Locking Pins - Spring Plugs and Cotter Pins - Shapes and Dimensions

Spring plugs and cotter pins (also called locking pin) are locking elements for different applications. They are often used for bolt connections as a simple and releasable locking element and are used to secure the axial force. The axial force is not the main load on the joint. The locking element is primarily used to hold the assembly together. The axial force is therefore often a smaller by several orders of magnitude than the radial force on the bolt. This article discusses the locking principles for bolt joints with locking pins, along with application examples of the various locking elements in mechanical engineering, and based on what criteria these elements can be selected.

What are spring plugs?

Spring plugs are specially shaped wire elements. As a spring-loaded elastic locking element (also available as a standardized), spring plugs have a straight part of the wire that is passed through a bore, and a shaped spring portion that can be wrapped around the bolt. The spring plug reliably prevents radial slippage and is secured on the bolt by its shape.

Spring plugs are often manufactured iaw. DIN 11024. There are several versions of spring plugs in different embodiments. Spring plugs are installed without tools.

Cotter pins are also shaped wire elements with simple geometry. Cotter pins consist of two straight portions that are parallel and in immediate proximity to each other. One end of the cotter is bent into a loop and provides a stop, the other end is open and can then be plastically deformed, usually with a tool. This secures the locking element against loosening.

Cotter pins are standardized in ISO 1234 - formerly DIN 94.

Alternatives to spring plugs and cotter pins

In addition to locking pins such as spring plugs and cotter pins, there are also special locking elements such as ball lock pins and thread locking bolts. These locking bolts are more complex to manufacture because these components are not made of a single material. In addition to the bolt, they also contain the locking element with a release mechanism.

Assembly is possible without tools because either the release mechanism made from a ball or thread is actuated by means of a switch or a spring mechanism with sufficient operating force can be overcome.

How to operate a ball lock pin:

① Release ball lock by pressing the button. (balls are retracted)
② Insert pin into appropriate bore and release button. (balls are pushed outward)
③ Re-pressing the button releases the ball lock. (pin can be pulled out)

Circlips, lock washers and snap rings are used to secure shafts or bores. These locking mechanisms are more robust at rotational speeds and have larger bearing surfaces. These locking elements can therefore be used even at higher axial loads. Circlips and snap rings can be used as axial locking elements from the inside in bores.

Hinge bolts are bolts with a fit on the gliding surface and are shaped for securing the locking element. They are the ideal counterpart to the locking elements and are configurable in the MISUMI shop in many different versions. There are versions with bores, threads, and cut-outs to secure the locking element.

Application example: Pin with cross hole: (1) Spherical plain bearing, (2) U-shaped hinge bearing block, (3) Hinge pin with cross hole

How to use spring plugs and cotter pin correctly

Spring cotter pins, splints and similar standardized parts are used for securing bolt connections and other applications. For bolt connections, the forces are absorbed by the bolt and there are no to very low forces in the direction of the bolt axis. The primary task of the locking elements is to secure the bolt against sliding out.

When are spring plugs and cotter pins used? And when are alternatives used?

Spring plugs are the correct choice if the connection needs to be changed more frequently. The spring plugs slide over the bolt and hold this position by pre-loading the geometry. This locking elements usually takes up more space than a cotter pin.

In mechanical engineering, spring plugs are used in the following applications:

• For securing interchangeable tools on machines
• For mounting covers to enclosures

Cotter pins are used when the connection is released very rarely to never. Cotter pins become brittle and fail faster with each assembly and disassembly. For this reason, cotter pins must be replaced after having been dismantled several times.

Splints are widely used in mechanical engineering, including:

• For securing operator controls on machine controls
• For securing hinge bolts in openings of housings
• To lock connections between partial machines, such as conveyors

Ball lock pins are preferred for applications requiring frequent, daily installations and modifications. These components are more difficult to manufacture and not as robust as spring connectors, but are appropriate for frequent and almost forceless disassembly and assembly due to the release mechanism. A spring mechanism is yet another embodiment of the ball lock pin. The balls of the ball lock pin are tensioned by a defined spring force. If the force during disassembly is sufficiently high, the balls are pushed into the ball lock pin, which can then be removed. The assembly works according to the same principle.

Ball lock pins are used in general mechanical engineering in the following applications:

• To secure fixtures in assembly operations
• To secure adjustable sub-assemblies
• To secure brackets in quality control

Thread locking bolts are used in mechanical engineering when a joint is exposed to high vibrations, for example:

• to securing clutches in drive systems
• to secure components in heavy-duty machines

Lock washers and circlips are mounted in turned grooves of the bolts and are used at higher rotational speeds as these elements distribute the mass more evenly around the bolt or shaft.

Typical applications in mechanical engineering are:

• Securing rolling bearings with lock washers
• Securing plain bearing bushings

Sizing and selection - spring plugs in bolt connections

Locking elements are selected based on the use case. The choice of locking element must already be taken into account when sizing and selecting the bolt.

The selection criteria include:

• Shaft diameter: The diameter of the bolt influences the selection of the standardized parts and the handling of the locking element. Spring plugs are more commonly used on small diameters, whereas ball lock pins and thread locking bolts more commonly used on large bolts.
• Frequency of installation: The interval of loosening the connection burdens the material and can be achieved without tools, e.g. on ball lock pins and spring plugs, or must be carried out with tools, for example on cotter pins.
• Axial force: The transverse force acting on the locking element is critical for selection. Small transverse forces can be absorbed by cotter pins, whereas large transverse forces require lock washers or thread locking bolts.
• Cost-efficiency: The costs decide between two possible variants, e.g. a spring pin can also fulfill many tasks of a cotter pin. Another of many criteria can be the cost of manufacturing the groove or bore.
• Material: The material is selected based on the previously determined shear force and the load expected to act on the locking element. The standardized shapes can be made of steel, stainless steel or with corrosion protection , depending on requirements.

At MISUMI, you will find the correct locking element for your cotter pin or bolt connection in the categories:

• Spring pins and cotter pins
• Hinge bolts
• Ball lock pins/thread locking bolts
• Circlips/retaining washers/snap rings

• Ball lock pins and thread locking bolt