Page 110 - Electromechanical Devices and Components Illustrated Sourcebook

72 Electromechanical Devices & Components Illustrated Sourcebook

Limit Switches
Limit Switches
Cam Buttons
Frame Plates

Spacers

Shaft Coupling
Drum Drum
Motor Mount
Frame Plates Cam Buttons
Synchronous Gear Motor
Side View Front View
Figure 4-105 Cam Programmable Synchronous Motor Barrel Timer

Taking the ordinary wall timer to the other end of the spec- and insulating tape, plastic sheeting, or heat shrink covers the
trum is the cam programmable barrel timer. These timers can screen. A series of brushes are constructed from brass strips
provide very sophisticated, complex, and precise control for and mounted so that they come in contact with the outside
all types of industrial equipment. A typical barrel timer has a diameter of the drum. To program the “on” cycles, holes are
drum with a series of buttons that trip limit switches as the cut through the insulating cover. A timer of this nature can be
drum rotates. The drum may have fixed buttons or may carry easily constructed to have as many on/off cycles and poles as
an array of holes so that the buttons can be placed in any loca- necessary for the application.
tion that is appropriate. The drum is generally driven by a
synchronous gear head motor. Figure 4-105 shows a cam pro-
grammable barrel timer. Resistors
An inexpensive barrel timer can be constructed as shown
in Figure 4-106. The drum is mounted on the shaft of the gear As is demonstrated in Ohm’s law, resistors can be used to
motor. A fine mesh brass screen is wrapped around the drum control voltage and current within a circuit. There are two
fundamental types of resistors, carbon and wire wound. Carbon
resistors use a measured plug of carbon for the resistive
Brass Strips element. The length and diameter of the carbon can be manip-
Frame ulated to build resistors with virtually any resistance that may
Pole 4 be desired. Wire wound resistors use a length of wire coiled
Pole 3 around some type of coil form for their resistive element.
Pole 2
Pole 1 Both of these types of resistors are available in a wide variety
sizes and configurations which are appropriate for virtually
Insulating any application.
Block
Figure 4-107 shows a sectional view of a typical carbon
Synchronous
Drum
Gear Head Motor resistor. The electrodes are impeded into the carbon plug and
the assembly is encased into a plastic housing. These are very
Cutouts
inexpensive items and are readily available.
Insulating Covers
Brass Screens The values of smaller-size resistors are generally identified
Figure 4-106 Four-Pole, Brush Contact Programmable Barrel with a four- or five- band color coding system. The color code
Timer system will clearly indicate the ohm rating of the resistor and