Roll case stresses

[albresco]

We have a simple roll case which will have the final roll centre start to crack once the chain starts to wear. The rolls are driven by a series of double/single sprockets. There is no provision for soft start. The cause is due to the slack in the drive chain necessitating the last roll to catch up as much a 1/2 a revolution before the slack is out of the chain with the effect of a very high acceleration and thus a high force. Has anyone done any analysis on this type of situation?

 

[ccw]

Hi albresco,

If I understand your description correctly, it appears that the rotational inertia of the set of rollers all react on the last roller getting it up to speed, The irresistable force meets the immovable object!

I would suggest a series of some heavy spring loaded and damped arms and idler sprockets or rollers located on the chain between the drive and driven sprockets in the roller set. This will take the static slack out of the stretched (worn) chain, so the last roller begins to move with the first roller. Note that this may place an additional accelerate-to-speed burden on your "across the line start" prime mover.

Another approach, would be to use a centrifugal mechanical or fluid clutch coupling on the last roller drive shaft to help absorb the start up energy. This approach may not be viable if the final roller speed is below that needed to fully activate the clutch to the torque required.

 

[albresco]

Thanks for your help. The information you have supplied will assist in a remedy. However, at this point, I am trying to quantify the forces exerted on the roll hubs. My client requires a report explaining why these rolls are failing.
I was wondering if anyone has done any calculations to determine the stresses in the hubs from the impact loading.

 

[ccw]

An analytical model to determine the forces would be complicated and would require estimates of compliances and nonlinearity considerations. Another alternative to determine the forces would be to run a series of tests with calibrated fusible links in the drive chain or in the torque connection between the sprocket and roller. In other words, arrive at the forces through testing rather than analysis.