Rigid couplings types

Rigid couplings connect two shafts such that there is no relative movement between them, forming a fixed and continuous joint. They transmit torque and rotation without allowing angular, radial, or axial misalignments; any alignment error generates high reactive forces on the shafts and supports. They are used when the positions of both machines can be guaranteed with precision and maximum torsional stiffness is required.

Types

Commercially available rigid couplings are classified into three main families:

Type	Description
Sleeve coupling	A hollow cylindrical sleeve that envelops the shaft ends, fixed with keys, pins, or set screws. The split or clamp version (split‑muff) divides the sleeve into two halves that are tightened with screws, allowing assembly without moving the shafts.
Flange coupling	Two hubs with external flanges joined by bolts. They incorporate a pilot shoulder to center the halves and often use standardized hole patterns (similar to ASME B16.5) to ensure interchangeability.
Clamp coupling	Uses a split ring that tightens the shaft ends by friction and possibly a conical seat; it is easily disassembled without the need for keys.

Standard Dimensions

The following tables list typical manufacturer dimensions (split‑muff and clamp couplings) and adapted dimensions from ASME B16.5 Class 150 flanges when used as flange couplings.

Split-Muff Coupling in Steel

Shaft Diameter	Outside Diameter (OD)	Length (L)
6 mm / 1/4 in	12.7 mm / 1/2 in	19 mm / 3/4 in
10 mm / 3/8 in	15.9 mm / 5/8 in	25.4 mm / 1 in
12 mm / 1/2 in	19.1 mm / 3/4 in	31.8 mm / 1-1/4 in
16 mm / 5/8 in	22.2 mm / 7/8 in	38.1 mm / 1-1/2 in
20 mm / 3/4 in	25.4 mm / 1 in	44.5 mm / 1-3/4 in
25 mm / 1 in	31.8 mm / 1-1/4 in	50.8 mm / 2 in
30 mm / 1-3/16 in	38.1 mm / 1-1/2 in	57.2 mm / 2-1/4 in

Rigid Flange Coupling (adapted from ASME B16.5 Class 150)

Equivalent Shaft Diameter (NPS)	Flange Outside Diameter	Bolt Circle	Number of Bolts	Bolt Diameter
12.7 mm / 1/2 in (NPS 1/2)	88.9 mm / 3-1/2 in	60.3 mm / 2-3/8 in	4	12.7 mm / 1/2 in
19.1 mm / 3/4 in (NPS 3/4)	98.4 mm / 3-7/8 in	69.9 mm / 2-3/4 in	4	12.7 mm / 1/2 in
25.4 mm / 1 in (NPS 1)	108 mm / 4-1/4 in	79.4 mm / 3-1/8 in	4	12.7 mm / 1/2 in
31.8 mm / 1-1/4 in (NPS 1-1/4)	117 mm / 4-5/8 in	88.9 mm / 3-1/2 in	4	12.7 mm / 1/2 in
38.1 mm / 1-1/2 in (NPS 1-1/2)	127 mm / 5 in	98.4 mm / 3-7/8 in	4	12.7 mm / 1/2 in
50.8 mm / 2 in (NPS 2)	152 mm / 6 in	120.7 mm / 4-3/4 in	4	15.9 mm / 5/8 in
76.2 mm / 3 in (NPS 3)	191 mm / 7-1/2 in	152.4 mm / 6 in	4	15.9 mm / 5/8 in
101.6 mm / 4 in (NPS 4)	229 mm / 9 in	190.5 mm / 7-1/2 in	8	15.9 mm / 5/8 in

Load Capacities

The torque capacity of a rigid coupling is primarily determined by the strength of the shaft‑hub connection (key, friction from clamping, or press fit) and by the coupling body’s shear and torsional strength. The values represent maximum continuous torque under static conditions for commercial grade carbon steels (yield strength ~250 MPa / 36 ksi).

Shaft Diameter	Steel split‑muff type (friction + key)	Steel flange type (4–8 preloaded bolts)
6 mm / 1/4 in	2.8 Nm / 25 lb·in	–
10 mm / 3/8 in	8.5 Nm / 75 lb·in	–
12.7 mm / 1/2 in	20.3 Nm / 180 lb·in	45 Nm / 398 lb·in
16 mm / 5/8 in	37.3 Nm / 330 lb·in	85 Nm / 752 lb·in
20 mm / 3/4 in	61 Nm / 540 lb·in	135 Nm / 1,195 lb·in
25.4 mm / 1 in	135 Nm / 1,195 lb·in	270 Nm / 2,390 lb·in
50.8 mm / 2 in	–	1,100 Nm / 9,735 lb·in
101.6 mm / 4 in	–	8,500 Nm / 75,200 lb·in

Note: The torques indicated for flange couplings assume grade 8.8 bolts and keys conforming to DIN 6885. The presence of shock loads reduces the allowable torque by a factor of 2 to 4.

Selection Criteria

1. Shaft alignment: Rigid couplings do not compensate for any misalignment. Radial alignment ≤ 0.05 mm / 0.002 in and angular alignment ≤ 0.1° must be ensured using shims and dial indicators. If any unavoidable misalignment exists, a flexible coupling should be chosen.
2. Torque to transmit: Select the coupling with a nominal torque at least 30 % higher than the maximum torque of the application, considering the shock factor.
3. Disassembly frequency: For frequent maintenance, choose a flange coupling or split-muff coupling. The split-muff allows disassembly without moving the shafts.
4. Rotation speed: Rigid couplings are insensitive to speed, but concentric assembly is critical to avoid vibration and fatigue at high speed.
5. Material and environment: In corrosive or food environments, use 304 or 316 stainless steel; in high-precision machines, anodized aluminum is used to reduce inertia.

Assembly and Maintenance

• Preparation: Verify the coaxiality of the shafts with a dial indicator. The radial eccentricity between both shafts must be less than 0.03 mm / 0.001 in.
• Split-muff coupling: Loosen the fastening screws, slide the two halves over the shaft ends, place the key if applicable, and tighten the screws to the specified torque (typically 10–15 Nm / 89–133 lb·in for M4–M6). Check concentricity with a dial indicator after tightening.
• Flange coupling: Clean the flange faces and the guide shoulder, assemble the two halves, align the centering pilot, and tighten the bolts in a diametral sequence in two steps, first to 50 % of the final torque then to 100 %. The bolt tightening torque follows standards such as VDI 2230; for M12 8.8 bolts the torque is approximately 90 Nm / 66 lb·ft.
• Maintenance: Periodically inspect bolt tightening and the presence of rust. In environments with vibration, re-tighten after the first 500 hours of service. Keys should be checked every 2000 hours; if play is present, replace the entire coupling.

Application Selection Table

Application	Recommended coupling type	Typical torque range	Remarks
Centrifugal pumps directly coupled	Rigid flange	200 – 5,000 Nm / 147 – 3,688 lb·ft	Allows quick motor disassembly. Use centering shoulder.
Spindle to encoder transmission	Split-muff (steel or aluminum)	5 – 50 Nm / 44 – 443 lb·in	Assembly without moving the spindle, light, low inertia.
Long one-piece drive shafts	Split-muff with key	100 – 2,000 Nm / 73 – 1,475 lb·ft	For joining two sections of a shaft that must be integral.
Servomotors in robotics	Clamp coupling, anodized aluminum	10 – 100 Nm / 88 – 885 lb·in	Good stiffness-to-weight ratio, maintenance-free, simple disassembly.
Heavy steel industry machinery	Rigid flange with grade 10.9 bolts	5,000 – 30,000 Nm / 3,688 – 22,125 lb·ft	Oversized flanges and multiple keys are used. Requires laser alignment.

Frequently Asked Questions (FAQ)

What is the maximum torque that a steel flange rigid coupling can transmit for a 25 mm shaft?

The maximum continuous torque reaches 270 Nm / 2,390 lb·in if standardized keys and grade 8.8 bolts are used. In applications with shock loads, this value must be reduced by half.

What is the difference between a split-muff coupling and a solid sleeve coupling?

The split-muff consists of two halves joined with screws, allowing the coupling to be disassembled without moving the shafts. The solid sleeve requires moving one shaft longitudinally at least a distance equal to the sleeve length.

What materials are used in rigid couplings?

The most common are carbon steel (C45 or AISI 1045), 304/316 stainless steel for corrosive environments, aluminum 6061-T6 when low inertia is required, and occasionally GGG-40 nodular cast iron in large couplings.

What alignment accuracy does a rigid coupling require?

The maximum allowable radial misalignment is 0.05 mm / 0.002 in, and angular misalignment must not exceed 0.1°. Exceeding these limits generates cyclic overloads that can reduce the life of the nearest bearing by 80 %.

Can rigid couplings be used at high speeds?

Yes, provided the shafts are perfectly aligned and the coupling is dynamically balanced. For speeds above 3,000 rpm / 3,600 rpm, a balance grade G 6.3 according to ISO 21940 and a radial runout below 0.02 mm / 0.0008 in are recommended.

How is a flange coupling safely assembled without inducing stress?

All bolts must be tightened progressively in an opposing diametral sequence. For an 8-bolt coupling, tighten first to 50 % of the final torque (e.g., 45 Nm / 33 lb·ft), then to 100 % (90 Nm / 66 lb·ft). After tightening, verify with a dial indicator that the radial runout does not exceed 0.03 mm / 0.0012 in.

References

• engineeringtoolbox.com: https://www.engineeringtoolbox.com/flanges-bolts-dimensions-d_464.html
• mcmaster.com: https://www.mcmaster.com/products/rigid-shaft-couplings/