When a pulley, sprocket, gear, coupling, or gearbox is mounted onto a shaft, something needs to transfer the torque from the shaft into the component. In many applications, that job is performed by a simple but critical component called key steel.
Despite its small size and low cost, key steel is often the difference between reliable power transmission and costly equipment failure.
What Is Key Steel?
Key steel, sometimes called a machine key, parallel key, or keyway steel, is a precision-machined square or rectangular steel bar that fits into matching keyways cut into both the shaft and the hub of a component.
When assembled, the key locks the two components together and prevents the hub from rotating independently of the shaft.
Common components that use key steel include:
- V belt pulleys
- Chain sprockets
- Couplings
- Gears
- Flywheels
- Conveyor drums
- Agitators
- Gearbox shafts
Without a correctly fitted key, the shaft may continue turning while the pulley or sprocket slips, causing wear, loss of drive, or complete failure.
How Does Key Steel Work?
Imagine a pulley mounted onto a shaft.
If the shaft simply sits inside a round bore, there's nothing to stop it spinning independently. By machining a slot (keyway) into both the shaft and pulley and inserting a piece of key steel, torque can be transferred directly from the shaft to the pulley.
The key effectively becomes the driving connection between the two components.
Common Signs of Key Steel Failure
Key steel is designed to be sacrificial in many applications. It is often cheaper and easier to replace a key than a shaft or pulley.
Common signs that key steel may be worn or damaged include:
- Pulley or sprocket slipping on the shaft
- Excessive vibration
- Clicking or knocking noises
- Worn or rounded key corners
- Rusted or damaged keys
- Movement between shaft and hub
- Difficulty maintaining alignment
If a key shows significant wear, it should be replaced before further damage occurs to the shaft or component.
Metric and Imperial Key Steel Sizes
Key steel is available in both metric and imperial sizes.
At RR Fisher, key steel is commonly supplied in convenient lengths (Usually 300mm) ready for cutting to suit the application.
You can search the range of Key Steel here.
Should You Reuse Old Key Steel?
In most cases, no.
Key steel is inexpensive compared with the labour involved in replacing a damaged shaft or pulley. If you're removing a component for maintenance, replacing the key at the same time is typically good practice.
A worn key can quickly damage a new pulley, sprocket, or coupling.
Key Steel vs Alternative Shaft Locking Systems
Many alternative shaft locking systems such as:
- Bi-Loc bushes
- Taper Lock bushes
- Shrink discs
- Keyless locking assemblies
eliminate the need for traditional keys altogether.
However, keyed connections remain one of the most widely used shaft locking methods throughout agriculture, manufacturing, food processing, quarries, sawmills, and general industry because they are:
- Simple
- Reliable
- Cost-effective
- Easy to machine and replace