Ball Screw End Support Configuration: Why the Same Screw Has Different Speed Limits
A customer called us last year about a 2005 screw that vibrated badly above a certain speed. The screw was fine. The nut was fine. Same part number as the machine next to it, which ran perfectly.
The difference was 400 mm of unsupported length.
A ball screw's maximum safe speed and maximum compressive load are not properties of the screw alone. They depend heavily on how the two ends are held. This is one of the least discussed parts of ball screw selection, and one of the most common reasons a correctly specified screw underperforms.
The Three Configurations
Supported – Supported
Both ends carry radial load only. Neither end fixes the screw axially.
Simplest and cheapest to build. Lowest rigidity, lowest critical speed. Fine for short vertical axes and light loads where the nut is doing little axial work. Rarely the right answer on a cutting machine.
Fixed – Supported
One end fixed for axial positioning, the other free to float.
This is the standard configuration and covers the majority of CNC and automation axes. It handles axial load properly at the fixed end while letting the screw expand freely as it warms. Typical implementation: BK at the fixed end, BF at the supported end.
Fixed – Fixed
Both ends fixed, usually with the screw pre-tensioned during assembly.
Highest critical speed, highest axial rigidity, best thermal behaviour over long strokes. It is also the most demanding to build - it needs correct bearing selection at both ends, controlled pre-tension, and tighter alignment than the other two.
One Point Worth Being Blunt About
Bolting a standard fixed-end unit at each end does not create a fixed-fixed system.
A designed fixed-fixed axis has a specified pre-tension. Two independent fixed ends without that pre-tension will simply fight each other - and as the screw warms and grows, it has nowhere to go. The result is rising torque, accelerated bearing wear, and sometimes a bent screw.
If you want fixed-fixed, it has to be designed as fixed-fixed from the start.
Critical Speed: Unsupported Length Dominates
Critical speed is the rotational speed at which the screw shaft enters resonance and begins to whip. Approaching it produces vibration, noise, and eventually damage.
The relationship follows this form:
Nₜ = k × (d / L²) × constant
where d is the screw root diameter, L is the unsupported length between bearing supports, and k is a factor set by the end support configuration.
Two things follow from that equation, and both matter in practice:
Length is squared. Doubling the unsupported length cuts critical speed to roughly a quarter. This is why the customer's two identical screws behaved so differently - 400 mm of extra span is not a small change.
The support factor k is configuration-dependent. Fixed-fixed carries the highest k value, fixed-supported sits in the middle, supported-supported the lowest. The same screw at the same length has a meaningfully different speed ceiling depending only on how it is held.
Alongside critical speed, check the DN value - nominal diameter in mm multiplied by rotational speed in rpm. Each nut and return system design has its own DN limit, and it is an independent constraint. A screw can be comfortably below its critical speed and still exceed the nut's DN rating.
Manufacturers publish critical speed and DN limits for their own products. Use the published figures for the screw you are actually buying rather than a generic formula, and keep a working margin below the stated limit.
Buckling: The Compression Problem
A ball screw in compression can buckle, and the governing factors are the same two: unsupported length and end fixity.
Buckling load depends on the screw's root diameter - not the nominal diameter - the unsupported length, and the end support configuration. Fixed ends resist buckling far better than supported ends.
Common industry guidance is to stay below roughly 80% of the calculated critical buckling force. Confirm the specific figure against the screw manufacturer's data for your model.
Buckling is mainly a concern on horizontal axes with high thrust and on vertical axes where the screw pushes rather than pulls. On a long horizontal axis, this is frequently the constraint that decides screw diameter - not the load rating of the nut.
What This Means When You Are Selecting
The sequence that avoids trouble:
- Start from stroke and layout. These determine unsupported length, which drives everything else.
- Check critical speed against your required rapid traverse rate, using the manufacturer's published figures.
- Check the DN value against the nut's rating - separately.
- Check buckling if the axis works in compression.
- Then choose the configuration and support units.
If critical speed comes up short, you have three options, roughly in order of cost:
- Reduce the unsupported length - add an intermediate support if the layout allows
- Move to a stiffer configuration - fixed-supported to fixed-fixed
- Increase the screw diameter
Increasing diameter is the most common instinct and usually the most expensive. Shortening the span or upgrading the configuration often solves the problem for less.
A Practical Note on Retrofits
When a customer replaces a screw on an existing machine, the support configuration is usually inherited from the original design. That is reasonable - but it is worth asking whether the original design was right, particularly if the machine has a history of vibration at speed or the previous screw showed uneven wear.
If you are quoting a replacement and the machine has always had a speed limitation the operator works around, the configuration is worth a look before ordering an identical part.
Information That Helps Us Help You
When you send an enquiry for a ball screw, these details let us check the configuration alongside the part:
- Total length and effective stroke
- Required maximum traverse speed
- Axis orientation - horizontal, vertical, or inclined
- Axial load, and whether it is tension or compression
- Existing support units, if this is a replacement
We will flag it if the requested screw is likely to run into a speed or buckling limit in your layout. Catching it before the order is placed costs nothing; catching it after the machine is assembled costs a great deal.
Tianjin OuNaiDa Transmissions Technology Co., Ltd. manufactures ball screws, support units, linear guideways, gears, racks, sprockets, and couplings. Established 2008, with 20 years of manufacturing and 15 years of export experience, working to CE and ISO 9001:2015 standards.
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