Ball Screw Life and Load Rating: L10, Dynamic Load C, and Static Load C0 Explained

Ball screw life is not a vague "how many years it will last". Suppliers quote by calculating the rated life L10 from the dynamic load rating C and your actual axial load. If the RFQ does not state load, speed, and life targets, the supplier can only estimate, which ends in either an under-sized screw that wears early or an over-sized screw that costs too much. This guide explains L10, C, and C0 and the four parameters your RFQ should carry.

What rated life L10 means: 90% survive 1,000,000 revolutions

L10 is the industry life benchmark: out of a batch of identical ball screws running under the same load and lubrication, 90% will complete 1,000,000 revolutions before material fatigue flaking appears on the raceway or balls. The subscript "10" means 10% failure is allowed, that is, 90% survival. It does not promise the exact life of one screw; it is a reliability figure for the batch:

Life in revolutions: L10 = (C / Fm)^3 x 10^6 revolutions.
C is the dynamic load rating (N) from the catalog; Fm is the equivalent axial load (N).
Life in hours: Lh = L10 / (n_m x 60), where n_m is the average speed in rpm.

Example: with C/Fm = 7.2, L10 = (7.2)^3 x 10^6 ≈ 373 x 10^6 revolutions; at an average 500 rpm that is about 12,400 hours. Compare any two duty cases by applying the same formula.

The cubic load law: a small load increase cuts life sharply

Life drops with the third power of the load, which is the most counter-intuitive part of ball screw sizing:

Change in axial load Fm	Relative life	Intuition
Base load	1x	Design baseline
Load x1.26	About 0.5x	Life halves with +26% load
Load x2	About 0.125x	Doubled load leaves about 1/8 life

So understating the load in an RFQ is risky: reporting 20% less load can overstate life by more than double, and the real part fails early. Include moving mass, cutting or clamping force, acceleration, and a reasonable impact allowance, and round the estimate upward.

Dynamic load rating C versus static load rating C0

Both C and C0 are printed in catalogs, but they verify different situations:

Item	Dynamic load rating C	Static load rating C0
Load condition	Rotating, continuous cycling	Static or very slow (≤10 r/min)
Failure basis	90% no fatigue flaking after 10^6 rev	No permanent raceway deformation
Check	Equivalent load Fm in the life formula	Fmax ≤ C0 / fs
Safety factor fs	Included in the cubic formula	Usually 2 to 5

Applications that "move little but push hard" - low-speed pressing, lift holding, clamping and locking - are governed mainly by C0; high-speed reciprocating feed is governed by C. Describe the duty in the RFQ so the supplier knows whether the axis cycles continuously or holds intermittently.

Put load, speed, and life into one numeric package in the RFQ

The supplier needs four numbers; with them the sizing result is reproducible:

Maximum axial load Fmax (N): moving mass, cutting/clamping force, acceleration, and impact.
Equivalent axial load Fm (N): weighted average when load changes along travel; for steady duty take Fm ≈ Fmax as a conservative estimate.
Average speed n_m (rpm): not the peak speed - the long-run average (example: 80% duty and 2000 rpm peak gives roughly 500 rpm).
Target life Lh (hours): 30,000 hours equals about 4 years of 20-hour daily operation.

Also state the mounting direction (horizontal or vertical) and duty cycle. Vertical axes are more sensitive to load and braking, see vertical Z axis load and brake selection.

Longer life is not always better: do not pay for life you will not use

The life target decides C: if C is too low, the screw diameter, nut style, or preload class must go up, raising price and lead time. Moving the target from 30,000 to 40,000 hours can push the screw from the 40 mm class to the 50 mm class in diameter, affecting both cost and installation space. Calculate the duty first, then set a life target with a reasonable margin; it is more professional than writing "as long as possible" and gets a more realistic quotation. For lead, speed, and accuracy trade-offs, see ball screw lead, speed, and thrust and ball screw accuracy class C3/C5/C7.

FAQ

Is workpiece weight enough as the load? No. The equivalent axial load should include table, fixture, workpiece, and all moving mass, plus acceleration and impact; on a vertical axis add the spindle assembly weight.

Is longer life always better? Not always. The life target decides the dynamic load rating C, which drives diameter, nut style, and price. Convert actual shifts and years into hours, then keep a 1.5x to 2x margin.

Should a low-speed pressing machine check C or C0? Mainly C0. For static or very slow duty, verify Fmax ≤ C0 / fs (fs usually 2 to 5) so the raceway is not permanently deformed.

Do preload and lubrication affect life? Yes. Excessive preload raises internal contact stress and shortens life; poor lubrication or contamination accelerates wear. The life formula assumes normal lubrication and sealing, so state those requirements in the RFQ.

Next step

Turn this guide into an RFQ

When the specification direction is clear, send the details below together with quantity, lead time, and packing requirements.

Include these details

Model, diameter, lead, accuracy grade, or target application.
Load, speed, travel, mounting method, and matching rail or support-unit needs.
Quantity, lead time, packing, and whether inspection records or shipment photos are needed.

Send load and life requirement View standard ball screws