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Rigid Coupling

Rigid couplings are used to connect rotating members such as shafts. They are compact, economical components for the timing, joining or aligning of shafts and provide transmission of torque and motion. They are used at lower speeds, particularly where zero backlash is desired. These type of couplings do not allow for angular or parallel misalignment. Rigid couplings are also not intended for use as a critical part of a drive line or as a substitute for flexible or universal joints. They are also not meant for other power transmission devices. They are basically used for aligned shafts only. They are the most cost-effective coupling option suitable for all rigid shaft connections.

These couplings comprise simple hollow cylinders having internal diameters of the proper size which can fit over the shaft ends being joined. The length of the coupling depends on the desired shaft gap between ends. For light applications, setscrews are used to attach the shafts to the coupling. This type of coupling do not accommodate for misalignment.

Materials used in Rigid couplings:
Some common materials used in making rigid couplings are as follows:
  • Brass
  • Aluminum
  • Cast Iron
  • Stainless Steel
  • Carbon Steel etc.

Buying Tips

Selecting a rigid coupling involves many different performance factors such as:
  • Torque
  • Shaft misalignment
  • Stiffness
  • Rpm
  • Space requirements, and
  • Others, that must all be satisfied for the rigid coupling to work properly.

Types of rigid couplings:
There are two basic types of rigid couplings. They are as follows:
  • Set-Screw Coupling: The rigid set-screw coupling is that type of coupling which is secured to the shaft by a set-screw that impinges upon the shaft.
  • Clamping Coupling: Clamping couplings are available in both one-piece and two-piece styles. One-piece clamping couplings are available with four tangential screws. These screws when tightened grip the shaft without marring it. The two-piece clamping couplings are available with eight tangential screws (four on each side). These types of couplings are assembled without removing other components on the shaft.
Installation of rigid couplings:
For all rigid couplings, the clamping screws are arranged in pairs. The rigid couplings can be installed by tightening the paired screws alternately. This can be done in several steps so that the tension is distributed evenly.
Rigid Couplings
Rigid Coupling

When one screw is tensioned, it tends to relax any tension developed by its paired screw. Hence, it is recommended not to tighten one screw completely before tightening the other as this could lead to an unequal distribution of tension. Again, when the screws are disassembled, tension should be relieved in a similar manner.

Features of Rigid couplings:
To ensure proper performance in any kind of motion control applications, several features of rigid couplings are required. They are as follows:
  • Rated torque: The maximum service torque for which the product is rated.
  • Rated speed: This is the maximum rated rotational speed of the rigid coupling.
  • Stiffness: This is expressed in degrees (torque per units of angular deformation).
  • Backlash: Due to a change, the rotational position loss is measured in degrees.
  • Honing: This corrects any residual distortions caused by stresses occurred during the manufacturing process, resulting in a round, precisely sized bore.
  • Important dimensions
    • Bore diameter: Internal diameter (ID) for mating to the shaft-end.
    • Coupling diameter: Outside diameter (OD), which includes the housing.
    • Design units are in inches (in) or metric measurements such as centimeters (cm).
    • Keyway width: This is an additional consideration for rigid couplings with one or more keyways.
Styles of rigid couplings:
There are various styles available in rigid couplings. They are as shown in the table below:

Set Screw Rigid Coupling

Set screw rigid coupling:

The most common and simplest style of rigid couplings is those with set screws to fix the coupling to the shaft through impingement.

One-piece Clamp Coupling

One-piece clamp coupling:

One-piece clamp couplings are solid. As a rule, one-piece rigid couplings can be used for applications up to 3000 rpm.

Clamp Style Rigid Coupling

Clamp style rigid coupling:

Another option is the clamp-style rigid couplings. These couplings wrap around the shaft to provide high torsional holding power without damaging the shaft and fretting inherent in set screw types.

Two Piece Rigid Coupling

Two piece rigid coupling:

There are two-piece styles rigid couplings which have the additional benefits of allowing for disassembly and maintenance without removal of other components. When a two-piece style is used, couplings can be used for applications up to 4000 rpm.

Multi-screw rigid coupling:

Multi-screw rigid couplings are also available. Here, all of the screws are inserted from the same direction.

Shrink discs rigid coupling:

Shrink discs rigid couplings are available with two external rings that tighten together axially and exert inward force on a third ring. This closes the inner ring onto the shafts which in turn provide a secure connection.

Key ways rigid coupling:

They include keys that can be used to stop the rotation of the shaft couplings.

Innovation in rigid couplings continues. Driven by customer needs, the future should bring new products that are more stronger, lighter, better, and more economical. By design, they still remain inflexible.

Benefits of rigid coupling:
The benefits of rigid couplings are as follows:
  • They are economical.
  • They have high torque capacity.
  • They have torsional stiffness.
  • They have zero-backlash.
  • Rigid couplings with honed bores and other features to assure proper performance are increasingly used in motion control applications where components are properly aligned.
  • In addition to motion control applications, they are also used to connect line shafting or other components such as a motor to a gearbox.
  • Since rigid couplings cannot accommodate misalignment, they are not recommended for use on mechanically fixed shafts. However, rigid couplings are ideal for overhung and extended loads like pumps, mixer paddles or long, unsupported shafts.