A Thomas flexible disc coupling connects rotating shafts through metallic flexible elements. For the simplified double-flex spacer geometry here, a parallel axis offset delta across flex-plane spacing L corresponds to geometric slope angle atan(delta/L); actual permissible misalignment is coupling- and application-specific.
Resources for separate linear-actuator installations
A rotating-shaft disc coupling and a linear actuator serve different functions. For a separate linear-actuator installation, guide the moving member with the machine structure and check its own force, stroke and mounting geometry.
For structural members, the actuator load should be applied through brackets that do not introduce unintended bending. Check both static load and the extra load created when the mechanism starts, stops, or binds.
For moving panels, hatches, guards, and slides, the actuator should not be the only support. Rails, hinges, bearings, or guides should carry side loads so the actuator shaft remains in push-pull service.
- Keep side load out of the actuator rod by guiding the moving member separately.
- Verify pin diameter, clevis width, bracket stiffness, and alignment under load.
- Check that deflection will not change the actuator load path during operation.
For FIRGELLI actuator projects, compare product families through the FIRGELLI actuator families and use the linear actuator replacement finder when replacing an actuator from another manufacturer. The selection should stay tied to real stroke, force, and mounting dimensions.
Thomas Coupling Interactive Calculator
Enter parallel shaft-axis offset, flex-plane spacing and an angle comparison limit. Results describe ideal geometry only and do not select, rate or approve a Thomas coupling.
Equation Used
- The two shaft axes are parallel and separated by the entered offset.
- The spacer is a rigid straight member between idealized flex-plane centers separated axially by L.
- Both ends accommodate the same geometric spacer slope relative to their parallel shaft axes.
- Disc compliance, torque, speed, axial displacement, combined misalignment, forces, fatigue, critical speed, tolerances and installation limits are omitted.
Metallic Flexible Elements and a Spacer
Regal Rexnord describes Thomas couplings as mechanical connections between rotating equipment shafts that use flexible discs to accommodate inherent misalignment while transmitting torque. Its XTSR52 spacer style has two hubs, a center spacer and flexible-element hardware at both ends.
The diagram is a two-dimensional geometric idealization. It shows parallel axes separated by delta and flex planes separated by L; it does not reproduce disc shape, bolt loading or a specific product size.
What This Explanatory Calculator Does
API 671 covers special-purpose couplings intended to transmit power while accommodating specified shaft displacement. Manufacturer manuals then provide model- and size-specific installation limits.
This calculator performs only a geometry comparison. It does not choose a coupling series or size, combine angular, parallel and axial displacement, or assess torque, speed, balance, fatigue or machinery loads.
Ideal Parallel-offset Geometry
delta_limit = L tan(theta_limit)
ratio = theta/theta_limit
Here L is the axial distance between idealized flex planes—not the overall coupling length. Both shaft axes are parallel, so the rigid spacer has the same slope magnitude relative to each axis. No factor of two belongs in this definition.
Worked Geometry Example
For delta = 0.5 mm and L = 114.6 mm, theta = atan(0.5/114.6) ≈ 0.250 degrees. Against an entered comparison limit of 0.25 degrees, delta_limit ≈ 0.500 mm, the ratio is about 100%, and the arithmetic margin is approximately zero.
This numerical match does not mean 0.25 degrees is the rating of any particular Thomas coupling.
Model Boundary
- Calculated: geometric spacer slope, offset corresponding to an entered angle, their ratio and an arithmetic offset difference.
- Required geometry: parallel shaft axes, rigid straight spacer and known flex-plane spacing.
- Not calculated: actual disc-pack deflection distribution, combined angular/parallel/axial misalignment, torque, speed, cyclic stress, bolt load, fatigue, balance, lateral critical speed, bearing reactions, thermal growth, installation tolerance, guard design, product selection, inspection interval or safety.
Frequently Asked Questions
Is the entered angle limit a Thomas product rating?
No. It is a user-entered comparison value; consult current data for the exact series and size.
Why is the angle atan(delta/L)?
The idealized spacer rises by delta over axial flex-plane spacing L.
Should the offset formula contain a factor of two?
Not with L defined as the full distance between flex planes and delta as total parallel axis offset.
Does the calculator include axial displacement?
No. Axial movement and combined misalignment are omitted.
Can it select a coupling?
No. Selection also requires manufacturer procedures and application torque, speed, service, bore, balance and displacement data.
Does a positive margin mean the installation is safe?
No. It is only a difference from the entered geometric comparison value.
References
- Regal Rexnord, Thomas XTSR52 Disc Couplings Installation and Maintenance: function, components and model-specific installation requirements.
- Regal Rexnord, Thomas Flexible Disc Couplings Product Catalog: coupling types, selection procedure and product-specific capacities.
- American Petroleum Institute, Digital Catalog, API Standard 671: special-purpose couplings for petroleum, chemical and gas-industry services.
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