Ball Screw Linear Actuator Sizing: The Six Numbers to Settle Before You Order
That is normal, and fixable. Sizing a ball screw linear actuator is not a talent, it is a checklist: six requirements, settled in the right order, each with a threshold or a calculation you can do on paper. This guide is that checklist. It pairs with our introduction to ball screw linear actuators, which covers what the units are and how they work; this one covers how to choose one.
The order, and why the order matters
Work the six in sequence: payload → motion profile → speed → precision → duty → environment. Each step narrows the field, and skipping ahead is what produces quotations for the wrong unit. Here is each step as a decision rule.
1. Payload: write the vector, not the kilogram
A payload is a vector and a lever arm, not a weight.
- Direction first. Horizontal: guides carry the load, the screw makes thrust. Vertical: the screw carries the load continuously, in compression — buckling and drop-braking enter the design, and the axis must be declared vertical at quotation time.
- Lever arm second. A center of gravity hanging off the carriage creates pitching and yawing moments (My, Mz). Every series publishes moment ratings, and off-center applications violate moments long before they violate payload ratings. Measure the CG offset before comparing models.
If the payload number in your head is only the workpiece weight, add the tooling, the gripper, and half the cabling. The rule of thumb we apply: if you cannot draw the load vector on the carriage sketch, the payload is not yet known.
For the underlying mechanics — why a catalog load rating is not the payload of a machine — see our load-capacity walk-through.
2. Motion profile: the datasheet the actuator cannot read
Convert the process into one cycle: distance, move time, dwell. From that single cycle come the three numbers everything else depends on — average speed, peak speed, and duty ratio. A machine that moves 300 mm every 2 seconds and a machine that moves 300 mm once a minute do not belong in the same actuator series, even though their "stroke" is identical.
The cycle also exposes acceleration: peak thrust happens during accel, not at constant speed. Note the accel time; step 3 needs it.
3. Speed: two ceilings and one inertia check
Peak linear speed must clear two independent limits:
- Critical speed — the screw's lateral resonance, which falls with the square of unsupported length. Long stroke + high rpm is where designs die; the worked numbers behind the limit are in our critical-speed explainer.
- DN limit — the nut's speed ceiling, which governs when the stroke is short and critical speed is not binding.
Then the check people skip: inertia matching. Payload and carriage mass reflected through the lead should sit within roughly 3–10× of the motor's rotor inertia. An actuator whose rated speed you never use because the motor cannot accelerate its own reflected load is a common and entirely avoidable mismatch. It is also why we ask for the motion profile rather than the model number: motor matching comes free with a quotation built from requirements.
4. Precision: buy the loosest grade that measures correct
Repeatability — returning to the same spot — and absolute accuracy — landing on the commanded spot — are separate specifications. Decide which one your process actually measures; production automation usually needs repeatability alone, and that realization alone often halves the budget.
The screw grade then follows from the number:
|
Repeatability target |
Typical grade path |
|
±0.05 mm or looser |
C7 rolled — the value tier |
|
±0.02 – ±0.01 mm |
C5 ground, or premium rolled at short strokes |
|
tighter than ±0.005 mm |
C3/C5 ground + preload class review |
Judge every grade claim at your stroke, because lead error accumulates over travel. The C5 vs C7 comparison breaks down what each grade changes — and what it doesn't.
5. Duty cycle: the number that decides the service life
Duty ratio — moving time over cycle time — drives heat. Heat thins the lubrication film in the nut and bearings, and wear follows. Two units of the same model, run at 20% and 70% duty, will age on different clocks. Declare the duty ratio honestly; it changes the recommended preload, lubrication interval, and sometimes the series itself. The maintenance side — grease type, interval, and the quiet way a dry nut fails — is covered in our lubrication guide.
6. Environment: decided at order time, not retrofitted
Dust, swarf, coolant mist and wood flour kill ball screws by contamination or by grease washout. Wipers, covers and bellows exist for each regime and cost a fraction at order time of what a contaminated nut costs at repair time. If the axis lives near a machining operation, specify the sealing class in the same sentence as the stroke.
Worked example: a vertical Z-axis, sized on paper in twenty minutes
Illustrative figures, not from any single customer project.
Requirements: vertical axis, lifting 25 kg fixture + 8 kg tooling, stroke 350 mm, up-and-down per cycle in 1.2 s, 2 s dwell → duty ≈ 0.55, repeatability ±0.03 mm, environment: light oil mist.
- Payload as a vector: 33 kg, vertical, in compression. Buckling check before anything else — at 350 mm + nut + bearings the column length stays short enough that a mid-diameter screw clears it with margin.
- Braking: a vertical axis must not drop on power loss → brake motor declared now, not at commissioning.
- Motion: 350 mm in ~0.8 s average → ~440 mm/s average, ~620 mm/s peak. A 10 mm lead at ~3,700 rpm peak is marginal; a 12–16 mm lead holds the peak under the nut's DN limit with room for the critical-speed check. The split between 5 and 10 mm leads — and why a middle value sometimes wins — is worked in the lead comparison.
- Acceleration: 33 kg at 0.5 g adds ~160 N to the thrust; peak thrust, not static load, sizes the motor.
- Precision: ±0.03 mm at 350 mm sits at the top of premium-rolled territory → C5 ground selected, consciously and with the price accepted.
- Duty & environment: 55% duty + oil mist → sealed-for-mist configuration, lubrication interval stated in the offer.
Six steps, twenty minutes, one defensible model — and a quotation that will not need a second round.
Five sizing traps, collected from real inquiries
- Quoting the model, not the motion — the inquiry that starts with a part number usually ends in a revised quotation.
- Vertical declared late — braking and buckling reshape the design; discovering it after the PO is the expensive version.
- Accuracy bought "just in case" — C5 everywhere when the process measures ±0.05 mm.
- Peak speed quoted, duty forgotten — the bearing outlives the warranty but not the shift pattern.
- CG offset ignored until the gripper drawing arrives and the moment ratings are already spent.
Frequently asked questions
What do I actually need to send for a sizing quotation? Payload with orientation and CG offset, one motion cycle (distance / move time / dwell), repeatability and accuracy targets separately, environment, and duty ratio. With those six, model choice and motor matching come back with the first offer.
Can stroke be customized on a catalog series? Yes — stroke is screw length and body length, not internal design. It is the easiest and cheapest customization in the category.
Does sizing change for multi-axis gantry builds? The per-axis sizing does not, but the Y-axis carries the X-axis actuator as additional moving mass — include it in the payload, and square the mounting surfaces.
What happens if I oversize "to be safe"? Cost, moving mass and motor size all rise, and the added mass works against your acceleration target. Right-sizing with margin beats oversizing every time.
General technical guidance — confirm all figures against the datasheet of the specific unit before design release.
Last updated: 2026-11-10
Send us the six numbers
Payload and orientation, one motion cycle, precision targets, duty, environment — and we will return a model recommendation with motor matching, from our single-axis linear stage range, or an honest recommendation of a bare ball screw where that fits better.