
B-65262EN/03 SPECIFICATIONS FOR THE αiS/αi SERIES 3.SELECTING A MOTOR
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Cautions in calculating the load torque
When calculating the torque, take the following precautions:
• Allow for the friction torque caused by the gib fastening force
(F
g
).
The torque calculated only from the weight of a movable part
and the friction coefficient is generally quite small. The gib
fastening force and precision of the sliding surface may have a
great effect on the torque.
• The pre-load of the bearing or nut of the ball screw, pre-tension
of the screw, and other factors may make T
c
of the rolling
contact considerable.
In a small, lightweight machine tool, the friction torque will
greatly affect the entire torque.
• Allow for an increase in friction on the sliding surface (F
cf
)
caused by the cutting resistance. The cutting resistance and the
driving force generally do not act through a common point as
illustrated below. When a large cutting resistance is applied, the
moment increases the load on the sliding surface.
When calculating the torque during cutting, allow for the friction
torque caused by the load.
Cutting force
Driving
force
Cutting force
Driving force
• The feedrate may cause the friction torque to vary greatly.
Obtain an accurate value by closely examining variations in
friction depending on variations in speed, the mechanism for
supporting the table (sliding contact, rolling contact, static
pressure, etc.), material of the sliding surface, lubricating system,
and other factors.
• The friction torque of a single machine varies widely due to
adjustment conditions, ambient temperature, and lubrication
conditions. Collect a great amount of measurement data of
identical models so that a correct load torque can be calculated.
When adjusting the gib fastening force and backlash, monitor the
friction torque. Avoid generating an unnecessarily great torque.