What is a Dog Clutch?

Paul Scott

A dog clutch is a mechanism used to connect and disconnect two rotating shafts or machine parts. It functions by mating a set of regularly spaced teeth or protrusions on one half of the clutch mechanism to a set of identical recesses on the other, rather than using the friction principle used in other clutches. This has the benefit of allowing both shafts to turn at the same speed without slipping and with minimal clutch wear. They can not, however, be used to control torque as is the case with a friction clutch. Dog clutches are commonly used in manual or stick shift automobile gearboxes, marine propeller drives, and on the drives of heavy machinery.

Bicycle hub gear assemblies use a dog clutch.
Bicycle hub gear assemblies use a dog clutch.

This type of clutch is typically made up of two round plates, one fixed to the drive shaft and one to the shaft being driven. One of the plates has a series of evenly spaced protrusions machined into its face, similar to the battlements on top of a castle wall. The other has an identically spaced series of like-sized recesses cut into its face. When the two plates are brought together, the protrusions mate or fit into the recesses, thereby effectively joining the shafts and transferring rotational motion from one to the other. This ensures a strong mechanical coupling between the two components with no slip characteristics.

Heavy machinery will often utilize a dog clutch.
Heavy machinery will often utilize a dog clutch.

Due to the lack of slippage, the clutch mechanism is not subjected to the amount of wear and tear that is encountered with friction clutches. It does not, however, allow for the gradual initiation of the drive process and resultant torque control that a friction clutch does as the drive engagement is immediate. This type of clutch arrangement allows for a strong, reliable transfer of rotational motion between components using a minimum of moving parts and constant maintenance of equal shaft speeds.

One of the most common uses of a dog clutch is the drive engagement of individual gears in a manual automobile gearbox. When different gears are selected in this application, a synchromesh system allows the drive and selected gears to mesh smoothly. Most marine stern drive and outboard engines use such clutches in their gear trains as do bicycle hub gear assemblies. Large arrangements are also commonly used in the main and auxiliary drive trains of heavy machinery, such as excavators where it's essential for there to be no clutch slippage. Another well known use of this type of clutch is the cord pull-start system on gasoline engines.

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