Straight Talk On Leadscrews

March 9, 2000
Reliable leadscrew applications depend not only on proper selection, but also on correct installation and alignment to eliminate off-axis load.

Robert Lipsett
Engineering Manager
Ball Screws & Actuators Co.
San Jose, Calif.

 

A self-aligning, single-rail stage has rail and screw locating pockets machined in one operation allowing the assembly to be installed without adjustment. A vertically floating lead nut compensates for misalignment and prevents binding.


High-precision, twin-rail stages use a fixed reference rail to align a parallel floating rail. The preloaded ground ball screw floats perpendicular to its axis in two directions during assembly. The screw ends are forced to center by the carriage then bolted down. The arrangement requires no measurements for alignment.


Leadscrews are an efficient and often cost-effective means to convert rotary motion to linear motion. Products incorporating leadscrews range from garage-door openers to precision laser-positioning systems. With such a wide range of applications, component selection can be a challenge, but this is only step one. Leadscrews must also be carefully installed and aligned.

Misalignment may cause problems such as binding, torque spikes, increased wear, vibration, audible noise, and lower accuracy. This is because leadscrews create only axial thrust and aren't able to tolerate lateral (side) or moment loads. Lateral loads act at an angle to the screw axis (center-line) while moment loads act parallel to but at a distance from the screw axis. Properly installed and aligned linear guides can help eliminate off-axis loads and moments, whereas misaligned guides may create them. What steps are taken to achieve proper alignment depend on leadscrew precision. To determine leadscrew precision, first look at lead-nut-to-screw clearances.

For lower-accuracy systems, the nut-to-screw radial (lateral) and axial clearances may each be about 0.005 to 0.010 in. To prevent misalignment-related issues, the nut must not deviate from coaxial alignment with the screw by more than half this amount. But trying to keep the nut from deflecting more than ±0.0025 in. with close-tolerance frames and guide rails may not be practical. This is because the tolerances of the support structure, guides, and movable carriage add together creating what's called tolerance stack up. One way to minimize the stack-up problem is to make the system adjustable, although this complicates assembly. Another involves eliminating tolerance stack up altogether by "designing out" certain bolted surfaces. Here, the support structure for both the guide rails and screw are machined from a single piece of material. Also, the carriage-mounted nut holder is machined directly into the carriage. Accuracy of such a system is limited only by the CNC machine tool that cuts it.

For some higher-accuracy systems, however, no measurable clearance between the lead nut and screw is permitted. In these cases, adjustable, tight-tolerance frames and guide rails alone may not eliminate screw binding and other misalignment problems. Assemblies are instead referenced to a single flat surface or edge such as one of the linear guide rails. Straightness of travel is then a function of how accurately a reference rail conforms to its mating surface.

One such system works like this: Linear bearings, rigidly attached to a translating carriage, force a second floating rail into parallel alignment with the fixed one. The screw assembly floats laterally with two degrees of freedom at each end. The lead nut (fixed to the carriage) forces the screw to center during assembly. Once aligned, the screw ends are bolted in position. When set up properly, the carriage will run parallel to the reference edge without binding.

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