Comparison · Coupling selection

Servo coupling selection: jaw vs. beam vs. bellows vs. disc vs. Oldham

Q: Jaw or beam coupling for a small servo?

For positioning accuracy (encoders, ball-screw drives) use a beam, bellows or disc coupling — zero backlash and high stiffness. For damping, larger misalignment and ordinary accuracy use a jaw coupling. The JM series with its pre-loaded spider is also backlash-free and makes a good compromise.

Q: Which coupling for a ball-screw axis?

Small-to-medium torque with a stiffness requirement: bellows. Higher torque or speed: disc. Encoder or light duty only: beam. What they share is zero backlash and high torsional stiffness, so the servo gain can be raised without oscillation.

Q: Does zero backlash mean no torsional deflection?

No. Zero backlash means no lost motion on reversal; torsional stiffness sets the elastic wind-up angle under load. Jaw and Oldham couplings are backlash-free but low in stiffness — a positioning axis needs both figures.

Q: Can you supply vacuum-rated bellows couplings?

Yes. Welded SUS304 or SUS316L bellows, vacuum-tested in-house to 10⁻⁶ Pa before shipment and individually sealed, for semiconductor and vacuum equipment.

A coupling is never just "a coupling": it is a four-way trade between misalignment tolerance, torque capacity, torsional stiffness and backlash. The five constructions common on servo and stepper drives each occupy a different corner of that trade. This guide compares them side by side and then gives a selection sequence that runs by positioning accuracy, misalignment type and torque.

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Comparison · Published 2026-09-12 · Wolfer Transmission

What each of the five does best

Jaw coupling (JM / LM): two jaw hubs with an elastomer spider between them — damps vibration and shock, simple, replaceable element, stiffness tuned by spider hardness (Shore A 80 / 92 / 98). JM is the zero-backlash servo series with a pre-loaded spider; LM is the GB/T 5272 general series and carries a little backlash (< 5 arc-min) and torsional give. Suits pumps, fans and general motor drives, and servo axes that need damping.

Beam / slit coupling (JT): one metal piece with helical slits as the flexure — zero backlash, compact, low inertia, compensates angular and a little radial misalignment, but low torque capacity; the standard part for encoders, small servos and light precision drives. Bellows coupling (JB): a metal bellows as the flexure — high torsional stiffness, zero backlash, compensates misalignment in every direction; the first choice for high-accuracy positioning, servo, metrology and semiconductor equipment, and available as a vacuum-rated welded stainless bellows.

Disc coupling (TM / DJM / SJM): single or double metal disc packs carry the torque — lubrication-free, maintenance-free, zero backlash, high-temperature capable, suited to high-speed, high-power servo and pump drives; a single disc pack takes angular and axial misalignment, and the double-disc versions add radial. Oldham coupling (JH): three pieces, two slotted hubs and a floating centre disc — the largest radial-offset capacity, zero backlash, replaceable centre, but lower torsional stiffness; for servo / stepper drives whose shafts have a noticeable parallel offset.

Weighing the four dimensions

Backlash decides positioning accuracy beyond what the encoder loop corrects — beam, bellows, disc, Oldham and the JM jaw are backlash-free, the LM jaw has a little. Torsional stiffness sets the servo's response and resonance frequency — disc and bellows are highest, beam in the middle, jaw and Oldham lowest (which is also where their damping comes from). Misalignment: typical capacity for Wolfer flexible couplings is axial ± 0.4 mm, radial ± 0.15 mm and angular ± 1°; for large radial offset look to Oldham or double-disc types. Torque and speed: select from the rated torque and maximum speed in the selector-page tables, with peak torque times a service factor kept below the rating.

The five servo couplings compared
Criterion	Jaw (JM / LM)	Beam / slit (JT)	Bellows (JB)	Disc (TM / DJM / SJM)	Oldham (JH)
Backlash	JM zero; LM < 5 arc-min	Zero	Zero	Zero	Zero
Torsional stiffness	Low (tuned by spider hardness)	Medium	High	Highest	Low
Angular misalignment	Good	Good	Good	Good (single / double disc)	Fair
Radial misalignment	Fair	Small	Good	Double disc	Best
Axial misalignment	Fair	Small	Good	Good	Fair
Damping	Good	Poor	Poor	Poor	Fair
Torque range in Wolfer tables	1.4–25 000 N·m	0.3–22 N·m	0.8–130 N·m	From 1.4 N·m up to very large sizes	0.7–33 N·m
Temperature / environment	Limited by the elastomer	All-metal	All-metal, vacuum-rated option	All-metal, high temperature	Limited by centre-disc material
Typical use	Pumps, fans, general servo	Encoders, small servos	Precision positioning, semiconductor, metrology	High-speed high-power servo, pumps	Servo / stepper with parallel offset

Torque ranges are taken from the size tables on the selector pages; rated / peak torque, maximum speed and misalignment capacity of a specific size are in its table.

The selection sequence

(1) Start with the axis's job: an axis with encoder feedback and a positioning requirement is chosen from the zero-backlash, high-stiffness group — beam for low torque, bellows for medium torque and higher stiffness, disc for high speed and power. (2) Then the misalignment: a clear parallel offset after assembly calls for an Oldham or double-disc coupling; angular-only misalignment is covered by a single disc or a bellows. (3) Vibration, shock and frequent starts (pumps, fans, general motors): a jaw coupling, with stiffness set by spider hardness. (4) Environment: high temperature — disc; vacuum or cleanroom — stainless bellows; corrosive — stainless parts. (5) Finally check the geometry: shaft diameters and lengths on both sides, outside-diameter and overall-length limits, clamp or keyed hubs — Wolfer machines the finished bores and keyways to your shafts and makes non-standard variants to drawing.

FAQ

Common questions on this topic

Jaw or beam coupling for a small servo?

For positioning accuracy (encoders, ball-screw drives) use a beam, bellows or disc coupling — zero backlash and high stiffness. For damping, larger misalignment and ordinary accuracy use a jaw coupling. The JM series with its pre-loaded spider is also backlash-free and makes a good compromise.

Which coupling for a ball-screw axis?

Small-to-medium torque with a stiffness requirement: bellows. Higher torque or speed: disc. Encoder or light duty only: beam. What they share is zero backlash and high torsional stiffness, so the servo gain can be raised without oscillation.

Does zero backlash mean no torsional deflection?

No. Zero backlash means no lost motion on reversal; torsional stiffness sets the elastic wind-up angle under load. Jaw and Oldham couplings are backlash-free but low in stiffness — a positioning axis needs both figures.

Can you supply vacuum-rated bellows couplings?

Yes. Welded SUS304 or SUS316L bellows, vacuum-tested in-house to 10⁻⁶ Pa before shipment and individually sealed, for semiconductor and vacuum equipment.

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