3 bearings vs 7 bearings

[Paddy_Crow]

quote:Originally posted by Benz Dr.

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

I agree, although above 4500 RPM it will probably be a little over 1 hP (I could probably calculate it...). But there is so much friction elsewhere in the engine, the difference in a few extra hydrodynamic bearings isn't going to be significant. Besides, the added support will allow you to produce so much more power that it will more than make up for the friction.

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

[glennard]

How about this- additional bearings might decrease the friction.  They eliminate the vibrations, amplitude, and the shaft rotates on a straight axis.  Harmonics, etc, you know??

[Paddy_Crow]

quote:Originally posted by glennard

How about this- additional bearings might decrease the friction.  They eliminate the vibrations, amplitude, and the shaft rotates on a straight axis.  Harmonics, etc, you know??

No. In order for a hydrodynamic bearing to work, the oil film must be maintained at all times. Once you get metal to metal contact, the bearing is toast. So bearing friction is primarily a function of the relative speed of the two surfaces and the viscosity of the oil in the film.

When the oil film breaks down and asperity contact takes place, friction increases exponentially. The heat from this friction causes metal transfer between the bearings and the crankshaft. Eventually the bearing shells stick to the crankshaft and spin. Catastrophic engine failure is imminent at that point. This all happens in the blink of an eye.

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

[glennard]

3 bearings = 1/3 distortion and 1/3 load on each.  7 bearings = 1/7 distortion and 1/7 load on each.  7 is better than 3.  The 7 bearing engine with lubrication will last a lot longer than a 3 bearing engine at 7000 rpm!  Turbines have more uniform radial loading than 8, 6, and 4 piston engines. 90, 120, and 180 degree bangs.

[Paddy_Crow]

quote:Originally posted by glennard

3 bearings = 1/3 distortion and 1/3 load on each.  7 bearings = 1/7 distortion and 1/7 load on each.  7 is better than 3.  The 7 bearing engine with lubrication will last a lot longer than a 3 bearing engine at 7000 rpm!  Turbines have more uniform radial loading than 8, 6, and 4 piston engines. 90, 120, and 180 degree bangs.

If crankshaft distortion is great enough to affect bearing friction, it will cause metal to metal contact and bearing life will be extremely short. Friction in a hydrodynamic bearing does not significanly vary with unit loading. The concept most of us have of dry or sliding friction is not applicable. More unit loading simply increases the pressure in the film. It will have a slight effect in that film thickness will be slightly reduced, but other variables are more significant in determining film thickness. Hydrodynamic bearing friction is completely determined by the shear stress in the oil film.

Parameters that will affect hydrodynamic bearing friction are the bearing diameter, length, etc.

Bearing life indeed can be affected, as the greater oil film pressure can lead to fatigue failure of the bearing shell.

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

[Charles 230SL]

I realize this is an old string but it appeared to be the most appropriate one to add my question to...

My 230SL is being restored by Bud's and I plan to have a new AC system installed. The car has the original M127 four-main engine. Has anyone had any bad experiences or know of problems with excessive main bearing wear (or broken crankshafts) resulting from added strain on the crankshaft?

thanks all, Charles

[pagoden]

You'll no doubt be getting what most of us think of as a "rotary" compressor with your new rig from Buds; it's what is commonly used these days.  Our cars originally had piston-type compressors which were a much greater load on the engine than the rotary type will be on yours, and I'm not aware of any reputation for failure associated with even the old style.  Not a direct or definitive answer but of some comfort, I hope. 

[And the modern compressors are not actually/literally rotary-type, but something involving a rotating "swash-plate", a sort of cam that drives multiple small pistons that do the compressing, but the nice result is that the whole thing is much smaller and lighter, has little rotating and (especially) reciprocating mass, and all of that is along the longitudinal axis of the car.  Uhunh.  Anyhow, it's a lot smaller, smoother, lighter and tidier -- and easier to power -- than the old style.  And everyone just usually thinks of it as a rotary compressor, 'for short' you know. ]    

[Charles 230SL]

got it, thanks for the quick response Dennis,,

The head had a recent rebuild w/valve job but I'm not sure when (if ever) the lower end was rebuilt. Since the original main bearings could likely still be in play, I was concerned. thanks again.

Charles

[ja17]

Hello Charles,

The main issues with the AC on the M127 engine is the fastening of the vibration dampner and front crankshaft pulley.  Many crankshafts have been damaged when the vibration, dampner/pulley assembly have come loose and ruined the front of the crankshaft. Later engines had an improved fastening arrangement for the dampner/pulley assemblies. A repair procedure was explained in a Feb. 1968 suppliment to the early BBB. A special crankshaft "stretch bolt", special spring washers and larger dowel pins are used to repair and improve the attachment of the crankshaft counterweight/pulley assembly.

Dealer workshop manuals (BBB)  were "loose leaf" so that suppliments could be added as problems and procedures evolve on new models. The final print editions of these manuals should have these suppliments added to them and are bound. If you have the latest edition of the BBB, this information should be found on pages 03-10/3 and 03-10/4 of the early BBB.

When the M127 engine is equiped with AC it increases the stress on this dampner/pulley assembly. At the vary least I would make sure that the new style stretch bolt and special spring washers are used to hold your dampner/pulley assembly in place. The original dowel pins should be adequate as long as the crankshaft is not damaged.  Using a "rotary style" compressor also reduces the strain on this assembly.


The M129 and M130 engines in the 250SL and 280SL had the improved crankshaft bolt and spring washers. In additon these engines had six bolts holding the pulleys to the vibration dampner. The 230SLs had three.

[Charles 230SL]

thanks Joe, I forwarded your response regarding the stretch bolts/washers to David at Bud's.
take care, Charles

[ja17]

In regards to  analysis of the 4 main and 7 main bearings earlier in this thread, it is not a simple comparison. The actual bearing design and the distribution system for the oil would also have a major impact on bearing life. In other words, a  poorly designed bearing configuaration and lubrication system on a 7 main engine, may not last as long as a well designed 4 main bearing system.

In general, the 7 main bearing engine, is considered stronger from the added support. Longevity is more a function of how well the engine bearings, and lubrication systems were designed. Mercedes has re-designed the bearings and oil distribution systems on the same series engines from time to time to improve durability. There is a multitued of different styles of crankshafts, bearings and oil pumps for the basic in line 6 cyl. Mercedes engine of the era. We can only assume that this was evolution of improvement in the design.

Major improvements in metal alloys have contributed to extreme longevity of many new modern engines.  Silicone enriched aluminum engine blocks, hardened camshaft and crankshaft journals  etc.

 Does anybody remember the GM Vega? ...............an example of one of the early failures of the aluminum engine block.

[pagoden]

You're welcome, Charles, for what I had to offer.  And true, as far as it went; the new compressors are a lot less strain all around.  Fine.  But there are people here -- and Joe is a prime example: present case in point -- who bring the breadth and depth of decades of working with these cars.  When I speak up to answer someone who's even newer at this than I am, I often mention that someone with a lot more knowledge will likely be along soon with more on the topic.  Sure wish I'd taken the time and space to do so this time.  But that's how it works around here; the power and value of the information available is just really neat, is what it is.  Best wishes.   

[Benz Dr.]

The only time I've seen broken crank ends were always on a 127 engine. The design simply isn't strong enough to take the added strain of a AC compressor. The newer rotary style pumps would be much better but I'm not a fan of AC in a 113 for any reason. You may stay cool while your engine deals with all of that extra heat; trying to get it out of an engine compartment blocked with all sorts of extra hardware.

If you ever need to replace the short hose between the thermostat housing and the water pump then you'll rue the day you ever thought about having AC. What should take an hour to change will likely take most of the day - at least 4- 5 hours more.

[ja17]

Hello Dan,

I have never run into the broken crank senerio but I agree with you on these overly complicated AC Brackets. It turns simple maintenance into real misery.   Your advice,  being a Canadian in cool climate, may fall of deaf ears to our Florida members.

I like the Sanden Compressor on the sedan AC  bracket.
 
Take care,
Joe Alexander
Blacklick, Ohio

[pagoden]

So do I, and thank you, Joe, for setting me on that path, but still 'tis not as simple as it looks.  The final fight fought was to slice the mounting ears off the space-squandering and overly thick -- $65.00! -- adapter plate, then shorten and weld them directly to the sedan bracket.  This eliminated contact with the air cleaner housing sheet metal and actually resulted in a generous clearance there.  Finally.

[Charles 230SL]

Attached is a photo of the kit that will be installed. I presume that's a rotary compressor in the picture.
I noticed that the air cleaner housing sheet metal on my car has been cut (butchered) to accommodate the previous compressor. Not sure what can be done at this point to cosmeticallly 'clean up' the hatchet-job.