Introduction
Metal bellows couplings connect two shafts through a thin-walled flexible bellows element welded or bonded to hub halves. The bellows flexes to accommodate misalignment while maintaining a compact, largely metal construction suited to servo, encoder, CNC and precision automation drives where low backlash and high torsional stiffness relative to outside diameter are priorities.
This guide helps engineers narrow bellows coupling options for a specific duty. It complements the broader Industrial Coupling Selection Guide. Rated torque, speed, bore, torsional stiffness and misalignment limits are manufacturer-specific — confirm on the relevant family catalogue before ordering.
When bellows couplings fit
Bellows technology is typically selected when:
- The drive requires low or zero backlash for positioning, feedback or encoder duty.
- High torsional stiffness in a compact envelope is needed relative to bore size.
- Operating torque and speed are within catalogue limits for precision flexible couplings.
- The installation can respect misalignment within catalogue values for the selected size.
- A metal, largely non-elastomeric flexible connection is preferred for temperature or chemical reasons.
Bellows couplings may be less appropriate when very high torque industrial pump duty, maximum vibration damping, or large parallel offset without size penalty is required. Compare elastomeric or disc technologies for those profiles.
Key selection factors
| Factor | What to establish | Why it matters |
|---|---|---|
| Torque duty | Continuous and peak torque | Bellows wall stress limits catalogue rated torque per size |
| Speed | Operating RPM | Balance and critical speed affect allowable operating speed |
| Bore | Shaft diameters; clamping, press-fit or flange mounting | Hub style affects assembly and torque transmission |
| Torsional stiffness | Control-loop or positioning accuracy requirement | Stiffness values are size-specific; compare on family page |
| Backlash | Lost-motion limit for encoder or servo feedback | Bellows designs target low backlash; confirm on catalogue |
| Misalignment | Angular, parallel and axial requirements | Bellows accommodates misalignment through flexing; limits are manufacturer-specific |
| Axial compliance | Thermal growth or end-float | Limited compared to some elastomeric designs |
| Temperature | Operating range | Metal construction suits many environments; confirm if special materials needed |
| Mounting style | Clamping hub, flange, hollow shaft, conical bore | Affects hub selection within a family |
| Envelope | Outside diameter and overall length | Bellows couplings are compact but length grows with bore |
Precision motion and backlash
Bellows couplings are commonly specified for servo motors, stepper drives, encoders and CNC ball-screw connections where lost motion would degrade positioning accuracy. Key points at principle level:
- Backlash — bellows designs are intended for low-backlash duty; confirm rating on the family page for the exact model.
- Torsional stiffness — compare catalogue stiffness for shortlisted sizes; stiffness affects system natural frequency and control tuning.
- Encoder duty — compact bellows couplings connect feedback devices; verify bore, speed and stiffness for the feedback shaft load.
Do not assume one bellows family substitutes for another. KTR TOOLFLEX S and R+W BK/BKL are separate product lines with distinct catalogue sizing, hub options and engineering data.
Trade-offs vs other technologies
| Compared to | Bellows typical advantage | Bellows typical limitation |
|---|---|---|
| Elastomeric couplings | Higher stiffness; lower backlash; metal construction | Less damping; typically lower misalignment per unit size |
| Disc couplings | Very compact on small bores; excellent stiffness-to-diameter on servo sizes | May not offer the same spacer or high-torque industrial layouts as large disc families |
| Rigid couplings | Accommodates misalignment that rigid designs cannot | Less stiff than one-piece rigid connections |
These are technology-level comparisons, not interchange or replacement claims between KTR, R+W or any other brand.
GBC family routing
| Application profile | Consider this family | Notes |
|---|---|---|
| KTR metal bellows, servo catalogue sizes | KTR TOOLFLEX S | Governed torque, bore, speed, dimensions and torsional stiffness on family page |
| R+W BK metal bellows | R+W BK couplings | Clamping hub, flange, hollow shaft and conical mounting options |
| R+W BKL zero-backlash bellows | R+W BKL couplings | Catalogue sizes with rated torque and dimension data on dedicated pages |
The R+W hub also lists elastomer (EK), miniature (MK) and linear (LK) products that are not bellows couplings — route bellows selection to BK and BKL families only.
Selection checklist
- Confirm torque duty — continuous and peak torque against catalogue rated torque.
- Confirm speed — operating RPM against catalogue limit for shortlisted sizes.
- Define backlash and stiffness requirements — compare catalogue values for exact model.
- Measure bores — shaft diameters and preferred mounting style (clamp, press-fit, flange).
- Assess misalignment — angular, parallel and axial against family data for selected size.
- Check envelope — outside diameter, overall length and guard clearance.
- Shortlist one family — open governed catalogue and compare size, torque, bore and stiffness.
- Request quotation — send duty data through engineering enquiry.
Next steps
- Review bellows families on the couplings hub and R+W couplings hub.
- Open the family catalogue that best matches your bore, stiffness and mounting requirements.
- Compare governed size, torque, bore and stiffness data for shortlisted models.
- Send torque, speed, bores, backlash/stiffness requirement and mounting style for engineering confirmation.
For broader technology context, return to the Industrial Coupling Selection Guide. For higher-torque flexible duty with damping, see the Elastomeric Coupling Selection Guide.
Scope and limitations
- This article provides bellows technology selection guidance only; family pages own catalogue data.
- Misalignment limits, stiffness values and speed ratings must come from manufacturer catalogue or product record data.
- No universal misalignment or stiffness numbers are stated in this guide body.
- KTR TOOLFLEX S and R+W bellows families are not presented as interchangeable.
- Cross-brand interchange or replacement claims are not made.