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What are the items that make up a ballscrew actuator's accuracy? (2)

The five items expressing a ballscrew actuator's performance

For ballscrew actuators, the following five items are defined to express their performance:

(1) Repeated positioning accuracy
(2) Positioning accuracy
(3) Traveling parallelism
(4) Backlash
(5) Starting torque

This article explains the items (4) and (5).

(4) Backlash

"Backlash", according to a dictionary is
1: Reaction, opposition, resistance
2: Gap, play, looseness
3: Snarl (of a fishing line)
With regards to ballscrew actuators, backlash is used with the meaning "2: gap, play, looseness".
See below for the method for measuring backlash in a ballscrew actuator.

The procedure to measure backlash is as follows:
(1) Move the slide block in one direction and stop.
(2) Apply load in the same direction as in the previous step and push the slide block.
(3) Remove the load.
(4) Measure the difference between the stop position of the slide block and the position of the slide block after the load is removed.

The value measured is the backlash of the ballscrew actuator.


Backlash appears as a gap in the axial direction of the slide block when the operating direction is inverted or an external load is received. However, its impact is small if the axial load is always from the same direction, (vertical load, for example) or no load is applied from the outside after stop. Note that in "repeated positioning accuracy and positioning accuracy" explained in What are the items that make up a ballscrew actuator's accuracy? (1), positioning was always performed from the same direction to measure accuracy by minimizing the impact of backlash.

(5) Starting torque


The next item is starting torque. This literally expresses the torque necessary at the instant the ballscrew actuator starts moving (at start).
This is measured by attaching a torque gauge to the shaft end of the ball screw (coupling connector)

Note that the starting torques described in the catalogue are starting torque measured using standard grease and without load. Therefore, be sure to add enough margin when choosing a drive motor.

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