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View Full Version : More OT, partly at least- rpm reducers



darryl
08-27-2011, 05:25 PM
Been looking at this high-speed motor, low speed output shaft requirement thing lately. There are a lot of high power, high efficiency, high power to weight ratio PM motors around now. One of the drawbacks to them is that the rpm range you often would want to adapt them to is much lower than the motor rpm. Even if you wanted to adapt one of them to a router for instance, you'd still need a speed reducer. Many of these motors run at 40 to 60 thousand rpm, which is one of the main factors in them producing so much power for their size.

At any rate, I've been looking for an elegant way to reduce that rpm to a usable range, which might be in the 1000 to 5000 rpm area. By elegant I mean smoothly, quietly, efficiently, compactly, and long-lived.

60,000 rpm is an awfully high input rpm for a gearbox, but there has been a design that did the initial rpm translation using rolling elements under pressure. The pressure was to increase as the desired torque from the motor increased, in order to prevent slippage in the parts. I don't know if this method proved to be worthy or not.

Virtually every cordless drill uses a motor which run at a high rpm, and the speed reduction is done with planetary gearsets. It's great that you can get balanced loading on the motor shaft, especially when there's lots of torque available and you can avoid having too much side pressure against what might be a relatively small bearing. They are compact, but limited in reduction ratio, thus requiring more than one stage to get to a workable rpm. Still, this must be one of the best methods, as it is used a lot.

There is the toothed belt, and the micro-V belt- I suppose you could set up a balanced loading by using two belts opposed, which would mean two output shafts which you might have to combine into one output, depending on your application.

This is the primary speed reduction need that I'm talking about, having to bring down a very high rpm to a usable one- what you do with that after is application dependent. Is there something else that works here?

MaxHeadRoom
08-27-2011, 06:17 PM
Not sure what PM motors you are looking at at that rpm apart from model aero brushless.
But there is an Australian guy selling a router control with precise feedback control down to 5krpm.
There is an extensive thread on the cnczone here it has collected alot of positive feedback or you can visit the site, SuperPID.
Max.

darryl
08-27-2011, 08:00 PM
Yes I am looking at model aero brushless. If you include the outer runner types, the rpm isn't as high, but still in the 12k and up range. In the lower power ranges, 600 watt and lower basically, the rpms are way up there. Most 'domestic' uses I would have would be in the 300 to 600 watt range, and the compactness of these motors is an asset.

Maybe I should start looking at the gearboxes that are made to fit some of these motors. A planetary, helical gearset, stackable- might be my best answer. One of the problems with electronic speed control is that when you dial the rpms way down, there goes your power as well.

becksmachine
08-28-2011, 12:42 AM
Can't think of the name right now, what is the one that uses one gear inside the other?

The internal gear has 2 or 3 teeth less on it's OD and is flexible enough to run against the outer gear that has teeth on it's ID. I guess that is sort of a planetary set up??

Ok, google is my friend! It is called a harmonic drive. Don't know if it would handle 60K but might check it out.

Dave

Guido
08-28-2011, 12:57 AM
http://www.globe-motors.com/home.html

Bring money and choose 'yer poison. Fits our pistol at 4,000 ft below Arizona desert, 150 F, to open and close tools inside 2 inch diameter boreholes.

--G

uncle pete
08-28-2011, 01:31 AM
Darryl,
It may not be even a consideration for your needs but at 60,000 rpm bearing and driven gear life will be rather short. Can you not find a lower rpm motor with enough HP and torque to do the job without the gear reduction? It takes massive amounts of power to just get to 60,000 rpm never mind doing work with it after that. The planetary drive will further rob horse power. Model Turbine jet engines run far faster but produce very high power to weight. Bearing life in those is very short too, Even with their total loss oiling and state of the art ceramic bearings. Maybe a bit more information about exactly what your trying to do would help.

Pete

strokersix
08-28-2011, 10:53 AM
Harmonic drives are very cool (not literally!) but watch out for efficiency. They work best at slower speeds. Efficiency drops off markedly above 1500 or so input rpm and don't run above 5000 or so input rpm. These numbers from memory so not exact.

strokersix
08-28-2011, 03:07 PM
I should also mention that a small harmonic drive unit retail cost is about $1000.

About the cheapest you will find planetary gearheads is around $200.

That leaves salvaging components from consumer devices such as drill motors. Or make your own. Or hobby stuff like RC cars which I think was mentioned above.

darryl
08-28-2011, 05:20 PM
The idea behind all this is to be able to use one of the new breed of small brushless motors in applications where the output shaft rpm needs to be a small fraction of the motors rpm. Because these motors are typically high rpm, and you'd want to keep the rpms up there so the motor can still develop lots of power, a first stage speed reduction unit would be needed. The main criterion is that it meets the specs I laid out- longevity, smooth, quiet, etc.

All I was asking is if there is such a technology out there. The closest thing to fit the bill seems to be planetary gear reduction, so I think the 'answer' is to have a two stage unit in a single housing, with helical input gears at least, even if the following stage is straight cut gears. I was asking to see if there were any better options.

The harmonic drive can give a large single-stage reduction ratio, which would be great, but if it can't handle the high input rpm it's not a suitable design in this case.

The planetary friction drive, being without gear teeth, would seem to be able to handle the high input rpms- thus I asked about that. In the model industry, there are gearboxes which use anodized aluminum gears- I can't imagine them having a very long lifetime, or being able to handle 300 watts of power, let alone 1000 watts.

darryl
08-28-2011, 07:01 PM
Found something interesting- what's called a split annulus planetary gear. It has the normal planetary gear arrangement, but the planet gears are each a stack of two gears- one with less teeth than the other. The primary ring gear is fixed, the motor drives the sun gear as usual, but the second deck on the planet gears drives a secondary ring gear, from which the power is taken off. It's a combination of planetary and harmonic drive. With the primary ring gear being fixed, the secondary ring gear rotates because of the different number of teeth in the second deck gears of the planets. If the difference in the number of teeth between the two decks is one, the second ring gear will rotate slowly compared to the fixed ring gear. Basically that makes for a high reduction ratio from the sun gear. Because there are no offsets involved, it's inherently balanced and could run at high speed. Because it's planetary, it has a balanced loading and thus no side forces acting on the bearings, and it has high torque capacity because at least three gears are transmitting the torque at all times.

Because the output torque is developed by gear teeth that mesh inside ring gears, the tooth contact area is greater than with normal spur gears. The only area where normal spur gear engagement occurs is with the sun gear, which is the lowest torque area being directly off the motor shaft. Also, you wouldn't have to make the sun gear super small in order to get a pretty decent reduction ratio, so the sun gear tooth loading can be minimized by making it a reasonable size.

If this method were integrated with the motor, the planet gear carrier could be centered by the motor shaft, and the output shaft could also have a bearing centered on the motor shaft. The output shaft would most likely need an outboard bearing as well, but we're down to a compact arrangement that seems like it would run very smoothly. Next issue is lubrication- but I think you could keep a small amount of oil sealed within this because it would either sink to a bottom side where there would be no gap to escape through, or be kept in a layer around the ring gear area by centrifugal force when the motor is running. The oil would not normally see the area where shaft seals are, except briefly during starting and stopping.

Seems like one pretty good answer to my quest-