Bearing loose on the crank? Anybody ever tried this trick?

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alexcagle

Cutoff Saw Specialist
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I've ran across a few Dolmar 7300 series crankshafts in the that the past, that bearings were not loose, but would slide off the crank pretty much by hand. Then when I'd go to install the new bearings, I could slip them all the way home by hand.
From time to time I'll run into an aftermarket crank that has the same problem.
I thought about peening the crank with a point in several places, but that didn't sound like it would keep it uniformly centered.
I went to Harbor Freight and got one of those "el cheapo" engravers a while back, so I ran a random pattern over the bearings press fit surface Then the same surface treatment to the inside of
the bearing. 1484210750108-1492015654.jpg
The microsopic divits did the trick. Maybe - like shot peening.1484211117511-591311277.jpg

For assurance, I put some loctite 638 on the mating surfaces, and pressed the bearing on.
It behaved just as a normaly clearanced press fit bearing.
If I ever get this saw back, then I'll report back on how it fared.
 
I have used the point chisel routine several times with great success, but is very time consuming. The engraver is a much better plan. With the chisel and point it takes skill to divot all the way around with consistency. I have three engravers and use them to make notes on the bar.
 
Neat idea.

Interesting in that the bearings on my clamshells all slide on by hand pretty much as you describe - not really sloppy but they'll pull right off. Clearly that leaves open the possibility that the crank could spin in the bearing, but I've never seen any sign of that happening.
 
That's an old method usually called "Pittsburging" by the old time machinists i worked with. If you're really interested in the blood & guts of bearing tolerances, charts like this link are used to measure shafts & bores Note that they're very precise (ten thousandths of an inch) :

http://www.nskamericas.com/cps/rde/...ng/CNSK_Pocket Fits06-Fits and Clearances.pdf
-and-
http://www.ntnglobal.com/en/products/catalog/pdf/2202E_a07.pdf

These charts are typically used in industrial equipment that are expected to run continuously under heavy loads (pumps, gears, etc) but the engineering is similar. The clearances are more than just holding the bearing on the shaft, the internal clearances after installation is what the interference fits are designed to achieve.
 
Curious why you are concerned about the crank journal being a slip fit in the bearing. What's the result of the crank having end play? Let me say 1st that I just play with saws when it's my dead time during the off season for kart racing, so I may be missing something relative to saws. The kart racing engines (both Japanese and Euro) I build all have a slip fit for the bearing and I (and most engine builders) build in 8 to 10 thou end play when the crank is cold in the cases. The answer to why it's different from saws to kart bottom ends could be that the kart engine crank pin bearings have thrust washers on the outside. I'm not saying anyone is wrong, I'm just throwing this out for discussion.
 
Curious why you are concerned about the crank journal being a slip fit in the bearing. What's the result of the crank having end play? Let me say 1st that I just play with saws when it's my dead time during the off season for kart racing, so I may be missing something relative to saws. The kart racing engines (both Japanese and Euro) I build all have a slip fit for the bearing and I (and most engine builders) build in 8 to 10 thou end play when the crank is cold in the cases. The answer to why it's different from saws to kart bottom ends could be that the kart engine crank pin bearings have thrust washers on the outside. I'm not saying anyone is wrong, I'm just throwing this out for discussion.
*
Yes, it is crucial to the durabilty of a chainsaw or cutoff saw.

At 10,000 plus rpms in magnesium housings, things that are loose become looser. Two different metals expand at different rates.
*
I actually pulled this crankshaft from a TS420 Stihl cutoff saw a couple months ago, after it ruined both halves of the crankcase from hammering the soft metal pockets out. Other than one bearing not having that press fit, the crankshaft is like new. So you can see why I am trying to use it. lol
 
Chrome or nickel-plating of >.001 inch thickness will need to be finally sized by grinding owing to the tendency of these metals to build up more thickly at the edges, than @ the centre

Copper plating can be easily cleaned to size (instead of grinding) by rotating it in a lathe chuck. Even though it doesn't last long, it is so convenient that it is often preferred to build up the surface rather than reshape it
 
Curious why you are concerned about the crank journal being a slip fit in the bearing. What's the result of the crank having end play? Let me say 1st that I just play with saws when it's my dead time during the off season for kart racing, so I may be missing something relative to saws. The kart racing engines (both Japanese and Euro) I build all have a slip fit for the bearing and I (and most engine builders) build in 8 to 10 thou end play when the crank is cold in the cases. The answer to why it's different from saws to kart bottom ends could be that the kart engine crank pin bearings have thrust washers on the outside. I'm not saying anyone is wrong, I'm just throwing this out for discussion.
Are we concerned with end play or side play? I thought it was the latter.
 
Are we concerned with end play or side play? I thought it was the latter.
I thought he was concerned with end play. The statement by the op was "crankshafts in the that the past, that bearings were not loose, but would slide off the crank"
383-crankshaft-end-play.jpg
 
*
Yes, it is crucial to the durabilty of a chainsaw or cutoff saw.

At 10,000 plus rpms in magnesium housings, things that are loose become looser. Two different metals expand at different rates.
*
I actually pulled this crankshaft from a TS420 Stihl cutoff saw a couple months ago, after it ruined both halves of the crankcase from hammering the soft metal pockets out. Other than one bearing not having that press fit, the crankshaft is like new. So you can see why I am trying to use it. lol

The 2 smoke kart engines I'm involved with turn max rpm's depending on engine and/or class from 12000to 17000 rpms. As I said the preferred end play is set between .008 and .010 inch. The bearings have a .003 inch interference fit in aluminum cases. In my world end play is necessary and maintaining the proper bearing fit in the cases is absolutely required for a fast and reliable engine, too tight is a restriction, too loose is destructive.
IMO the rpm's and material the cases are made of is not the difference. As I mentioned I originally thought the thrust washers on the crank pin was the reason but back in the day I built many 5hp Briggs Lhead (flathead) engines and part of the process was slip fitting the bearings on the crank on both ends on the dual bearing types but those engines usually maxed out around 7g's and had a conventional crank and no thrust washers.

I suspect the difference could be the stresses on the crank and or the heat build up in the bearing pockets.
 
I understand the need for an interference fit of the outer bearing races into the case, and end play is another subject, but I'm still not clear why the crank and inner race cannot be a slip fit. Especially since it actually is in several different saw designs I own.

On those saws I doubt there is any more clearance between the crank and inner race than there is between the balls and the races of the bearings, so I don't see how this clearance allows the crank to move and pound against the outer race, therefore damaging the bearing pockets. Now some of those saws are the Poulan design with rubber coated bearings, so that would damp out any vibration before damaging castings, but not all of them are made that way.

The only issue I can see still is the possibility of the crank spinning in the inner race, but I have not see any sign of this.
 
I thought the crank was hardened in the bearing area - will a knurling tool work on hardened metal like this? Anyone ever tried it? I may have an old 7900 crank I can try.

i suspect that a knurling tool is harder than a crankshaft, but you wouldn't know until you try. also, i think it would be easy to over do the knurling and make it impossible to press a bearing on. really the best solution would be a new crank. i think that resins (loctite) are a waste of time. i remember when hombres used to put sawdust in a noisy differential before they sold the car, guaranteed for 30 days or 30 seconds, whichever comes first.
 
I understand the need for an interference fit of the outer bearing races into the case, and end play is another subject, but I'm still not clear why the crank and inner race cannot be a slip fit. Especially since it actually is in several different saw designs I own.

On those saws I doubt there is any more clearance between the crank and inner race than there is between the balls and the races of the bearings, so I don't see how this clearance allows the crank to move and pound against the outer race, therefore damaging the bearing pockets. Now some of those saws are the Poulan design with rubber coated bearings, so that would damp out any vibration before damaging castings, but not all of them are made that way.

The only issue I can see still is the possibility of the crank spinning in the inner race, but I have not see any sign of this.

the most common mode of bearing failure that i have observed in my short sweet life is when the bearing starts to fail and the inner race slips and rotates on the shaft. it causes the shaft to wear and requires both bearing and shaft to be replaced. a pressed, interference fit postpones that failure mode.
 
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