October, 2016
www.us-tech.com
Page 53
Switch Reliability: How to Choose the Right Metal Dome
By Nicholas Wojtusik, Marketing Manager, Nicomatic E
lectrical switches are necessary in almost every HMI (human-machine interface) and are used in everything from kitchen microwaves to
military vehicles. Many companies rely on the activation life
advertised by manufacturers, but the data is com- piled in a strictly-controlled testing environment. Every electrical switch is used in a different way and in a different environment. Numerous circumstances affect the lifecycle of metal domes and it can vary when used in a real-world application vs. the testing environment.
Choosing the Right Dome When searching for the right
dome for a particular application, ask the following questions. Will the switch last for the life of the product? How many cycles will the dome actual- ly see? What is the force applied for testing standards and how does that compare to the final application? What changes during the design phase affect the tactile ratio of a dome? There are a variety of standard
tests required for metal domes. The one that focuses on the lifecycle of a metal dome does nothing but depress and release a dome until it reaches a predetermined number. Before, dur- ing and after the testing the dome must be examined to see if there are any changes in characteristics. For testing, first determine the
actuation force needed to depress the dome. After determining the maxi- mum force necessary, the industry standard is to set up to depress the dome with 10 percent higher force than the actuation needed to reach contact closure. The dome is then placed on a
hard, flat surface perpendicular to a testing probe, the surface is milled or tape is applied to avoid movement. A typical testing probe has a diameter of approximately 50 percent of the dome being tested, typically made of a non-elastic material (e.g. PTFE) and is aligned to the center of the dome. The testing probe is then set to depress the dome for a specific num- ber of actuations during a specific amount of time. The dome is then examined at certain intervals to check its integrity and to determine if any characteristics have changed. Depending on the application,
many items affect the lifecycle and performance of a dome, including the size and strength of the user, how centered the dome is, the shape of the dome, whether it is dimpled or non- dimpled, if it has higher or lower trip forces, the dome’s size, and whether a dome is round, double-round, oblong, triangular, or four-legged.
With outside forces affecting the life of the dome, the best course of action is to submit the dome to a test with an actuation similar to the use it will receive.
With outside forces affecting the
life of the dome, the best course of action is to submit the dome to a test with an actuation similar to the use it will receive. If a dome is being used in an
office setting and only used once in the morning to turn a machine on and once in the evening to turn the machine off, then lifecycle testing is not the most important factor. If a dome is being used in a harsh environ-
ment, like a switch for entry of an armored vehicle, where the users are large and use much more force then in an office setting, the testing criteria must be adjusted. The lifecycle testing for the armored vehicle
Metal domes used in switches to facilitate an electrical connection.
will need to have a higher actuation force than the standard 110 percent, some companies expose
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