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Interesting article on roller tappets

E

eatenbyagrue

Active Member
Slightly off topic, but I figured the airforce crew might be interested. A new article came out today that talks about roller tappets vs. sliding tappets. It's about cars, not airplanes, but gave me at least some insight into what's going on in the engine http://www.economist.com/blogs/babbage/2010/11/tribology

Getting close to engine ordering time, so learning more about engines has been a top priority. Whether or not roller tappets are worth it has been confusing. One engine guy told me that they're more corrosion prone than sliding tappets, and a waste of money...
 
Interesting article. While my (yet-to-be-flown) Superior IO-360 has roller tappets, I don't know too much about them. Anecdotal story: A tech at Superior did comment to me several years ago when they were still making engines that when they switched to roller tappets their warranty claims on cams went way down.

I do have some experience with roller tappets in small block Ford V8's. In that case they are considered to be nearly indestructible, at least for street use. Half a million miles or more on cam and lifters is pretty normal. As for the oiling thing, I'm not sure I buy it, or even that it matters. Because there is no sliding contact, they are much less sensitive to the wear that can take place on initial startup, before oil has a chance to bathe the cam lobes.
 
my first engine wore out due to cam lifter making metal at 3,150 hours. ill take my chances with a roller lifter this time. will let you know in about 8 years what happens. fly often. change the oil. keep out the rust.
 
While I don't profess to know any spectacular details between roller and flat on aircraft, I've done more than a few engine rebuilds and modifications on many a hot rod. I could care less about body work, but I'll do engine work all day until the cows come home.

I do like roller tappets, but not for longevity. I think they will help, but frankly, I've got a 200,000 mile car that's certainly got flat tappet and never been torn down. It's all about RCM (Reliability Centered Maintenance). Excellent set of articles in recent Sport Aviation mag. Roller tappets allow me to have a much smoother running high HP hot rod engine. The profile of the roller allows it to follow the cam perfectly. Where the flat version sort of "falls" off the top of the lobe. This makes the cam sort of ride on the edge of the tappet at bit.

At the same time, I can only describe the spectacular failure if a roller bearing fails to be well lubricated. I've pulled out a tappet that looked more like an oval as the cam just ground away the roller.

I can see where both have their pros and cons with all in life. In no doubt it will help with friction, and if you pit a lobe of the cam, you'll likely be able to keep going. The problem with that pit is that's where everything bad will always settle. So the cam can/will still become a point of failure, regardless of tappet.

My plan (with my flat tappet IO-320 rebuild) is to change the oil often, fly the airplane often and keep up with oil analysis. Ignored little problems become big problems, and big problems become expensive. That $100 oil change doesn't look so bad any more, heck it costs more than that for one of my sports cars these day.
 
My engine, which was built from a Lycoming kit by Barrett, has roller tappets.

If you decide to go with a Lycoming kit, they come with the roller tappets. I don't think the old style cam push rod assembly is available with new engines. Also, the Lycoming roller tappets are not available for older engines. It is a different system with a different case. Clone engines, I believe, do not use the Lycoming roller tappets but a different system.

So for better or worse, if you go new Lycoming it is with roller technology (except the 0235). I suspect the change is good but only time will tell. So far, a bit over 100 hours, the engine runs well, uses a quart of oil in 17 hours and the oil remains very clean. I suppose some would say the oil is not doing its job of cleaning the engine, but maybe the engine isn't dirty. No leaks so far, just a smooth running motor rolling along on its rollers. :)
 
I'd like to know why the mechanic thinks roller tappets are more corrosion prone. They might have more parts to corrode, but once a tappet face begins to spall it's all over for the cam.

I will say that when I started my IO-390 (roller cam) on oil analysis it was flagged for having the iron as too LOW.
 
not for me

Roller tappets are not field-overhaulable. At overhaul they must be replaced and they are $250 apiece...mandatory replacement. That's an extra $2000 at overhaul on a 4-cyl engine. No additional horsepower gain.

Had the valve cover off of my VW TDI at 200K miles. The cam has a very aggressive profile, and I was pleasantly suprised to find zero wear (flat tappets). The engine has had nothing but Mobil 1 Turbo Diesel Truck oil in it, which is high in ZDDP, changes at 10K intervals. I intend to try running this same oil in my Rocket.
 
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http://www.cpgnation.com/forum/new-tech-bulletin-flat-tappet-camshaft-10.html

Scroll down to the part about "lubrication".

Roller lifters have been around for many years, the driving force behind their use in automotive applications used to be for high performance engines------they allow a much more aggressive cam profile than a flat faced lifter. As the duration of a cam increases, it gets to a point where the flat lifter will not follow the profile of the lobe without hitting the corner of the lifter. Also, the "ramp" of the lobe, which is basically how fast the valve opens, is another factor in the mix. The benefit of lower friction of a roller in almost nil-----the main power gain is that a roller allows the cam profile to be optimized for more power.

Now when you look at the cam profile in typical air cooled an aircraft engine, the cam is extremely mild in profile than an automotive unit. The need for a roller lifter to follow the cam lobe is just not there. The benefit of a roller lifter in an aircraft engine is two fold, less wear due to rolling action instead of sliding action, and the ability to survive on modern oils that have had the critical wear controlling compounds removed from them.

When you couple this with the issue of infrequent operation, and the accompanying oil drain off of the cam, and surface rusting----even a minor quantity of rust----the roller is able to shine in this environment.
 
Oil additive

rocketbob said:
Had the valve cover off of my VW TDI at 200K miles. The cam has a very aggressive profile, and I was pleasantly suprised to find zero wear (flat tappets). The engine has had nothing but Mobil 1 Turbo Diesel Truck oil in it, which is high in ZDDP, changes at 10K intervals. I intend to try running this same oil in my Rocket.
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Off topic, I use BG MOA (motor oil additive) in my oil changes. 1 pt to 5qts Aero Shell 15W-50. At 500 hard acro hours (SC-IO-540-C4B5). I?ve continued to have excellent oil analysis results. Personal results, I had a high time plane with a 0-320, After a TBO overhaul and break in. I start using the BG MOA. The plane was in a flying club and then later sold the plane with 1900 hrs SMOH to another flying club on the field. Last heard was that it had over 3700hrs since SMOH and still had excellent comp. & oil analysis.

Thomas S.
 
clean oil

David-aviator said:
My engine, which was built from a Lycoming kit by Barrett, has roller tappets.

If you decide to go with a Lycoming kit, they come with the roller tappets. I don't think the old style cam push rod assembly is available with new engines. Also, the Lycoming roller tappets are not available for older engines. It is a different system with a different case. Clone engines, I believe, do not use the Lycoming roller tappets but a different system.

So for better or worse, if you go new Lycoming it is with roller technology (except the 0235). I suspect the change is good but only time will tell. So far, a bit over 100 hours, the engine runs well, uses a quart of oil in 17 hours and the oil remains very clean. I suppose some would say the oil is not doing its job of cleaning the engine, but maybe the engine isn't dirty. No leaks so far, just a smooth running motor rolling along on its rollers. :)
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After changing black oil at 50 hrs on other Lycomings without roller tappets, I am amazed how clean the oil is on my roller tappet engine at the 50 hr oil change. I am asking myself, why am I changing this oil at 50 hrs. I am wasting money. I could cut my oil changes in half. Is the 50 hr oil change a hard requirement or suggested? How about changing at 100 hrs.? Anything wrong with that?
 
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Steve Melton said:
After changing black oil at 50 hrs on other Lycomings without roller tappets, I am amazed how clean the oil is on my roller tappet engine at the 50 hr oil change. I am asking myself, why am I changing this oil at 50 hrs. I am wasting money. I could cut my oil changes in half. Is the 50 hr oil change a hard requirement or suggested? How about changing at 100 hrs.? Anything wrong with that?
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Getting the popcorn ready. Looking forward to the response you get on this one. I am thinking primer wars, tip-up/slider, etc. but with perhaps more passion! :D
 
I'm lost.....

Steve Melton said:
After changing black oil at 50 hrs on other Lycomings without roller tappets, I am amazed how clean the oil is on my roller tappet engine at the 50 hr oil change. I am asking myself, why am I changing this oil at 50 hrs. I am wasting money. I could cut my oil changes in half. Is the 50 hr oil change a hard requirement or suggested? How about changing at 100 hrs.? Anything wrong with that?
Click to expand...

So, the cam makes the oil dirty? I always thought it was blow-by, unburnt fuel/carbon and stuff like that. I think you just have a really good cylinder/ring seal with this engine vs. your previous engine.
 
The real reason to change the oil at 50 hrs is because it's getting loaded full of combustion gas byproducts. Even if it still looks clean, it might contain corrosive stuff dissolved/suspended in it. Oil is cheap. Change it.
 
Neal@F14 said:
The real reason to change the oil at 50 hrs is because it's getting loaded full of combustion gas byproducts. Even if it still looks clean, it might contain corrosive stuff dissolved/suspended in it. Oil is cheap. Change it.
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but if the stuff is dissolved/suspended, wouldn't you see it?
 
chrishalfman said:
So, the cam makes the oil dirty? I always thought it was blow-by, unburnt fuel/carbon and stuff like that. I think you just have a really good cylinder/ring seal with this engine vs. your previous engine.
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I think the sliding cam causes the oil to become black.
 
Steve Melton said:
cam friction and the resulting heat causes the oil to form micro coke and become black color
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I guess it would require a spectrographic or micrographic analysis to confirm that coke particles are indeed the source of the black color. I'm not sure about the chemical mechanisms for petroleum coke formation. I am somewhat familiar with coke production in thermal processes involving coal, but not petcoke formation. I'd be interested in seeing any references you might be able to cite.

Are you proposing that high local temperatures and pressures at the cam/lifter interface could cause coking? I have been involved with operation of high pressure reciprocating pumps and haven't seen coking in the lubrication oil, but of course the crankcase temperatures in my application were considerably lower than an IC engine, however, at a bulk oil temperature of less than 200 degF i wouldn't expect thermal degradation. I'd imagine that local temps would have to exceed 500-600 degF with a very rapid ramp rate to cause formation of coke in the time frame encompassed by cam/tappet contact.

Typically formation of coke from coking coals can be prevented in thermal processes by pre-oxidizing the coke forming constituents. Don't know about coke forming in lube oil.
 
a cam is a wedge. the oil film is moving in three directions as it compresses. in a perfect condition the oil film would be a constant thickness over the surface but it's never perfect and very small localized area could actually see a very high pressure as the oil distributes causing high temperature, 600F or so, causing oil micro coking. I don't have any references to cite.
 
I am certainly not an expert, but the ones I have talked to (engine builders and engineers) all blame oil darkening to blow-by of combustion byproducts. Just a datapoint for you.
 
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Another datapoint maybe: The oil in my Pratt-Whitney R-1340 turns black within one or two hours after oil change. That's 10 gallons of oil, and roller-type lifters. Low compression engine, large cylinder diameter.
 
Blow by

Yes it is usually caused by blow by. Interestingly enough, and much to my consternation, my Lycoming (Mattituck) IO-540 in the RV-10 turns the oil black and the Continental R-680 in my Stearman does not. Both have roller tappets.

Vic
 
well, there goes my hypothesis down the tubes.

hy?poth?e?sis
a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation.
 
That article should be considered entertainment, not something to make decisions on. Yes, ZDP is the additive for slipper follower cam-tappet systems. Push rod, not for bucket tappet, or for rocker followers. Automotive used chilled iron followers and phosphate, cast iron cams for low cost. Until 1971, a GM small block cost about $200 in production cost, complete with ignition and carb.

The aero design uses a nitrided steel cam for much better fatigue strength, but if abrasives are present (like diesel particulates) then the wear life is compromised. We don't have that, so the life is pretty good. Chilled lifters are another matter.

OIL: Oils serve a lot of functions as you know. Hydrodynamic lubrication, additives for boundary lubrication (too slow for hydrodynamic lifting), cooling, acid neutralization, particle suspension, and more.

Oil degradation is generally due to the same factors. Oxidation is typically the cause of blackening. This is from the areas where it gets hot. Around heads, exhaust ports zones, piston under crown, and the cycle of wiping off the cylinder wall from exposure to combustion. The bulk temperature of the oil also contributes to oxidation. Just take some and pour it into a frying pan and heat it up. It will begin to smoke as lower fractions evaporate then begin a slowly darkening. Time at temperature is a factor.

Lead is a suspended contaminant. If you take a used sample and centrifuge it, it will become more clear as the lead settles out. Dispersants and other additives keep it from plating out in the engine or settling into a gooey layer in the sump and low areas.

Sulfur in the fuel and exposure to combustion yields acids. The TotalBaseNumber, (TBN) is a measure of the ability of the oil to neutralize this. It is unseen and generally ignored, but can be reported in an oil analysis.

Particle contamination is from wear. Anything that slides. Everything that slides. This is reported in the oil analysis.

These are a few things about oil off the top of my head. Blackstone as much better explanations of the total oil picture, but not on the engine design related aspects. Oil formulations and engine design go hand in hand.

Guinness Book of World Records would like to see a pre 1985, push rod, engine that went 200k miles and did not have serious cam/follower degradation.
 
http://www.economist.com/blogs/babbage/2010/11/tribology[/url]

This is an excellent article which explains what I have known for years and the reason I use 15-40 Diesel Engine Oil in my cars....living in California a slightly heavier viscosity oil is not as much of an issue as other areas and helps me to explain why I get such great mileage from my vehicles whether autos or aircraft.
 
Rv10inoz

Do you know any reason for rocket to rethink that? I have used Castrol GTX20w50. In my lycs for a good while.
 
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