Determining bevel gear clearance

[Henro]

I don't know 100% for sure but I think the sliding mainly occurs in mixed or boundary lubrication regimes- direct metal to metal contact - no or very little oil film. The surface speed needed to form a full hydrodynamic film does not exist. That is certainly true in the case of heavily loaded spiral bevel and hypoid gears.

Dan

That makes sense. BUT if you put oil on a surface and slowly move your finger across it, it is still slippery.

So I would imagine, even with slow moving gear surfaces immersed in oil, that there would be a lubrication effect, regardless of what the speed difference was between the surfaces...

Spoken by a guy with an electrical background, not mechanical...so just a gut feeling.

Granted, a hydrodynamic oil film between surfaces would reduce wear to zero...

 

[TheOldHokie]

That makes sense. BUT if you put oil on a surface and slowly move your finger across it, it is still slippery.

So I would imagine, even with slow moving gear surfaces immersed in oil, that there would be a lubrication effect, regardless of what the speed difference was between the surfaces...

Spoken by a guy with an electrical background, not mechanical...so just a gut feeling.

There is but its not full film. Press your finger down real hard and that slippery oil film mainly disappears.

Dan

 

[Henro]

There is but its not full film. Press your finger down real hard and that slippery oil film mainly disappears.

Dan

Now we are just splitting hairs...

Something is almost always better than nothing...LOL

Fun talking details though...educational on my end. And hopefully for some others here as well.

 

[TheOldHokie]

Now we are just splitting hairs...

Something is almost always better than nothing...LOL

Fun talking details though...educational on my end. And hopefully for some others here as well.

We have been splitting hairs from the very start when the topic was still meauring lash......

Dan

 

[ejb11235]

Now we are just splitting hairs...
Something is almost always better than nothing...LOL

Actually, I think the thickness of the film gets to the very heart of the matter. Surfaces are not flat ... they may be "globally" flat, but there is always a grain structure/roughness (think about scraping techniques to make surfaces flat). So the the thickness of the oil film and the properties of the oil, especially at contact points sliding under pressure ... this is wear (ha ha pun intended), ahem, where, the action is.

 

[ejb11235]

This concept of sliding is a bit hard to describe in English.. As previously shown the spur gear slides for the entire length of mesh with the exception of the pitch point. What changes is the delta in the velocities of the moving parts at the contact point. So "sliding" is not increasing" or decreasing. - the surface speed of the slide is increasing and decreasing. Henro picked up on that right off the bat

I think this is what I meant but honestly not 100% sure. Sounds similar to the speed of a wave travelling through water. The wave has a speed, but the water is only moving up and down. Speaking of water metaphors, I think this is about as deep as I can go in this round. Thanks for an interesting conversation.

 

[NorthwoodsLife]

So, because of this post's tangent; I racked my personal memory from my 1980's college days.

I'm just a hack. I've got a little background in math and physics. But I'm a hack at this late stage in life. But I know a little...

As a Civil Engineering Major, the math was there, but the metallurgy... Not so much.
But, as a visual learner, I can be sure that there is friction in any situation where gears are involved. Anytime a linear force goes through a direction change, there is friction. Even the distance of a linear force, such as a long PTO driveshaft, there's power loss through the driveshaft mass, or a a middle bearing carrying the load.

That being said, any, ANY, change of linear projection IS going to produce friction and power loss. Even if it's through a common universal joint or thru a ball bearing (CV) constant velocity joint.
Gears, at any degree of change to the linear, will create friction. The gears in a common automotive Ring and Pinion (90 degrees), and gears in a simpler bevel gear process ( < 90 degrees) in Kubota front axle for example. Is going to produce power loss due to friction.
Every gear system, of any kind, results in the gears sliding against each other at the toe or heel. And usually almost always within the midrange.

Ring and Pinion gears have a machined curve. That's because the force energy can be better distributed across a greater surface area between the pinion gear and the ring gear. Yes .. it's sliding. It's most definitely sliding.
As for the comments about oil viscosity and this application. Folks greater than I have determined through Research and Development ( R & D) what type of oil works.

My hat is off to each of you. Especially those who researched or found in their memories the formulas quoted.