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How to figure gear size/diameter

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How to figure gear size/diameter

Post  batjac on Tue Jul 14, 2015 11:34 pm

I didn't want to clutter the other thread, so I'm starting this one.  Looking at the three jug engine below, I was wondering how to calculate the size gears to use.



I know it has something to do with modulus this, and radius that, and pitch the other thing, but how does one figure out the diameters of gears if one knows the distance between the center of the two shafts from each other?  I know the inner gear (the prop hub) will be larger to slow the prop, but I don't know how to take the distance between the two, factor in the gear ratio, and then find the right size gears to use.

Any suggestions on a tutorial or website?

The Gear Loose Mark
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Re: How to figure gear size/diameter

Post  1/2A Nut on Wed Jul 15, 2015 1:38 am

You can count the teeth on the spur gear say 32
Pinion gear say 14 and divide in the numbers for 2.2857:1

You want a usable rpm given the prop pitch desired and dia.
I would go (prefer for speed) with a 1.3:1
7x7 prop
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Re: How to figure gear size/diameter

Post  batjac on Wed Jul 15, 2015 2:00 am

1/2A Nut wrote:You can count the teeth on the spur gear say 32
Pinion gear say 14 and divide in the numbers for 2.2857:1

You want a usable rpm given the prop pitch desired and dia.
I would go (prefer for speed) with a 1.3:1
7x7 prop

I understand how to get the gear ratio I want. It's the size and diameter of the two gears I'm not sure how to calculate. I looked at McMaster-Carr to try to figure it out, but there were several columns for each gear to select. Once I have the diameter, the tooth pitch should be easy to figure out.

Mark
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Re: How to figure gear size/diameter

Post  1/2A Nut on Wed Jul 15, 2015 2:27 am

Yes I understood you, I couldn't answer that bit.  

Deduce the magic:
Teeth design pitch size etc varies. A spur gear big enough bore to accept a 1/4in drive shaft and 1/8in pionions bore will give you the spacing you need. He used 48 pitch gears.

Seeing what was done:
The gear set shown is a 28 tooth spur and 14 tooth pinion by the looks of it has given the builder the spacing he needed to crowd the engines in nice and tight. 2:1 ratio for a deep pitch big prop.

Scale the pic down to a .049 engine
Spur gear is around 22mm and pinion is 13mm or 1/2in dia.

Also for better front support a cage design should help in a big way.



Last edited by 1/2A Nut on Wed Jul 15, 2015 2:53 am; edited 1 time in total
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Re: How to figure gear size/diameter

Post  1/2A Nut on Wed Jul 15, 2015 2:42 am

http://www.ebay.com/itm/Vintage-COX-3-CYLINDER-Tri-Motor-Gas-Engine-Model-Airplane-Engine-Ralph-Barnet/131556410055?_trksid=p2047675.c100010.m2109&_trkparms=aid%3D555012%26algo%3DPW.MBE%26ao%3D2%26asc%3D32475%26meid%3D2b40c94987a744cfb8339877e6582d76%26pid%3D100010%26rk%3D18%26rkt%3D24%26sd%3D181797438185

I asked the owner to count the teeth for the spur and pinion. You can eyeball the teeth size (48 pitch) when you make the order it is a standard type of tooth seen on many RC cars and model projects.

I made a gear box for a TD .049 back in the late 80's the added torque made for a very fast RC outrigger hydro and allowed me to use more standard size Octura props.

The standard brass fittings made this easy for the builder to retrofit but heavy, out of aluminum would be perfect

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Re: How to figure gear size/diameter

Post  1/2A Nut on Wed Jul 15, 2015 2:57 am

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Re: How to figure gear size/diameter

Post  pkrankow on Wed Jul 15, 2015 7:41 am

You are constrained by packing size of the engines. The engines have to be a minimum distance apart. Use this to calculate a ration of diameters, then look for gears that are close to these diameters. The safety margin is probably adequate. The loads are light enough that steel gears should be strong enough, brass too.

There might be torque information in the selection of gear information that relates to how strong the teeth are. Otherwise it is calculating the tooth load and if a tooth will permanently deform (plastic deformation)

Phil
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Re: How to figure gear size/diameter

Post  1/2A Nut on Mon Jul 20, 2015 1:46 am

Ok I have the difinitive answer for your question from the current owner - searchnstein


Tri Motor Gas Engine Model Airplane Engine by Ralph Barnet

"Hi Brad, there's 22 on the large and 11 on the small, sorry it took so long!"

Thanks, Don
- searchnstein


Looks like my count from the photo was on the money he has a 2:1 gear ratio for 48 pitch gears.
If you order that you should be able to make it work with a 3/16 bore spur gear and 1/8th inch pinion.
3/16 is typical 5mm bore like most .15 size engines.



Again for me I would want more rpm. 20 / spur and 13 / pinion for 1.53846 to 1 ratio.

Say 17k rpm at the engine (loaded) for 11k at the prop. 8x6 prop would work. 62.5 mph theoretical
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