Circlips and pins provide simple mechanical methods of retaining, locating, aligning or securing components. Although they are often grouped together as small machine components, their functions are quite different and the correct choice depends on what the component is required to do.
This guide explains the principal differences between the circlips and pins covered by RoyMech and helps identify the appropriate technical reference for dimensions and associated standards.
| Requirement | Typical Choice | RoyMech Reference |
|---|---|---|
| Retain a component axially on a shaft | External circlip | External Circlips |
| Retain a component axially inside a bore or housing | Internal circlip | Internal Circlips |
| Lock a nut, pin or similar component against accidental withdrawal | Split cotter pin | Split Cotter Pins |
| Locate, align or secure two components using a tapered interference fit | Taper pin | Taper Pins |
The first selection decision is whether the assembly requires axial retention, locking or location and alignment.
Circlips are retaining rings installed in machined grooves. They are primarily used to prevent axial movement of a component relative to a shaft or bore.
Pins pass through or into holes in mating components. Depending on their form, they may be used for locking, locating, alignment or securing an assembly.
The fundamental distinction between the two circlip types covered by RoyMech is whether the retaining groove is located on a shaft or inside a bore.
External circlips fit into a groove machined around the outside diameter of a shaft. They provide a shoulder which prevents an adjacent component from moving axially beyond the circlip.
Typical applications include retaining bearings, gears, bushes, pulleys and other components assembled onto shafts.
External Circlip DimensionsInternal circlips fit into a groove machined around the inside diameter of a bore or housing. They provide an internal shoulder which restricts axial movement of the retained component.
Typical applications include retaining bearings, bushes and other components installed inside housings.
Internal Circlip DimensionsA circlip and its groove work together as a retaining system. Selecting the correct circlip diameter alone is therefore not sufficient.
The groove diameter, groove width and location determine how the circlip is supported and how axial load is transferred between the retained component, circlip and shaft or housing.
The designer should consider:
The distinction is determined by the location of the groove rather than the component being retained.
| Feature | External Circlip | Internal Circlip |
|---|---|---|
| Groove location | Outside of a shaft | Inside a bore or housing |
| Primary purpose | Prevent axial movement along a shaft | Prevent axial movement within a bore |
| Installation | Expanded over the shaft | Compressed into the bore |
| Typical retained components | Bearings, gears, bushes and pulleys | Bearings, bushes and internal components |
Assembly and servicing requirements should be considered when designing the groove and surrounding components.
External circlips normally need sufficient radial access for the circlip to be expanded during installation or removal. Internal circlips require access within the bore so that the ring can be compressed.
Nearby shoulders, housings or other components should not prevent the appropriate installation tool from engaging correctly with the circlip.
The two pin types currently covered by this RoyMech section perform different functions. A split cotter pin is principally a locking or retaining device, whereas a taper pin can provide positive location and alignment between mating components.
Split cotter pins pass through a cross-hole and are secured by bending the two legs apart after insertion.
They are commonly used to prevent nuts, clevis pins or similar components from becoming detached or withdrawing from an assembly.
Split Cotter Pin DimensionsTaper pins are fitted into matching tapered holes and can provide accurate location and a secure mechanical connection between mating components.
The taper produces progressively increasing contact as the pin is inserted, allowing the components to be positively located relative to each other.
Taper Pin DimensionsA split cotter pin is normally used as a secondary locking or retaining device rather than as a precision locating component.
A common arrangement uses a split pin through a drilled bolt, stud, clevis pin or shaft. After insertion, the legs are bent to prevent the pin from passing back through the hole.
When selecting a split cotter pin consider:
Taper pins are useful where two components need to return to a defined relative position after assembly. The matching tapered hole provides positive seating and location.
Applications may include locating machine components, hubs, levers, fixtures and assemblies where repeatable alignment is required.
Important considerations include:
Where a pin transmits mechanical load rather than simply locating or locking an assembly, the designer should consider the actual load path. Depending on the joint arrangement, relevant failure modes can include shear of the pin, bearing stress at the hole and deformation or failure of the connected components.
The dimensional tables should therefore not be interpreted as load-rating tables unless a load rating is specifically provided by the applicable standard or manufacturer.
Hole and shaft tolerances can be important where accurate location, controlled clearance or interference is required. This is particularly relevant to locating pins and other precision mechanical assemblies.
For guidance on the ISO system of limits and fits, use the RoyMech ISO tolerances reference and selection guide.
Consider whether the joint will be assembled once or dismantled regularly during service.
Circlips provide convenient removable axial retention where suitable tool access is available. Taper pins can provide repeatable location but require appropriate access for removal. Split cotter pins are simple locking devices and should be inspected and replaced as appropriate when an assembly is dismantled.
| If you need to... | Consider... |
|---|---|
| Retain a component on a shaft | External circlip |
| Retain a component inside a housing or bore | Internal circlip |
| Prevent a nut or pin from withdrawing | Split cotter pin |
| Locate and align mating components | Taper pin |
Dimensions and associated reference data for external circlips fitted to shaft grooves.
External CirclipsDimensions and associated reference data for internal circlips fitted to grooves in bores and housings.
Internal CirclipsDimensional reference for split cotter pins used for locking and retaining mechanical components.
Split Cotter PinsDimensional reference for taper pins used for locating, aligning and securing components.
Taper Pins