Published:2026-10-19 00:00:00
Updated:
Linear actuators · Servo connections
From motor rotation to a settled carriage
A smooth positioning move depends on the complete drive chain. Looking at the coupling between the motor, screw and load gives stiffness, damping and alignment requirements a practical context.
01 / Duty and trade-offs
Start with the station cycle
Picture a dispensing axis: it accelerates, approaches the part, settles and begins dispensing. The coupling transmits acceleration torque and contributes to the torsional response. Several couplings may fit the same shaft diameter, yet behave differently under changing load, alignment error and settling requirements.
Torque and speed
Identify continuous, accelerating, reversing and braking conditions, including peak duration and cycle count.
Stiffness and damping
Evaluate the process point together with the screw, supports, load and servo settings.
Interfaces and alignment
Check shaft diameters, grip length, shaft-end separation and adjustment space before accepting misalignment limits.
This guide concerns servo-drive connections for linear actuators. Confirm the coupling model and supply scope separately; the actuator family alone does not specify them.
02 / Design comparison
The two designs place flexibility in different elements
A disc coupling accommodates specified misalignment through elastic flexure of metal disc packs and is commonly considered when torsional stiffness matters. A jaw coupling transmits torque through an elastomer between its hubs. Elastomer material and hardness affect stiffness, damping and temperature suitability. Distinguish preloaded, backlash-free positioning designs from ordinary jaw couplings with clearance.
| Aspect | Disc coupling | Jaw coupling |
|---|---|---|
| Torsional response | Typically high torsional stiffness, but finite elastic twist remains. | The elastomer provides torsional compliance and damping; stiffness depends on design and conditions. |
| Backlash | Confirm both disc connections and shaft-to-hub connections. | For positioning, verify a preloaded backlash-free design; this is not universal to jaw couplings. |
| Misalignment | Single- and double-flex designs differ. Parallel offset generally requires a double-flex arrangement. | Check each permitted direction and the combination rules for the actual model. |
| Environment and care | Inspect disc packs, fasteners and connections within fatigue and alignment limits. | Check temperature, fluid compatibility, wear and replacement conditions for the insert. |
| Selection emphasis | Compare actual stiffness and complete-axis dynamics when the twist budget is tight. | Compare elastomer options and measured settling where shock or vibration needs attenuation. |
These are design characteristics, not a universal ranking. Compare actual model data under the same duty: stiffness, inertia and displacement limits vary within each family.
03 / Worked example
Backlash-free still allows elastic twist
For small deformation and approximately linear stiffness, transmitted torque produces a relative angular displacement. With the coupling directly connected to a screw in a 1:1 drive, this can be converted into equivalent linear displacement:
θ = T / Kt δx = pθ / (2π) = pT / (2πKt)
T: transmitted torque, N·m; Kₜ: torsional stiffness, N·m/rad; θ: relative angle, rad; p: screw lead, mm/rev; δₓ: equivalent linear displacement, mm.Take p = 10 mm/rev and T = 3 N·m. The following stiffness values are hypothetical. They do not represent the two coupling families or HZMotion product ratings.
| Assumed stiffness Kₜ | Relative angle θ | Equivalent displacement δₓ |
|---|---|---|
| 1,000 N·m/rad | 0.003 rad | About 4.77 μm |
| 3,000 N·m/rad | 0.001 rad | About 1.59 μm |
The example turns a request for more stiffness into a displacement budget. If changing external load changes torque, the same approximation gives Δδx = pΔT/(2πKt). Real elastomers can be nonlinear, and catalog static and dynamic stiffness values are not interchangeable.
A motor encoder usually does not directly measure all elastic displacement downstream of the coupling. Linear feedback can correct some deviation within its bandwidth and control limits, but dynamic and stability constraints remain. Screw, bearing and structural deformation and other transmission ratios are excluded here.
04 / Selection sequence
Meet operating limits, then compare dynamics
| Step | Inputs | Decision |
|---|---|---|
| 1 · Torque | Duty cycle; acceleration, reversal and braking torque; peak duration and cycle count. | Follow the selected coupling maker’s rules for continuous, peak and alternating duty, including applicable temperature and service factors. |
| 2 · Speed and inertia | Highest actual speed; inertia of hubs and connecting parts. | Check speed limits for the bore and connection type; include coupling inertia in motor sizing. |
| 3 · Stiffness and settling | Load variation, displacement budget and process-start criterion. | Estimate elastic twist, then verify settling and vibration using the actual motion profile. |
| 4 · Misalignment and reactions | Radial, angular and axial displacement, cold and running. | Check combined displacement and additional bearing loads. Catalog limits are not alignment targets. |
| 5 · Interfaces and environment | Both bores, fits, shaft extensions, keys and spacing; temperature and fluids. | Verify grip, tightening and removal access, plus insert or disc-pack suitability. |
If two candidates satisfy torque requirements, record settling at the process point using the same load, speed, acceleration and initial control settings. Then optimise each candidate and retain the settings. Coupling type alone cannot guarantee cycle time. Read this alongside servo-motor inertia matching and short-stroke cycle time and settling.
05 / Installation
Leave each shaft in its intended position
Put the complete dimensional chain on one interface drawing: motor shaft extension, actuator shaft extension, bracket length, coupling grip length and specified shaft-end separation.
- Verify before clampingCheck hub positions, bore fits, insertion lengths and shaft-end separation. Prevent the shaft ends from bottoming against each other; do not pull misaligned parts into place with clamping screws.
- Align and recheckMeasure radial and angular alignment using suitable instruments; set axial spacing to the instructions. Apply the requirements for the actual single-flex, double-flex or elastomer design.
- Use the specified tightening methodFollow the selected coupling’s sequence and torque. Shaft surface condition, keys and clamp screws have different requirements; do not transfer instructions between connection designs.
- Validate from low speed to working dutyFollow the machine commissioning procedure to check starting torque, rotational resistance, temperature and vibration. Verify reversals and process-point settling, and retain assembly and tuning records.
If rotational resistance increases after tightening, revisit the interfaces, shaft-end separation and alignment. A more flexible coupling cannot replace correction of installation errors. See the motor-bracket interface checklist and shaft-alignment inspection guide.
06 / Technical brief
Prepare the inputs for a useful comparison
| Item | Provide |
|---|---|
| Product interfaces | Complete actuator model, lead and bracket drawing; motor model, shaft diameter, extension and keyway. |
| Motion task | Orientation, load, travel, speed, acceleration, start frequency and complete cycle. |
| Performance | Process-point tolerance, settling requirement, feedback arrangement and current commissioning results. |
| Environment and service | Working temperature, fluids, enclosure space, removal method and spare-part needs. |
| Confirm explicitly | Coupling model, insert or disc-pack configuration, permitted misalignment, tightening instructions and supply scope. |
Use the same brief for a new axis or a replacement motor. Interface details and duty data together identify configurations that can actually be installed and operated.
07 / Questions
Coupling selection questions
Does a precision actuator always need a disc coupling?
No. Review the actual model’s stiffness, backlash, torque, misalignment capability and complete closed-loop behaviour. Acceptance at the process point is more meaningful than the coupling family name.
Do all jaw couplings have backlash?
No. Positioning designs can use a preloaded, backlash-free insert, while still exhibiting elastic deformation. Confirm the actual insert, hub connection and assembly conditions.
Is a larger misalignment allowance an installation target?
No. It is a capability with operating limits. When multiple displacements coexist, use the selected model’s combined-misalignment rules.
Does higher torsional stiffness always shorten settling?
No. Stiffness changes system dynamics, while settling also depends on inertia, damping, other flexible elements and servo settings. Validate the complete cycle.
Does the actuator model define the supplied coupling?
Not by itself. Confirm both shaft interfaces, connecting length, load and environment, and record the coupling model and supply scope in the technical agreement.