Thursday, July 12, 2012

Limit Switches Explained

 The automatic operation of machinery requires the use of switches that can be activated by the motion of the machinery. The repeat accuracy of the switches must be reliable and the response virtually instantaneous. The size, operating force, stroke, and manner of mounting are all critical factors in the installation of limit switches due to mechanical limitations in the machinery. The electrical ratings of the switches must be carefully matched to the loads to be controlled. In general, the operation of a limit switch begins when the moving machine or moving part of a machine strikes an operating lever which actuates the switch.
The limit switch, in turn, affects the electrical circuit controlling the machine and its movement.
Limit switches are used as pilot devices in the control circuits of magnetic starters to start, stop, speed up, slow down or reverse electric motors. Limit switches may be used either as control devices for regular operation or as emergency switches to prevent the improper functioning of machinery. They may be momentary contact (spring return) or maintained contact types.

 Micro limit switch:

Another type of limit switch often used in different types
of control circuits is the micro limit switch or micro
switch. Micro switches are much smaller in size than
the limit switch , which permits
them to be used in small spaces that would never be
accessible to the larger device. Another characteristic
of the micro switch is that the actuating plunger
requires only a small amount of travel to cause the contacts
to change position. The micro switch has an activating plunger located at the top of the switch. This switch requires that the plunger be depressed approximately 0.015 inch or 0.38 mm.
Switching the contact position with this small amount of
movement is accomplished by spring loading the contacts as shown . A small amount of movement against the spring will cause the movable contact to snap from one position to another.
Electrical ratings for the contacts of the basic micro switch are generally in the range of 250 volts ac and 10 to 15 amps depending on the type of switch. The basic micro switch can be obtained with a variety of different activating arms as shown.

Subminiature Micro Switches

The subminiature micro switch employs a similar spring contact arrangement as the basic micro switch The subminiature switches are approximately 1/2 to 1/4 the size of the basic switch, depending
on the model. Due to their reduced size, the contact rating of subminiature switches range from about 1 ampere to about 7 amperes depending on the switch type. Two different types of subminiature micro switches are shown above.
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