KMT031NGJLHS Activation Force

I have not received a reply back from either C&K or Littelfuse

Questions:

  1. The tolerance given on page 2 is ± 25%

  2. Applying this tolerance gives:

  3. 2.55 N, minimum

  4. 4.25 N, maximum

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[Q on KMT031 NGJ LHS.eml|attachment](upload://ihrma9Tt7e1vdCJrP2CmTenTj5i.eml) (204.6 KB)
  1. We used an Instron to measure the activation force on 5 individual button components. The average button force measured was around 3.11N, and all measurements were within ± 0.2N of the average (all were within the tolerance specified in the datasheet).

  2. We have a piece of plastic that goes from the top of the switch to outside our enclosure.

  3. We are testing both the switch by itself and in the final enclosure.

  4. Joe W. is working on contacting the button vendor to understand why the measured button force is consistently below the nominal value, instead of a normal distribution around the nominal value like we were expecting.

  5. Darrell measured the button force on one of the DCS SUDs we have in Arden Hills. The flat button had a measured button force of 2.58N and the concave button had a measured button force of 2.84N, which matches what was seen during the DCS testing in MA.

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  1. What happens to the activation force after x # of activations?

  2. Does it drift either up or down?

  3. It is dependent of temperature or humidity?

  4. We are having a cold weather for the last 2 weeks.

  5. We’ve been shipping the parts and assemblies from MN to MA in that weather.

  6. The operating life is listed on Page 3 as 300,000 cycles.

  7. Can I assume a normal bell curve for the distribution?

Joe

Hello joseph.wilger,

Welcome to the DigiKey TechForum.
I’m glad all the switches were within tolerance for the operating force.
The questions about any change to the force over time, should be answered by the manufacturer. The datasheet has the operating temperature from -55C ~ 85C, and it has an IP68 rating, so it would be waterproof and can be immersed in water, so humidity would not affect it.

It should be understood that the nominal value represents a design target, with upper/lower limits serving as acceptance criteria. No representation regarding the statistical distribution of measured values is made or should be inferred; only that measured values will be within the stated range when measured as prescribed by the manufacturer.

This differs from situations where information is generated via a post-hoc analysis of a product and its production process, e.g. “characterization”. The “typical” value of (for example) an op amp’s offset voltage found in the datasheet represents the manufacturer’s observed statistical mean for that product trait. The “min” and “max” values in those same characteristics tables usually indicate 3-sigma limits of the same attributes.

The deformation of the mechanical dome element in such switches obviously needs to remain within the elastic limits of the material in order to enable 6-digit cycle lives. Without claiming expertise in the matter, my presumption would be a relatively flat or declining force requirement over the course of service life, due to formation of micro-cracks in the material and eventual fatigue failure.

I’d think it fair to ask what criteria constitute “failure” for purposes of determining the stated cycle life; it’s one thing to define it as some performance parameter no longer meeting initial specification, quite another to define it as a complete mechanical failure. Barring better information, I’d think it reasonable to interpret the stated cycle life as an L50 figure to catastrophic failure.

Temperature affects most everything, it’s just a question of degree… Again, not an expert on the specific topic but as a first guess I’d expect actuation forces to vary linearly with temperature-induced shifts in the Young’s modulus for the materials in question. A couple quick searches suggests that might be in the territory of a few hundred PPM/°C.

What distribution? As mentioned above, one can expect the actuation force of delivered product to be between stated min/max, but there is no representation or warranty of statistical distribution within those limits.