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#134 Mastering Ball Screws - 4 : Increase in Ball Screw Speeds

Category : Linear Motion Components
October26, 2012

For chip mounters, robots and CNC machine tools, achieving higher ball screw speeds is an important agenda. Speed optimization techniques will be explained in this discussion.

(1) High speed optimization of ball screws

Since a ball screw is a mechanical element that converts motor's rotary motion into linear motion, there are two following means to increase the speed.

(1) Increase the linear motion distance per one rotation → Use of large leads
(2) High rotational speeds → Increase RPM (simultaneously attaining high accuracy)

Since the accuracy increase is also needed at the same time as the ball screw speed increase is needed, (1) and (2) must be satisfied simultaneously.

(2) Issues of increased rotational speeds

Ball screw speed increasing is associated with the following high level difficulties.

a) Orbital speed of the steel balls: Speed limitations based on dmn value --- dm: Ball diameter, n: Rotational speed
b) Critical speed of a ball screw
c) Thermal deformation increase by heat rise
d) Increased vibrations and noises
e) Load (inertia, etc.) increase due to high speed motion and high accel./decel.

a)As the steel ball's orbital speed (dmn value) increases, damages to the ball recirculation circuit components, such as tangs of return tubes, due to repeated shocks will become nonnegligible.

[Fig.1] Example of return tube damage

b)When the ball screw is rotated at high speeds, it will reach a critical speed where the natural frequency of the long narrow shaft suspended at both ends will brings a resonance vibration phenomena at a certain speed.
c)Ball screw temperature will rise as the speed increases due to the friction torque increased between the steel balls and the shaft. This temperature rise causes heat expansion and the accuracy will be degraded.
d)Ball screw nuts have complex recirculation paths with internal discontinuities (not smooth). As the ball travel speed increases, associated vibrations and noises will also increase.
e)Since the ball screw imparts high speed and high acceleration motion on objects, the loads on the ball screw increases.