Page 154 - Electromechanical Devices and Components Illustrated Sourcebook

116 Electromechanical Devices & Components Illustrated Sourcebook

Translating Rotary Motion Guide Bearing Preload Spring
Antibacklash Element
Drive Nut
to Linear Motion Lead Screw

Motion control most often means controlling linear motion.
Since motors produce rotary motion, some sort of linear
conversion must take place. The most common method is to
couple the output of the motor to a threaded shaft, or lead
screw.
For high load applications, recirculating ball screws are
usually used. The nuts used in these types of screws use ball
Tooth Belt
bearings to engage the threads. The balls are free to rotate and End Plates
are fed back into the front of the nut via an external conduit Guide Rods Motor Shaft
as they roll out of the back. In this way a threaded shaft and
nut can be produced with minimal friction and, therefore, Motor Mount
minimal wear. These types of screws are commonly found in Stepper Motor
computer controlled machine tools. Figure 6-43 shows a pow-
Figure 6-45 Rotating Antibacklash Nut
ered recirculating ball screw with a fixed nut.

Clamp Collar
Recirculating Ball Nut
Shaft Coupling Switching the motor to forward or reverse either extends or
Bearing Ball retracts the actuator.
Moving Table Screw
Servo Motor For applications that have low loads, an acme lead screw
with an antibacklash nut can be specified. Figure 6-45 shows
a linear actuator that uses an acme lead screw and antiback-
lash nut. In this configuration the lead screw is fixed and the
nut is rotated. This arrangement is very popular for low load
Fixed Base
applications that incorporate stepper motors.
Figure 6-43 Powered Lead Screw
Another method to achieve liner motion is to use a toothed
belt, as shown in Figure 6-46. This is a very common way to
control computer equipment such as disk drives and printer
heads. In a belt arrangement the positional resolution of the
Motorized linear actuators, as shown in Figure 6-44, are belt is dependent on the circumference of the drive pulley and
common devices for adding motion control to all sorts of the accuracy of the drive motor.
equipment. These units can be found on retractable awnings,
convertible automobile tops, satellite antennas, and hospital
beds. They are typically a cylinder fitted with a hollow piston.
The bottom of the piston has a bronze nut with a threaded
steel shaft. The threaded shaft is powered by a DC motor. Toothed Belt
Motion

Toothed Belt
Idler Pulley
Drive Pulley Stepper Motor
DC Motor
Figure 6-46 Linear Belt Drive
Nut
Guide Housing
Piston Head
Piston Tube
Threaded Shaft
For extremely high load applications a rotary actuator
may be specified. These actuators are generally built using a
worm gear drive. Worm drives can produce very high
Bushing
Base mechanical advantages and, as such, are used in the most rig-
Driven Pulley orous applications. Figure 6-47 shows a worm drive rotary
Figure 6-44 Motorized Screw Thread Actuator actuator.