Limit Switches and Other Pilot Switches

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Figure: 1 (a) Fork lever roller yoke limit switch. (b) Roller arm limit switch. (c) Top roller limit switch. (d) Wobble lever actuated cat whisker limit switch. (e) Side roller limit switch.

(a) Fork lever roller yoke limit switch. (b) Roller arm limit switch. (c) Top roller limit switch. (d) Wobble lever actuated cat whisker limit switch. (e) Side roller limit switch.

A variety of pilot switches is available for use in control circuits. One of the most popular is called a limit switch (see Figure right). These switches are mounted physically in the machine so that motion of the machine or the parts around the switch will cause the switch to activate. Figure 1a shows a yoke limit switch. The yoke has a roller on each section of its activator arm. This arrangement is used where machine motion is in two directions. When the machine moves in one direction, it strikes the right-hand roller and causes it to switch to the right. This action causes the right roller to be pressed downward, and the left roller snaps to the up position where it's in position to detect machine motion when it moves back to the left. The yoke arm on switch will continue to detect motion to the right, and then to the left. This type of switch is useful for surface grinders where the part being finished is moved back and forth under the grinding wheel. Each time the part moves to the end of its stroke, the switch is activated to the other direction.

Figure 1b shows a limit switch with a single roller arm. This type of switch detects motion in only one direction. When the machine moves past the roller arm, it will move the arm to the right so that the switch is activated. Figure 1c shows a top roller limit switch. This type of switch has a small roller mounted on a plunger arm. When the machine moves past the roller, it causes the plunger to depress to activate the switch. When the machine moves past the switch, a spring causes the plunger to move back upward into place to be reactivated. This type of switch can detect motion in either direction. A variation of this type of switch uses a blunt end on the plunger, which means the switch can only detect motion that is directed down on the plunger. This type of switch is generally mounted at the very end of machine travel, and when the machine reaches the end of its stroke, it depresses the plunger.

Figure 1d shows a wobble lever actuated limit switch that is sometimes called a cat whisker limit switch. This type of limit switch can detect motion in any direction. That is, any movement of the cat whisker can cause the switch to activate. This type of switch is also much more sensitive than other types of limit switches. Figure 1e shows a side roller limit switch. This type of limit switch is mounted in such a way as to detect end of travel. When the machine reaches the end of its travel, it will depress the plunger and cause the switch to activate.

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Symbols for Limit Switches and Other Motor Control Devices

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