milling machine you may have to get the job done professionally.

When fitting keys, make sure there are no burrs on the key or keyway. Parallel keys and Woodruff keys are tapped into position on the shaft and the wheel or pulley is then placed in position, with its keyway lined up with the key in the shaft, and pressed home. If you have no means of pressing the wheel onto the shaft and it has to be hammered home, use a piece of wood between the hammer head and the wheel to protect the wheel. To remove a Woodruff key from its keyway tap it gently at one end so the end swivels upwards; you can then prise the key from its keyway. When taper keys are used, the wheel and shaft can be assembled with the keyways in line; the key can then be driven into place.

Instead of keys, you can use tapered or parallel pins to fix wheels and shafts. Making an accurate tapered pin and its mating holes is likely to be beyond your resources, but you should be able to make and fit parallel pins. The hole in the shaft and wheel needs to be slightly smaller in diameter than the pin to be used, but no more than light hammer taps should be required to drive the pin into position.

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