Roller bearing tensioner.

[Gordon Raymond]

Just curious if anyone else has seen a chain tensioner for an older NORD engine that used a roller bearing in the chain tensioner sprocket? I had one once, but used it for some ones engine rebuild. Anyone know which engine, or engines, used these? The plain bronze bushing version is the common one, probably due to better wear characteristics. The roller version probably would handle shock loading better, but as there shouldn't be any on the tensioner, why the roller bearing variation?

[Tifosi]

Lower rolling/rotational resistance for race purposes?

[Gordon Raymond]

Well.. since ONLY my friend Darren will comment, here is some more, with pictures. I don't think this was intended as a race part. First, roller bearings have more frictional loss in some applications due to the actual greater area of the rollers as compared to a plain bearing. I learned this when I was working at OMC in the racing division many years ago. Most 2 cycle engines used these as crank bearings and rod bearings, due to their ability to withstand pounding, shock loading and variable lubrication at speed. As we know, Porsche used this on 4 cyl Carrera engines years ago. These Porsche engines worked great at speed, but low RPM use chewed them up, perhaps from lubrication failure. Many of these engines today have been converted to plain old bearing shells.
OK, back to this question. I found some pictures of this roller unit in my old Giulia parts manual. It lists the use "up to car # 02980". That's nice, because I have seen plenty of cars before # 02980 that have the plain bearing type, and as mentioned, I've only noticed ONE of these over the years, that as a part in my collection of spares. I remain curious.

[Clayton105]

Hmmm, your thoughts about power losses are contrary to my understanding on bearings. Ball and roller bearing are considered to be frictionless because of the rolling action and not a sliding action. There is no oil shear! Therefore, minimal power losses are attributed to their use. Also, the actual contact area is quite minimal compared to plain bearings. Each roller transfers the force via a thin "bearing" edge.

Plain bearing can carry far higher loads because the actual pressure is far lower for a given force.

The reason they are commonly used in small two stroke engines are that the fuel/oil mist in the crankcase is sufficient to lubricate them.

Their use in older racing engines is for two reasons: Lower frictional losses and lower pressure required for lubrication (lower parasitic losses...).

From my understanding, their demise in racing engines (crank bearings, that is) has been caused by increasing requiring complex built-up crank (that are less ridgid and durable), and ultra low viscosity synthethic oils negating the pumping losses.

Can anyone add/comment on this?

As for the tensioner bearing... never actually pulled one apart!

[Alex]

Gordon,

I copied this to my files quite some time ago, and didn't make any further notes as to its origin. Will this be of any help?!

Alex.

PS: my father had a quite special 356 with one of those roller-bearing engines and Abarth exhaust, which he adored despite it being temperamental and awkward on cold starts (only handled well with a bag of cement in the front compartment, too ...). Many pre-war Bugatti engines are now being restored using plain shell bearings for reliability reasons, too. Not everyone is fastidious enough to fit (somehow) a hidden block pre-heater or drain and warm the oil in a big pan before refilling the sump!

[Gordon Raymond]

Clayton,
Guess I need an engineer here. As a mechanic, "THE ENGINEERS" told me in any bearing, there should be no contact, the whole thing should float on the oil film. The total area of each roller combined was greater than the area of the plain shell. (I have never measured to find out.) However the oil shear to float all the rollers is greater than with a plain bearing. (?) You said:
"Also, the actual contact area is quite minimal compared to plain bearings. Each roller transfers the force via a thin "bearing" edge."
I think, by contact area, you are actually talking about the roller float area. I am also thinking of the volume of oil required to keep all this floating on an oil film.
I admit, it has always appeared that roller and ball bearings have less frictional resistance than plain bearings, as you state. This is probably a lot more complex a question than I thought!
The photo is better than my parts book example. I have now discussed these tensioner bearings with three older experienced Alfa mechanics here in the US. All of them admit to never noticing if the bearing was plain or roller, as they never took them apart. When wobbly or loose, they, like me just replaced the whole thing!
Thanks for your comments.

[Tifosi]

Quote:

Originally Posted by Gordon Raymond Guess I need an engineer here. As a mechanic, "THE ENGINEERS" told me in any bearing, there should be no contact, the whole thing should float on the oil film. The total area of each roller combined was greater than the area of the plain shell. (I have never measured to find out.) However the oil shear to float all the rollers is greater than with a plain bearing. (?)

[Clayton105]

I am an engineer (but first was a motor mechanic and fitter/machinist, so unlike a lot of engineers, have a lot of practical experience in the actual application and maintenance of these designs).

It’s well over a decade since I finished my studies, but I do remember some of what I learnt in my mechanical design unit. I remember calculating frictional losses and bearing areas/loads with formulas and recommended maximum pressures etc, but those are the brain cells I must have ruins with too many brews since then…. This is also not my area of expertise (I work in motor vehicle certification and regulation) since I finished my studies so I apologise if I am supplying any inaccurate information.

On a microscopic level: ideally yes, there should never be actual metal to metal contact in either form of bearing, but there inevitably is at some times.

The action of a roller bearing and a plain bearing is quite different when you think about it:

In a roller bearing, only the very bottom two or three actual bearings are carrying load at any one time. Oil must be supplied for a couple of reasons. On a microscopic level, the oil is there to provide a cushion between the two similarly hard surfaces to roll over each other. Oil must also be provided to conduct heat away from the bearing… the bearing components will generate heat as they flex and deflect with the load (a bearing will deform as only the outer surface is hardened) and some sliding does occur between the actual bearing and cages.

A plain bearing has a purely sliding action between the two surfaces, and therefore the oil must be there to lower the coefficient of friction to a workable level (the bearing materials are chosen for other mechanical properties). The actual film of oil forms a wedge under the “contact area” and is literally drawn into the load carrying area by the movement of the bearing and is squeezed out the sides. With low speed engines, the pressure requirements can be so low, that splash fed crank bearings will suffice. With very heavy engines (like engines that power large ships), maintaining oil pressure is so critical, that special oil pumps are used to avoid metal to metal contact, even when the engine is NOT running.

The advantage of load carrying capability of a plain bearing over a roller bearing comes from the similarity of the inner and outer diameters of the bearing surfaces. With a roller bearing, not only are the diameters very different, but the inner race curves in the opposite direction to the curvature to the rollers: this not only creates a smaller wear surface on the inner race, but increases the point pressure for this part of the bearing (next time you replace a bearing, if you look, you will find that 9 out 10 times, the inner race will have worse pitting than the outer race.

Sorry to bore you all, but I have found over the years, that the more I understand these basic theoretical principals, the better I can apply them practically.

Oh, Alfa probably went to the bush... because it was cheaper!

[Gordon Raymond]

Clayton & Darren,
THAT'S what I needed! I have printed out your posts to re read and memorize, as you CAN teach an old dog new tricks (it may take longer). This is not dull reading for me, I assure you both. My background is in market analysis economics, but I've spent much more than 45 years working with this stuff. When a question like this comes up, it's great to know I can find and learn the real answer quickly, without a research hunt. What you both have described answers more than the original question. Thanks!

And the final answer is, Alfa discontinued this system for reasons of COST, just like they stopped lightening the idler sprocket, (leaving that to me) after the 1300's!

[Clayton105]

That's loverly!

I have lightened these, but mine have never been quite so artful. Ettore Bugatti: Eat your heart out!

I remember reading that Lancia used to use needle roller bearings in the front suspension in the Flaminia. Such engineering purity is devine, but in the modern world where cars are sold more by marketing and a gadget tally, such engineering extravagance is only for a few makers, and those that want to go bankrupt.