# Shaft couplings

Rigid and flexible shaft connections, torsional behavior, misalignment, hub interfaces, and a bounded ROTEX GS sizing example.

Reviewed: 2026-10-31

Tags: components, mechanical, couplings, power-transmission

At assumed steady **2.0 kW** and **1,500 rpm**, shaft torque is **12.73 N·m**. Applying an illustrative factor of **2.0** gives **25.46 N·m**, below the cited size-24 KTR spider's **60 N·m** rating and the type-2.5 hub's **45 N·m** friction torque at a **20 mm** bore. Check transients, shaft fit, temperature and combined misalignment for the assembly. [KTR instructions, pp. 4–8, tables 1, 2, 4 and 5](https://www.ktr.com/fileadmin/ktr/media/Manuals/45510en000000.pdf#page=4).

Torque passes through the intermediate member and both shaft–hub joints. Also check positioning error, bearing reactions and replacement access.

## Rigid and flexible families

**Rigid couplings** connect shafts without a deliberately compliant intermediate member. Huco offers one-piece and split two-piece clamping designs; the split version permits removal without dismantling the machine. Rigid construction depends on the shafts remaining aligned. Huco cautions that fixed-axis shafts can move relative to one another through thermal changes, creep, or settlement, even after initial alignment. [Huco rigid couplings](https://www.huco.com/products/couplings/rigid-couplings); [Huco zero-misalignment guidance](https://www.huco.com/design-guide/selecting-for-zero-misalignment).

**Flexible couplings** use different mechanisms rather than one interchangeable form of flexibility:

| Family | Mechanism and selection consequence |
| --- | --- |
| Jaw with elastomer spider | Deformation between metal hubs accommodates displacement; stiffness and damping depend on the selected element. |
| Bellows | Corrugated metal flexes across shaft offsets while retaining high torsional stiffness. |
| Disc | Thin membranes flex between driving and driven attachments. |
| Beam or slit | Slots machined into a single member provide compliance. |
| Oldham | A central disc slides across hub tenons to accommodate offset. |

These descriptions follow [Huco's selection guide](https://www.huco.com/design-guide/selection-criteria). Its Oldham description includes preloaded designs intended to eliminate backlash: the family name alone does not establish backlash, material, life, or suitability for a particular duty.

## Torsion is different from misalignment

Axial displacement changes shaft-end separation; radial displacement offsets the axes; angular displacement tilts one axis relative to the other. Torsional deflection is rotation under transmitted torque. A coupling can therefore be flexible across shaft displacement and stiff about its rotational axis. R+W describes precisely this combination for metal bellows, while emphasizing that sizing and handling remain necessary. [R+W bellows coupling basics](https://www.rw-america.com/bellows-coupling-basics/).

Backlash is lost rotational motion when direction reverses; elastic twist can occur even without that clearance. For a linearized torsional stiffness `Cₜ`, the engineering estimate is `θ = T / Cₜ` in radians. Do not substitute a dynamic stiffness or a catalog stiffness measured at a particular load into every operating condition. R+W also considers drive/load inertia and excitation frequency when assessing acceleration and resonance. [R+W sizing discussion](https://www.rw-america.com/bellows-coupling-basics/).

## A specific jaw-coupling envelope

**KTR ROTEX GS size 24, 98 ShA-GS spider, double-slot clamping hubs type 2.5**; KTR-N 45510 EN, **edition 26, 2025-09-01**:

| Property | Value and boundary |
| --- | --- |
| Spider rated torque `TKN` | 60 N·m |
| Spider maximum torque `TKmax` | 120 N·m; distinct from the rated value |
| Type 2.5 maximum finish bore | 28 mm |
| Overall length `L`; hub lengths `L1`, `L2`; spacing `E` | 78 mm; 30 mm each; 18 mm |
| Hub outside diameter `DH` | 55 mm |
| Friction torque `TR`, type 2.5 at a 20 mm bore | 45 N·m |

Spider torque and shaft-hub capacity differ. Type 2.6 has a keyway; it is a different interface. [KTR instructions, pp. 4–8, tables 1, 2, 4 and 5](https://www.ktr.com/fileadmin/ktr/media/Manuals/45510en000000.pdf#page=4).

The separate, undated **ROTEX GS catalog extract, printed pp. 118–120**, lists size-24 type-2.5 speed as **6,950 rpm**, versus different limits for other hub types. It lists the 98 ShA-GS spider's permanent temperature range as **−30 to +90 °C**. These are catalog-specific boundaries, not permission to combine maximum speed, torque, temperature, and displacement. The extract directs readers to updated product data; its legacy hazardous-area statements do not establish current assembly approval. [KTR catalog extract, pp. 119–120](https://www.ktr.com/fileadmin/ktr/media/Tools_Downloads/kataloge/03_backlash-free_jaw_couplings_ROTEX_GS.pdf#page=2).

## Worked torque screen

Assume an illustrative steady **2.0 kW mechanical output at 1,500 rpm**, with no transmission loss between that measurement point and the coupling. From `P = Tω`:

`T = 2,000 / (2π × 1,500 / 60) = 12.73 N·m`.

An **assumed screening multiplier of 2.0** gives `25.46 N·m`. This is below both the 60 N·m spider rating and the 45 N·m catalog friction torque for the stated 20 mm hub bore. The comparison illustrates why the lower interface capacity matters; it does not select a complete drive. The multiplier is an example assumption, not a KTR-prescribed service factor. R+W's bellows selection discussion uses load factors according to dynamics, demonstrating why factors belong to a manufacturer's selection method rather than a universal coupling rule. [R+W sizing discussion](https://www.rw-america.com/bellows-coupling-basics/).

Motor peak/deceleration torque, reversals, emergency stops, actual shaft tolerances and engagement, temperature corrections, and simultaneous displacement still need evaluation. The 120 N·m spider maximum cannot become a continuous operating allowance or override the weaker hub connection.

## Alignment, installation, and protection

KTR table 14 gives **0.10 mm radial** and **0.9° angular** displacement for this size/spider, at load up to `TKN` and **+30 °C**. Simultaneous radial/angular offsets use proportional allowances. Accurate alignment remains necessary. [KTR, pp. 27–28](https://www.ktr.com/fileadmin/ktr/media/Manuals/45510en000000.pdf#page=27).

Use the supplied hub's bore, insertion, spacing, and tightening instructions. Flexibility does not replace a rotating guard: KTR requires contact protection before start-up. Neither catalog fit nor this example establishes machinery-standard compliance or hazardous-area approval. [KTR, p. 29](https://www.ktr.com/fileadmin/ktr/media/Manuals/45510en000000.pdf#page=29).

## Related reference

[Production planning references](../../production/planning/index.md)