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Rotate
10-07-2002, 11:31 AM
More brainstorming...

Is it possible to make gears without tooth so that it's one steel wheel driving another. May be sandblasting the surface so that the two won't slip. Obviously this type of gear won't transfer much power, but I thought where extremely accurate motion control is required this would be a workable solution. I'm thinking of instrumentation applications may be, where the phases of the two gears must maintain a fix ratio all the time.

Albert

Forrest Addy
10-07-2002, 12:37 PM
I'd think there would always be some microscopic error that compounded over thousands of revolutions would screw up timing of any fixed ratio friction rotary drive.

Kinda hard to beat gear drives for timing precision on any scale.

Rotate
10-07-2002, 01:07 PM
The reason why I was thinking about this was because I realized that involute gears are very good at maintaining synch over millions of revolutions, but because of the complex profile of the tooth, the gear ratio would not be maintained very accurately over small rotary motion. In other words if the gear ratio was 1:10, then rotating the pinion 1 minute arc should cause the gear to rotatae 1/10 minute arc, regardless of which part of the tooth was currently engaged. Unless the involute tooth have been precisely matched, I would image that there would be jitter in the ratio during the motion of tooth engaging and disengaging. Again, I'm thinking of instrumentation applications, where the gears wouldn't turn more than a couple of times.

Albert

SGW
10-07-2002, 01:32 PM
Actually...one of the advantages of the involute tooth form is that it gives uniform rotary motion. There is the the question, of course, of "accurate tooth form," but if you're worrying about that level of precision, you can worry about the accuracy of the tooth form as well as anything else.

Weston Bye
10-07-2002, 06:40 PM
Albert:
How many revolutions do the gears have to turn? Some computer hard drives use a steel band pinned in the center and looped around a drum on the head positioning stepper motor shaft. Then each end is pinned to circular portion of the pivoting arm that carries the read/write heads. Some floppy drives do this also to provide linear positioning of the heads. The band is held in tension. If the drums have no runout and the steel band does not stretch, the only theoretical error would be due to thermal expansion. If the drums are wide enough and the band were wound in a spiral so that it did not cross over itself, several turns of one and possibly both drums would be possible. Note however, that this arrangement is only good for a limited number of the revolutions.

Wes

bdarin
10-07-2002, 08:11 PM
Seems I recall reading something about toothless gears in a paper from NASA back in the late 70's/early 80's. Don't remember any details, as the paper was long and boring to read. But you're on the right track. Maybe nasa.gov has more info. Then again, maybe the whole idea was abandoned cuz it didn't work out. But it did exist.

Dick Payne
10-07-2002, 08:55 PM
Rotate:
If you put a rubber band around one of the steel discs (like a woodworkers band saw wheel) you can transfer a lot more power with less slipping and also take up the error in dimensions at the same time.

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Dick

NAMPeters
10-07-2002, 10:28 PM
Dick is on the right track. Careful control of the diameter of the hard wheel and tweeking the center-to-center distance will give you what you want. Some means to adjust the the c-to-c distance will provide the ability to adjust for wear and tear etc.

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Neil Peters

Oso
10-08-2002, 12:29 AM
obviously not a precision application, but some large old lathes, pit lathes, actually, in the 50 foot or more swing class, were powered by friction wheels.

That should dispose of the power transfer problem...

Thrud
10-08-2002, 01:02 AM
Oso:
Friction wheels were used in the pit lathes as a safety measure - it is difficult to managage a 50' swinging mass safely without slipage to protect everything in the driveline. This is much like when we allow our lathe belts to slip if overloaded - the best feature of the old flat leather belts.

Excellent observation.

Albert

In some applications such as one way clutches on precision ground shafts the ability of the one way bearing to resist turning was accomplish with a very toxic oil called "Gold Oil". It gives extreme high friction to the metal to metal interface yet allows smooth lubricated function in the freely rotating direction. The Gold Oil functions like this, the higher the pressure, the greater the friction. It was commonly used in old laser printers and fax machine paper feed systems. You may be able to find other "traction oils" but this one was the best and can no longer be purchased as it is a known carcinogen.

A hard Ureathane (over 90 shore) will last a fairly long time but it too eventually fails from torn chunks out of the tire or excessive wear.

As SGW has rightly pointed out correct involute form gears precision lapped are the way to go. These must be properly hobbed or cut from a involute form cutter made for the exact tooth count required for proper mesh.

In the pursuit of precision there are not many short cuts that can be taken.

Paul Gauthier
10-08-2002, 07:54 AM
I used to ride a Honda CX 500 motorcylcle. I still have it, it is in a pile out behind the barn. I got pretty intimate with the motor after tearing it down 3 times in three months. Anyway the starter motor on this bike is not connected via gears or chains, it uses 3 hardened rolls around a central shaft. When starting the rolls are jammed against the shaft and thereby grip it very tight causing it to rotate and the motor will start. Once the rpm's increase beyond the speed of the starter motor the three rolls them disengage. It has been way to long to remember this perfectly but if I remember correctly the three rolls are contrained in a tapered slot. So that when the starter engages the rolls are forced to small end of the slot. And when rpm's pick up they are pushed to the large end??
Make sense????????????

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Paul G.

metal mite
10-08-2002, 10:49 AM
How bout travel dials?
They have a hardened spherical driver with axiel finish marks that are supposed to make their own track in your soft lathe bed.
Don't know how they last in the long run but are accurate if properly cared for used.

Koppers / Kopflex used to make a variable speed transmission based on friction drive and some special oil? May be what you guys are talking bout?

mite

Ragarsed Raglan
10-08-2002, 12:46 PM
Paul G,

That'll be a 'sprag clutch' then! I've seen this design similarly, but slightly modified, used in race car differentials. These diffs were known as ratchet diffs.

Great experiance driving a Mini Cooper S with one of those in it!

RR

Jaymo
10-08-2002, 01:21 PM
What about knurled wheels? Just a thought.

Oso
10-08-2002, 02:15 PM
Raglan:

Old Saab models 95 and 96 also had those, with a choice of hard engagement or clutch action.

With the clutch action, coasting right along was easy, quiet and saved gas. Of course they are illegal now.

BUT, if you wer going around a curve, you had to keep the pedal down, because if the clutch disengaged, the turning radius tightened up fast! (Front wheel drive in teh 1960s).

Rotate
10-08-2002, 05:22 PM
Oso,

What do you mean they are illegal now?

Albert

Dave the Nave
10-08-2002, 06:02 PM
Rotate,perhaps you may want to check out www.varispeed.com (http://www.varispeed.com) This might be the same thing that you've described earlier.I believe this is a cvt (constant velocity transmission)adapted to a bicycle,which I've longed to invent for many years but someone beat me to it.I just wonder how heavy this thing is for mountain bike applications?