Page 78 - Electromechanical Devices and Components Illustrated Sourcebook

40 Electromechanical Devices & Components Illustrated Sourcebook

Switch Actions Actuator Actuator Pivot
Stop (On)
The action of a switch is the mechanism by which the con- Off (Open)
tacts are opened and closed. There are two basic actions that
are commonly found in switches. The knife switch is consid-
Actuator
ered a cam action. The contacts of a cam action switch open Stop (Off)
and close in direct relation with the actuator position. Because
Spring
of the slow speed at which the contacts open and close, arc- Actuator Arm
Fixed Contact
ing can be problematic. To compensate for the damage that
On (Closed) Floating Contact
arcing may cause to the contacts, cam action switches gener-
Arm Stop
ally use heavier duty construction. Pivot Point
Terminal
Figure 4-10 shows a typical cam action switch mechanism. Cradle
When the actuator is pushed to the left, the cam opens the Terminal
contacts. When the actuator is pushed to the right, the cam Figure 4-11 Snap-Action Switch Mechanism
allows the contacts to close. Many cam action switches have
flats in the cam to provide some holding action for the posi-
tion of the actuator.

The most common switch action is a hybrid of the cam
and snap actions. The pseudo-snap-action switch is a cam
On Flat action switch with a snap action actuator. This type of
switch has a number of attributes that make it an attractive
solution for many applications. The simplicity of the design
Off (Open)
provides relatively low manufacturing costs, minimized
contact bounce, fast open/close cycle, and excellent tactile
feedback.
Terminal
Figure 4-12 shows a pseudo-snap action switch mecha-
nism. Take notice that the contact and actuator design is very
Actuator
similar to the cam action shown in Figure 4-10. The principal
Actuator Pivot Recoil Arm
Off Flat difference is the actuator ball and spring, that are intended to
Recoil Spring
provide the snap action.
Contacts A specialized type of pseudo-snap action switch is the
On (Closed)
drum switch. These switches are generally designed to pro-
Terminal
Arm Spring vide a forward-off-reverse function for low horsepower, three
phase motors. Figure 4-13 shows a typical commercial drum
Terminal
Arm
switch.
Figure 4-10 Cam Action Switch Mechanism
Snap action switches are designed to provide a very fast
open/close cycle. Snap action switches incorporate a mecha-
nism that stores the energy of the actuator and releases it to Off Stop On Stop
the contacts at a single moment. This system is primarily On Flat
intended to minimize arcing between contacts. Snap action
switches can be rather small in size in reference to their cur-
rent carrying capabilities. In addition to their small size, the Off (Open)
snap-action switch also provides excellent tactile feedback to
the operator.
One drawback to snap action switches is contact bounce.
When the switch is closed, the contacts are forced together at Actuator
a very high rate of speed and the moving contact may recoil, Actuator Pivot Arm
or bounce, off the fixed contact. Generally, switch bounce is Arm Spring Off Cam Fixed Contact
only a concern on circuits that have extremely sensitive Floating Contact
switching requirements. Actuator Ball
On (Closed) Spring Terminal
Figure 4-11 shows a snap action mechanism that might be
found in a high-quality switch. The actuator energy is stored in Terminal
the spring. As the spring crosses the pivot point, it pulls both the Fixed Ball
contact arm and the floating contact up into the fixed contact. Figure 4-12 Pseudo-Snap Action Switch Mechanism