Does preload affect lead accuracy?

No. Preload removes axial clearance and raises stiffness; it does not change lead accuracy itself. Positioning accuracy comes from accuracy grade plus preload plus mounting coaxiality.

How much heating after preload is normal?

A steady-state rise of 20–35 °C is normal. Above 50 °C or too hot to touch, the preload is too heavy or lubrication is inadequate — step down or improve cooling.

Is P0 actually usable?

Yes, and many applications should use it: pure drive, continuous high-speed running, no reverse positioning accuracy required. P0 has the lowest heat, the longest life and the lowest cost.

Can I still do reverse positioning after preloading?

Yes, but accuracy is limited by the preload class and mounting stiffness. P3 and above can reach the 1–2 µm range for repeatability, depending on coaxiality and support-unit stiffness.

Ball Screw Preload Selection: P0–P4 Classes, Heat and Temperature Rise

How to choose ball-screw preload: P0–P4 axial clearance classes, recommended preload ratio (CNC ≤ 8% of Ca, automation ≤ 5% of Ca), the effect of preload on heat and temperature rise, and a decision flow by duty cycle.

Preload removes axial clearance by interference, buying stiffness at the cost of heat and torque. More preload is not always better — picking the right class for the duty cycle is the cheapest way to extend screw life.

1. P0–P4 preload classes and axial clearance

Classes P0–P4 run from none to heavy: as preload rises, axial clearance drops, stiffness rises, and so does heat generation.

Class	Level	Axial clearance (mm)	Typical use
P0	None	—	Pure drive, high speed
P1	Very small	< 0.005	General positioning
P2	Small	< 0.01	General positioning, medium speed
P3	Medium	< 0.02	High-precision positioning
P4	Large	< 0.03	Heavy cutting

Clearance figures are reference values; use the maker's preload table for exact numbers. A slight axial warm-up after preloading is normal.

2. Recommended preload ratio (preload ÷ Ca)

Preload is set as a percentage of the basic dynamic load rating Ca. Too much shortens life and raises temperature.

Application	Recommended ratio	Note
CNC machining centre	≤ 8% of Ca	Balances positioning accuracy and heat
Automation / semiconductor	≤ 5% of Ca	Thermal control first; stiffness just needs to be enough
General drive / positioning	≤ 5% of Ca	Cost and life first
Heavy cutting / high stiffness	8–12% of Ca	Short-duration heavy duty only; improve cooling

Above roughly 12% of Ca, friction heat rises sharply, grease ages faster, and life drops instead of rising.

3. Effect of preload on heat and temperature rise

Friction power from preload scales roughly with preload force × sliding speed. Higher preload at higher speed heats faster.

P ≈ Fp × v

Fp is the preload force, v is the sliding speed.

Speed	P0 temp rise	P2 temp rise	P4 temp rise
500 rpm	≈ 5 °C	≈ 8 °C	≈ 14 °C
1000 rpm	≈ 8 °C	≈ 13 °C	≈ 22 °C
2000 rpm	≈ 13 °C	≈ 21 °C	≈ 35 °C

Temperature rise figures assume 25 °C ambient with natural convection. Actual values vary widely with grease, cooling method and housing conduction.

4. Preload selection flow by duty cycle

First ask whether positioning accuracy matters. Pure drive, speed only → P0; no preload saves both cost and heat.
Need backlash removed → start at P1/P2; try P2 first and move to P3 only if accuracy demands it.
Thermal limits matter (automation / semiconductor) → keep preload under 5% of Ca; accept slightly lower stiffness rather than exceed the temperature budget.
Heavy cutting needing high stiffness → P3/P4, but confirm cooling is in place (oil or forced air) and limit heavy-duty dwell time.
Finally fold preload into the equivalent load Fm for life calculation — the heavier the preload, the shorter the effective life.

5. Matching screw preload to support-unit bearing preload

Screw preload and support-unit bearing preload are different: the first removes nut-to-screw clearance, the second removes fixed-end bearing clearance. Both are needed, but under overload the fixed-end bearing fails first.

Fixed-end bearing axial stiffness is listed in the bearing load chart: 2.9 N/µm (EK-6) to 27.5 N/µm (BK-40).
With heavy preload, verify the fixed-end bearing preload too — do not let the bearing become the weak link.

FAQ

Does preload affect lead accuracy?: No. Preload removes axial clearance and raises stiffness; it does not change lead accuracy itself. Positioning accuracy comes from accuracy grade plus preload plus mounting coaxiality.
How much heating after preload is normal?: A steady-state rise of 20–35 °C is normal. Above 50 °C or too hot to touch, the preload is too heavy or lubrication is inadequate — step down or improve cooling.
Is P0 actually usable?: Yes, and many applications should use it: pure drive, continuous high-speed running, no reverse positioning accuracy required. P0 has the lowest heat, the longest life and the lowest cost.
Can I still do reverse positioning after preloading?: Yes, but accuracy is limited by the preload class and mounting stiffness. P3 and above can reach the 1–2 µm range for repeatability, depending on coaxiality and support-unit stiffness.

Precision Grade C3/C5/C7 Lead Accuracy Sizing L10/Nc/Pc Mounting Torque & Pre-stretch Bearing Load Chart Full Model List

Preload classes per JIS B 1192 and common ball-screw design practice; preload ratios and temperature figures are standard engineering references.

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