Building a Light Clay Straw Tumbler

A light clay straw tumbler is a tool for coating straw with clay slip to create light clay straw which is then typically used for infilling wood framed walls. While making light clay straw by hand is easy if you want to make it in bulk (say for a house) I recommend building a tumbler.

Several years ago we built a tumbler to infill the walls of our cabin. We used around 80 bales of straw. After the walls were packed the tumbler migrated around the yard until finally succumbing to scavenging.

We built another tumbler mostly from scratch a couple of weeks ago to infill the walls of the Earth Berm Cottage(EBC) and I documented the process. Our tumbler is based on the Riverstone Studios design.

The Frame

The frame is a two feet by six foot rectangle. I used two by fours for everything except the legs which are 4x4s. The tumbler needs to withstand a lot of vibration so I put in a lot of diagonal braces. I made half laps joints in the legs and fastened them to the frame with lag screws.

The barrel cradle is another two foot by six foot rectangle. I hinged it on one end of the frame to make it easier to adjust the angle of the barrel.

The Barrel

The barrel consists of three 55 gallon metal drums fastened together to form an 8-9 foot long barrel. First I used my angle grinder to cut off the bottoms of the drums. I went through several cheap cut off wheels but it went well.

Once the ends were off I cut tabs into ends of two the drums. This way one drum can insert into another. The tabs or 2 inches deep and initially I made cuts 8 inches apart. I had trouble with the second drum so the cuts are 4 inches apart on that one. Keep in mind that you want the finished (non-cut) ends of the drums on both of your openings. Before fastening the drums I needed to make the “fingers”.

The “fingers” are one foot long pieces of flat steel with several long nails welded to it. They manipulate the straw, separating it as it tumbles through the barrel. This was the only welding I did. The fingers could be replicated using long bolts and sections of slotted steel.

The “Hands”

I had two three foot long pieces of flat steel so I welded on the nails and cut them into 1 foot sections later. After using this tumbler for a while I’m really liking the flat steel. We used angle iron in our previous build that tended to trap clay slip.

Keeping with the fingers metaphor we’ll call the one foot sections of flat steel and nails “hands”. The hands are attached to the barrel with self tapping screws.

Here are a some placement considerations:

1) Locate the hands no closer than 12 inches from either opening. I put mine 18 inches away from each opening.

2) Put each hand about 120° of out alignment with it’s neighboring hands. That seems to work well to separate the straw.

3) Install the hands as you connect the drums. This way you to don’t have to crawl too far into the barrel to make your attachments.

4) Make your attachments mid span between the ribs of the drums so they don’t interfere with your casters and drive belt which will be close to the ribs.

5) Work with a buddy. Attaching the hands took me about four times as long it should because I was doing it by myself.

The barrel sits on two sets of 5 inch rigid casters. Since the caster location is dependent on the ridge configuration of the barrel I mounted each set on a piece of 2×4 that was not tied into the cradle structure. I put the casters about two inches from each end so I could easily attach the 2×4 to the cradle.

Each set of casters should be located just below a rib of the barrel. In our case that put the two sets of casters 5 and a half feet apart. The barrel is about two and a half feet longer than the cradle. I’ve found it useful to have more of the overhang on the lower side.

A 5-10 degree barrel angle seems to be the sweet spot for getting the clay slip and the straw to do their dance. I created a spacer using an 8 inch piece of two by four with a slight angle on one end attached to 15 inch 2×4.

The Drive System

The tumbler rotation is driven by a 1/3 horse power electric motor salvaged from an old dryer. I think any electric motor that size or larger would work. This motor didn’t have any good mount points so I built a small plywood box to fix it in place.

Dryer Motor in Plywood box

This motor spins at around 1700 rotations per a minute. We want the tumbler to spin at 20-35 rotations per a minute. We put a 1 1/2” drive pulley on the motor drive shaft so if we connected it directly to the 24” diameter barrel the barrel would spin at 1/16th of 1700 rotations per a minute or 106 rpm which just push the straw and slip against the sides and not allow it to tumble.

To slow it down we made a secondary drive shaft with two drive pulleys, one 6” and one 2 inch. The primary drive 1 1/2” shaft connects to the 6 inch pulley so the secondary shaft spins at ¼ of the speed of the motor or 425 rotations per minute.

The 2 inch drive pulley on the secondary shaft is connected to the 24 inch barrel so it spins a 1/12 the speed of the of the secondary shaft or 36 rotations per minute.

To make the secondary shaft I used a pair of pillow blocks and 5/8” solid steel rod. Initially, I used a threaded rod but the pillow blocks couldn’t grip it well and the rod would slowly walk the drive pulleys out of alignment.

I used a 3/8” wide 30 inch v-belt to connect the primary shaft to the secondary shaft and a 3/8” wide 96 inch belt to connect the secondary shaft to the tumbler. I would buy extra 30 inch belts as it takes more of a beating than the longer belt.

To tension the 30 inch belt and align the pulleys I mounted the motor and secondary shaft assembly on a platform made from two boards connected by two half inch threaded rods. The pillow blocks are slightly wider than a two by four so in my final build I replaced one of the two by fours with a 1×6 ripped down to five inches.

We’ve processed nine bales of straw so far with the new tumbler and it’s working well.