Roller-Bearing Cranks; Again! 1

Not so much a question, as a comment. Thread108-76629 was interesting but the bulk of the comments were negative towards the use of rolling element bearings for crankshaft use in conventional engines (as opposed to 2-strokes), and particularly for diesels. The opinions seemed to be that the high firing loads of a diesel would be damaging to the bearings, causing them to Brinell or otherwise fail.

I suspect it very much depends on how the engine is designed in the first place. Thread108-76629 was a question based on a conventional plain-bearing SI V8, where one assumes the rollers would be expected to run on the existing journals. In other words, an adaptation of a plain-bearing design. But if you were to design an engine from the outset for a roller-bearing crank, in what ways would the engine be different? Perhaps it would be instructive to look at such an engine, and a diesel no less.

After the Second World War, Maybach in Germany designed a family of 185 mm bore diesel engines for marine and rail traction use, and in the late '50's, the MD series were licensed to Bristol-Siddeley in the UK who built these engines for British Rail amongs others. Rated at up to 1,800 rev/min, these were magnificent engines (from the specification, anyway) with welded steel "tunnel" crankcases, roller bearing disc-webbed cranks, blade and fork con-rods, 6 valves per cylinder, DOHC, and unit injectors, and offering much higher specific power and power to weight ratios than competitive engines. They were available in 4, 6 V8, V12 and V16 configurations. The roller-bearing crank design essentially allowed a more compact and thus lighter engine than conventional practise allowed.



I have a paper and a brochure on these engines, and the first page of the brochure is shown here. The crankshaft has a 200 mm stroke, and the rollers run between the outer diameter of the crank webs and a race located in the crankcase tunnel. When overhaul is needed, the crank track is ground, and the outer race is renewed, and oversize rollers are then required.

All in all, they were amazing engines, and it would appear that this is what you would need to do to design a good roller-bearing crank engine. Interestingly however, they did not appear to live up to their technical promise in rail-traction service at least, and indeed although it looks robust, some of the in-service problems were with the crankshaft. Go to:


The MD engine is down the page a way.

PJGD

I have to say I like the concept that the main bearing is also the crank arm and the counterweight. It shortens the crank by the width of the main bearings, or frees up that space to give bearing area to the big ends while keeping their periferal speed down.

It greatly increases the contact area for the rollers.

The rollers are probably necessary because of the huge periferal speed of the mains, rather than to reduce friction.

I expect that it could use a much smaller oil pump than a conventional design, thereby saving a bit of power.

It would also have a significant windage advantage I would expect.

I would sooner see a complete assembled roller bearing pressed onto the crank, instead of running in the the rollers in the crank, then the crank and bearing assembly bolted into place by saddles, similar to very large conventional bearing caps.

Having said all that, I still prefer plain bearings with a pressure fed oil supply.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Note that you have knocked a lot of strength out of the block as well.

Still, that is a neat design in many ways, I wonder why the 2 stroke boys haven't used it? Much nicer than building the crank up.



Cheers

Greg Locock

PJGD:

That engine has roller bearing mains and journal bearing rods.

I like the 1-piece lower end though (no main caps)!

Regards,
Terry

Nice catch, that's why the 2 strokes don't do it.

Cheers

Greg Locock

How is oil supplied to the rod journals? Splash?

strokersix:

Most likely, the oil is pressure fed thru one end of the crank.

Regards,
Terry

Terry is right, the lube oil is fed into one end of the crank through an on-center rotating connection. In turn, the crank has generous through-drilled galleries which are plugged as necessary.

A cross-section of the engine appears here:–



Note that this illustrates the front end of the crank which also features a huge friction-type TV damper. The (nodal) output end of the crank takes care of the accessory drive which is all gears, right up to the DOHC. Other features not previously mentioned include central pre-chamber combustion chambers, and steel crown oil-cooled aluminium pistons.

PJGD

PJGD:

That's quite an engine. V12, Fork and blade rods, and what appears to be 6 valves/cylinder.

Thanks for the pictures. I've never seen a CI engine configured like that before. Kind of like a Rolls-Royce Merlin.

Terry