Simple bearing friction equations 1

[Aled87]

iM Trying to calculate friction loss in a camshaft journal bearing based on a basic coefficient of friction of 0.1, what equations do i use to calculate friction in a bearing?

I know that the change in velocity at a constant speed is zero therefore work done is zero? Although there must be a friction loss surely?

any help would be good, thanks

 

[pontiacjack]

Umm... I can't help wondering why you're asking that question on this engineering forum. It appears to me that the "help" you need is a basic high-school-level physics course.

 

[pontiacjack]

Sorry, I guess my response sounded a little bit rude. I merely wanted to point out that the header on these forums reads "for engineering professionals".

 

[dicer]

I would say there are too many variables to accuratly calculate the losses. There maybe some computer simulation programs that may come close. I would say this is not a high school physics or math question.
The variables include number of lobes, number of supporting bearings sizes of bearings (area) , spring pressure on tappets, type of tappets. Or is this just the shaft itself?

 

[patprimmer]

If pushrod, cylinder layout can also be an influence.

Regards
Pat
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[BrianGar]

How exact do your calculations need to be?

Answer will vary between simulation and real world testing.

A new engine will have higher bearing friction due to high spots present on bearing surfaces leading to surface contact and therefore boundary/mixed lubrication. For the first while, figures will drop, and then level off once everything is run in.
This is also the case for any follower mechanism.
Keep in mind that the opposite is true with reference to piston rings, and bore sealing. This will have a direct effect on cylinder pressures.
Depending on what you want to test, and how accurate you want to be, you could do a bench test. Rotate cam and measure load.
End result will depend on how much stuff you add to the test rig, what temperature its at, what the oil temperature is, if you have springs fitted, etc. These will have to be taken into account, as will cam to follower friction loads.
Then you have cylinder pressure, and the force it exerts on the exhaust valve at the point where the valve cracks it open.

Timing belt load is not to be ignored either, journals nearest the timing belt pulley will show increased loads. Same is true if its a twin cam, and one cam drives the other.
Dont forget any sundry devices also driven off the cam via lobes, or gears.

There is software out there for calculating theoretical values, I have no doubt that it is expensive, and totally useless if you dont know what Inputs to clock in.
I know Ricardo Uk did or still does case studies on all the above. Im not sure if they do software packages also, but I remember reading studies years on on engine friction that they had carried out, so perhaps dig through their archives. I do remember they always carried out actual bench tests to verify the calculations.

As mentioned by Dicer, lot of variables...and not a ten minute calculation.

Brian,

 

[tbuelna]

Aled87,

As BrianGar points out, journal bearing losses are a complex analysis. And calculating losses in an engine camshaft journal bearing are made even more complicated by the fact that the loads vary both during a single rotation and with regards to engine speed. Ideally the camshaft journal contact should be hydrodynamic in nature, so the losses are mostly due to shear in the oil film. Using a simple coefficient of friction is probably not a valid approach if you wish to obtain an accurate result.

If you know the specifics of your journal bearing geometry, lubricant, and operating conditions, there are lots of good technical references available that should give you the necessary equations. Here's a link to one I would recommend:

http://hdl.handle.net/2060/19840023570

Good luck.
Terry

 

[pontiacjack]

I apologize if I'm off-base, but:
The whole original post seemed to me to be totally uninformed. His "known" coefficient-of-friction of 0.1 is at least a couple of orders of magnitude higher than any acceptable real-world value for oil-fed cam journals. And his second paragraph ("... work done is zero...") is complete gibberish.

 

[BrianGar]

Pontiacjack, even so, no harm to point out from the start, which he seems to be at, that it is not that simple as you know,

Brian,