Circlips

The vast majority of my articles on the subject of fasteners have been specifically on the threaded types. They fulfil a critical role in any engine, and require a degree of understanding and some calculation to work successfully. There are many other fasteners used in a typical race engine, however, that are not so demanding of the engineer. One such fastener that is used widely is the circlip.

In this article though we are not going to deal with round wire circlips, as commonly used for retaining piston pins; they were discussed in a recent piston article. Here we will deal with the more common flat circlip, and look at some specific variations that can be useful.

Circlips are inexpensive components designed to be elastically deformed to allow them to fit into specially machined grooves provided in shafts or housings. The amount of deformation required for them to fit requires that they are made from materials with a certain amount of elastic deformation. Moreover, they operate in an elastically strained condition, and the amount of radial load must not diminish markedly in service.

Circlips are generally mass-produced by stamping from sheet metal, and are usually steel, although some are available in other materials such as stainless steel or beryllium copper. Other manufacturing techniques such as machining or laser cutting can be used, and lend themselves to special designs where required.

Most circlips are of a standard pattern, although for racing there are some interesting variations on this. For rotating assemblies, where a circlip is used on the rotating parts, a ‘balanced lug’ circlip may be used, which is designed to be ‘in balance’ when fitted – that is, the centre of gravity of the circlip is coincident with the axis of the hole into which it is fitted. That is not true for standard circlips and, depending on the speed of rotation, significant out-of-balance forces may occur, requiring the assembly to be balanced to take account of the circlip out-of-balance forces. This isn’t a problem in itself but, if the standard circlip is ever removed and refitted, it does mean the circlip has to be fitted in precisely the same orientation as before, to avoid putting the assembly out of balance.

Bowed circlips are a deliberate attempt to provide a degree of pre-load and to take up small amounts of axial play in an assembly. In its free state the circlip has a precise amount of curvature so that, when flattened or partially flattened in its installed state, it provides a load on the adjacent components.

Bevelled circlips are a variation on a standard flat circlip. A shallow angled bevel is applied to the outer diameter on one side of the circlip (assuming that this is an internally fitting circlip) by careful sizing and positioning of the groove; the circlip does not go fully ‘home’ in the groove but is forced axially, owing to the reaction between the circlip bevel and the corresponding surface of the circlip groove, which has an angle equal to that of the circlip bevel. The bevelled circlip has the advantage over a bowed circlip of a much higher stiffness, as it doesn’t rely on elastic axial deflection of the circlip to take up any axial play.

Fig. 1 - Circlips come in many shapes and sizes, with some designed for specific tasks

Written by Wayne Ward