DO GEAR TEETH ROLL?

Where is the truth to the assertion that properly designed gear teeth only make rolling contact?

If this is the case, why lubricate?

Sam

Hi, Sam. Involute gears roll where their pitch circles touch; they slide above and below this point. Lubrication avoids (continued) metal to metal contact, it allows sliding between parts, once an oil film has formed, as in plain bearings.

In the case of pick-off (or change) gears for screwcutting, etc., some oil makes the gears a little quieter.

In higher speed gears, as in closed gear boxes, the oil also helps to remove heat caused by friction.

Bill

As Bill says, the teeth slide, so all that lubricant wasn't an unnecessary complication.

An ideal pair of revolving gears would show constant distance between centres and also constant relative angular velocity. They would thus be pretty quiet although lubrication would be advisable. Testing gears for accuracy is done by meshing them (with a preload force) with precision "master gears" and measuring the variation in centre distance as they turn.

Murray

Thanks Bill and Murray

That is where my confusion resides. It is easy (for me) to see that rolling takes place the instant both pitch circles coincide but …

Statements I have read seem to imply that rolling takes place immediately two teeth make contact. Yet any two points on opposite teeth which are about to make contact will (relatively speaking) be travelling in different directions and at different velocities as they begin to merge or separate.

Or am I missing something?

Sam

Can o' worms. What assertions that there is not sliding? Some guy's forum post on the internet? What is the definition of "properly designed" gear teeth. There are other considerations to "proper design" beside lack of sliding. EG, ease of manufacture, tolerance to variations in centre distances and other machining variations, strength under load etc etc.

In fact, one of the problems with lubrication of involute gear profiles is there is no sliding at the infinitely small point where contact is at the meeting of the two pitch circles, in theory. So lubricant is not squeezed ahead of the contact point, so lubrication fails at this non-sliding point, not at the point of maximum sliding as one might otherwise think.

There are moves afoot  to come up with better alternatives to the involute curve, see here for a starting point **LINK**

Edited By Hopper on 22/04/2018 07:47:40

Question: Why are roller bearings lubricated? When you think about it, the second question in the original post will be seen as a bit unnecessary.

If everything is perfectly made, if the surface is perfectly smooth, if there will be no micro-welding under the theoretically infinite forces of infinitely rigid gear teeth surfaces, then no lubrication is needed.

In reality, the wheels are not infinitely stiff, the wheel is not perfectly smooth and not perfectly made, either. All this imperfection leads to small sliding movements that degrade the surface, and lubrication reduces the degradation rate to acceptable levels. If the two were perfectly made etc, then there is likely to be micro welding, which degrades the surface and starts wear - so lubrication is used to reduce the probability of micro welding to acceptable levels by leaving a non-weldable molecule between the two surfaces.

Taking this limit explanation shows why it's needed.

Regards,

Richard.

Thanks for your replies Hopper, NDIY and Richard,

If only I had the time and stamina to read the outstanding intelligence in Hopper’s `Gear Technology’ link.

Including gestation, I’m on my 83^rd trip around the sun, I was hoping for a simple answer. It rather serves me right for being Devil’s Advocate.

In summary, and as I imagined, who ever started the ball rolling (pun?) i.e. that correctly designed gear teeth roll over each other was wrong, and that sliding (albeit it minimal?) takes place; other than where the pitch circles meet.

In acknowledging Richard’s first-rate explanation, does the frictional heat generated from sliding, account for the need to submerge the gears in lubricant including heat dissipation?

Sam

Another function of the oil in gearboxes (etc) is the prevention of corrosion. Another still is the timely removal of wear debris from the sliding or rolling surfaces. Another is the reduction of noise.

Useful stuff, oil

Cheers, Tim

Wikipedia has a good article on involute gears and confirms that they only "roll" at one point in the contact cycle.

I believe it is true that perfect cycloidal gears would always roll, but actually they are nearly impossible to make. Clock gears usually approximate cycloids by straight lines and circles, and how close they come to rolling I'm not sure that anyone knows.

Thanks Tim and John (H),

The animation here ... https://en.wikipedia.org/wiki/Involute_gear#cite_ref-1

other than throwing me back to constructing an involute in prac. drawing at secondary school, satisfies my curiosity completely.

Have fun,

Sam

Sam,you looking for a simple answer?, I discovered long ago that NOTHING in life is simple,not even gears.

As Tim say,s "useful stuff,oil"

Quite right Larry.

The more we know the more we don't know.

Then there's the stuff that we don't know, we don't know.

And can't even find out!

Have fun,

Sam

Some company down here in Oz advertises "Oils ain't oils"

It seems to me that things are further complicated by the number of teeth engaged at any moment and if a rolling contact on one of them takes the strain off a sliding one.Colin

I think for a straight cut gear, most of the time only one tooth is engaged except at the start and end of a tooth cycle, when both (i.e. the old and new) teeth are sliding. A classic book on this is "Gears For Small Mechanisms " by W O Davies. Situation is different for helical gears which are designed so that as you move axially along a tooth it changes at any instant from "just engaging and sliding" to "engaged on the pitch point so both gears and rolling" to "just disengaging and sliding".