TORQUE REQUIRED TO ROTATE BRUSH UNDER WATER

[iDAMECH]

Hello! I wondered if anyone has any experience with designing machinery using brushes. I am a machine designer and have been asked by a customer to estimate the torque required to turn a long, cylindrical, spiral wrapped brush (about 24" OD, 160" long, 6" pipe core). The brush will be mostly submerged in water, will be inside of and in contact with a larger steel drum that is rotating in the opposite direction, and will have potatoes tumbling in the mix. I wondered if at the very least there is some data on resistance of brushes to turn in water and to turn against different surfaces. I was hoping to figure the total torque required by adding up the contributing torques, ie. torque from water + torque from drum + torque from product.

Any thoughts?

Thanks!

 

[handleman]

You can look up the value in the CRC handbook tables. Check the section for "Brushes, partly submerged, steel, tumbling potatoes". Be sure to read the "Opposite direction" column instead of the "Same direction" column.

 

[iDAMECH]

I'm not familiar with the CRC handbook tables. Can you provide a link to these? Is there really something that specific in there?

 

[SnTMan]

OK, I gotta ask: Why are potatoes tumbled?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[iDAMECH]

They are tumbled to clean them.

 

[iDAMECH]

Here is a link to a video of the machine:

https://www.youtube.com/watch?v=YI-ZAJ21wPY

Obviously there isn't a brush in there yet. Image a large brush in the same location as the buildup of potatoes in the drum so they are between the brush and the drum. I've attached a sketch taken from a shot in the video.

 

[3DDave]

I think handleman was making a joke. The chance that there is general knowledge of such a specific case is low.

I suspect that to get formulas to describe that condition would involve millions to tens of millions of dollars in testing; not a thing that is likely to happen.

I would start with a guess that, with a perimeter of about 6 feet and length of 14 feet at a lever arm of 1 foot, one might expect between 1 lbf and 10 lbf of shear per square foot, so between 84 and 840 lbf-ft of torque. I would probably double the average estimate to order a motor/gearbox of 900 lbf-ft.

Post installation, either look at the power required to drive the motor at the desired rotation rate, which will give a better indication of the torque being supplied or measure the torque directly by putting the motor on an instrumented platform or install a shaft torque instrument.

The worst outcome if the estimate is too high is that the motor/gearbox costs more than required. If the estimate is too low, then not only does the testing get delayed, a new motor/gearbox has to be acquired, the structure possibly needs redesign/reinforcement/ or scrapping and replacement. The cost of underestimated and overestimated drawings and structural analysis are the same, so there's no savings in wimping out. The cost of assembly and fabrication will be about the same.

Best of luck - cleaner potatoes for a better world!