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Vibration in Dead Weight Calibration Machine - Solved!!!
2

Vibration in Dead Weight Calibration Machine - Solved!!!

Vibration in Dead Weight Calibration Machine - Solved!!!

2
(OP)
in thread404-5069: Vibration in Dead Weight Calibration Machine, Scott Wertel asked:

We have a Morehouse 550lb Dead Weight Calibration System. When the system loads between 400 to 500 lbs, the weight rack begins to sway which imparts loads into our test element, making it very difficult to obtain a steady reading. We are currently designing better fixtures so the sway does not directly translate into our test element.

Has anyone experienced this "weight shake" problem on this or any other dead weight system? If so, how did you solve it? We have several ideas, but proving the ideas would take many hours of trial and error to narrow down the specific cause and we can not afford the down-time right now.
Also, does anyone have any schematics on this system in case we are forced to try our more difficult and less preferred ideas?

I know this is in reference to a 23 year old post, but I am an engineer working at the same company Scott Wertel worked at and I recently solved the problem with this machine.

The dead weight calibration system is like an upside down Christmas tree suspended from the top by means of a load cell (the load cell we are calibrating). The machine provides mechanisms to let you automatically add or remove known test weights to platforms radiating out from the central spine of the Christmas tree. This machine allows you to apply any desired load to the load cell by selecting a combination of different test weights are applied to the central spine.

Physically this upside down Christmas tree acts like a pendulum and the natural resonant frequency of this pendulum is dependent on the length of the spine. Changing the applied load does not change the resonant frequency of the pendulum. In addition, the spine of the Christmas tree is a solid steel rod which also has a natural resonant frequency (like a tuning fork) that is inversely proportional to the applied load (more load, lower frequency). The problem is if these two independent resonant frequency ever coincide then bad things happen that make the system unusable at that load. The solution I used was to increase the pendulum length enough so that two resonant frequencies only coincide for loads that are above the max weight limit of the system (I chose 600 lb).

Every person I tried to explain this to claimed BS, we have been using this system for decades and nothing has changed. The truth is we use a variety of different adapters to fit different load cells each of which change the length of the pendulum meaning we only see the problem for specific combinations of adapters and target dead loads. The problem has been there forever, but most of the time it wasn't noticed because we didn't happen to be using the correct combination of adapters and target dead loads to let the two resonant frequencies coincide.

Lesson I got from this is chasing down a problem in an existing system don't give too much weight to other peoples opinions. Follow up on what you see, data informs theory, not the other way around. Due to personell change, nobody at my company were aware of Scott Wertel's investigation back in 2001.

Phil Ouellette, BSEE

RE: Vibration in Dead Weight Calibration Machine - Solved!!!

Quote (Phil)

that is inversely proportional to the applied load (more load, lower frequency)

This sounds backwards. Thinking of any string musical instrument - more tension --> higher frequency.

Anyway, good job at solving a mystery.

RE: Vibration in Dead Weight Calibration Machine - Solved!!!

Quote (Greg)

It's a bending mode
idea

RE: Vibration in Dead Weight Calibration Machine - Solved!!!

That was the "Thinking about it until the lightbulb of understanding comes on" icon.

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