Accuracy Standards

The precision of the SKR is determined by repetitive positioning accuracy, positioning accuracy, backlash, and travelingparallelism.

Repetitive Positioning Accuracy

Repeat the measurement seven times from the samedirection to a certain point. Divide the maximum difference by two. Conduct the same test at three points, the “center”of the stroke, and on both the approximate maximum andminimum positions of travel. Add
t 1 ± to the largest difference.This accuracy is generally measured with a laserinterferometer and sometimes with a dial-gauge.(Taken from THK Accuracy & Measurement Standards.)
t 2 t 3 Figure. 8 Repetitive Positioning Accuracy

Positioning Accuracy

The maximum stroke is taken as the reference length, andthe maximum error between the actual distance traveled from the reference position and the instructed value isexpressed as an absolute value.
(+)(-) A A Instructed value A Distance traveled Referenceposition A=| Distance actually traveled – Instructed travel distance value(Error)0 Figure. 9 Positioning Accuracy

Backlash

Lock the actuator’s carriage into a fixed position via theactuator’s drive mechanism. Do not lock the actuator’scarriage by “fixing” it rigidly. Push the carriage from one direction with a predetermined external force using a push/pull gauge. Zero out the dial-gauge while the axial force isbeing applied-release the external force and read the dial- gauge. Measure at three separate points along the stroke,at the center and the end of travel positions. Backlash isthe maximum measured value.
BacklashReturnScrew feedLoadLoad displacement (including elastic displacement) Figure. 10 Backlash

Travel Parallelism

A straightedge is placed on a leveling plate mounted withthe SKR, and parallelism is measured over almost the entiredistance traveled using a test indicator. The maximum errorin the reading within the distance traveled is taken as themeasurement value.
Straightedge Figure. 11Travel Parallelism

10