roller cam follower lubrication?

[brashear]

Hi,
Just wondering why OHC with roller followers need lubrication, if it's rolling contact everywhere? In theory, if we remove the valve guides from the equation, does an engine with OHC roller followers even need to have the top lubricated?

 

[evelrod]

If it was still 1918, you might have a point. However...

E.g., my Ford based Esslinger 2.0 that is in my 23 T revs to 9000 rpm and the little roller rockers really like that shot of lubricant! ;o)

Seriously, the cam gear, springs, valves, lifters, rockers, etc. generate an enormous amt of heat and require a steady flow of oil for cooling and lubrication.

Rod

 

[patprimmer]

A 1902 Oldsmobile has a roller cam that is outside any engine covers. It is in the mud and the blood and the beer so to speak. It obviously has no lube. It peaks out at less than 1000rpm and has a very gentle cam profile, to the point that you can easily compress the valve springs with yourbare hands.

Even if the followers survived with no oil (which they won't as Rod points out, everything Rod says, plus cam bearings will fail.

Regards
Pat
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[brashear]

Sure, from experience it is not good practice to run the roller cam followers outside the cover. I could've told you that myself - but why? What is the cause behind it?

 

[patprimmer]

Because, even if the rollers can take it, everything else in the system can't.

Regards
Pat
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[Tmoose]

Even rolling contact has errors in geometry. Ball/roller bearings and gears ( all rolling contact, to some degree) need genuine ElastoHydrodynamic lubrication to achieve decent life when the loads are worth mentioning.

 

[franzh]

Just look at some of the old radial engines on WWI aircraft, with their exposed rocker arms. Amazing they held up at all, and explains why the pilots wore goggles!

Take a look at Henry Fords original racer with Barney Oldfield. That massive engine had exposed rocker arms and top end. Of course, there was no warranty and engine life was in hours, not years.

Franz

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[drwebb]

Does the roller spin on an axle with a sliding contact?

Reduced sliding contact in roller follower valvetrains is enough to change the anti-wear performance requirement for the lubricant that is used.

The implication that valve springs generate enough heat to require liquid cooling is surprising.

 

[evelrod]

drwebb---With dual (triple) spring interference fit and valve lift at <.5" with engine rpm ~9000...You do not see the potential for heat generation in the valve springs?

Rod

 

[patprimmer]

I have seen carbonised oil on valve springs. That indicates to me something is getting hot there. The exhaust side could be partly explained by heat conducted from the valve and guide boss, but hysteresis and friction between spring or dampers in multi component systems is substantial in high performance engines.

Regards
Pat
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[YvesLLewelyn]

Roller followers have small needle rollers between the outer race that contacts the cam and the central axle and these certainly need to be lubricated.

 

[dicer]

If there is no need for lubrication on a so called frictionless bearing, then why do they fail if they are not lubricated?
There should then be no reason to pack wheel bearings with grease etc.
As a youngster I ran a 2 cycle gasoline engine without lubricating the bottem end, ie the crankcase, the piston had enough oil the way I was introducing the fuel, the big end with the needle bearings gave way.
The end answer is there is always friction, if not you could spin a perfectly balanced shaft with rollers and it should perform like its in orbit around the earth, with zero friction losses. Well I guess we need to get away from gravity too. In the end all friction less metal bearings need lubrication. I suppose ceramic do as well.

 

[brashear]

This was a theoretical question. I think a lot of you are missing the point by stating past conventions and experiences.

I think lubrication in ball bearings are for the cage to ball contact, no? Otherwise I'm still having difficulty grasping why you need lubrication for rolling motion - except for maybe shock absorption?

 

[evelrod]

Postbrashear (Computer)
10 Feb 10 19:11
This was a theoretical question. I think a lot of you are missing the point by stating past conventions and experiences.

What else do I have to go on? Sixteen years of school and fifty two years of experience is all I have.

Theoretical? Missing the point? I don't think so. Even the most basic concept of "ball bearings" accept the actual non rolling friction part. No non lubricated bearing will actually "roll" 100% of the time. Especially with heavy and variable loading. Your missing some basics in trying to create a hypothetical situation that in the real world just cannot exist. HEAT REQUIRES COOLING if you want your bearing to live. In most cases that means LUBRICANT.

Sorry for the rant. The question is silly.

Rod

Rod

 

[YvesLLewelyn]

I suppose theoretically you are right - if all the motion was pure rolling you probably could get by with no lubrication. But, as has been pointed out, neither the motion of the follower on the cam is pure rolling (a lot of sliding and bouncing, especially at high RPM) or the motion of the needle rollers in the follower itself is pure rolling.
The roller follower uses uncaged needle rollers and they are all side-by-side with little gap in between. Bearing people call this a "full complment" and it gives the follower a higher load-bearing capacity than the lesser amount of needles in a caged type. It also means that the sides of the needles touch each other and they are moving in opposite directions - very much a case of sliding and not rolling contact - so they need lubrication.
Engines have very elaborate lubrication systems - it may be possible to not lubricate the cam and follower system but there is no advantage in doing so.

 

[patprimmer]

The point I missed was there was no point to the question, so I should not have bothered to answer it.

Don't worry, it won't happen again.

Regards
Pat
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[dicer]

brashear, simply put real world examples prove that lubrication is needed.
And what about, the cause of bearing failure related to surface wear created by microscopic “cold welding” (look up the definition of cold welding) of ball material to the race. The cold welds break as the bearing rotates, creating surface roughness that leads to heat generation and bearing failure.
Lubricant provides a microscopic film between the rolling elements to prevent abrasion and skidding. In addition, it protects surfaces from corrosion, and protects the contact area from particle contamination.
So in the end especially metalic type frictionless bearings need some form of lubrication, period. No ifs ands or buts.

 

[brashear]

So to summarize:
Even though roller cam follower systems are rolling contact, there is still a fair bit of sliding contact which requires lubrication. These come from:
1. needle rollers have friction on the sides
2. the motion between the cam and follower has non-negligible sliding when subjected to high accelerations
3. Cold welding of balls to races due to extreme point and line contact pressures create roughness that causes premature wear
Furthermore, the friction caused by the above mechanisms generates heat, which can be dissipated by the flow of oil which acts as a coolant.

These answers were very helpful and exactly what I was looking for. Thank you.

The answers that I said were not as helpful were along the lines of:
"I drove an old tractor once that had unlubricated cam followers and it didn't last for more than 100 hours, so you shouldn't run your cams unlubricated and it'll never be possible!"

Thank you for all your help. I hope I didn't offend anyone.

 

[dicer]

Well the old tractor and all that, comes into the simple logic and common sense factor, the other information is a given, when the former is used.