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	Ballscrews   vs. Leadscrews When comparing robotic assembly drivescrews, one must take into account the following pieces of information: pitch, rolling error, runout error, resolution, repeatability, and absolute positional accuracy. Pitch is the frequency of the screw thread. For example, if the pitch were 0.025", then a complete revolution of the screw would translate into 0.025" of linear movement. Runout Error is the total deviation accumulated over the length of the screw. Rolling Error is the deviation from nominal at any position along the screw. Resolution, the minimum increment of linear movement, is determined by the pitch of the screw and the step angle of the stepper motor driving that screw. Repeatability is a measure of the capability to re-position the nut to the same coordinate time after time. Absolute Accuracy is a measure of the capability to position the nut to an exact coordinate as determined by a standard. Precision lead screws are available with runout errors of +/-10 µm/100mm and rolling errors less than 10% of runout error. Because the rolling error is much less than the runout error, the absolute accuracy of a lead screw stage is easily improved via software correction tables. ISO/DIN Class III ball screws are available with runout errors of +/-8 µm/100mm for the shorter lengths to 2.1µm/100mm for the longer lengths. Total error can not be corrected via software since the rolling error varies widely and is typically 70-100% of the runout error. The following plot shows a typical precision leadscrew in red and a typical Class III ballscrew in blue. Over 300mm, the leadscrew's absolute accuracy could be off by 30µm whereas the ballscrew's absolute accuracy could be off by only 6.3µm. More typical values are show in the graph. Through the use of software correction, runout error is easily compensated for. However, the rolling error of the leadscrew is considerably less than the rolling error of the ballscrew. This makes the leadscrew a better choice for computer-controlled precision linear positioning even though the ballscrew runout error is considerably less that the leadscrew runout error. As seen above, the absolute accuracy of the precision leadscrew can be made nearly perfect with TriTek's software compensation package.