Designing a mechanical stopper

[DRSEngineer]

I need to design a mechanical stopper with a spring feature but before I do this I need to caculate the force for which a gimbal will strike it.

The gimbal is cylindrical in shape and not uniform in shape and rotates about the x axis, its long side. Rotates 120 degress in a 1/4 second and the total mass of it is .415lbs.

I am having a hard time trying to figure out what equations I need to determine the force that will strike the stopper.

The overall length of the gimbal is 3.14inches, the motor is .591 inches from the left side. The point of impact is .466 from the left and .125 from the motor.

I pretty much have the equations I need to design the spring like stopper so the only thing holding me back is trying to derive the force of the gimbal.

Thank you!

 

[desertfox]

Hi DRSEngineer

A picture speaks a 1000 words, can you please upload a sketch and we might be able to help further.

desertfox

 

[DRSEngineer]

Here is a rough sketch. I included the intended stopper design.

 

[desertfox]

Hi DRS

Sorry but I am having trouble understanding your sketch, is the x axis as you of drawn it coming at from the paper or is it meant to be parallel? also in the top skecth what are the hidden detail lines? I thought it was the cross section of the disc, ie to show the disc shape.
The bottom sketch is that a view from the lefthand side of the top sketch?

desertfox

 

[DRSEngineer]

Sorry I had to go to a meeting right before I drew the sketch. Ignore the hidden lines, thats a second axis point of rotation. We have a lens in there that rotates in the Z axis.

The x axis is inline with the paper. The bottom sketch is a cross section of the left hand view.

 

[desertfox]

Hi DRSE

Thats not an easy calculation, however all I can suggest is use an energy balance to get you started.
So if the gimbal rotates 120 degrees in .25 seconds you can work out the velocity and hence the kinetic energy which the gimbal strikes the spring loaded stop.
Now if your spring loaded stop is on a compression spring it possible assuming no energy loss to equate the kinetic energy of the gimbal to the stored energy of the spring ie:-

kinetic energy gimbal= 0.5 * k * x

where k = spring stiffness

x = spring defl

So if you now how much deflection of the spring you can afford after contact with the stop you can determine spring stiffness and hence solve the above.
A word of caution though, what have you got in the way of damping this device, because after you compress the spring,the spring will try to release its energy back on the gimbal or in a nut shell the gimbal will bounce back.
Thats the best I can do with the info you have posted.
You won't be able to obtain a force as this is an impulse problem amd you would need to know how long the impulse lasted in split seconds before you could even start comtemplating a force.

desertfox

 

[unclesyd]

Instead of clanging and banging around your device could you incorporate a C-Flex bearing to control the motion. The word mirror scared me.
You will still need the numbers from the calculation format posted byDesertfix{

http://www.c-flex.com/#self

 

[zekeman]

I would use a cam oe a geneva type mechanism to control the "stopping" in view of the optical system you have.
However, if you feel confident about using a spring stopper, then use the energy method proposed by desertfox.

 

[IRstuff]

Generally, whacking stops is usually not desireable, as there is no controlled motion. Usually, stops are intended to counter some undesired and unintended motion, and are not supposed to be integral to the normal functionality of the system.

Isn't your servo controller supposed to decelerate the gimbal BEFORE it hits the stops? Has the systems engineer blessed the notion of the gimbal bouncing and shocking itself every iteration of motion? I don't know of many optical systems that tolerate that type of behavior from its gimbals, as that usually results in misalignments and poor reliability. If you reduce the shock to the gimbal, the gimbal will bounce like crazy, and if you preven the bouncing, the shocks might be excessive for proper optical performance.

TTFN

FAQ731-376

 

[DRSEngineer]

The bounce back is something I will have to look into, didn't even think about that.

I inherited this project so the entire gimbal design has been completed so I don't have many options or much information about the servos controlling the motion. I will have to raise some concerns about shock and reliability issues. Hopefully I can find a good solution for this.

Thank you to all this has been very enlightening.

 

[DRSEngineer]

It looks like my supervisor didn't give me all the information.

The stoppers are only going to be used if the gimbal loses power mid cycle and the mechanism they have in place to slow it down fails.

Thank you again for all your help.

 

[IRstuff]

THAT makes way more sense... Thanks for filling us in.

TTFN

FAQ731-376

 

[desertfox]

Hi DRSEngineer

Your welcome, glad you managed to sort it out.

desertfox