How to Build a Motorized Trommel—And Why on Earth You Would Want To

Dec 08, 2023

I feel bad for my neighbor.

Brian's the earth-biscuit type, with a flop of blond hair and a kayak rack on his Jeep that he actually uses. He's a community-garden aficionado and a yard farmer who could talk compost for hours—mostly because there's a massive heap of it in his backyard. Brian's compost pile is the Everest of our neighborhood. It is robust of scent and full of twigs, old pineapple rinds, his Australian shepherd's buried rawhide chews, and gigantic mounds of last year's oak leaves. And buried deep inside is some of the best compost Mother Earth has ever cooked.

Amazing stuff, if you can get to it.

A few years ago Brian built a manual compost sifter, just a big screen within a frame, and he shook small batches of compost through it, separating the fine material from whatever hadn't finished breaking down. He used the rich matter to top-dress his lawn, which improves moisture retention and soil structure, and to make his flower and veggie beds go nuts. He reduced the size of his compost mound and made room for the fall leaf drop in our neighborhood.

"When you just haul away your leaves, you're losing that whole year's worth of solar energy stored as carbon," he says.

See? That's how Brian talks. He's committed to the organic and sustainable life. Problem is, Brian has a bad back. Hours of manually sifting heavy compost ran up his chiropractor bills.

Then Brian unearthed an old rock bed left behind by a previous owner and thought about how great those rocks would look on the other side of the yard—but first the bed would need to be sifted and cleaned. The very thought of putting it all through his manual sifter nearly put him in traction. So he hit the Internet to find a better solution.

Brian decided to build a motorized trommel, a rotating cylindrical screen that separates fine material from rough. It's especially good at shaking out fine compost from a heap of rot and leaves.

When Brian first told me about this trommel project, he was so stoked about it that he got me excited too.

"I'll be able to shovel in a few loads and sift out the good stuff, then put the rest back to keep cooking," he said. "It'll be great!"

But when I arrived at his house to check it out, he took me to a dark corner of his backyard.

"There it is," he said, a little sheepish now, pointing to a screen drum, lying on its side near the compost heap. "I haven't gotten very far. I've hit a roadblock."

You know how it goes. You start a project, then halfway through Saturday you're surrounded by tools and a half-finished mess. Brian had watched hours of YouTube videos by guys who'd successfully built mechanical trommels before him—guys like Paul Miller of La Mesa, California.

He watched as Paul framed a basic cylinder with bike rims and screening, then mounted it on a wooden frame with smaller wheels turning the sieve within the rims. Atop the structure he mounted a motor. The whole thing sat at an angle, so when Paul shoveled rough material into the higher end, the cylinder dropped fine material below and dumped chunky debris into a wheelbarrow or hopper.Brian got to work on the cylinder first:

1. Use three 24- to 26-inch bicycle rims for the cylinder frame. Brian grabbed his from the local bicycle collective. When I interviewed him, Paul said a friend who fixes bikes donated his. You get the idea.

2. Remove spokes from the bike rims with wire cutters, which leaves you with three hoops.

3. Roll hardware cloth or chicken wire (sized according to the gauge of sifting you want to do) into the hoops to form a cylinder shape. Brian pop-riveted his into place but you can also wire it.

4. Choose a motor. Brian bought a ½-hp electric motor at a tool shop. Paul said he found his ¼-hp electric motor for $25 on Craigslist.

Brian got stumped because he wasn't sure how to connect the motor to the trommel to get it to turn at a speed appropriate for sifting compost. How would he connect it to the cylinder?

He wasn't sure. Other projects filled his workshop. The trommel took a back seat. Eventually, Brian moved the cylinder into the backyard, where he felt bad about it for two years. The compost pile grew and grew.

Like I said, Brian's green-living credo is pretty infectious. I wanted to help him finish his trommel, so I figured I'd start at the heart of the problem: the motor. Lucky for us, our other neighbor, John, is a mechanical engineer for a major international manufacturer.

"The goal here is not to slow down the motor but to control a properly sized energy source," John says. The rotational speed of the trommel is critical for safety. About 20 or 25 rpm would be plenty. Plus, lowering the machine's speed would increase its torque—the twisting force that creates rotation—allowing Brian to sift larger piles of compost.

A basic ½-hp electric motor spinning at 500 rpm is obviously too fast to couple directly to a trommel. Additionally, that same motor creates about 5 lb-ft of torque, which is not enough to do the job. To make the motor work, John says the easiest solution is to purchase a speed reducer. These affordable, mass-produced units are readily available from industrial distributors and many websites. Essentially a speed reducer is a gearbox.

"In addition to reducing the speed to a manageable level, it'll increase the torque," John says. "The neat thing about gears is that when you arrange them such that the output speed is reduced, the torque increases inversely." For example, if you connect a 500-rpm motor to a speed reducer, and the output speed is now 25 rpm, or ½0 of the original speed, your torque now increases by a factor of 20.

"Your 5 lb-ft of torque is now 100 lb-ft at the gearbox," he continues. Most motors can be readily adapted to a wide variety of speed reducers through standardized flanges and coupling.

"Industrial supply houses and motor distributors can help you put a nice little package together," he says. "Ask an electrician to make sure your circuit is wired correctly to withstand the load from the motor."

Alternatively, you can reduce the speed using a series of pulleys.

Instead of using a speed reducer, Paul, the YouTube guy, rigged up a machine using the trommel's center rim for speed control. "I used a 21-inch bike rim as a pulley wheel to step down the rpms of the motor," he says. Other trommel builders use a 1,750-rpm motor with a 2-inch pulley (A) going to a 10-inch pulley (B), then a 2-inch pulley (C) going to the 25-inch trommel frame (D).

With that information you can easily calculate the output rpm with this incredibly simple equation:

Once Brian figures out his motor system, he can build the frame. And then he can finish his trommel.

1. The size of the frame will depend on the size of your cylinder and the position of the wheels you use to turn the cylinder within it. Brian planned to mount his trommel on caster wheels from the local Habitat for Humanity ReStore, but you could also use small wheels with an axle from a home store.

2. Frames are best made from 2 x 4 material with a plywood top that's sturdy enough to attach the motor to. (One YouTube builder made his motor mount adjustable for height because his pulley belts stretched out over time and he wanted to be able to tighten them.)

3. Screw the wheels directly to the frame to turn the cylinder. Paul Miller recommends just screwing the caster into the middle of a 2 x 4 and lining up the caster wheel with the middle of the bike rim, repeating on all four sides at each end. "There was no planning or measuring involved," says Miller. "I just basically built a square around the rims."

4. Extend frame legs to the ground or attach wheels to the base to make a more mobile unit.

5. Here's where you can customize. Some of the YouTube builders added a piece of sheet metal as a guard on one side of the trommel so it doesn't fling dirt and debris all over the yard. Some builders crafted different drums for different purposes—smaller screens for composting, larger screening for rock jobs. Others made the trommel contraption high enough that it could be directly positioned over raised garden beds to reduce the amount of shoveling required.

6. Position the trommel at a slight downward angle so, as it turns, discarded debris falls away. Use a wheelbarrow or similar hopper to catch the debris coming out the end of the trommel.

7. Enjoy, as Brian says, "a revitalized and amazing living soil structure with highly organic material!"

• Operate a motor-driven trommel from a circuit with a ground-fault circuit interrupter.

• Cover the machine; store it out of the weather.

• Build the machine with a large, accessible on/off switch. Don't rely on unplugging it to disconnect it.

• Don't allow kids to play with it.

• Shield belts and pulleys to keep loose clothing and hair from being entangled.

• Don't wear loose clothing, and tie back hair when operating it.

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