High power gear drive 5

[MightyGNU]

Hi, I'm new to this forum and not technically an engineer (at least professionally) but I hope you all can help me with this.

I'm building a helicopter, I have a 125hp engine the produces a maximum of about 130 lb/ft of torque at about 5500rpm. I need a 15:1 drive in order to keep the rotor tips subsonic. I've read a lot of documentation from the industry, most notably this nice catalog from Boston Gear (

https://www.altraliterature.com/-/media/Files/Literature/Brand/boston-gear/catalogs/p-1930-bg.ashx)

and I don't understand something. Even large, hardened helical gears (see # HS636L) are only rated for about 15hp at 3600rpm. I'm an auto mechanic, and manual transmissions use similar sized helical spur gears to handle as much as a few thousand hp at 10,000rpm. What's the difference? And how should I go about building this system?

Thanks,
-Dan

 

[mfgenggear]

@mightyGNU
Disclaimer this is flight article.
Where it is a safety issue.
Please consult a licensed Professional engineer. (PE) for the proper design.

Now that is done here is some info.
Guide line
Helicopter gear box is very complicated.
And takes many years of experience.
Even as a qualified designer.
Helicopter gear box has to be compact, light weight, with at least one input shaft. And two out put shafts.
Main rotor and tail rotor.
Gears shafts and gears are special design
To horse power, torque, and RPM.
Gears are designed DIN or AGMA standards.
Material is chosen to the high stress requirements. And are normally carburized
(Case harden gears) that are normally Xtra thin walled and light.yet the gears
Have to with stand the above requirements.
For unlimited cycles.
Off the shelf gears will not work properly. Gear teeth will prematurely wear,or fail. Can't have that on flight article. Lastly there many different gear combinations, helical, spur, and spiral bevel gears.

 

[MightyGNU]

I appreciate the response but you didn't exactly answer my question. My car (1988 pontiac firebird) had an engine that made 135hp. The transmission (tremec t5), had fairly small helical spur gears (less than 1" face) and easily handled that 135hp, complete with the very large shocks and torque spikes that come with automotive applications, for over 200,000 miles and it's still in very good shape (I took it out to upgrade). I would imagine a helicopter has fairly even load without the huge shocks you get in a car. Is Boston Gear just very pessimistic about gear strength or is there a difference in construction between them and the gears in an automotive transmission?

I understand that the design is more complicated than stated in my question but I was attempting to ask something very specific in minimal terms.

 

[mfgenggear]

Helicopter gears are high strength premium
Steel such as AISI 9310, when case harden
The surface has high hardness, for surface hardness, and high strength core for toughness. The off shelf gears unless are
Of the above material will not hold up.
Automotive gears are generally heavy duty
Alloyed material. but are more robust
Because there is not a weight critical as helicopter Gear box.
These gears can be designed to 7000 to 10000 HP Input of 10000 to 30000 RPM and reduce to 1500 RPM.

 

[mfgenggear]

So the point is with your requirements
The gears would have to be designed
To easily handle HP, Torque, and RPM.

 

[spigor]

The difference is that the stock gears from that catalog are not carburized and ground to high accuracy.
This gear calculator should let you play with the material, process and the lubrication and see the rating difference between different setups:

https://khkgears.net/new/gear_calculator.html

 

[MightyGNU]

Thanks for that information. Does carburizing/nitriding/hardening really make the metal 100 times tougher though? If I were to get them carburized would I be able to use them?

 

[spigor]

100x does not look realistic, but you should be able to see yourself with that gear calculator.
To achieve a desired result, all the factors have to be in place - e.g. material selection, heat treatment, surface finish. There's a ton of information behind that all.

 

[MightyGNU]

OK, a few more questions. What's the best way to lubricate them? Fill the case with gear oil? Or Grease? Any additional thoughts or things I missed?

Also where can I buy nitrided gears in small quantities cheaply?

 

[mfgenggear]

Hire an engineer

 

[drawoh]

MightyGNU,

I did not look at your link. Gears are sized by torque. The faster the gear turns, the more power it can transmit. This is basic mechanical engineering you need to understand if you are to design this stuff. You are concerned about vibration, wear and cooling. You also care about the total mass of the transmission.

On turbine powered helicopters, the output shafts run at very high speed, reducing the torque, and allowing smaller, lighter transmission components.

Have you considered that if your transmission fails, you will likely die?

--
JHG

 

[MightyGNU]

drawoh,

Most gears are rated only to a certain RPM, and more importantly the engine runs at a fixed RPM. I have played with turboshaft engines but they are expensive to build and difficult to make a reasonable amount of power with, especially at this size. My current plan is to get the largest helical gears I can and build a pressurized oiling system for them.

With regard to everyone's safety concerns, I'm not worried. There is a saying I once heard about aircraft, "It's a good idea to always stay 2 or 3 mistakes above the ground" the idea being that in the event of a problem, there is time to either try to regain control or crash in a controlled manner. If the engine or transmission fails it will be possible to disengage the rotor clutch and perform an autorotation, even if the swashplate breaks or the rotors seize, the rotors have a drag coefficient nearly that of a parachute. I think 'likely die' is an exaggeration here.

 

[mfgenggear]

The surface has high hardness, for surface hardness, and high strength core for toughness. The off shelf gears unless are
Of the above material will not hold up.

I could of not spelled out any better or any clearer.
No one here will be responsible for your injury or death.
Spend some money and get a prebuilt system or hire an engineer.

 

[drawoh]

MightyGNU,

Make it big and forget it.

This is an excellent idea most of the time, but it does not work in aviation, especially with helicopters. To save weight, you must design components to operate at high stresses. This requires a clear understanding of the forces on your components, skill at structural design (note mfgenggear's remarks above), and you need to do very high quality manufacturing. Highly stressed parts have finite lifespans. At[ ]200, or[ ]400 or 1500[ ]hours, your parts must be removed and scrapped. A qualified engineer/designer will achieve your required lifespan.

Imperfections on high speed gears cause vibration, which causes fasteners to come loose, and it causes metal fatigue. Do you understand that gears have tangential forces which result in torque, and normal forces which force the gears to separate?

--
JHG

 

[MightyGNU]

A qualified engineer/designer will achieve your required lifespan.

I'd appreciate it if people would stop saying this. If I were willing to let someone else design things for me (and entrust my safety to them), I would not be asking here. I assume people here understand how expensive these components can be 'off the shelf', buying them that way is just not an option for me.

Highly stressed parts have finite lifespans.

This is very true, I will make a note to inspect the gears every 10 hours or so.

 

[drawoh]

MightyGNU,

We keep saying this because it is true. Are you going to strip your transmission and inspect each and every gear tooth for cracks every ten hours?

--
JHG

 

[drawoh]

MightyGNU,

Aerospace components are expensive because high quality fabrication, inspection and tracking are required to assure stress requirements are being met.

If you design a 1000lb transmission for your helicopter, you will be safe because...
[ol a]
[li]...you are using lots of material. The stresses will be low, and crappy materials and manufacturing won't matter.[/li]
[li]...your helicopter will be too heavy to leave the ground.[/li]
[/ol]

If you don't know what you are doing, you design with large safety factors. This results in something heavy.

--
JHG

 

[mfgenggear]

An improperly designed system could have catastrophic failure within 5 minutes.
Gears are designed to mate with each other.
For issues like
Interference
Vibration
Wear factor
Pitting
Scuffing
Tooth bending
Spiral bevels are required to transmit
Power at a right angle
It would be possible it will self destruct
At start up during testing.
Now you spent $$$$ for a pile of junk.

Read all the literature possible.

 

[MightyGNU]

Are you going to strip your transmission and inspect each and every gear tooth for cracks every ten hours?

Yes that is what I meant.

An improperly designed system could have catastrophic failure within 5 minutes.

I'm aware of this, I will already need to build a test stand for the rotors to make sure they don't come apart under stress so I will be able to test both at once, If they don't show damage after an hour or so at %150 maximum rpm they should be OK for flight.

I seem to be spending all my time talking about objections to my idea. Does anyone here actually feel like helping? Many of you suggested consulting someone with experience, but that's why I'm here and not building stuff...

 

[3DDave]

Buy and build a helicopter kit. See how that goes. Then you'll have access to a number of people who will know exactly what you are dealing with. Branch out from there.