PSN-L Email List Message

Subject: Re: Real time traces
From: Brett Nordgren brett3nt@.............
Date: Fri, 21 Aug 2009 19:01:59 -0400

Hi Karl,

Nothing particular at the ends.  The stress builds from the center of the 
spring as you move toward the ends by maybe 25 or 30%, but nothing terribly 
exciting happens at the clamps.  However there is another problem related 
to creep that we did address.  By using relatively thin (0.012") spring 
material we kept the max stress level down to roughly 70-75,000 psi, while 
using material with a yield strength of 280,000 psi.  By working at 1/4 of 
yield, creep is a non-issue.  If you missed it, I ran through some numbers 
in my 8/16 message to Barry Lotz.

In general, I feel that most home designs use springs which are much too 
thick and hence too highly stressed and prone to creep, etc.  Of course if 
you use a thin spring, the mass has to go down too, but in a feedback 
instrument, that's all to the good. (up to a point, of course).

Actually, Dave was the one who designed the spring.  He may want to add 


At 01:57 PM 8/21/2009 -0700, you wrote:
>Hi Brett,
>Thanks for the reply.
>I have a mechanical design question about your sensor. Did you do any 
>thing special with the spring anchor / clamp mechanism?
>For some time I've been focused on creep when leaf springs are clamped and 
>stressed. The thought is that when the spring gets bent, the stress in the 
>outer fibers of the spring will be beyond what the clamp can retain. This 
>is especially true right where the spring emerges from the clamp.
>In some of my experiments this has caused noticeable hysteresis and creep. 
>I surmise the same problem might occur when clamping flexures although the 
>stress is bound to be much lower than in a leaf spring.
>Possibly some of this can be dealt with by stressing the spring and then 
>clamping it, and maybe even artificially aging the assembly after clamping.
>Ideally the spring would be made with thicker ends for clamping but this 
>is obviously impractical for our purposes.
>I'm curious if you did anything to help mitigate this effect.

Watch our wiggles

or watch some very very good wiggles


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