Cams and Lubrication

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Go to page 117 and look from there.

First article uses capacitor type behaviour to examine the oil film between cam and followers...high capacitance means oil film, conductance means contact.

Has some osciliscope traces of new cams wearing in, you can virtually see the asperite contact diminishing to mostly full film lubrication
 
Ok professor
smile.gif

What is a hertzian stress factor??

Is this an expression that involves all of the kinematic factors, dynamic factors, physics, and chemical or elemental factors that an lubricant experiences in addition to what the parts experience as well?
 
Consider a train on a track, or a roller bearing on it's race.

In theory, both of those are a line of contact (or a point), either of which have zero surface area, so the contact pressure must be infinite.

It's not the case, however, as the materials deform. The ball/train wheel becomes a little bit flat, and the rail/race takes on a little bit of a dimple.

That's the area of "Hertzian" stress. The material makes sure that the pressures and stresses aren't infinite.

If it's too high, then it can lead to fatigue on the surface of the materials, and spalling.

A lubricant in that zone can spread the load.

e.g. a roller bearing that runs dry of grease/oil can be badly affected.
 
Makes sense. Can that load be expressed as psi?? Or another value?? Friction modifiers would than be the part of the equation that spread out the load right if its not hydrodynamically lubricated??
 
"One of the conclusions of Williamson et al is worthy of recording, namely "for all the oils tested there was always a lubricant film present between the cam and follower surfaces". The oils had a dynamic viscosity variation from 5 to 23 mPa.s at 100C."

Maybe it's time I stop defaulting to thicker oils.

Also, I want this book.
 
I hate reading for hours from a screen too.

Interesting point, that the radius of the lobe favored the aggressive one and not the rounded-off lobe.

So I starred into space, ran the pointy lobe in my head and came up with; the rotation is stopped at cam-lobe tdc, but not with the rounded lobe.

Whatta all think, does that dog hunt?
 
roller cams and folowers use a lobe shape much more like cam A...

So, could MoS2 in a colloidal suspension (with 0.3to1µm particle size) offer a friction redcution or wear reduction benefit in the areas where the oil film is at it's thinnest?

Polar fluids would also offer benefits there?
 
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Originally Posted By: Shannow
Go to page 117 and look from there.


Has some osciliscope traces of new cams wearing in, you can virtually see the asperite contact diminishing to mostly full film lubrication


That doesn't appear to be quite the interpretation offered in the article, and its quite hard to see how it would happen over the timescale reported.

It goes from full to partial contact (presumably mixed lubrication) in 15 mins, but its still mixed after an hour.

Contact is much reduced after 24 hours, but this is not interpreted as a shift to "mostly full film lubrication" (how could that happen, unless perhaps the surfaces were wearing smooth?) but the development of a non-conductive surface film.

I'd guess this is due to the activation of anti-wear stuff like ZDDP, though this is not specifically stated.
 
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Originally Posted By: Wampahoofus
"One of the conclusions of Williamson et al is worthy of recording, namely "for all the oils tested there was always a lubricant film present between the cam and follower surfaces". The oils had a dynamic viscosity variation from 5 to 23 mPa.s at 100C."

Maybe it's time I stop defaulting to thicker oils.



I'd have liked to see a bit more on that, since, without getting into engineering, oil selection is the only parameter under my control.

I'm unclear how to interpret the practical significance of "always a lubricant film present". The conductivity experiments suggest it can't mean "always in hydrodynamic lubrication", and it would have been interesting to see how contact varied between oils.
 
It would have been interesting to see contact traces for a re-start with a conditioned cam after a long rest period, to see how the initial start-up contact differed, especially if oil retention was quantified and/or varied by washing it off.

Any non-conductive surface film could perhaps (?) be removed by electrolysis, (?) which shouldn't (?) be erosive to the steel.

This could perhaps be usefully combined with direct surface-roughness measurements.
 
Originally Posted By: SteveSRT8
I'm still waiting for Turtle boy to come along and correct you guys...

Why wait for him when you can present your ridiculous views?
 
Originally Posted By: ExMachina
Originally Posted By: SteveSRT8
I'm still waiting for Turtle boy to come along and correct you guys...

Why wait for him when you can present your ridiculous views?


would that be the "more like penetrating oil"...ermmm...oh, wait.
 
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