Introduction: Convert a Petrol Weedeater to Electric

When my Ryobi weedeater* died and I could not repair it myself, my mechanic advised that it wasn't worth him trying either. This was unfortunate, as I had a bunch of attachments for it: long-reach chainsaw, scrub cutter, leaf blower, and line trimmer.

We considered buying a cordless weedeater, and they are especially good for the 800m2 (0.2 acres) we call home, but I realised I probably had everything I needed to get the old one going again. What's more, a decent cordless one costs around NZ$600...

Much to my wife's despair, I keep the occasional thing in case it might come in handy some day. This is less of a problem, by the way, if you keep such things tidily stored and have a better reason than "it might come in handy some day". Well, this Instructable is a prime example of the "handy some day" principle. Everything I needed to make this work was already on hand in my recently tidied-up workshop.

Planning, thinking and test-fitting bits together took months, on and off; the original build only took a long afternoon with another short afternoon to redesign the clutch shaft. Keep in mind that although there was some design in this, if you can think of more efficient ways to do any step, please leave a comment. I've done a number of searches on the net but cannot find anyone else who has done this, which seems bizarre: if you have any links, please share them.

Here's how I did it.

* line trimmer, whipper snipper, weed eater, etc

Step 1: Preparing the Motor

This motor came from an old washing machine left out on the kerb in the days before the city council banned such things. It was a bit rusty and dusty, with various attachments unneeded for this project (micro-switch, capacitor, and the like).

I stripped the motor, blew out the dust with compressed air, cleaned and greased everything, and reassembled it. I couldn't get the V-belt wheel off the shaft, but the front bearings show no sign of wear.

Stripping a motor is a subject of its own, so let me refer you to such excellent 'ables as:

https://www.instructables.com/id/Starter-Motor-Repa... by liquidhandwash

https://www.instructables.com/id/Dismantling-a-Broo... by qthurtle

https://www.instructables.com/id/How-to-disassemble... by tundrawolf

Step 2: Preparing the Engine Casing

One could, perhaps, connect the electric motor directly to the drive shaft, but with all the support mounts and shaft conversions that would be needed, why bother when one already has all those in the original petrol engine case?

Removing the old engine, the trigger assembly, and the cables was simply a case of undoing a few screws. The "come in handy" philosophy dictated keeping the screws and some other bits.

Because I needed a flat face to attach an adapter plate to, I cut off the curved part of the cowling. In my case, the cowling had a moulding line I could follow.

I used my "Renovator" multitool, but you could probably do just as well, if not better, with a fine-toothed handsaw or the like. Once the cowl was removed, I cleaned up the edges with a Stanley knife and sandpaper.

Two of the screw holes had locator flanges, so I cut those off with the knife also.

Step 3: Make the Clutch Shaft - Version One - Principles

This was the trickiest bit.

I had saved the centrifugal clutch which screwed onto the shaft of the old engine. As you can see from the first picture (not from my weedeater), the clutch screws on to a threaded shaft which has a smooth length (a "pin") below the clutch. The pin fits into a hole in the drive drum to keep the clutch centred, and as the clutch spins, it expands to grip the walls of the drum.

The clutch needs to turn in whatever direction makes the "wings" trail. This means that if you are looking at the side with words on, the clutch should turn clockwise. In my setup, the motor turns clockwise so the clutch needed to have its face closest to the motor. I hadn't taken into account the gearing in the cutting head, so the head actually turns in the wrong direction. This is easily remedied by reversing the power wires and turning the clutch upside down, which I'll do eventually.

I got the clutch off the engine easily enough, but could not do so with the original shaft. This meant I had to make one.

Step 4: Make the Clutch Shaft - Version One - Measuring the Shaft

I had a coach bolt in my pan-o-bits which had a 100mm long 7.5mm thread that fitted nicely into both the drum hole and the motor's V-belt pulley. Unfortunately, the clutch had a larger thread, but I thought to get around that by clamping it tight between two nuts.

I started by fitting the clutch and two nuts to the bolt, and checking the fit into the drum. This looked good.

To mark the "pin" part of the shaft, I took the clutch off again, threaded one nut on further than needed, fitted the shaft into the drum hole until it touched bottom, and wound the nut down until it touched the drum. I took the assembly out and marked the hole depth with a marker pen.

Measuring the length of shaft needed was trickier, and I made some judgement mistakes that I fortunately got away with.

Resting the engine cowling in my vice, I sited the motor so that its legs were resting on the cowling. My first mistake here was not taking into account the thickness of the adapter plate (4mm). By inserting a tape measure, I was able to estimate the length of the shaft (+- 60mm) which I then checked with a sliver of wood short enough to fit inside the assembly. My second mistake was not allowing for the depth of the thread in the V-belt pulley.

Step 5: Make the Clutch Shaft - Version One - Cutting the Shaft

I measured 60mm up the thread from the first mark and made another one. You can see in the photos that I nearly screwed up (ho, ho) by initially measuring 60mm from the end of the thread, instead of the bottom of the lower clutch nut. As it was, I hadn't taken the depth of the pulley thread into account ...

Clamping the bolt in the vice, I hack-sawed the thread off at the correct mark.

Using two adjustable spanners, I kept the lower clutch nut in place and tightened the upper nut tight against the top of the clutch.

Step 6: Make the Clutch Shaft - Version One - Fitting the Shaft

The thread in the V-belt pulley was a slightly different pitch than the shaft's thread. Fortunately I had a nut with a similar thread that I was able to wind down the shaft: this altered the shaft's pitch to make it like the pulley's.

I got the shaft started into the pulley with an adjustable spanner ("Crescent wrench"), then held the lower shaft nut in the vice and tightened the pulley onto the shaft with adjustable pliers.

Using the spanner and pliers together, I tightened the pulley nut tight up against the pulley.

The shaft was not in line with the motor axis, but gentle tapping with a hammer fixed this. Using the cut off cowling as a gauge, I slowly turned the shaft, with gentle hammer re-corrections, until it turned without wobbling.

This was an important step as it helped determine the location of the motor on the adapter plate.

Step 7: Drawing and Cutting the Adapter Plate

I happened to have a piece of 4mm-thick aluminium-perspex-aluminium sheet, but you could just as well use solid aluminium, a thin steel plate, or even a good piece of plywood. As already mentioned, I didn't account for this thickness in the drive shaft length either, sigh.

I traced around the engine casing then drew a wider border around where the new motor was to go. A jigsaw made short work of the cutting.

As always, double-check the fit!

To mark the cowling screw holes, I carefully lined the cowling up with the outline I'd drawn, then used a hammer and screw to punch four location marks.

I'd kept the original screws for these holes, so simply drilled four holes the size of the screws' shanks. Driving the screws into these holes forced them to create their own threads.

Step 8: Cutting the Centre Hole in the Adapter Plate

With the cowling lined up with its outline, I poked a long thin screwdriver down the drive-shaft hole and tapped the end with a hammer to mark the centre. You can see in the photo where I'd tried locating the centre by using outside marks.

I found a jar lid that was a little bigger than the V-belt pulley and used that to draw a circle centred on the screwdriver mark (I drilled a hole in the lid's centre so I could see the centre mark).

Putting the plate in the vice, I drilled a pilot hole, then cut out the hole with the jigsaw. If you had a suitable-sized hole saw, you could use that.

Step 9: Marking the Motor Mounting Holes and Fitting the Motor

This was a bit difficult to take photos of, but here's how it works:

I supported the cowling and the attached adapter plate upright in the vice. You could support it any way you wish.

With the clutch shaft attached to the motor, I lowered the motor over the adapter plate until the clutch was properly located in the drum, with the "pin" fully engaged in its hole. This more or less centred the motor immediately, but I twisted it a bit until I was happy with the hole placement.

I drew around the mounting feet with with a felt pen* and used the same screw-and-hammer hole marking technique from Step 7 to mark the holes.

I happened to have three bolts with washers and nuts that were exactly the right length and thread diameter to screw into the motor's mounting holes. Nice! I drilled out the three new holes in the adapter plate, and in doing so discovered that two of them came out right at the edge of the cowling. >:(

This was solved simply by marking where the bolt heads met the cowling, then nibbling and filing to make a recess for each bolt. This in turn means that one has to bolt the adapter plate to the motor first, so one can use a spanner or socket on the bolt head, then screw the cowling onto the plate.

Voila! The motor is fitted.

* "Vivid" marker, indelible marker.

Step 10: Make the On/off Switch and Wire Every Thing Up

24th July 2016: UPDATE: Do not use the weedeater's original switch! It turns out that it is not designed for 240v and gave me a "tingle" last time I used the machine. I will need to rethink this component.

Ideally, one would use the throttle lever from the original setup to activate a microswitch that would kill the power as soon as the lever was released, but while I have the necessary parts, I haven't figured out how best to put them together yet.

I still had the on/off switch and throttle housing, so I used those.

I desoldered the original spade connectors from the switch and resoldered them to one end of both wires in some house lighting wire I had on hand.

As I reassembled the throttle housing, I led the wiring down through the throttle hole, then connected the other ends to the motor wires with a terminal block.

I just happened to have a cord and plug I'd rescued from some defunct household appliance or power tool, so wired that in series with the switch wire via a terminal block, connected the green earth wire to one of the motor bolts, and taped it all up so no live electrical parts were exposed. A future plan is to create a custom motor housing.

This is the step were one thinks carefully about which way to connect the wires. I mentally worked back from the clockwise-turning cutting head and decided the motor should spin anti-clockwise: somehow, in spite of all the mental agony, I still got it wrong and the head turns the wrong way - it doesn't matter too much as weedeater line, like Gloria Estefan's love, is like a knife that cuts both ways. I'll correct this in Electric Weedeater Mk2.

Step 11: Test It!

Yay, it works well! Oh, wait ... why has the cutting head stopped turning while the motor is still running?

(Removes cowling to inspect drive shaft). Oh. The upper clutch nut has wound itself all the way up the shaft and the clutch has disassembled itself...

(Reassembles clutch with some difficulty)

Step 12: Make the Clutch Shaft - Version Two - Fitting a Sleeve to the Clutch

Part of the problem with the clutch disassembly, I think, was that it didn't fit tightly onto the shaft thread.

I happened to have a short length of aluminium tube in my pan-of-bits, from who-knows-where, that was a slightly larger diameter than the clutch thread. I cut off a piece as long as the clutch depth and used sidecutters to slice up the length of it and nibble away at the circumference until it was a tight fit when hammered into the clutch thread.

A 7.5mm drill was the right size to drill out the sleeve until it was just open enough to take the cut-off end of the original bolt when used as a tap to cut a thread in the sleeve. This bolt worked OK as a tap in this instance, but you may want to check out these two links:

https://www.instructables.com/id/Make-an-Acme-Tap/ by pfred2

https://www.instructables.com/id/Make-a-tap-from-a-... by Fixit guy

Step 13: Make the Clutch Shaft - Version Two - Soldering the Upper Clutch Nut

In an attempt to stop the clutch flying apart again, I found two large washers in the pan-of-bits. One appeared to come off the weedeater itself, and the other was a penny washer* that I drilled out to take the shaft diameter. As the clutch core is slightly thicker than the two shoes, these washers would prevent auto-disassembly without interfering with the shoe movement.

Instead of having a lower clutch nut, I wound two nuts onto the shaft followed by the now-threaded clutch. I positioned the clutch so that it took the original place of the lower nut, then wound the two upper nuts down on to it. Using two spanners, I then tightened the two nuts against each other.

To make sure everything remained in place, I decided to solder the nuts to the shaft. This was made easy by using the little butane torch I'd got by redeeming some "FlyBuys" loyalty points. These torches are the things chefs make crème brûlée's with.

After reattaching the cowling, it was time for another test...

* A fender washer (US), penny washer or 'repair washer' (UK) is a flat washer with a particularly large outer diameter in proportion to its central hole.

Step 14: Final Test

Yay! It not only works perfectly, there is less vibration now. :)

(Proceeds to weedeat the front yard while his wife marvels at his ingenuity).


There are a few points to note here.

1. Always use an isolating transformer or residual current interrupter (I use HPM's version). Always use personal protective gear (PPE).

2. You cannot safely use this in the rain.

3. Point 2 means you can justifiably keep watching TV - or writing Instructables - until the ground has dried out again...

4. The motor is not big enough. While it works, it only gives enough power for doing light trimming. However, I also have a bigger motor (from a food processor), so later on I'll try that out and update this Instructable.

This website indicates a washing machine motor's watts: https://nz.answers.yahoo.com/question/index?qid=20...

"The mechanical power of a normal domestic washing machine motor is probably between 200 and 400 watts. The motor speed is probably about 1500 revolutions per minute (RPM). The common motor speeds for AC motors operating on 50 Hz power are about 1000, 1500 and 3000 RPM. They run about 3% slower than that when fully loaded. Torque (Newton-meters) = 9.549 X Power (watts) / RPM."

The Sunbeam Oskar food processor motor is 600 watts which is equivalent to 0.8hp.

This conversion website http://www.rapidtables.com/convert/power/watt-to-h... indicates that even an 1100W motor is only 1.5hp, which is way less than the Ryobi's 25hp.

I think I'm missing something about motors and watts and horsepower. Let's get some feedback from the audience...

(... A few hours later, a message from Slowpoke made me realise the petrol motor was 25cc not 25hp!. Duh. He says: "Ryobi's specs for the 26cc motor is between 0.7 & 0.9 Hp" which means my current motor is understandably way too small, and the Sunbeam motor will be about right. Thanks Slowpoke.)

Comments

author
finton (author)2016-01-01

Slowpoke says I can add his original message:

Original message sent 5 days ago by Slowpoke

Hi,

Great Instructable. I have been planning to do something similar too.

You mentioned Horsepower of your electric motor and the difference between the electric and the petrol motor. Ryobi's specs for the 26cc motor is between 0.7 & 0.9 Hp, so your electric motor is a little less power. I think that your biggest problem would be RPM. I could not find the Ryobi's average RPM but it would have to be over 3000. Considering the noise these things make, the average RPM is probably much higher (>6000).

Try the food processor motor and see how it goes. I'm going to try a 1800 Watt vacuum cleaner motor. Being a series wound motor the top speed is determined by load. Induction motors such as washing machine motors their speed is determined by the number of poles less slip factor (but I see that you already know that - sorry).

I'll let you know how my conversion goes if you are interested. Apart from the coupling to the motor shaft, I can use almost all of your instructable. Brilliant. Thank you.

author
mrgw98 (author)2015-12-28

What went out on the thing? I might be able to help you fix it back to original petrol glory.

author
finton (author)mrgw982015-12-28

I never did find out pmcflufington, and I've now thrown out the motor as part of a big workshop cleanup (stupid, stupid).

I certainly appreciate your offer though - from your profile, you seem to know about these things...

author
mrgw98 (author)finton2015-12-29

I know more about things than I realize sometimes. I retain knowledge very well and something very small can spark my memory of that knowledge. I am very creative, innovative, and imaginative. Thanks and glad you at least have something figured out to use.

author
Prfesser (author)2015-12-28

Very good instructable, nicely detailed! If/when my Ryobi motor quits, I'll be looking up your words again!

Be aware that a food-processor motor is probably designed for intermittent duty rather than continuous duty. OTOH, DC treadmill motors are often 2-3 HP intermittent duty and may operate continuous duty at ~1 HP. However, their RPM may be rather high for a weedeater. If you see someone throwing away a complete treadmill, the speed control & motor might be adapted to your application. (If not, they'll surely be good for *something* along life's journey!)

author
finton (author)Prfesser2015-12-28

Thanks Prfessor! While I don't wish failure on your Ryobi, I'd be very keen to see your take on converting it.

I take your point on intermittent duty and the RPM. Still, what can go wrong, right? :)