Page 51 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 2 Basic Mechanics 13

2" Travel 1" Pull
Motor

Two Stage
Vee Belt drive
Fixed Pivot Traction Spool
Solenoid
Pulley
Cable
Counter Weight
Figure 2-15 Doubling Solenoid Throw
Cable

To calculate the motion generated (MG), divide the motion
applied (MA) by the ratio. Load (Elevator)

12" (MA) 6 2" (MG)
Figure 2-17 Traction Drive
Figure 2-15 shows how a single moving pulley can be used
Another variation of the pulley is the spool. The spool is
as a motion multiplier in conjunction with a solenoid. The
most commonly used on winching equipment. The spool is a
pulley is mounted to the plunger and when the solenoid is
pulley that continuously wraps the cable around the drum, as
activated, the motion is multiplied by 2.
shown in Figure 2-18. Normally, the spool is driven through
One of the most common uses for pulleys is vee belt drives,
the axle via a reversible electric motor and reduction drive.
used in power transmission. Figure 2-16 shows a typical vee belt
When the motor is operated in forward, the cable extends and
drive. The ratio of the drive can be determined by dividing the
when the motor is reversed, the cable retracts.
diameter of the driven pulley by the diameter of the drive pulley.
The revolutions per minute (RPM) of the driven pulley can be
determined by dividing the motor RPM by the drive ratio. Drum
Spool
Axle
1725 (motor RPM) (6 3) 862.5 (driven pulley RPM) Drive
Cable
6" Diameter
V-Belt
3" Diameter Lift
Figure 2-18 Cable Spool
Drive Pully
Rotating Axle Cable actuation systems are most commonly found on air-
Motor craft. Cable actuators can offer an exceptionally lightweight
method to transmit power and motion. Figure 2-19 shows the
basic elements of a cable actuation system. Notice that the
Driven Pully
return spring is located at the opposite end of the cable from
Figure 2-16 Vee Belt Drive the solenoid. This is necessary to maintain a tension on the
cable at all times.
Figure 2-20 shows how a cable and pulley actuation sys-
tem may be approximated by a cable within a sheath. A com-
The traction drive is another method of using the pulley. mon example of cable and sheath actuators can be seen on
These drives are most commonly found as the lift mechanism most bicycles.
for elevators. In a traction drive (Figure 2-17), a simple pulley
is set up. The load is placed on one end of the cable and a
counter-weight is placed on the opposite end. To move the The Inclined Plane
load, the pulley, or traction spool, is driven by an electric
motor and reduction drive. A traction drive has the advantage The inclined plane is a simple machine that can provide us
of applying a uniform load on the drive motor, regardless of with a significant mechanical advantage. Inclined planes are
the length of the cable. most commonly found on and around loading docks where