Nut Housings and Couplings: The Two Parts That Decide Whether the Axis Runs Smoothly
The screw is right. The support units are right. The axis still runs rough, the motor draws more current than it should, and the coupling has started to make a noise after three months.
Two components usually explain it, and both tend to get specified last: the nut housing that connects the screw nut to the moving carriage, and the coupling that connects the motor to the screw.
Neither is glamorous. Both are where misalignment either gets absorbed or gets converted into wear.
The Nut Housing
The housing transfers axial force from the screw nut into the load, and it does one other job that is easy to overlook: it defines the relationship between the screw axis and the carriage.
If the housing's bore is not concentric with the screw, or its mounting face is not square to the guideways, the nut is forced to travel along a path that does not match the screw. The nut cannot argue with the geometry, so it absorbs the error as a cyclic side load - once per revolution, for the life of the machine.
What that looks like in service: running torque that varies with position, uneven wear inside the nut, noise that appears at certain points in the stroke, and premature failure of a screw that was correctly specified.
Housing types
Maintenance-free housings incorporate a lubrication reservoir, useful where the axis is hard to reach or the maintenance schedule is unreliable.
Steel housings offer higher rigidity for heavier loads and are the usual choice where the axial force is significant relative to the carriage stiffness.
In both cases, the number that matters is not the housing's load rating - it is the concentricity and squareness of its machined faces relative to your mounting surface.
Mounting the Housing Correctly
The sequence matters more than the torque values.
- Establish the screw axis first, using the fixed-end support unit as the reference.
- Bring the carriage to the nut - do not force the nut to the carriage.
- Leave the housing bolts loose, run the axis through its full stroke, then tighten with the nut in a mid-stroke position.
- Recheck running torque across the full stroke.
Tightening the housing at one end of the stroke locks in whatever misalignment exists at that point. Tightening at mid-stroke distributes the error.
Smooth, consistent torque end to end is the acceptance criterion. A torque reading that climbs near one end tells you the axis is fighting itself there.
The Coupling
The coupling exists to transmit torque while absorbing the small misalignments that are unavoidable between a motor shaft and a screw shaft. How much it can absorb, and how stiffly it transmits torque, depends on the type.
Jaw / spider couplings use an elastomer element between two metal hubs. Good vibration damping, tolerant of modest misalignment, and forgiving in general-purpose applications. The elastomer introduces torsional compliance, which is not ideal where positioning stiffness matters.
Single diaphragm couplings are torsionally stiff with essentially zero backlash. They accommodate angular misalignment well but very little parallel offset.
Double diaphragm couplings add a second flexible element and a spacer, which lets them handle parallel offset that a single diaphragm cannot. This is the usual choice for servo-driven positioning axes where both stiffness and some alignment tolerance are needed.
Parallel slit (beam) couplings are machined from a single piece with a helical slot. Compact, zero backlash, inexpensive, tolerant of several misalignment types - but torsionally softer and limited in torque capacity. Common on small and miniature axes.
Oldham / cross couplings handle substantial parallel offset through a sliding centre element. Useful when the motor mount cannot be aligned precisely, at the cost of a wearing part.
Choosing Between Them
A short decision path that covers most cases:
- Servo positioning axis, tight accuracy → double diaphragm. Zero backlash and high torsional stiffness preserve the resolution the screw gives you.
- Stepper axis, general automation → parallel slit or jaw, depending on torque. Both are cost-effective and forgiving.
- Miniature axis → parallel slit, usually the only type available in the size.
- Shock loads or vibration present → jaw with elastomer, accepting the torsional compliance.
- Motor mount alignment is poor and cannot be improved → Oldham, and plan to replace the centre element periodically.
A coupling does not fix misalignment. It survives it. The distinction matters, because every coupling type has a published misalignment limit, and exceeding it transfers load into the motor bearings and the screw's fixed-end bearings.
The Backlash Question
If your axis needs repeatable bidirectional positioning, a coupling with backlash will show up in the measurement. An elastomer jaw coupling has some compliance; a worn Oldham has real lost motion.
Diaphragm and slit couplings are zero-backlash by construction. If you have specified a C3 or C5 screw and preloaded the nut to eliminate axial play, do not then connect it with a coupling that reintroduces lost motion at the motor end. It happens more often than you would expect, usually because the coupling was chosen on bore size and price.
Sizing
Select on continuous torque with a service factor, not on peak torque alone, then confirm:
- Bore sizes at both ends match the motor shaft and screw shaft end
- Keyway or clamp style suits both shafts
- Overall length fits between motor face and screw journal
- Published misalignment limits exceed what your mounting can realistically achieve
- Torsional stiffness is adequate for the servo bandwidth you intend to run
That last point is the one most often skipped on servo systems. A coupling that is too compliant limits how high you can set the gains before the axis becomes unstable - the machine works, but never quite as well as the specification promised.
Specify Them Together
Screw, support units, nut housing, and coupling form one mechanical chain. Specifying them separately from different suppliers is where the interface dimensions stop matching.
Send us the axis rather than the part: stroke, load, speed, motor frame and shaft dimensions, carriage mounting interface, and accuracy target. We will confirm the whole chain matches before anything is machined.
Tianjin OuNaiDa Transmissions Technology Co., Ltd. manufactures ball screws, support units, nut housings, couplings, linear guideways, gears, racks and sprockets. Established 2008, with 20 years of manufacturing and 15 years of export experience serving over 300 overseas distributors, to CE and ISO 9001:2015 standards.
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