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Motors and Generators | Abb AC Servo motors
5
Ballscrew drive
The load parameters have to be reflected back to the motor shaft for this type of drive as well. The inertias which have to be considered
include the ballscrew as well as the load. If the ballscrew inertia is not readily available, the formula for a cylinder may be used. Figure 5
presents the formulas for determining reflected inertias.
As an example, a 200 lb (90.6 Kg) load will be positioned via a ballscrew which is 0.5 inch (1.27 cm) in radius and 44 inches (111.7 cm)
long. The pitch is 5 rev/inch (1.96 rev/cm). The total load and ballscrew inertia would be:
calculating reflected load inertia:
calculating ballscrew inertia:
The total inertia which would be connected onto the motor shaft would be the sum of these:
J
=
J
1
+
J
1s
=
0.00052
+
0.00313
=
0.00365 lb-in-s
2
(Metric = 4.21 gm-cm-s
2
)
For precision positioning applications, the ballscrew is sometimes preloaded to eliminate or reduce backlash. If preloading is used, the
preload torque must be included since it can be significant. The ballscrew’s efficiency must also be considered when finally determining
torques.
Abb AC Servo motors