calculated VS measured axial deformation on shafting

[nickjk]

In a current design I am applying a axial compression force onto a shaft. During testing I was suprised at the difference between the calculated axial deformation and the measured axial deformation. Before I can move forward in correcting the design I need to have a better understanding where my calculations or measuring equipment failed. Any help would greatly be appreciated.

Formula used Delta = (F x L)/(A x E)
F=axial force applied, L=axial length of steel tube, A=effective area of tube cross section, E=modulas of elasticity for steel

To simply the test I used a simple tube shape with the following dimensions O.D. = .938 in. I.D. = .618 in. length = 2.990 in. E for steel 30,000,000 psi

A 2000 lbs axial load is applied slowly to the tube.

From my calculations the deformation should have been .00051 in.

The testing machine is showing a deformation of between .00115 and .00120 in.

The testing machine ram is 1.7500 diameter traveling thru close tolerance bushing to prevent angularity in vektors.
The transducer is mounted between the top of the ram and the coupler that connects to the hydraulic cylinder. The indicator readings are taken at the bottom of the ram.


Thank You
Nick

 

[EdStainless]

Do the test again, this time load to 1000#, unload to 100# and zero everything, then load to 2100#. My guess is that your biggest errors are zero offsets.
You are fairly close. There may be a problem in doing this with a tube. You will see diameter change (remember Poison) that is greater than with a bar sample.
You may want to use a better E for the material. If this is a low carbon steel my handbook lists 27.9*10^6

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[nickjk]

I would like to express my thanks for your support.

I spent most of today checking the accuracy of the test stand. I was able to verify the load cell readings by using hydraulic pressure readings with the know cylinder bore. I was able to verify the indicator with the use of gage blocks on the test stand. I verified the stability of the test stand by measuring deformation of the base at 3000 lbs the value was .00005 which I feel is fairly accurate. As you stated after a load is applied I would remove pressure and make sure indicator would return to zero then apply and remove the load at least three times with the value returning to zero before I would make my measurements. There may be something funny happening at the end of the ram where there is a 5/8 hex hardened steel rest button. I am looking at revising the design of the ram next week to make sure I dont have stress issues increasing the strain (this is reaching)

You are right, I am seeing measurments on the side of the tube indicating growth in compression which is expected.

Do you know of any calculation that would give me a more accurated result or what type of accuracy I can expect from using these calculations?

I guess in past projects minimum deformation has not been such a issue. You calculate your stresses and include a safty factor and normaly are good to go. The sucess of the project I am working on is highly dependent on controling this axial deformation.

Thank you again, any help would truely be appreciated

Nick