Machine Design


Movement differential:For basic and plunger-actuated switches, this is stated as a range. The low limit indicates the smallest differential obtainable. For switches with leaf or lever actuators, the movement differential is generally the maximum movement of the actuating device that ensures operation of the switch in both directions of actuator travel. Movement differential should be specified as a maximum value instead of range.

Overtravel: This is the distance the actuator moves after snap-over without causing damage. Minimum overtravel that occurs after snap-over is normally not specified. But good practice is to provide for a minimum of 0.002 to 0.003-in. overtravel of the basic switch to ensure adequate contact pressures and some mechanical stability.

Operating force: For most applications it is sufficient to ensure that the actuator force exceeds the upper limit of the operating force range of the switch.

Total overtravel force: This is the force necessary to reach the operating limit position of the switch actuator. For most snap-action mechanisms, overtravel force generally exceeds operating force by two times. This value is usually not specified by manufacturers. If actuator force is limited, the switch might not be driven far enough into overtravel for reliable operation. On the other hand, overtravel force in excess of three times the operating force may overstress the spring mechanism, resulting in decreased mechanical life.

Releasing force: This is the applied force at which switch contacts snap back from operating to normal position. The spring mechanism of the switch should never be used to restore an external actuating device to its normal rest position. If more restoring energy is needed, add auxiliary springs.

Contact force: This is the amount of force holding movable and stationary contacts together. It ranges from 5 gm in the coin switch to more than 150 gm in heavy-duty motor-load switches.

Repeatability: This typically is expressed in terms of operating position, movement differential, and operating force. Maximum repeatability is obtained when the switch is not required to make or break an electrical load. Here, the operating position of a general-purpose switch is repeated within 0.0005 in. for thousands of operations. High-sensitivity snap-acting switches are capable of repeating the operating position within 0.0001 in., and the movement differential within 0.0003 in., even after 5 million operations.

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