gear noise -special case

[fxlarry]

Hi all, I've got a simple transmission: two seperated parallel gears with an idler in between that slides in and out of contact with both gears, thus transferring power from one gear to the other through the idler.

I'd like to lower the noise of the system, and I'm considering the advantages of helical gears. However, the worst noise happens as the idler comes in and out of engagement -when there's not full contact of the gears, and the teeth mesh noisily.

Will helical gears help at all in this special noise case?

 

[Roach]

Not that its any of my business, but how to assure alignment of teeth for engagement. Maybe I am not understanding the system?

 

[fxlarry]

(If you have the Mechanisms amd Mechanical Devices Sourcebok, it's in the section on overrunning clutches, pg 304 in the 2nd edition)

The idler slides in a slot that can be concentric (or even only roughly so) with the drive gear. Therefore, it always remains in some engagment with that gear (kind of like a planet gear). The motion of the drive gear drives the idler down the slot into engagement with the driven gear. There is no alignment feature, so sometimes the idler must turn a small amount before finding a space between teeth on the driven gear. This transmission acts as a one-way clutch, in that if the drive gear is stationary, but the driven gear is still rotating, the idler gets driven up out of the way.

You can find this mechanism in a lot of toys.

 

[diamondjim]

Have you tried using progressively modified
tooth forms like 50 percent long on the drive
gear and 25 percent long on the intermediate
gear and -25 percent on the driven gear and
see how this sounds? I think it may be harder
to engage the helical intermediate gear, but
even then modified forms would probably help.

Check out some recess action gearing or long
and short addendum systems for concept.

 

[Spurs]

Fxlarry

Is the arrangement that you are describing suitable for spur pinions that need to slide into engagement as in a starter motor application.

I am having a problem with the pinion jamming against the mating gear as it slides laterally into position during the start cycle. It happens about 1 time in 500 starts.

If not - any advice for my problem - or a reference on where I can find the info I need?

 

[fxlarry]

Spurs,

I get something similar happening too, when the idler rides in a straight slot. In my case, I think the teeth of the idler happen to fall on top of the teeth of one of the gears, rather than in the space between teeth. What happens then is that no driving happens, the drive gear spins, but cant get engagement with the driven. I can imagine it could also jam if the idler is forced all the way down its slot, regardless of whether the teeth are in the right position or not.

I changed the setup so that the idler rotates in a small arc around the drive gear like a planet, rather than riding in a straight slot tangent to the gears. This ensures constant contact with at least the drive gear. I seem to have eliminated my non-driving problem, as the idler will always (eventually) be spun into the right place to engage the driven gear. I wonder if this might help your problem as well.

Tell me more details if you can, this is interesting!

-fxlarry

 

[Spurs]

fxlarry

There is not much more to tell...

Does anyone else have any ideas on how to improve our problems?

 

[fxlarry]

DiamondJim,

I'm unfamiliar with progressively modified tooth forms, can you tell me more about them, or link me to a website? Thanks!

-fxlarry

 

[diamondjim]

If you can get a copy of the fairfield gear program and input the percent modification instead of entering the tooth thicknesses starting with the drive gear operating against the pinion. Then run this pinion information
with the final driven gear. This is kind of like a special planetary system as far as concept. If you have fixed center distances, it takes some special handling to get all of this to come out correct.

 

[Spurs]

Diamondjim

I am very familiar with modification as you describe,
however,
how would modification of this type fix a problem related to the pinion not engaging laterally into the driven gear in a starter application as described?

I believe what fxlarry is saying is that the application is a starter motor. During the start cycle, the pinion which is mounted on a helical thread moves along the thread (ie. along its axis) and the tooth of the pinion is supposed to engage in between the teeth of the driven gear. However, what is happening is that the pinion tooth does not always engage cleanly and creates noise as it is trying to find the engagement position.

I believe in his application this pinion and driven gear is also a parallel axis helical gear set.

Is there a way to design the pinion / driven gear combination to guarentee clean engagment everytime?

 

[diamondjim]

I guess the best help might be to have
some crowning towards the starting side
which essentially would be providing more
clearance at the start of engagement.
It would have to be a server crown or an
extreme radii along the full starting
side of the pinion. Sorry for the late reply,
but off on vacation for a few days.

 

[sacem1]

On starter motors installed in cheap engines usually they do not take the trouble of shaping the side of the tooth in a way that it would help guide the teeth into engagement every time.

I had a problem like this in a small starter for a very light pick up and the solution was to grind a small sideway bevel into every tooth in the flywheel and leave the side with rounded "shoulders" between teeth so that there would not exiost any "land" were the tooth of the entering gear could be stopped as it would allways engage.

In bigger engines this usually does not happen as the starter motor is bigger and the solenoid that drives the engagement pinion has enough energy as to move slightly the flywheel rotating it so that the gears get into the right position.

 

[plasgears]

Consider building in some clearance in the engaging pinion. I call this wobble clearance, which allows easier engagement. This is commonly used in small engine starters.

 

[fxlarry]

Just wanted to thank everybody for their enlightening feedback. Unfortunately, I can't tell you if any or all of these solutions worked, beacuse we've ended up going with a completely different transmission method (sprag clutch)! But keep the discussion going -and look out for questions on sprag clutches ! Thanks again, everyone.

-larry

 

[Spurs]

Plasgears

Can you give me an idea of how much clearance would guarentee engagment.

My application does not have a solenoid to pull the pinion into position.

On some variations, I have no problem with engagement. On other variations, I have a failure about once every 500 starts.

Is there some way of guarenteeing engagement every time? I have tried the approach of rounding the interface between the face and the flanks of the teeth on one side, but it doesnt seem to help much.

Im looking for any help anyone can give me on the topic

Thanks

 

[plasgears]

On a pinion of 3 in. dia., consider a wobble clearance of about .050 on the diameter. Then check the success rate. Also try other clearances. This should help you zero in on a good clearance range.

 

[Spurs]

Plasgears

Do you mean on a 3" pitch diameter gear you would allow .050" of backlash, or do you mean you would estend the center distance by .050" above the standard center distance?

 

[plasgears]

Add this clearance to the mounting so that the pinion is able to wobble in its mounting. This is what we use in small engine starters to maximize engagement and minimize abutment.