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High power gear drive
5

High power gear drive

High power gear drive

(OP)
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/Lite...) 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

RE: High power gear drive

@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.

RE: High power gear drive

(OP)
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.

RE: High power gear drive

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.

RE: High power gear drive

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

RE: High power gear drive

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

RE: High power gear drive

(OP)
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?

RE: High power gear drive

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.

RE: High power gear drive

(OP)
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?

RE: High power gear drive

Hire an engineer

RE: High power gear drive

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

RE: High power gear drive

(OP)
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.

RE: High power gear drive

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.

RE: High power gear drive

MightyGNU,

Quote (Murphy's Law of Mechanical Engineering)


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

RE: High power gear drive

(OP)

Quote (drawoh)

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.

Quote (drawoh)

Highly stressed parts have finite lifespans.
This is very true, I will make a note to inspect the gears every 10 hours or so.

RE: High power gear drive

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

RE: High power gear drive

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...
  1. ...you are using lots of material. The stresses will be low, and crappy materials and manufacturing won't matter.
  2. ...your helicopter will be too heavy to leave the ground.
If you don't know what you are doing, you design with large safety factors. This results in something heavy.

--
JHG

RE: High power gear drive

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.

RE: High power gear drive

(OP)

Quote (drawoh)

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.

Quote (mfgenggear)

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...

RE: High power gear drive

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.

RE: High power gear drive

(OP)
I think you are optimistic about my budget. The AW95, the cheapest DIY helicopter kit I am aware of, costs over $10,000 USD and is barely functional as a helicopter.

RE: High power gear drive

MightyGNU,

I have gotten into carpentry as a hobby. A friend of mine is also into it. He lives in a condo, so he takes advantage of some do-it-yourself carpentry shops here in Toronto, Canada. There are all sorts of work benches and tools. There is an old guy who looks over your shoulder and gives you advice, apparently, whether you asked for it or not.

He advises people to not build chairs. Chairs are like aircraft. They require good structural design, and a standard of workmanship beyond that of many (most?) hobbyists. I am of course, building a chair here at home. smile

You are not understanding my point about lifespan. A qualified engineer designs an airborne transmission to be light weight, and to have some specific life, say 500 hours. The casing, the shafts, gears and bearings are designed close to the limits and then evaluated for ultimate stress, yield stress and fatigue. The transmission is built, installed and flown for 500 hours. After that, it is removed from the helicopter and beaten into a plowshare.

--
JHG

RE: High power gear drive

@MightyGNU
MightyGNU (Mechanical)
(OP)
9 Jun 20 23:19
(I think you are optimistic about my budget. The AW95, the cheapest DIY helicopter kit I am aware of, costs over $10,000 USD and is barely functional as a helicopter.)

Because the volume is low and are not off the shelve Gears, each gear because of the geometry, bearing surfaces, carburizing, Non destructive test , can and will
cost in excessive of $2-5 K each. these type of gears are very expensive, and that's not including the engineering design of the details, gears, shaft and 3 housings, main rotor, tail rotor shaft,
gears , tail rotor gear shafts, and the purchase of the bearings, plus the expense of the cooling and lubrication system.
It will be in excess of $10k before it is done. and that is if there no design errors.
like spigor said earlier these gears are of high quality and close tolerances..
to design, fabricate and assemble it will take lots dollars.
when a KIT is purchased, it has been designed and tested, but not full proof. but the odds are in favor, plus all the tools and engineering has been tool proofed.
it is a proven design. you are head strong and want to build gear boxes. you may have to totally redesign and rebuild three boxes before you get it right.
fact that you have lack of engineering skills in building such a design puts you at awkward situation.
The reason I said hire an engineer, is because he or she has gear box design,and will be a good mentor for you, and has seen and under stands what works, and what does not.
the dollars spent for his help will cut the learning curve significantly. build your gear boxes, just get engineering consultation in the process.
It will put you way a head of the game, and you will learn in the process. now I may have given you the worst case scenario. it may in up not being as
difficult or as complex for a small one person helicopter. but every response has been leads for you to do your home work and read and study.
you can not expect to design a gear box here. first of all the structure of the helicopter has to be know, the engine that will mate with the gear box,
how it will be mounted and installed, the amount of maximum weight it has to be, gears are designed to S/N curves, to know exactly when maintenance is required.
gears and bearings will require replacement. all parts have to be re mechanically inspected, and re non destructive inspected for defects, cracks, excessive wear,
or what ever.
The advice given is for your benefit to under stand the requirements and the cost.
we have given you more than normal advice.

RE: High power gear drive

Quote (MightyGNU)

I need a 15:1 drive (..)

Quote (MightyGNU)

(..) manual transmissions use similar sized helical spur gears to handle as much as a few thousand hp at 10,000rpm
A typical manual transmission has two shafts and a set of gear pairs, each pair with the same center distance- are those gears not what you are looking for?
A 15:1 ratio could probably be achieved with 2 or 3 stages.

RE: High power gear drive

(OP)
Manual transmissions generally have the entire shaft with gears machined as a single piece. I guess I could cut one up though... I would add that most transmissions have a maximum ratio of about 3.5:1, meaning at least 4 stages. (Also it turns out I did a calculation for the rotor wrong and the drive only has to be 10:1)

RE: High power gear drive

3 stages:
3.5 x 3.5 x 3.5 = 42.88

The 10:1 should easily be possible with 2 stages.

RE: High power gear drive

(OP)
You're right, I was too tired to be doing math last night 😑️.

RE: High power gear drive

gears for helicopters are among the highest loaded gears you may come across. they have to be able to withstand the stresses due to the load and in practice they are quite frequently inspected at regular intervals and parts need to be exchanged before wear reaches a critical stage. in other words: there is no comparison with automotive gears.

automotive gears are designed for a defined limited life span. in fact, every individual gear is designed for a different life span (for reverse a lifespan of 20 minutes at full load suffices), where higher forward gears need to last much longer. based on a perceived type of use the lifespans of the individual gears add up to the lifespan required. that is a quite different situation: in a helicopter drive every component needs to last as long as the design life....

another difference will be the load. in automotive use full load is seldom applied, most times the load is substantially lower, say about 30 to 50%. in a helicopter drive the average load on the gears will be quite a lot higher in terms of percentage of maximum load. so, the applications are quite different.

weight also is far more important with aircraft components - high load together with light weight will also make attaining a suitable design life much more difficult.

lubrication of helicopter gears is quite critical due to the high load conditions that prevail. the preferred lubricant is a fluid and not a grease. grease would cause churning losses and thus generate a lot of heat. most times a rather thin oil is used based on a synthetic type of fluid. that fluid is changed at regular intervals, whereas in automotive gearboxes there no longer is a need for changing the oil - the lubricant lasts long enough for the foreseen life of the gearbox.

RE: High power gear drive

(OP)
That may be true of stock cars but I'm thinking of race vehicles. Those cars are driven at maximum load almost constantly and the load applied when launching or downshifting is in excess of twice the maximum output of the engine. Those shocks are what most frequently kills automotive transmissions. I would imagine helicopters, which are subject to fairly steady loading, would not need to be as durable (within the given lifespan anyway.)

RE: High power gear drive

Durability is a choice the designer makes. Some things need to last longer then others and that usually is reflected in the design.

In certain applications durability more or less is overruled by safety requirements. A car transmission failure may be awkward, a failure of a helicopter transmission can be fatal and will get you in the headlines in ways you prefer not to. Generally speaking critical aircraft components are designed not only for a defined lifespan (which is a statistical outcome, some components will fail earlier, others will last much longer) but also require frequent inspection to make sure that at any given moment it will at least last until after the next flight. That usually results in maintenance instructions in terms of (every x hours, no more then a certain amount of wear acceptable, exchange parts every y hours even when the component appears still fully functional. The overriding principle is "better safe then sorry" - for good reason. That, together with the requirement of "as light a weight as possible" put helicopter transmissions in a quite different category then other transmissions.

RE: High power gear drive

3
MightyGNU- I fully understand why you may be upset that people are not just pointing you toward a catalog where you can buy what you need... but there's a couple of reasons for that.

I applaud your enthusiasm. As a licensed helicopter pilot myself, I can appreciate why you'd want to build one. They are a unique type of fun that you can't really replicate in any other man-made moving object.

With that said- these people are responding the way they are for a reason.

Helicopters, even well-engineered, 8-figure airframes, are very dangerous. That danger is mitigated only through two things: very careful, meticulous operation by pilots, and very very detailed engineering by designers.

If you're already a licensed helicopter pilot, I may be telling you things you already know. But a helicopter is NOT an airplane. They are fundamentally different, and in a helicopter it is very, very easy to put aircraft into a situation that feels fine but is actually extremely dangerous.

If you are able to build a functional helicopter, and it flies, and you start buzzing around with no training, I can almost guarantee you that isn't going to end well. Helicopters have a number of very unique responses to flight conditions that are NOT intuitive and can quickly result in an unscheduled high speed appointment with the ground. Any helicopter, let alone one without FADEC or aircraft state monitoring systems, can be put by the pilot into a state which is extremely precarious- and without training, you will not know how to respond. A helicopter which has entered a vortex ring state, for example, will kill you on the first try if you don't know how to identify it and how to respond.

The gearbox of a modern helicopter is literally one of the most intensely engineered components of any mechanical system currently in existence. A helicopter gear box has to do a lot of things all at once - it has to transmit a lot of power without generating a lot of heat or vibration; in many cases it has to help handle a significant portion of loads transmitted from the rotor to the airframe, which can be gigantic; it has to be absolutely reliable, and if it does fail, it has to do so in a way that gives the aircrew and cargo or passengers the best possible chance of avoiding a fiery death.

Designing a mechanical assembly that can do all of that is exceedingly difficult.

You will not be able to buy the right assembly off the shelf for cheap. And unless you plan to spend a very long time performing testing and paying for very expensive one-off parts, and very expensive non-destructive testing of those parts, you will not be able to build the right assembly yourself.

If you build a gearbox yourself, it is very likely that you will experience a failure- and if that happens at more than a few feet off the ground, or if it happens on the ground at anything above idle, this failure will kill you.

Quote (MightyGNU)

If they don't show damage after an hour or so at %150 maximum rpm they should be OK for flight

This test, were you to run it, would tell you nothing without very expensive NDT that you can't perform at home- unless you are set up for x-ray and dye penetrant testing- which I'm pretty sure I can safely assume you aren't. As a side note, you should never test a flight critical rotating component at 150% of anything. Centrifugal force is a square function- meaning if you test rotors at 150% of rated RPM, they are very likely to come apart, or suffer damage such that when they are rotating and loaded at full power, they will come apart. Don't do that. Please.

Quote (MightyGNU)

Yes that is what I meant.

I'm not sure you understand what he is saying - inspecting the gears of a helicopter doesn't mean opening the case and looking at them for a few minutes. It usually means completely disassembling the entire gearbox, and having the gears, shafts, bearings, and sensors sent off for very detailed dye penetrant and/or (usually and) x-ray testing to look for cracks. This is expensive - and if you don't do it, at some point you will experience a catastrophic failure.

As trained engineers, most of us have seen what happens when engineered systems fail. I would hesitate to ever tell anyone that what they want to try to build is impossible. But if $10,000 for a kit is an impossible extension of your budget, than it seems to me that your current circumstances are not likely to permit you to do what you're telling us you want to do. Ultimately we can't stop you- but what we can do is try to make it clear, from our experience, what you are up against and what risks you are taking that you may not be considering. That's what we do for a living, and that's what we are doing here.

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