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How to figure out gear stregnth?

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Jake2465

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I have always wanted to make a couple redrives for automotive / air conversions. I figured I could run some beefy spur gears to make it happen. One of the first questions that entered my mind was how to figure out just what size of gear would be a safe bet for the required torque to the prop? The next question was what kind of steel to use. I thought that perhaps some pack carburized 1018 would be a good starting point? I tried to get some information from people I knew, but none of them make gears. The only advice I got from someone was that he suggested that the mating gears be lapped together and polished so the contact surface would be as perfect as possible.

I made a quick sketch of what came to mind as a pinion gear that the crank would run. The addendum is 3.75" diameter with a 25deg pressure angle and 4 pitch. it is about two pounds worth of steel. Also, I have a gear image of a RR Merlin that would appear like they went with some hefty spur gears as well. One retired AP mentioned to me that those gear boxes that the Merlins ran would outlast the engines they were bolted to.

The only other indication I got was that perhaps people use pressure angle and the gear root cross section to calculate shear strength. Only issue is that would think that whatever answer is given from that would have to have more strength because the gears are not static.
 

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T Bredehoft

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#2
Merlin? Spitfire? how about roller cams to reduce friction/need for strength?
 

Asm109

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You want to design and build a gear reduction box for an airplane correct? Model plane or full size that carries people?
If the latter.
My advice? Get a Mechanical Engineering degree so you understand strength of materials and fatigue etc, etc. Then pay somebody who all ready does this kind of work to design it for you. The degree will allow you to converse intelligently with the designer.
 

Bob Korves

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I have always wanted to make a couple redrives for automotive / air conversions. I figured I could run some beefy spur gears to make it happen. One of the first questions that entered my mind was how to figure out just what size of gear would be a safe bet for the required torque to the prop? The next question was what kind of steel to use. I thought that perhaps some pack carburized 1018 would be a good starting point? I tried to get some information from people I knew, but none of them make gears. The only advice I got from someone was that he suggested that the mating gears be lapped together and polished so the contact surface would be as perfect as possible.

I made a quick sketch of what came to mind as a pinion gear that the crank would run. The addendum is 3.75" diameter with a 25deg pressure angle and 4 pitch. it is about two pounds worth of steel. Also, I have a gear image of a RR Merlin that would appear like they went with some hefty spur gears as well. One retired AP mentioned to me that those gear boxes that the Merlins ran would outlast the engines they were bolted to.

The only other indication I got was that perhaps people use pressure angle and the gear root cross section to calculate shear strength. Only issue is that would think that whatever answer is given from that would have to have more strength because the gears are not static.
Jake, I suggest you research automotive to aircraft engine conversions and find the ones that have the best reputation for quality and reliability, and then buy one ready to mount to the aircraft. I have done about 800 hours of flying in experimental exhibition and racing category aircraft, including test flights, and know quite well that you really want to have everything except the ONE design change you are testing to be fully proven safe and reliable. Flying an aircraft with multiple unproven modifications is a very dangerous thing.
 

T Bredehoft

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You lost me at roller cams.
It appears you're designing gears to drive the over-head cam shafts. If the Valve lifter/cam interface is a roller bearing, instead of a flat wiper, friction will be reduced considerably, reducing load on the gears.
 

Jake2465

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It appears you're designing gears to drive the over-head cam shafts. If the Valve lifter/cam interface is a roller bearing, instead of a flat wiper, friction will be reduced considerably, reducing load on the gears.
They are shaped like that for higher pressure angle which will provide the gear tooth with greater strength than a profile with something like 14.5 deg pressure angle. Trade offs are always around. Higher pressure angles impart more radial loads on the bearings that support the gears and also generate more noise. BUT, I am no expert :).

Flying an aircraft with multiple unproven modifications is a very dangerous thing.
Bob, you are absolutely right. Having several unproven components wrapped up into one airframe is quite the roll of the dice and I am not a particularly lucky fellow. However I do feel that good systems start with asking questions about just how something will need to be accomplished to give the best possible outcome. If the answers are hard ones, then that is just the nature of it.

For gears I could simply go through a machine design hand book and look up the couple of chapters that involve gear design. It is a pretty lengthy process that involves several formulas and transferring of data from graph to graph to arrive at some acceptable answer. And all of this is dependent on the actual condition of the material being used. So, I am sure that whatever answers are had will be accurate enough, but not 100%. That's where empirical testing comes in to double check.

I think I asked about the gears because I value the knowledge that took others a career to gain. I was hoping that perhaps others would say they know what has worked and what has not. All I keep hearing from other fellow pilots is that those gear boxes cant be trusted and just stick with Lycoming and they probably heard that from someone else who heard it from some other. And often when I start to press them as to what exactly is not good enough with whatever it is, they don't have an answer :(. Doesn't do me much good when they don't know either.

Just curious, did you ever go to Reno? I have always wanted to go to an air race but never have.

Here is what I learned to fly in :D.
 

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Bob Korves

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It appears you're designing gears to drive the over-head cam shafts. If the Valve lifter/cam interface is a roller bearing, instead of a flat wiper, friction will be reduced considerably, reducing load on the gears.
Correct, Tom, but with aircraft engines the three things you need most is reliability, reliability, and reliability. The engines are built to simple designs and use components what will fail soft, not catastrophically. A failing engine is bad enough, but you want it to keep running until you can make a safe landing. There are no tow trucks cruising the sky to help you out. Basic flat tappets, made as light weight as possible, and to "best" quality standards are typically chosen for IC aircraft engines.
 

Bob Korves

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Just curious, did you ever go to Reno?
Yes, I went to the Reno air races just once. Coolest thing for me was the F104 Starfighter demo flight. It is the plane I most wish I could have a chance to fly. All of my personal aircraft racing experience has been in sailplanes, where there is no engine to fail, and it is a lot quieter but perhaps even more busy though strategically as much as tactically, and just as demanding of good judgement. I have flown quite a few types of aircraft, but after sailplanes my favorites were (and are) old taildraggers and aerobatic planes. Had to give it all up 10 years ago after some medical issues. Still hang out with my glider buddies...
 

Jake2465

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Nice! I have some time in an ASK-21 and the Super Blanik. Being able to hear the metal skin flexing and popping with the thermals took some getting used to on that Blanik.

The F-104 is definitely one fine piece. I live about 8 minutes from the Air force Museum. They have one sitting on a stick in the front yard.
 

Bob Korves

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#13
At a major soaring award event:
https://en.wikipedia.org/wiki/Flying-M_Ranch
https://en.wikipedia.org/wiki/Barron_Hilton_Cup
Also got to spend a quiet morning sitting on the porch steps talking with Gene Cernan, last man to walk on the moon
Took actor Cliff Robertson for a ride in our Duo Discus glider, he and Barron bought one after the demo flight
I was walking around the ranch property early one morning, and while admiring the Hughes MD500E helicopter Barron walked up and asked me to go with him on his morning ride, which he did nearly every morning at the ranch. How can you say no to that? :)
I am certainly not a groupie, this was a truly fun event, great people all. I only got to go because a soaring friend of mine was a winner, and his wife didn't want to go. I was his guest...
Had also landed out at the ranch a few years earlier than that and was invited to dinner with Barron and his guests while waiting for my crew to arrive with the glider trailer. I was the center of attention, ??? Go figure, a hack like me with a death defying story... :eek 2:
 
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