Author Topic: 3 bearings vs 7 bearings (Read 19021 times)

blue230sl · « **on:** November 20, 2007, 16:55:36 »

have tried to search for threads but my pc keeps timing out - so questions are:
what functions do the bearings perform;
what is the difference between 3 vs 7?

john
64 230sl
89 190e 2.6

Paddy_Crow · « **Reply #1 on:** November 20, 2007, 19:36:27 »

The bearings support the crankshaft. In essence, by spacing 7 bearings over the same span as 3, it's much stronger. Think of how easily a stick supported on both ends will bend. Now add a third support in the center, it takes a lot more force to bend it.

Democracy is two wolves and a lamb voting on what to have for lunch...

blue230sl · « **Reply #2 on:** November 20, 2007, 21:44:27 »

thanks...wonder why they went from 3 to 7 and not 4 or 5...

john
64 230sl
89 190e 2.6

Paddy_Crow · « **Reply #3 on:** November 20, 2007, 22:11:13 »

Because of the spacing of the cylinders. I believe the 230 actually have four bearings. Two on the ends, one between cylinders 2 & 3, and one between cylinders 4 & 5. That gives you two pins betwen each main. When the displacement was bumped up, more support was required so they went to 1 cylinder between each set of mains.

Democracy is two wolves and a lamb voting on what to have for lunch...

CraigD · « **Reply #4 on:** November 20, 2007, 23:35:00 »

So in real historical evidence, have the 7-bearing engines lasted longer than the 4-bearing engines?

Craig
'70 280SL Euro, Manual, Leather
Silver/Black

Paddy_Crow · « **Reply #5 on:** November 21, 2007, 08:10:18 »

I'm not sure about historical data.

From a design perspective, the 4 bearing design is going to limit the amount of load the crankshaft can carry. So if you bore out and stroke a 4 bearing block, essentially turn it into a 280, it probably won't last very long at all if you actually use the added performance. (Assuming you could find a crankshaft that would allow you to stroke it.) Chances of a broken crank or spun bearing would be high.

If kept within its design limits, there's no reason that a 4 bearing I-6 engine can't last indefinitely.

Democracy is two wolves and a lamb voting on what to have for lunch...

al_lieffring · « **Reply #6 on:** November 21, 2007, 08:36:17 »

quote:
Originally posted by CraigD

So in real historical evidence, have the 7-bearing engines lasted longer than the 4-bearing engines?

Craig
'70 280SL Euro, Manual, Leather
Silver/Black

My experience is that the lower end of the 4 main bearing engine is as durable as durable as the 7 main.
One reason could be that the same volume of oil from the pump is distributed to fewer bearings resulting in less drop of pressure.

I prefer the 230 engines because the larger cylinder bores of the 280s leave thinner block walls, resulting in higher internal tempratures, especialy in cylinders 4 and 5. In 280 motors that I have taken apart these two cylinders always show significantly more wear than the others.

When I rebuilt my 230 motor at 100,000 miles I was able to use the standard size bearings, there was almost no wear and the oil pressure never comes down off the 45psi mark even when driven hard on hot days.

This motor was not the original to the car, When I bought (the car) the block and head were in the trunk. the original motor was damaged by a wrist pin failure that made the original crank and block unuseable.

Al Lieffring
66 230SL

ja17 · « **Reply #7 on:** November 21, 2007, 15:56:28 »

Hello,

My guess;

All Mercedes engines of the era are very robust. Non of them were prone to failure because of their design. The seven main bearing crankshaft looks like it is easier to balance and potentially had more capability to handle future power requirements.

It is a heavier engine with more internal "drag" (friction) due to additional bearings.

Download Attachment:

231.JPG
24.11 KB

I would suspect that higher rpms as found in more modern engines greatly increases internal stresses. As I recal as rpms double internal stress increases to the third power! (cubed)

Joe Alexander
Blacklick, Ohio

wbain · « **Reply #8 on:** November 21, 2007, 19:01:39 »

Here is a pic of my M180 which is being rebuilt. (I need some 80.5 mm pistons, 1st oversize, cheap.)

Thanks

Warren Bain 1965 220S, 1989 300SE, 1989 420SEL, 2002 Ford Crown Vic Police Interceptor

Paddy_Crow · « **Reply #9 on:** November 21, 2007, 22:31:46 »

Inline sixes balance naturally regardless of the number of bearings.

There is no easy method for calculating bulkhead stresses in cylinder blocks. Good thing, if it was easy I wouldn't be nearly as well compensated.

Democracy is two wolves and a lamb voting on what to have for lunch...

ja17 · « **Reply #10 on:** November 22, 2007, 06:46:47 »

Hello Paddy,

If you look at the massive counterweight in the middle of the four main crankshaft compared to the somewhat uniform balancing weights on the seven main, you will realize that designers had to be a little more radical when balnacing four main engines. In addition the flywheel and front balancers also had radical couterbalances to off set the spinning weight of two rods and two pistons between each
main bearing on the four main engine.

When you ever have a seven main crank assembly re-balanced they can simply balance each individual part or balance everything as a unit.

If you re-balance a four main engine the parts can only be balanced as a assembled unit.

Counterweighted replacent flywheels and front counterweights on four main engines must be matched to the old parts so counterbalancing forces are the same when replacing theseparts.

On seven main engines, since all the parts are symetrical and balanced individually and as an assembly, you are able to switch and relplace flywheels and front weights without any balancing.

Think of a crankshaft like a bridge, one with four supports (bearings) the other with seven.

Joe Alexander
Blacklick, Ohio

Paddy_Crow · « **Reply #11 on:** November 22, 2007, 08:00:35 »

I'm speaking from an engineer's perspective (having spent the past 25 years designing engines). Balancing an inline six cylinder engine is not difficult compared to other configurations. V-6 is notoriously difficult to balance, for example. With the I-6, the primary and secondary forces and couples naturally cancel without requiring a balance shaft.

This is not to say it can't be done wrong. It sounds like you're saying the seven main engine is internally balanced and the four main is externally balanced, which is certainly an advantage to the guy working on them in the field.

Democracy is two wolves and a lamb voting on what to have for lunch...

ja17 · « **Reply #12 on:** November 22, 2007, 20:15:42 »

Hello Paddy,

It's nice to have another "motor head" on board, I am sure we will have some additional questions you can help us with soon!

Joe Alexander
Blacklick, Ohio

ja17 · « **Reply #13 on:** November 22, 2007, 20:22:30 »

Hello Paddy,

What's your thoughts on the Mercedes 5 cylinder engines. Sounds like a nightmare to design, but I suspect with a crank throw every 72 degrees it is smoother than the v8 with two crank throws every 90 degrees?

Joe Alexander
Blacklick, Ohio

wbain · « **Reply #14 on:** November 22, 2007, 22:03:48 »

Here is a pic of my crank being polished, in my class.

Thanks

Warren Bain 1965 220S, 1989 300SE, 1989 420SEL, 2002 Ford Crown Vic Police Interceptor

Naj ✝︎ · « **Reply #15 on:** November 23, 2007, 02:38:02 »

While we are on the subject, is it true that the early 250 engines had main bearing lubrication problems?
Apparently some regrinding of the cranks was required to solve the problem...

naj

68 280SL

glennard · « **Reply #16 on:** November 24, 2007, 17:57:24 »

Another answer to the questions. The bearings keep the axis of the shaft rotating in a 3 dimensional straight line. The piston forces make the shaft flex and bend. Bearings are designed big(wide) enough to contain these forces. 7 bearings place a containing counter force on both sides of each piston. 3 or 4 bearings leave 2 or 3 pistons between bearings. Bigger engines and turbines have bearing oil pumps that lift the load off the bearing before the shaft turns, thereby lubricating the shaft at all times. Supposely 90% of gasoline engine wear is on the first few strokes before oil pressure gives lubrication. Happy motoring.

Benz Dr. · « **Reply #17 on:** November 24, 2007, 19:27:21 »

The engine wear on start up is kind of a myth started by big oil companies trying to sell some new additive. Some wear can happen on start up but there is no load on the engine until it fires and then oil pressure should be there quickly. Lighter or synthetic oils solved a lot of these problems.

The real culprit is dirt. Even pices as small as 2 or 3 microns can scratch engine bearings. Most oil filters only filter down to about 10 microns so a lot of wear can happen when oil gets dirty.

Dan Caron's
SL Barn
benzbarn@ebtech.net
slbarn.mbz.org
1 877 661 6061

ja17 · « **Reply #18 on:** November 25, 2007, 21:27:49 »

Hello Naj,

The 250 Series engines had two different version crankshafts and bearings. I believe that by the time the 250SL came along, they all had the later version. The 280 SL engine also had two differnt style bearings but used the same crankshaft on both.

Joe Alexander
Blacklick, Ohio

Paddy_Crow · « **Reply #19 on:** November 25, 2007, 22:26:29 »

quote:
Originally posted by ja17

Hello Paddy,

What's your thoughts on the Mercedes 5 cylinder engines. Sounds like a nightmare to design, but I suspect with a crank throw every 72 degrees it is smoother than the v8 with two crank throws every 90 degrees?

Joe Alexander
Blacklick, Ohio

I have to confess that I'm not very familiar with MB's line of engines. 5 cylinder engines aren't very common (although there seem to be more of them these days), but inline engines are generally even firing and not difficult to balance, so they do tend to be smooth running. V-8 engines with 90 degree vee and shared pins will also be even firing and balance easily, so they also run very smoothly.

Democracy is two wolves and a lamb voting on what to have for lunch...

Chad · « **Reply #20 on:** November 25, 2007, 23:04:34 »

Are you all now talking about the durable OM617 5 cylinder diesel engines?
Were there 5 cylinder gasoline engines from this marque?

1967 230SL

glennard · « **Reply #21 on:** November 26, 2007, 09:17:00 »

Peugot and/or Reaunalt 'featured 5s, not so?

ja17 · « **Reply #22 on:** November 26, 2007, 11:21:32 »

Yes Chad, these were the Mercedes diesel engines. I think Audi also used a five cylinder gasoline engine also.

Joe Alexander
Blacklick, Ohio

Paddy_Crow · « **Reply #23 on:** November 26, 2007, 12:35:43 »

I know GM and VW both sell an I5 gasoline engine. Ford/Mazda/Volvo have announced one, but I don't know if it's actually in production yet.

Democracy is two wolves and a lamb voting on what to have for lunch...

Benz Dr. · « **Reply #24 on:** December 03, 2007, 00:02:28 »

I talked to my machinist over the weekend and asked him to give me his take on extra friction from three more bearings. He looked at me like I was from a another planet which only confirms what I already know.

It might take a tenth of a horse power, maybe....

Dan Caron's
SL Barn
benzbarn@ebtech.net
slbarn.mbz.org
1 877 661 6061

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Author Topic: 3 bearings vs 7 bearings (Read 19021 times)

blue230sl

Paddy_Crow

blue230sl

Paddy_Crow

Paddy_Crow

al_lieffring

wbain

Paddy_Crow

Paddy_Crow

wbain

glennard

Paddy_Crow

Chad

glennard

Paddy_Crow