The decisive factor for positioning accuracy that makes the difference between simple movement and precise positioning.
Axial play (also backlash or lost motion) describes the small axial movement of the screw or nut that is possible without any rotation of the drive shaft – the “dead travel” when the load direction is reversed. While uncritical for many standard applications, it is a major source of error in precise positioning tasks and thus a quality criterion for performance-oriented drives. (ZIMM specifies the axial play for ball-screw-based screw jacks in hundredths of a millimetre, e.g. 0.06 mm or 0.03 mm depending on series/size.)
Causes of axial play
Axial play is not a manufacturing defect; it results from necessary tolerances between the thread flanks of screw and nut. This small clearance ensures smooth running and prevents jamming/seizing under load and with temperature changes.
Design measures for minimisation
1) Trapezoidal screw drives (TR): low-backlash / adjustable nuts
- Concept: Anti-backlash nuts (split double nuts) are often used. Two nut halves are preloaded axially against each other via springs/adjusting elements.
- Function: Both flanks are in contact – axial play is eliminated; the design is often adjustable to compensate wear over the service life.
- Application benefit: Suitable when self-locking of the TR is desired while at the same time improving positioning accuracy.
2) Ball screw drives (KGT): preload
- Concept: KGT achieve freedom from play by preload – an internal, defined force that eliminates backlash from the outset.
- Methods: Double-nut preload (two nuts preloaded against each other, e.g. via precision ring), ball selection/oversize preload (slightly larger balls create defined preload in a single nut).
- Advantage: Preload eliminates play and increases stiffness – essential for dynamic and high-precision applications. (The fundamentals of backlash and preload in ball screws are described in standards and common industry practice.)
Practical relevance & design (ZIMM)
- Know and specify axial play: ZIMM specifies axial play for each ball-screw series/size (e.g. ZE/ZE-H/GSZ with 0.06 mm or 0.03 mm), which simplifies the positioning accuracy budget in axis design.
- TR vs. KGT: TR + anti-backlash nut = robust & self-locking with reduced play; KGT + preload = backlash-free & stiff for highly dynamic positioning tasks. (KGT are not self-locking – plan a brake for holding.)
- Consider system effects: Play interacts with compliance (shafts, couplings, bearings) and control system to determine repeatability and positioning accuracy. Assembly quality, lubrication and cleanliness are crucial to minimise micro-movements and wear.
Short conclusion
- Goal = precise positioning: Measure/define axial play and choose an appropriate play-reduction strategy (TR: low-backlash/adjustable, KGT: preloaded).
- ZIMM advantage: Specified axial play values for each product as well as matching system components (shafts, couplings, bearing arrangements) make it easier to design for lasting precision.
