Traction drive transmission research
 

For the engineering guys,

NASA did several studies on traction drive for helo transmissions.
Traction or friction drive is gears without teeth, or in other words, rollers that transmit power. Some locomotives used friction drive.
Sounds odd, I know, but it may have some advantages like long life at high speed and low noise.
I see several of the studies are on the web but none that could be downloaded.

Anybody have knowledge of what the current thinking is with respect to traction drive?
Thanks,
slowrotor

Hi Slow,
i don't have a partiular knowledge about that but these guys may have :
www.rotrex.com
they use it for their gears.

Victor

For friction drive to work, there must be a considerable force to hold the wheels against each other. (the friction coefficient "mu" and the usually gravitational force, g) This produces big forces on the axles or hubs and the driveshafts. bearings will need to be super strong. But for a light car or a Robinson R25 (the beltless version) the effort might be worth it.

A gear will make the other gear turn with a minimum of force (other than the rotational torque) - just enough to stop the gears from moving apart.

sprague clutches and freewheel units already do the job in most transmissions or drive trains. no gears there.:}

Very interesting technology but what are the advantages of this over the belt drive system used by Enstrom and others? E.G. Is it cheaper and does it absorb less power in it's operation?

On a brief scan through the data on this system it is for high speed applications, possible over and beyond piston engined helicopter applications

Just so we are talking about the same thing...
 

Friction drive uses 2 surfaces in physical contact under pressure, to transfer torque from one surface to the other.

Traction drive, uses a special synthetic oil between 2 surfaces that are NOT in physical contact, to transfer torque from one surface to the other. The "gap" between the surfaces is precisely controlled (through high quality manufacture), and the fluid used has a property where it becomes virtually solid when exposed to significant sheer pressure. So it's actually the fluid that transfers the torque in traction drive, NOT friction.

Traction drive therefore does not require the bearing loads that friction drive does. Traction drive however always involves a small amount of slippage, because the fluid cannot demonstrate the solid property, until there is a certain amount of sheer pressure.

Headturner,
I think the advantages of traction drive might be:
1) low noise
2) easier to make
3) higher ratio output
4) longer life
5) no gear tooth fatigue

disadvantages:
1) high bearing load (as mentioned by Ascend Charlie)
but not in the planetery configuration that is apparently self adjusting.
2) slight slippage (about 1%)

Still wondering if any helo engineers have considered using traction drive.

Flight Safety,
Your post went in while I was I typing. Very good info. I thought friction and traction was the same. Thanks for the clarification. Do you know if any helos use traction drive?

slow
did you have a look at ROTREX superchargers ?
they use it, and explain it very wel BTW they manufature their planetary gears.

Slowrotor thanks for the clear response.

If there is a continuous 1% slippage then the application could be suitable for reduction gears between engine and rotor head.

There must be other disadvantages, e.g. cold weather ops, that preclude the application in helicopters otherwise it would seem a way to avoid those Chip Light indications, noise (a sensitive factor) and apparently the manufacuring costs are less.

I have not seen this type of gearing employed by motor manufacturers and they are generally very quick to take up good technology ideas. We're in the motorcar business and our engineers are going to look int this concept to drive alternators and waterpumps and other functions if suitable.

However, getting back to helicopters, what or where would this be type of gearing/torque transfer be applicable?

Strange coincidence?
Have just finished reading Capt Eric Brown's book entitled: "The helicopter in Civil Operations".
One of the topics where he expressed some optimism in the further acceptance of helicopters was the 'breakthrough' made by NASA researchers who had been working on a Multiroller Traction Drive for the past 10 years.
The book was published in 1981!!

The advantages, back then, were deemed to be:

1. Quiet, almost vibrationless means of transmitting power.
2. Simpler and less expensive to manufacture than helical or planetary gears.
3. Lighter and smaller than conventional gear boxes.
4. As efficient as toothed-gear system.
5. Higher reliability and are less susceptible to breakdown and wear than conventional gear boxes.

It was envisaged that roller drives would enter the realms of in-service practability in the mid-1980s!!!!!

So who's got them?

How would this work in a helicopter, where the drive has to turn through 90 degrees or 43 degrees?

Gears do that quite simply, but I can't see how this traction drive would do it.

Didn't Nissan produce a vicsous coupling differential for their pulsars some years ago? I always wondered why they discontinued it as the improvement in the traction & handling - albeit when pushing hard - was well worth it, even if it wasn't my car....

As for AC's point about turning the direction of drive, I'm sure Dave Jackson could adapt his double universal joint for this or other purposes, as I assume he's not making double jointed supermodels full time.

Even if it wasn't possible to eliminate gears completely, the idea of reducing the weight, complexity & maintenance requirements of transmissions is highly attractive, so long as it's stability could be maintained. Friction or traction, or even a combination, are fine so long as used to best effect and the desired result is achieved.

Zeeoo,
I went to www.rotex.com Good explanation about traction drive.
Still wondering if it has been used for a helo rotor transmission.

EESDL,
Interesting. I remember reading about the research at NASA way back in the 80's I guess. What became of it is the question.
Maybe its under wraps.

AC,
I think it would work in the same way a 45 degree bevel gears works. Just simpler.

Flight safety,
Are you sure about the "gap" between the rollers? And traction fluid that becomes solid in the gap? Very weird if true.

Thanks for your interest
slowrotor

Traction drive transmission research
 

Traction, friction., they both involve extreme pressure whether it be pressure on rollers or in the case of traction I assume pressure on fluid retaining seals. The Masquito boys in Belgium tried a hydraulic tail rotor drive gearbox but changed it to a cable drive because if I remember correctly they either could not get it to hold oil for long enough , or maybe the whole thing got too hot. Getting too hot is not an uncommon problem when using a viscous medium to transmit power especially when attempting to gear up. As this is being looked at regarding its possible use in very light rotorcraft, one has always to bear in mind that the CAA and I assume FAA, like to see components that have some track record, and so any innovative ideas might not sit well with them.The Masquito M80 also had originally a 12 foot dia main rotor. I was present with them during discussions where they were trying to explain how the autorotational quality was up to the required standard, but for the life of me I could not understand how given the high disc loading, they achieved this. Development of the M80 is I believe on hold for the moment, which is a great pity as these chaps had really given it their all.

Back in the early and mid 1970"s a bunch of work started on trying to make roller transmissions, but the extreme pressures and the need for close tolerances with new welding techniques proved their downfall, I think.

The first program I worked on at Sikorsky was a roller gearbox one.

Modern transmissions with double helix and split torque are very light and very reliable. For helicopters, which favor relatively constant speed, the advantage of roller or belt drives is somewhat lost.

The automotive variable ratio transmissions allow one to do away with clutches and gears, because they allow almost continuous change in gear ratio while still driving, so that the car's wheels can be driven by the engine throughout the speed range, in spite of the fact that the engine wants to be in a narrow rpm band. The rollers or belts are shifted down variable diameter shafts so that smooth continuous drive is had, a real advantage.

Wear and maintenance issues are still an issue, but materials are coming that might solve this. Of course, electric drive also is transmission-less, and might take over.

Slowrotor, here's a link to a company called Arch Technology that manufactures a product line of fluids for traction drives called Santotrac, that display the properties I mentioned.

Santotrac Fluids

Just fyi, here's a research project for an automotive CVT transmission using a Santotrac traction drive fluid, Santotrac-50.

University of California Davis CVT research project

Interesting technology.

Seems the traction drive is better suited to variable ratio automotive transmission. No reports of advantage or any use in a helo.

I think I will look more closely at friction drive. Rubber drive wheels. The advantages would be:
1) looser fit is tolerated
2) absorbs torsional vibration from engine
3) open, light frame (no gear case)
4) no clutch needed
5) smooth and quiet
6) easy to inspect

Friction drive is probably only practical for a very low power helo. Even if the transfer efficiency is relatively poor, the cost and simplicity may be more important.
Look at all the problems with the Rotorway drive line... belts, chains, oil leakage, secondary shaft failure...
There must be a better and lighter system for small sport helos.

slowrotor

slowrotor,

What about the V-belt (not cog-belt). It does not have the efficiency of gears but it is 'one stop shopping' for; speed reduction, axial, radial & torsional vibration absorption, misalignment, and soft-start.

It appears to work well in the R-22.

The 3VX Gripnotch by Morse and the Poly-V by Browning are two considerations.

Dave

Dave,
Belts have a number of advantages. But one problem is the large diameter wheel required on the engine because a belt cant bend around a small shaft. I want a large speed reduction ratio in one stage to avoid the problems with secondary shafts.
The R-22, S-300 and Enstrom all use belts, then they employ a gearbox for reduction. Seems redundant to me.
Enstrom has a wide poly V (I think) (about 8" wide), looks like about 1 to 1 ratio. So the belt is not used for speed reduction.

For my low rpm rotor I will need about 18 to 1 speed reduction. Easy huh!

slowrotor

Dave, here are some interesting links:

Wedge Roller Traction Drive Unit

This traction drive transmission idea combines power transmission and sprag clutch functionality, which has helicopter potential, would be very nice for your application.

Different Wedge Roller Traction Drive

This unit also explores the wedge rollor traction drive idea (used on an electric motor) with a fixed reduction ratio of 6:1. The sprag clutch function is also incorporated.

As a product

Here are the above concepts as products.

1982 traction drive study - 137 hp, 14:1 reduction, gas turbine

You have to buy this one.

Aerospace quality traction drives

Abstract of study - 240 HP fixed ratio traction drive transmission

Click on "Citations" for more information.