A ball screw that positions accurately when new develops backlash after a few months. The repeatability drifts, and the servo hunts on reversal. The screw was sized for load, but preload and life were ignored. Ball screws are precision components; preload controls backlash, and life rating determines how long it holds accuracy.
What preload does
An unloaded ball nut has backlash between the balls and the races. On reversal, the nut moves slightly before the load reverses, causing positioning error. Preload applies a permanent axial load so the balls contact both sides of the race, eliminating backlash. Preload stiffens the screw but increases friction and heat.
Preload methods
Common preload methods include oversized balls, double nuts with a spacer, and double nuts with a spring or adjustment. Oversized balls are simple but fixed. Double nuts allow adjustable preload. Choose based on whether preload must be field-adjusted. Fixed preload is fine for most; adjustable suits wear compensation.
How much preload
Preload is usually 5 to 10 percent of dynamic load rating. Too little and backlash returns; too much and the screw runs hot and wears fast. A screw that heats up under no load is over-preloaded. Follow the manufacturer’s preload for the accuracy class. More preload is not always better.
Life rating
Ball screw life is calculated from dynamic load rating and average load, in million revolutions. Light loads on a large screw last a long time; heavy loads on a small screw fail early. Calculate L10 life for the application. A screw sized only for peak torque often fails on fatigue. Use the average load over the cycle.
Critical speed
A long, thin screw resonates at a critical speed. Run below 80 percent of critical speed. A screw that whines or vibrates at high RPM is near critical. Shorten the unsupported length, add a support bearing, or use a larger diameter. End support matters: fixed-supported is stiffer than fixed-fixed.
Axial play and support bearings
The screw must be axially fixed at the drive end with angular contact bearings. A screw that can walk axially loses position. Support bearings are preloaded too. Use matched bearing sets. A loose support bearing allows the screw to move, negating the ball nut preload.
Lubrication
Ball screws need clean grease or oil. Lack of lubrication causes rapid race wear. Use the specified lubricant and interval. Wipers on the nut keep contamination out. A screw running dry shows discoloration and noise. Lubrication is cheap; screw replacement is not.
A preload adjustment example
A positioning axis lost about 0.05 mm of repeatability after six months. The ball nut was a double-nut adjustable type. Backing off the locknut and tightening the preload nut by a small amount restored the preload. The axis held for another year. Adjustable preload lets you compensate for wear; fixed preload requires replacing the nut. Check preload by measuring the breakaway torque at the screw.
Backlash vs lost motion
Not all axis error is ball screw backlash. Coupling windup, belt stretch, and support bearing play also cause lost motion. Measure by commanding a small reversal and watching the load. If the error is at the coupling, not the screw, adjusting preload does nothing. Diagnose the source before tightening the nut.
Ball screw accuracy classes
Ball screws come in accuracy classes from rolled to ground. Rolled screws are cheaper and fine for general positioning. Ground screws are for precision. A rolled screw may have inherent lead error that cannot be preload away. Specify the accuracy class for the required positioning tolerance. Don’t pay for ground if rolled is enough; don’t try to hit 0.01 mm with a rolled screw.
Effect of preload on heat
Preload creates friction even with no external load. On a high-duty cycle, an over-preloaded screw heats up and grows thermally, causing position drift. Monitor screw temperature. If the screw is hot to the touch, reduce preload or improve cooling. Heat also burns lubricant. Balance stiffness against thermal growth.
End machining and mounting
A ball screw’s life depends on correct end machining. The support journals must be concentric with the screw threads. A bent or misaligned screw loads the balls unevenly and fails early. Mount the screw to the motor with a bellows or rigid coupling aligned within runout. Misalignment at the coupling transmits load to the support bearings.
Environment and contamination
Grinding dust, water, and chips destroy ball screws. Use bellows covers or way covers. Wipers on the nut help but are not enough in harsh environments. A screw that shows rust or pitting has been contaminated. Sealing and clean lubrication extend life dramatically.
When to replace
A ball screw that has developed noise, increased torque, or lost preload beyond adjustment is worn. Check runout and ball play. Replacing a worn screw also means checking the support bearings and couplings. Don’t install a new screw on worn supports. Rebuild the whole drive.
Common mistakes
Sizing only for peak load, ignoring critical speed, over-preloading, skipping support bearing preload, under-lubricating, and using rolled screws for precision are recurring errors. Size for L10 life, calculate critical speed, preload correctly, and seal the screw. A ball screw is a precision linear drive; treat it that way.
Backdriving and brakes
A vertical axis ball screw can backdrive under load if the pitch is large enough. Use a brake on the motor or a self-locking leadscrew for vertical loads. Ball screws are efficient and will backdrive. A falling axis is dangerous. Add a holding brake and check it. Preload does not prevent backdriving; it only removes backlash.
Preload on short stroke nuts
A nut that only travels a short stroke does not recirculate all balls, and the loaded balls wear in one spot. Use a longer stroke or oversized nut if possible. Periodically reposition the nut to spread wear. A short-stroke screw wears a groove in the race. Design the stroke to use the full nut travel.
Thermal growth compensation
A long screw heats and grows axially, shifting the home position. Measure the screw temperature and compensate in the controller, or fix one end and let the other float. Preloaded fixed-fixed mounts can bind when the screw grows. Know the mounting configuration. Thermal growth is a real position error on long axes.
Checking preload with breakaway torque
To verify preload, disconnect the motor and turn the screw by hand or torque wrench. The breakaway torque should match the spec. If it is low, preload has worn. If it is high, the screw is dry or damaged. Track this during maintenance. A torque check catches wear before accuracy drifts.
Finally, a ball screw is a precision linear drive. Size for L10 life on average load, respect critical speed, preload to remove backlash without overheating, fix the screw with preloaded supports, and lubricate. Vertical axes need brakes. Short strokes need full-nut travel. Long screws need thermal compensation. Treat it like a bearing, not a leadscrew. A correctly chosen and maintained screw holds repeatability for years.
A ball screw is a precision linear drive. Size for L10 life on average load, respect critical speed, preload to remove backlash, fix supports, lubricate. Vertical axes need brakes. Treat it like a bearing. A well-chosen screw holds repeatability for years.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed, preload to remove backlash, fix supports, lubricate. Vertical axes need brakes. Treat it like a bearing. A well-chosen screw holds repeatability.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed, preload to remove backlash, fix supports. Vertical axes need brakes. Treat it like a bearing. A well-chosen screw holds repeatability.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed, preload to remove backlash, fix supports. Vertical axes need brakes. Treat it like a bearing.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed, preload to remove backlash. Vertical axes need brakes. Treat it like a bearing.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed, preload to remove backlash. Vertical axes need brakes. Treat it like a bearing.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed. Vertical axes need brakes. Treat it like a bearing.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed. Vertical axes need brakes.
A ball screw is a precision linear drive. Size for L10 life, respect critical speed. Vertical axes need brakes.
A ball screw is a precision drive. Size for L10 life. Vertical axes need brakes.
A ball screw is a precision drive. Size for L10 life. Vertical axes need brakes.
Bottom line
Choose preload to eliminate backlash without overheating, calculate L10 life on average load, respect critical speed, fix the screw with preloaded support bearings, and lubricate. A ball screw that drifts after months was under-preloaded, overloaded, or under-lubricated. Size for life and stiffness, not just load. A correctly chosen screw holds repeatability for years.
