Building a Soil Sifter / Rotary Trommel

Spoiler alert! If you can't wait for the final scene in this instructable, check out this video for the completed trommel.

This instructable is a companion to my previous Bike Spoke Forged Knife. In that instructable I used the bicycle spokes, here I've used the wheel rims.

Back story

Earlier in the year I processed a ton of apricots and discarded the stones in the compost heap. Months later when my wife wanted to use the compost, it was apparent that the stones were decades away from naturally decomposing. And I knew I needed a method to remove the stones (and other detritus).

I did some research and found a rotary trommel was the way for me to solve this problem. There are many examples of rotary sifters on the web but this instructable chronicles my build where I re-purposed most of the material. The only new components were castors, wire mesh and the vee pulley (which I barted for with some beer).

3 x 26" Bicycle rims (actual inside rim diameter is 21 inches (550mm)

8 x castors

5 mm wire mesh

cable ties (zip ties)

Steel box tube (although timber would work too)

motor

vee pulley

vee belts (assorted sizes until I figured out what I needed - one large belt to drive the mesh tube)

bolts and nuts

2 x hinges

2 x bicycle forks with wheel attached

solid sheet or similar to deflect soil downwards

I cut a length of sheet metal to attach to the inside of the wheel rim. This probably helps to support the mesh but you may be able to get away without using it. I cut it 4 inches (100 mm) wide* then carefully removed the burrs with a file. I used the existing spoke holes in the rim to temporarily attach the sheet metal using lacing wire.

I calculated the inside circumference of the wheel rims and cut the mesh with 6 inch (150 mm) overlap**. Stand the mesh on its end and drop two wheel rims over it. At the top fix the third rim in place with clamps*** then begin to fix the rim to the mesh using the existing spoke holes. You should aim to use every hole twice and I found it useful to drill out the hole a little to fit in my 5 mm cable ties. At this stage, decide where the 'head' of the cable tie will be...inside or out. I chose inside so it wouldn't interfere with the castors (but since discovered the castor rides above the cable tie). It would be better to have the head outside of the chamber...one less thing to block material.

Fix both ends with cable ties then begin work on the center rim. I used a length of timber to ensure this rim was evenly centered in the tube. It helped to have a second person assisting placing the cable ties from the outside while someone is reaching into the chamber...it's quite awkward. Pro tip: Begin your first cable tie at the edge of the mesh then continue around the mesh. I didn't do this (I began in the middle) and ended up with the mesh pulling in slightly. Happily it hasn't affected performance.

With all three rims cable tied into place, begin lacing the mesh overlap together. A second person here will also help.

Trim off the excess cable tie tales.

Changes I would make:

* If you install a sheet metal liner beneath the rim, keep it to the width of the wheel rim and no wider. Any wider (as I found out) blocks filtered material from passing through. See the exception to this below ***

** Limit the mesh overlap to maximum 2 inches (50 mm) as this double layer hinders filtered material from passing through.

*** While using the trommel I wish I had just a little more of the mesh protruding from the frame so that I could more easily place a wheelbarrow beneath it where it catches the junk. I would suggest installing this rim about 6-8 inches (150-200 mm) back from the end of the mesh, and this should be braced with a wide section of sheet metal that is attached to the rim and extends to the end of the mesh.

Measure your mesh tube and cut steel to suit. Plan for the tube to be contained within the frame, although I would suggest the exit end (where the rocks and waste come out) protrudes 6-8 inches beyond the frame.

Weld up the frame and place the tube inside so that mount points can be established for the castors.*

I placed my castors in such a way that if I want to remove the tube, I can easily wind the threads into the frame, thus opening up the gap and allowing the castor to be eased from the bicycle rim.

The box steel I used was thin walled so I welded nuts in place for the castors to thread into. There is no need for an additional locking nut as once the castor is embedded in the wheel rim, it cannot turn. It is unlikely that the thread would wind itself up or down over time.

Pro tip: it's useful to hang the large drive vee belt around the tube prior to installing all of the castors. If you don't, you'll have to loosen your castors to install it later.

Changes I would make

* After I had completed the frame, I realized one end rim did not line up with the frame, hence in the photos you'll see I made small weld-on extensions that the castors were fitted to. It hasn't affected performance but took more effort to rectify. It would have been much simpler to mount all castors in the frame.

This motor is 1/4 hp and cost me $15 bucks, with the previous owner gifting some used vee belts and a 2-sided pulley housing. The motor rpm was too high so I found a large vee pulley that geared the belt right down to a usable speed. Your motor and vee pulley combo's will infinitely vary so some trial and error will be required.

I placed the motor and housing on a bench to establish direction of motor and size of belt I needed.

I then (precariously) clamped the motor and vee pulley housing on timber blocks while I figured out placement and position to obtain belt tension from the motor to the housing, and from the housing to the tube.

My large drive belt (about 2 m in circumference) needed the pulley housing to be lifted (to take up excess slack) so I made up a steel frame. To create tension, I fitted a sturdy hinge to one end and bolted (rather than welded) this to the frame. To recess the nuts, I drill a large hole into the frame, hover the nut flush then carefully weld and grind it flat. Use a sacrificial threaded bolt to hold the nut in place so you don't mess up your good bolts with weld splatter. You may need to tap the thread if any weld sneaks onto the threads of the nut.

The motor was mounted independently so I could adjust the motor-housing belt tension, again by using a hinged mount. At the other end of both mounting frames, I fitted a long bolt that lifts or lowers the frame to adjust tension. Note a simple way to make a turning knob out of a bolt.

Considerations:

Plan on a height that will allow a wheelbarrow to sit underneath from either the side or the rear (exit).

You also want to plan a gradient for the soil etc to drift down. I can't tell you the exact angle I mounted my frame but it's probably 4 inches (100 mm) lower at the exit end. If you don't have a gradient, your sifted siftings will sit in the sifter!

You don't need to make this a mobile trommel (though I highly recommend it). It's not too heavy so two people could carry the trommel and place it on saw horses or similar at the job site. However, mounting it on wheels means one person can easily move it from place to place.

Yes, stub axles would have been tidier and prettier, but I took the simple option of welding the forks to the frame. This worked just fine. Place some timber under the frame to figure out your tipping/balance point and place the forks a little in front of this. Otherwise it could tip forward when being wheeled around or during use.

The rear legs I made to swivel in three positions: pointed down with feet on the ground; pointed horizontal where they act as handles (think wheelbarrow); straight up for storage (though in hindsight this position probably won't be used).

Mount a solid sheet of something (I used ACM) to the frame to stop soil etc being sprayed out the side of the trommel. I mounted this panel on one side only but it could be useful on both sides. Definitely needed on the drive side though! If you had access to those soft plastic hanging industrial door entrance strips, they'd make great curtains.

With a bit of trepidation, I wheeled the trommel out to our compost heap. Those pesky apricot stones were about to be dislodged!

I really didn't know how the machine would perform. Was the mesh too small...the rotation speed wrong; the angle of the frame incorrect...lots of decisions I'd made were about to be tested.

Well it was a brilliant feeling to see the trommel operational and doing a most excellent job of sorting out the good from the bad. Looking at the video and photos, you can see how efficiently it removes rocks, bits of plastic and glass, apricot stones and even weeds. The bad stuff ends up in the wheelbarrow, while the rest piles up beneath the trommel.

The machine was now worthy of a paint job so I applied a protective rust converter then a top coat to the frame.

Aint she a beaut!

About the legs/handles. I planned to wheel the trommel by adjusting the legs into a horizontal position then wheeling the trommel around. But it wasn't long until I realized a broom handle and wooden dowel slotted nicely into the frame and became great handles. No longer any need to adjust the leg position. Use what you have lying around.

This freshly rotary hoed section of lawn needed sifting so bring out the trommel (to be honest I wouldn't have even considered doing this job without the sifter).

You can see how much junk and large rocks lay beneath the surface of our grass. We ended up with about 8 wheelbarrow loads of rocks, stones and glass fragments from that one strip of lawn.

This is an amazing machine and I'm very pleased to have it available. It should give long service and working parts are easily replaced.

Thanks for watching.