obtaining force/displacement values?

[PH163RD]

Hi all,
im currently involved in the redesign of a major component at our plant and am required to gather data on the forces acting on the machinery.

the piece of equiptment is a roller which is pneumatically powered to rise and lift a sheet and then fall via quick exhaust acting as a quick outlet for the pressure.

im aiming to calculate the bending moment and shear stress acting on the clevis brackets holding the roll in place.

any ideas on how i can obtain some acceleration, force values using instrumentation?

regards

 

[MintJulep]

Accelerometers and load cells or strain gauges.

 

[solider]

I agree with MintJulep, as a first stab.

If you want to get displacement directly, then you could attach an LVDT to the base/cylinder and roller/piston.

If you are going to use an Accelerometer, make sure its a DC Accelerometer - then you can integrate to get velocity and displacement.

Finally, how about monitoring the air pressure throughout the cycle - a poor second to the load cell.

For all this you should acquire data at something like a kilohertz - as a guess, not knowing your system in any detail.

 

[powerjunx]

I agree to MintJulep and solider.

 

[electricpete]

Does the stress of concern occur at the end of the fall? If so, do the brackets come to rest on a mechanical stop to halt the momentum of the entire assembly?

If not, is there a particular part of the cycle you are concerned with?

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[PH163RD]

yes the stress concern occurs at the end of the fall. the brackets come to a rest due to the piston of the pneumatic cylinder bottoming out!! they problem occurs where the inertia force of the roll on descend causes a bending force on the clevis brackets.

what data will the accelorometer actually display?

 

[sreid]

Would not be a good solution be to add pneumatic or hydrolic snubbers to spread the impact force over time?

 

[GossamerWing]

You can create a simple accelerometer with a spring of constant (k), a block of mass (m), & a linear potentiometer set to measure displacement in the same axis as the rise and fall of your roller.

For the block, the mass (m) is the weight divided by the acceleration due to gravity (g).

For the spring F = kx.

And by Newton's 2nd F = ma . . . so ma = kx, and a = kx/m.

Mount the potentiometer on the bracket, the spring on the potentiometer shaft and the block on top of the spring.

Each time the roller falls, the potentiometer will record a max displacement (x).

From this you'll get acceleration without spending a fortune for equipment that won't be used long term.

 

[PH163RD]

i have an accelerometer which displays the voltage amplitude over time. is there a simple way to convert these units to force or something more useful?

 

[GregLocock]

Well, if you know the mass of the roller, and the acceleration, you certainly know something to do with forces, or my name isn't Mr Newton. Which it isn't.

Cheers

Greg Locock

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