Keys look simple.
A small rectangular piece between a shaft and hub can transmit a surprisingly large amount of torque.
But a key should not be selected by habit alone.
## What does the key do?
The key prevents relative rotation between the shaft and hub.
Torque passes through the key and creates forces along its contact surfaces.
That means the key and surrounding shaft material need to be checked.
## Check key shear
A simplified design check compares the transmitted load with the allowable shear capacity of the key.
The exact equation depends on the key geometry and design standard.
The important point is that the key has a finite load capacity.
## Check crushing or bearing stress
The contact pressure between the key and the hub or shaft can also become critical.
A key that is safe in shear may still have excessive bearing stress.
That is why both failure modes matter.
## Do not forget the shaft
Cutting a keyway removes material from the shaft.
The resulting geometry can affect local stress and fatigue performance.
This becomes especially important for shafts under cyclic loading.
## Fit and assembly
The key should provide the intended rotational drive while allowing the hub to assemble as designed.
Check:
– Key size – Keyway dimensions – Hub length – Shaft diameter – End clearances – Assembly method – Retention
## A practical engineering lesson
A standard component is not automatically a safe component.
Standards give you dimensions and established design practices, but you still need to check the actual application.
Torque, speed, material, duty cycle and fatigue all matter.
That is the difference between selecting a key from a table and engineering a keyed joint.