In mechanical engineering, a parallel key is a type of key used to connect a rotating machine element (like a gear, pulley, or coupling) to a shaft, ensuring that torque is transmitted without relative rotation between the shaft and the component.

Key Features of Parallel Keys:

Shape: Rectangular in cross-section.

Mounting: Fits into matching keyways (slots) cut into both the shaft and the hub of the component.

Function: Prevents relative rotation by locking the shaft and component together.

Standard Sizes: Defined by standards like ISO, DIN, or ANSI; typical proportions are width (w) × height (h), e.g., 10×8 mm.

Types of Parallel Keys:

Rectangular (or flat) parallel key:

Uniform rectangular cross-section.

Most common type.

Load is distributed over the entire surface.

Square parallel key:

Width and height are equal.

Often used in smaller components.

Gib-head key:

A rectangular key with a head (like a tab) at one end to help with removal.

Applications:

Used in gearboxes, couplings, pulleys, and flywheels.

Found in machines, engines, and rotating equipment.

Advantages:

Simple to manufacture and install.

Good torque transmission capability.

Disadvantages:

Requires precise machining of keyways.

Can introduce stress concentrations, leading to fatigue over time.