Max RPM - who cares?

[ShoerFast]

As we all know,,,,,

Detonation, dirt ingestion and inadequate lubrication are three common causes.

Frank
As we all know that "dirt ingestion" would effect all the bearings somewhat the same. Throw in piston / cyl wear with inadequate lube.

Detanation would take a well engeneered engines wrist-pin bearings a little quicker then the big end bearing,,,, as the rod would act a a slight anval with the little end taking the brunt.

It's so strange to see some of the ideas that are out there!

Kevin

 

[bwalker]

As we all know that "dirt ingestion" would effect all the bearings somewhat the same. Throw in piston / cyl wear with inadequate lube.

My expierance has shown me that the big end rod bearing is the most sensitive to dirt ingestion. Maybe because many have coated steel rollers instread of plain steel like the crank case bearings.

 

[ShoerFast]

Reading the owners manual ,,,,,

Reading the owners manual could help with that dirt indgestion! ,,,in my experance!

Kevin

 

[Crofter]

ShoerFast;

I found some info on failure in high speed planetary gear assembly needle bearings that, though they do not have needle rotation reversals that a big end bearing sees, still fail at high speeds. Apparently the g forces caused break out off the separater fingers of the cage with clear fatigue patterns on the break. Yet with purely concentric rotation much higher speeds caused no problems. I have not had any saw cranks apart to see whether the journal was "splined" by beating of the power pulses or scarred by skating of the needles as they are stopped and changed direction by being grabbed by the rod and journal. Running more oil in the mix seems to be an aid in staving off big end failure but I am not sure why in this case. Is it lubrication or cushioning the rattle of the needles in the cage. That is my guess. I am sure if they could safely and happily rev higher the manufacturers would be glad to have it.

 

[Lakeside53]

Frank
As we all know that "dirt ingestion" would effect all the bearings somewhat the same. Throw in piston / cyl wear with inadequate lube.

Detonation would take a well engineered engines wrist-pin bearings a little quicker then the big end bearing,,,, as the rod would act a a slight angle with the little end taking the brunt.

It's so strange to see some of the ideas that are out there!

Kevin

EDIT : sorry, I pushed this topic slightly of course from just big end, but...

Not so sure... Wrist pin and big end bearings are needle roller type. Mains are "mostly" deep groove ball bearing bearings. The roller type are very strong with a high resistance to wear because of their larger surface area. Ball bearings have small point contacts on each ball/race that can wear or be damaged more easily.

I see some awful pistons/cylinders/cranks with ever sloppier big end bearing wear, but few (other then a spectacular welded big end on my 088) failures of the big end bearing. Almost all failures I see are main bearings.

The main bearing failures I see are from 1) dirt ingestion particularly on concrete or abrasive cutting saws, and 2) cage collapse. Cage collapse can be for many reasons, some of which are excessive temperature with plastic cages, possible softening of the carrier from fuel additive, and wear from dirt.

I've never seen a carrier collapse on a bearing that has a riveted steel cage (where there is a rivet in between each ball bearing), only in plastic cages. In concrete saws where bearing wear is always a problem Stihl uses riveted steel cages, the bearing can be shot, but the race cage is intact and the saw still runs! I'm constantly amazed at how bad these can be and still run well.

I have put riveted steel cage bearing in a number of saws - mainly the flywheel side where it's not a proprietary type. No problems...

Had an 036-pro yesterday with a reported rewind problem.. sure... the rewind was jambed, but... the flywheel side main bearing has collapsed, and the crankshaft BROKE off between the flywheel and the bearing. Nice... My guess - they ran it long after the bearing was bad resuting in cage failure, etc..


If anyone is looking for some interesting reading on mounting bearings and various failure modes :"Mounting and Dismounting of Ball and Roller Bearings" from FAQ Bearings Corp, Publication number 80100ED. 113 pages. I'm sure it's been updated since my copy so the Publication number may have changed.

 

[Lakeside53]

My experience has shown me that the big end rod bearing is the most sensitive to dirt ingestion. Maybe because many have coated steel rollers instead of plain steel like the crank case bearings.

It may have a lot to do with the "range of motion", The wrist pin bearing just oscillates a few degrees in place, whereas the big end rotates fully. Maybe the wrist pin bearing just doesn't get dirt into it like a big end - once it's down the gap in the crank sides, the dirt is just waiting to tear into the ends of the big end rollers or rod housing.

 

[ShoerFast]

Engine engineers,,,,,

Lakeside

Just the way they (engineers) need to stop side play, deep groves work well for that, needle bearings can carry an extreem amount of load.

It seem what Frank stumbled on has more to do with the quanthem dynmantics, or some other kind of forgen language I know less then nothing about.

If there is a tie to the range of motion and the place were the bearing stops, and goes the other way, and the exacite spot it stops each time, it will set up wear patterens?

If that spot was differant each time new, but worked to the same spot with a little wear, then it would fail early,,,,, but this is way out of my leage!

Kevin

 

[SawTroll]

....
Other that as a tuning tool, I don't understand why it matters if one saw can do 11,000 RPM, where another can so 13,000 RPM. From the few checks I've done of test reports, saws all seem to hit max power around 9,000 to 10,000 RPM, and max torque around 6,000 to 7,000 RPM, so these are the ranges where you would get the most out of your saw when cutting. So why does it matter how fast a saw can run out of the cut?

It is the chain speed that produces cutting, not the hp or nm - those are just needed to keep the rpms and chain speed up.
In small wood the resistance that the wood/chain/sprocket size combination produce, often will not be enough to get the rpms down into the power band you referred to, but cutting will be faster even though you are not utilizing the power potensial of the saw.
Higher rpms mean faster initial cutting, and then all the other factors will decide how fast the speed will drop off.

 

[bwalker]

Reading the owners manual could help with that dirt indgestion! ,,,in my experance!
[/QUOTE That may be true in some cases, but in cerain operating conditions it doesnt matter what maintenancepractices you use. I have gone so far as to use womans pantyhse as a prefilter on my bikes when ran in the sand dunes, but this still doesnt top all dirt ingestion.

 

[Lakeside53]

I have gone so far as to use womans pantyhose as a prefilter on my bikes when ran in the sand dunes, but this still doesn't top all dirt ingestion.

Hmmmm, that explains your avatar!

 

[klickitatsacket]

over the years I have seen several saws that have had stretched connecting rods on the big end. Every one of these saws came from guys who liked to "tune" their own saws. 2 of these saws came from the same guy (064 and a 084) who would poke holes in his intake boot with a screw driver to get higher RPM The others seam to have been run lean and at high RPM. The large end of the rod would be egged out and show signs of severe heat. I think I might even have one in the back still. If I find it I will throw a picture of it on here.

 

[Lakeside53]

The large end of the rod would be egged out and show signs of severe heat. I think I might even have one in the back still. If I find it I will throw a picture of it on here.

These "hot" enough for you? I paticularly like the heat gradient up the conrod

From 2003 088. It's WELDED in place.

 

[ozflea]

Looks nicely heat treated to me and well tuned ?????

Mc Bob.

 

[Lakeside53]

Looks nicely heat treated to me and well tuned ?????

Mc Bob.

Just run by idiots!