# Completely stuck - Attempting to construct a trommel

## Main Question or Discussion Point

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Hi

I am trying to construct a firewood trommel and have been going in circles (no pun intended) in order to calculate the max torque and HP of the electric motor that might be required. Any help would be greatly appreciated.

I know the following

Circumference =229cm
Length of trammel = 263 cm
Weight (inc load of material) = 345kg
Target rpm = 15
Drive wheel (D) = 16cm

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anorlunda
Mentor
I had to look up the word trommel. I learned that it is a revolving cylindrical sieve for cleaning or sizing ore. Is this close to what you mean?

anorlunda
Mentor

berkeman
Mentor
I'm getting dizzy...

Spot on anorluda. I moved the post to the mechanical engineering thread as I thought that was more appropriate.

berkeman
Mentor
I moved the post to the mechanical engineering thread as I thought that was more appropriate.
In the future, if you want your thread moved to a different forum, just click the "Report" link in your post, and ask the Mentors to move it. That way we don't end up with duplicate threads all over the place. Thanks!

berkeman
Mentor
I am trying to construct a firewood trommel
So now that we know what a trommel is thanks to @anorlunda, why do you want to tumble firewood in one? Is it for cleaning or texturing or something? Thanks.

russ_watters
Mentor
I am trying to construct a firewood trommel and have been going in circles (no pun intended) in order to calculate the max torque and HP of the electric motor that might be required. Any help would be greatly appreciated.
This is a really difficult problem because it depends on the amount of firewood in/passing throught the trommel and how it interacts with the wood. For example, if there are baffles that spin the wood (like those inside a clothes dryer), that adds torque. And there's just no easy way to model it.

My suggestion is to see if you can find existing trommel designs and match them. The good news is you can vary rpm and flow rate to vary torque and hp.

So now that we know what a trommel is thanks to @anorlunda, why do you want to tumble firewood in one? Is it for cleaning or texturing or something? Thanks.
In order to sift out splinters and fines. Customers don't want anything too small.

This is a really difficult problem because it depends on the amount of firewood in/passing throught the trommel and how it interacts with the wood. For example, if there are baffles that spin the wood (like those inside a clothes dryer), that adds torque. And there's just no easy way to model it.

My suggestion is to see if you can find existing trommel designs and match them. The good news is you can vary rpm and flow rate to vary torque and hp.
The trommel weighs 245kg and I estimated that at any one time there would be max of 100 kg of firewood travelling through it. It doesn't have any baffles. It is to all intents and purposes 2 wheel rims (either end) with 30 lengths of steel set at 50mm gaps for the fines to fall though.

anorlunda
Mentor
This is just a rough guess. Assuming that via belts or wheels the trommel rotates about 10 RPM and the motor at 1500 RPM, then it reminds me of a washing machine. 1 to 1.5 HP is adequate for most washing machines.

Are you the DIY type? There are several past thread here on PF for how to find a used washing machine motor and how to wire it up.

phinds
Gold Member
... gaps for the fines to fall though.
What are fines?

Bystander
Homework Helper
Gold Member
What are fines?
Any small particles, twigs, dust, rock flour, ....

phinds
Gold Member
Ah. Gotcha.

Baluncore
When rotating, the wood will be carried part way up the wall. The distance will be determined by how much wood is being processed at the time. The energy input will lift the wood approximately one trommel radius per half turn of the trommel. Estimate the motor power requirement from the weight being processed, radius and time to rotate a quarter of a turn.

Baluncore
DAY1865 said:
I know the following Circumference =229cm Radius = 36.45cm
Target rpm = 15 Drive wheel (D) = 16cm
I estimated that at any one time there would be max of 100 kg of firewood travelling through it. It doesn't have any baffles.
15 RPM = 4 seconds per turn = 1 sec per 90 degrees.
100 kg will be lifted to a height of about 0.40 m every second.

Potential Energy = m·g·h
100 kg * 9.8 m/s² * 0.4 m = 392. joules, every second = 400 watt.
Call it 745 watt = 1 HP.

A 16 cm drive wheel on a 73 cm drum gives a final drive ratio of 4.55
The 16 cm diameter drive wheel must turn at 15 * 4.55 = 68.25 RPM.
A motor might turn at about 1650 RPM.
So you will need an additional reduction of 1650 / 68.25 = 24.2
That will probably need a two stage gearbox on the motor.

You may have problems with the drive wheel slipping unless you use a roller chain drive. That would allow a smaller chain drive sprocket on the motor.

Sap from wood may stick to dry metal, so you may need to spray a fine mist of water on the metal grid bars to prevent buildup of sap and then debris that might block the grid.

Tom.G
A 16 cm drive wheel on a 73 cm drum gives a final drive ratio of 4.55
The 16 cm diameter drive wheel must turn at 15 * 4.55 = 68.25 RPM.
A motor might turn at about 1650 RPM.
So you will need an additional reduction of 1650 / 68.25 = 24.2
That will probably need a two stage gearbox on the motor.
Or a jackshaft with a reduction ratio of 4.8± from motor to jackshaft and jackshaft to drive wheel.

Disadvantage: needs guards so you don't lose body parts

Cheers,
Tom

You may be overthinking this. A lot of the sorting for gravel is done with sloped screens. Given the big size difference between what you want to keep vs discard, putting it on a simple conveyer and running it down a 2" mesh grate is probably sufficient.

I think trommels are used to separate material where it's smaller, and somewhat stickier, or where the division between sizes is small.

Well, this might be a very disappointing attitude but if everything else fails ... well, it still works, you see
Pick an existing trommel from somewhere and just check the engine power...

(Source is some random trommel-distributor from an online marketplace)

Last edited:
Baluncore
You may be overthinking this.
That is engineering.
A lot of the sorting for gravel is done with sloped screens. Given the big size difference between what you want to keep vs discard, putting it on a simple conveyer and running it down a 2" mesh grate is probably sufficient.
The trommel breaks up lumps of feed-stock by dropping and tumbling them against each other. Because it rolls short lengths of firewood, it actively removes and then separates bark from the logs being processed. The bark removal process is not satisfied with a simple screen.

Pick an existing trommel from somewhere and just check the engine power...
Good data. The smallest is 50 tonne per hour, with 3 Kw. But the OP is building a smaller system, so the 1 HP motor seems like a reasonable solution that should not stall.

anorlunda
Mentor
The bark removal process is not satisfied with a simple screen.
When he said remove "fines", I get the idea that it is just removing the small pieces that fall off easily and that might litter the trunk of the consumer's car on the trip home. Not a complete debarking. Otherwise, I agree with everything you said.

I still suspect that a washing machine size motor would be adequate.

Baluncore