MOTION SYSTEMS HANDBOOK
Ballscrew update
Fixed-free arrangements provide the least rigidity.
Ballscrews include a screw and a nut housing recirculating balls to carry the axis load. Though recent years have seen more use of leadscrews in designs that once employed only
A fixed-floating arrangement has bearings at both ball screw ends, but a thrust bearing only at one.
ballscrews, four application requirements still necessitate a ballscrew.
A fixed-fixed arrangement provides maximum rigidity.
Vup = Permissible travel variation within useful travel V300p = Permissable travel variation within 300 mm I0 = Nominal travel V2πp = Permissible travel variation with 2π travel ep = Tolerance on useful travel Cup = Travel compensation LEAD I1 = Axial thread length Iu = Usable travel Ie = Excess travel
LEAD
POSITIONING BALLSCREW VALUES I1 Iu
Ie
Ie
300 +
GROUND VERSUS ROLLED BALLSCREWS
V300p C
V2πp Vup
Travel deviation
_
Io
2π rad
Vup
ep ep
BALLSCREW NUT TUBE RECIRCULATION The standard and most economical option is tube recirculation.
BALLSCREW NUT DEFLECTOR RECIRCULATION
BRIDGE DEFLECTOR
16
Heavy loads: With recirculating steel balls to support the load, ballscrews have a higher load capacity than comparably sized leadscrews. Leadscrews with bronze nuts can drive heavier loads — but as the load increases, so does friction … and that lowers duty cycle. Note: Related to sizing is the L10 bearing life equation, which provides a statistically proven estimation of the screw’s life in meters or rotations traveled. The wear characteristics of a ballscrew make exact life hard to predict. Leadscrews with plastic nuts can be selected based on PV values, but these give pressures and velocities the screw can withstand — not an estimation of life. High accuracy requirements: Unlike leadscrews (which exhibit backlash between the nut and the screw) ballscrews can be and often are preloaded to remove backlash. This is typically done by using balls that have a diameter that’s slightly larger than the space between the raceways of the screw and nut. (Some leadscrews have nut designs that eliminate backlash, but these typically add friction and reduce efficiency.) Plus ballscrews are classified by standards dictating lead deviation, so choosing the proper ballscrew accuracy class is straightforward. High efficiency: Rolling balls in a ballscrew rely on point contacts to support a load. Point contact produces less friction than the line contact found in leadscrews, which boosts efficiency. More specifically, ballscrew efficiency is almost always 90% or better, while leadscrew efficiency is normally 50% or lower. High efficiency translates to lower required motor torque, so often a smaller motor (and components) may be used. Demanding duty cycle: The lower friction of ballscrews means they produce less heat than leadscrews … so can withstand higher duty cycles. In fact, duty cycle is only considered in ballscrew selection when determining the travel the screw will achieve in its calculated life. In contrast, duty cycle and heat must always be considered when selecting a leadscrew.
DESIGN WORLD — MOTION
Another comparison made in industry is that between ground and rolled ballscrews. What’s more important than this choice are the following three distinctions. DIN/ISO and JIS specifications define two ballscrew accuracy types: P is for precision and T is for transport, and lower numbers indicate better accuracy. Precision classes ranging from P0 to P5 and transport classes from T5 to T9 (T10 for JIS). JIS specifications denote accuracy classes with the prefix C for precision and Ct for transport. A common misconception is that accuracy class specifies manufacturing method, but the two are not intertwined. Rolled screws can be P5 and even P3 accuracy, and some ground screws only have T accuracy values. More important is whether lead error V300 accumulates over the screw length. P accuracy classes don’t allow lead error accumulation, while T accuracy classes do. Geometric tolerances are also specified by DIN/ISO and JIS standards: For ground ballscrews, both thread grinding and journal grinding are done using the same reference centers, making it easier to minimize radial runout and keep the screw threads and end journals concentric. When screws are manufactured by rolling, usually the end journals are machined and ground after the threads are rolled, so maintaining concentricity and runout is more difficult. Even so, if a ballscrew is manufactured to DIN/ISO or JIS standards, it will meet those lead accuracy and geometric specifications ... regardless of whether it was manufactured by rolling or grinding. Processes produce different surface finishes: Rough surface finishes cause increased friction and accelerated ball wear. Grinding and rolling both produce smooth surfaces, but rolled screws must also be polished to remove oxidation. So when considering surface finish, the comparison is actually between ground and polished rolled screws. The quality of the polishing step determines rolled-screw finish quality. Ground screws are usually necessary where P5 or better accuracy is required ... but both rolled and ground screws can deliver P5 and even P3 accuracy — as well as DIN/ISO 7 and 9 (JIS 10) accuracy. 8 • 2020
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