1. Introduction

A coupling is a mechanical device that permanently connects two rotating shafts to transmit power from a drive end (such as an electric motor) to a driven end (such as a pump or gearbox). Its primary purpose is to join two pieces of equipment while allowing for a degree of misalignment and/or end movement.

2. Key Functions of a Coupling

Power Transmission: The fundamental task is to transmit rotational power (torque).

Accommodate Misalignment: Perfect alignment between two shafts is often impossible to achieve and maintain. Couplings can compensate for:

Parallel Misalignment: The shafts are parallel but not collinear.

Angular Misalignment: The shafts are at a slight angle to each other.

Axial Misalignment (End Float): The shafts move closer together or farther apart along their axis.

Absorb Shock and Vibration: Flexible couplings can dampen shock loads and vibrations, protecting both the driver and driven equipment.

Provide a Safety Function: Some couplings act as a mechanical fuse by slipping or breaking under extreme overload conditions, preventing damage to more expensive components.

3. Main Types of Couplings

Couplings are broadly categorized into two families:

A. Rigid Couplings

Description: Used when precise shaft alignment is required and maintained. They do not tolerate any misalignment.

Applications: Used primarily in applications where shafts are already perfectly aligned, such as connecting sections of a long line shaft.

Examples:

Sleeve or Muff Coupling: A simple cylindrical sleeve that fits over the ends of two shafts.

Flanged Coupling: Two flanged hubs are keyed to the shafts and bolted together.

B. Flexible Couplings

Description: Can accommodate various types and degrees of misalignment. They contain flexible elements.

Applications: The vast majority of industrial applications (pumps, fans, compressors) where some misalignment is inevitable.

Examples:

Jaw Coupling: Uses a synthetic spider element between two metal hubs. Excellent for general-purpose applications.

Gear Coupling: Consists of two geared hubs meshing with a sleeve. Handles high torque and minor misalignments.

Disc Coupling: Uses a series of flexible metal discs to transmit torque. Excellent for high-speed applications with no backlash.

Beam Coupling (Helical Coupling): A single-piece, helical-cut design that accommodates misalignment and is often used in precision motion control.

Oldham Coupling: Uses a floating disc to accommodate parallel misalignment.

Universal Joint (U-Joint): Used for large angular misalignments, commonly found in automotive drive shafts.

4. Key Selection Criteria

Choosing the right coupling involves considering:

Torque and Power Requirements: The coupling must handle the application's torque without failing.

Type and Degree of Misalignment: The coupling must be capable of compensating for the expected misalignment.

Shaft Sizes and Keyways: Must physically fit the driver and driven shafts.

Speed of Rotation (RPM): High-speed applications require couplings with good balance, like disc couplings.

Operating Environment: Factors like temperature, presence of chemicals, or dust can influence material selection (e.g., stainless steel vs. aluminum).

Backlash: The amount of "play" between components. Zero-backlash couplings are needed for precision motion control.