# Source gears for 20...35hp engine reducers

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#### jbiplane

##### Well-Known Member
Does anyone have an idea where to source good quality gears to use in light engenes reducers?
Yes it is possible to produce, but using of shelf components says of moto could cut the cost.
For this power range helical gears with module 1.5 1.75 2mm will work fine. Big gear can have 56...60 teeth
and small 17...20

#### pictsidhe

##### Well-Known Member
KHK.
Otherwise, car gearbox gears. Differential gears from small FWD transaxles may be worth a look.

#### Dana

##### Super Moderator
Staff member
Many industrial suppliers, e.g. Martin, Boston Gear, Browning, etc.

#### rotax618

##### Well-Known Member
For low HP (<30) reduction, poly Vee belts will be the lightest and easiest to build.

##### Well-Known Member
Log Member
stock drive products

#### BoKu

##### Pundit
HBA Supporter
For a while it was common to see PSRUs built from planetary gear sets taken directly from automotive automatic transmissions. However, that is a lot less common these days, so it is a reasonable assumption that it might not have worked as well as intended.

Edit add: Overview of PSRU Products

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#### jbiplane

##### Well-Known Member
For a while it was common to see PSRUs built from planetary gear sets taken directly from automotive automatic transmissions.
We did one such reducer of Toyota 1,6l engine salvage, but customer used it to few to understand if quality is good enough and it will last long.

For low HP (<30) reduction, poly Vee belts will be the lightest and easiest to build.
Yes, we produce this, but want try gears and fit to low budgets.

#### Armilite

##### Well-Known Member
COG & Mico v Raw Gear Stock is available, some short pieces also show up on eBay.

My Manta Belt Drive I just got has a 2.230" PTO Micro V Pulley. Top Pulley is 4.5" / 2.23 = 2.01 Ratio.

The most commonly used Rotax Prop Bolt pattern is the (6) Bolt based on a 75mm Bolt Circle using 8mm Bolts. The largest (6) Bolt Pattern is based on a 101.6mm Bolt Circle. So 101.6mm = 3.999988" so a Top Pulley Hub say 4.0" will hold the most common used 75mm Bolt pattern. A 120mm/4.724395" Hub is big enough for the largest Bolt Pattern.

Using 2.5" PTO = 63.5mm. Some common Ratios used.
2.5" x 1.8 = 4.5" Top Pulley ----> Most Honda/Clones use this 1.8 Ratio.
2.5" x 2.0 = 5.0" Top Pulley
2.5" x 2.25 = 5.625" Top Pulley
2.5" x 2.35 = 5.875" Top Pulley
2.5" x 2.58 = 6.45" Top Pulley ---> Most popular Ratio used on Rotax's up to 50hp!
2.5" x 2.62 = 6.55" Top Pulley ----> Most popular Ratio used on Rotax's 65hp+.

Using 2.75" PTO
2.75" x 1.8 = 4.95" Top Pulley
2.75" x 2.0 = 5.5" Top Pulley
2.75" x 2.25 = 6.1875" Top Pulley
2.75" x 2.35 = 6.4625" Top Pulley
2.75" x 2.58 = 7.095" Top Pulley
2.75" x 2.62 = 7.205" Top Pulley

Using 3.0" PTO
3.0" x 1.8 = 5.4" Top Pulley
3.0" x 2.0 = 6.0" Top Pulley
3.0" x 2.25 = 6.75" Top Pulley
3.0" x 2.35 = 7.05" Top Pulley
3.0" x 2.58 = 7.74" Top Pulley
3.0" x 2.62 = 7.86" Top Pulley

There is Micro V Tooling Inserts in (2), (4), (6), (8). I have Belt Drive Plans for an 80hp Engine that uses a 16 grove Micro V pulley. I'm assuming 80hp/3 Belts = 26.6hp per Belt which the Belts are probably rated for a little higher 28-30hp, and they use the 4th Belt to lower the strain, 80hp/4 Belt = 20hp on each Belt. Micro V Belts are limited to 6500rpm, I don't know about Cog Belts. Like a 277 26hp@6250rpm could probably use (1) 4v belt if rated for 28-30hp., but for Safety, you would use (2) 4v Belts.

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#### jbiplane

##### Well-Known Member
I have everything related belts. Want try gears from serial products which are cheap. Have a look screenshot from taobao
Suzuki AX100 clutch with big gear below 7USD. Looking for corresponding "small" gear.

#### pictsidhe

##### Well-Known Member
I have everything related belts. Want try gears from serial products which are cheap. Have a look screenshot from taobao
Suzuki AX100 clutch with big gear below 7USD. Looking for corresponding "small" gear.View attachment 103956
Motorcycle clutch baskets look like they may be ideal to put a TV damper inside

#### Dana

##### Super Moderator
Staff member
Motorcycle clutch baskets look like they may be ideal to put a TV damper inside
It might already have one... I converted a Kawasaki KX100 engine for paramotor use, there were rubber dampers between the gear and the clutch basket... I retained them but discarded the clutch plates. I bolted the prop shaft directly to the clutch basket. With the original crankshaft gear I had a 3.4:1 reduction ratio.

#### rv6ejguy

##### Well-Known Member
For a while it was common to see PSRUs built from planetary gear sets taken directly from automotive automatic transmissions. However, that is a lot less common these days, so it is a reasonable assumption that it might not have worked as well as intended.

Edit add: Overview of PSRU Products
Jack Kane from EPI falls into the same trap as many others discussing Load Models on the automotive cycle, assuming because a car only takes 30-50 hp to cruise at highway speeds, that's all that the engines and transmissions can take for extended periods. He should know better. The validation tests of modern automotive transmissions and engines are well documented these days at very high duty cycles. Many modern auto engines also output a lot more power than typical GA engines. They aren't littering the autobahn with their broken carcasses after cruising at 100-120+ mph.

Jack also told us that aircraft gearboxes couldn't be reliable with bath lubrication- pressure lubrication was needed. 18 years and 441 hours down the road and my bath lubed Marcotte gearbox has never been apart. I don't know of one EPI gearbox which has that time on it without coming apart.

Car/ truck transmissions and differentials use bath lubrication and work just fine for thousands of hours with zero maintenance- hundreds of millions of them.

I'll take the real world examples over theory any day. While EPI has an excellent technical site for the most part, not everything there can be believed.

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#### Bille Floyd

##### Well-Known Member
It might already have one... I converted a Kawasaki KX100 engine for paramotor use, there were rubber dampers between the gear and the clutch basket... I retained them but discarded the clutch plates. I bolted the prop shaft directly to the clutch basket. With the original crankshaft gear I had a 3.4:1 reduction ratio.
I was looking into doing that with a CRF250r, or KX250f ; both engines
have EFI's after certain dates !!

What % weight did you take off from the stock engine ; when
ya cut the transmission off ?

Bille

#### Dana

##### Super Moderator
Staff member
I was looking into doing that with a CRF250r, or KX250f ; both engines
have EFI's after certain dates !!

What % weight did you take off from the stock engine ; when
ya cut the transmission off ?

Bille
I don't recall exactly. Actually I didn't cut the transmission off, I just removed all the gears and shafts and plugged the holes. The prop shaft bearing carrier replaced the original clutch cover. I think the finished engine weighed a bit under 30#, I don't recall if that included the exhaust. I still have it but haven't messed with it in years.

#### WonderousMountain

##### Well-Known Member
I was taken in by EPI's site years ago,
It's very well done and a wealth of info,
But the pet theorys don't check out. One
about piston slap & part life was especially
troublesome.

Supposedly the wear is determined by the
Velocity & area pressure on the skirt from
Piston mass rotating around the wrist pin.
So it happens that piston mass gravitates
above the pin, because that's where it has
to seal the combustion charge & carry most
rings. Basically ignore this & reduce weight.

Of their own Engines, very few fly, performance
parts are out of place in 2,500hr overhaul period.
As for gears, I would think anything properly
treated, profiled, and correctly sized will work out.

Stay Keen,
LuPii

#### Bille Floyd

##### Well-Known Member
I don't recall exactly. Actually I didn't cut the transmission off, I just removed all the gears and shafts and plugged the holes. The prop shaft bearing carrier replaced the original clutch cover. I think the finished engine weighed a bit under 30#, I don't recall if that included the exhaust. I still have it but haven't messed with it in years.
DANG ; thumpertalk (.) com said the KX100 engine, will put out
28-hp ; and you got a really low weight , on your finished engine !!
You bet, i gave ya a , (Like) on your post !

28hp would make both my Atos and Exxtacy fly ; only thing holding
me back is that it is a 2-stroke.

Both the engines i mentioned, are 4-strokers, and in the low 50-Lb
range ; was hoping i could loose about 15% of that, by dicing the
transmission off ? Complete rebuild kits, for both those engines , can
be had for in the $700 range ; and that included new crank-shafts and pistons and bearings. I would go for a belt reduction from a known source if i went that rout ; that's what interested me , in this Thread ! Can get a Polini Thor 202 , for a tad over$3k ; it's a 2-stroke , and
it might be a problem in some states in another few years ?

Bille

#### Dana

##### Super Moderator
Staff member
I was guesstimating 25HP as the performance was identical to a friend who had a 25HP Aerocorsair motor, he weighed the same as me, and flew the same paraglider. I daydreamed about designing a CNC crankcase for it but never pursued it.

It had narrow powerband... a little too much prop pitch (I had a ground adjustable prop) and it bogged down around 8000 rpm and never get on the pipe, a touch less and it'd go right to 12000. I think it would have done better with a slightly smaller prop on a faster aircraft.

#### Armilite

##### Well-Known Member
Finding off the shelf cheap gears, especially since you need 2-3 different Ratios!

A New Rotax B Gear Box 2.58 Ratio is $835 if you Shop around. Leading Edge Airfoils has the Best Price I have found. Many New Belt Drives cost more than that. Be we get Screwed here in the USA on Prices. I know a Rotax C & E Gearbox is way cheaper$400-$500 in Itlay than what we Pay here. I never checked on the B Gearbox. There was a guy in one of the Countries on the North Side of Africa that had New B Gear Drives for$550 a Year ago.

JLO, SACH's, Kawasaki, and about 20 other brands of Snowmobile Engines made many different Singles 100cc to 396cc. JLO L297 would make a nice Ultralight Engine.

100cc/7cc= 14.3hp@6500rpm! For 100cc to make 25hp you would need to turn it 11,000rpm!

25hp x 7cc = 175+cc is needed for 6500rpm!

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#### EzyBuildWing

##### Well-Known Member
Polini-Thor 250cc 2-stroke gearbox is good for 36HP...... maybe Polini will sell you a gear-set?
Why not just buy THREE Polini Thors (18kg each) and configure them as pushers in a Moyes Dragonfy?

#### rv7charlie

##### Well-Known Member
For a while it was common to see PSRUs built from planetary gear sets taken directly from automotive automatic transmissions. However, that is a lot less common these days, so it is a reasonable assumption that it might not have worked as well as intended.

Edit add: Overview of PSRU Products
I don't consider that a reasonable assumption at all. Ask the rotary engine guys; virtually all of them are running planetaries successfully. I suspect that the real drivers (pardon the pun) have more to do with physical configuration and ratios needed, than 'not working well'.

The rotary's shape begs for the prop shaft to have near-zero offset from the engine's E-shaft ('crankshaft'), and it's an inherently high rpm, low torque engine, requiring a wide ratio reduction; easy with a planetary. And heavy truck planetary gearsets are really inexpensive, off the shelf, to use as the core of the drive.

Most piston engines (even the Subaru) actually need some shaft offset to get the prop shaft up near the top of the engine (or the top of the intake system on the Subie), to get the thrust line in the proper place while keeping the engine within the cowl (unless flying a Piet is your thing). And because most of them operate at in a much lower rpm range than the rotary, they need a narrower ratio reduction, which is tricky to achieve with a planetary gearset.

It's all about the right tool for the job. My RV6 makes a relatively poor motorglider, but the typical motorglider won't cruise at 170 kts and allow me to taxi it into my 30' wide hangar after a flight.

Charlie