Crankshaft end play shim options

[HBRag]

I've got a Chinese 82mm crank, that is .005" to long on the flywheel end. I've confirmed the bearing and the flywheel depth, both are spot on. Yet, I have .050" of end play with out shim. Three thick shim and I still have .007" end play. So I am considering my options.

Option 1 - Grind the Crank
This is the right answer, however for the cost to extract the dowel pins and grind, I might as well get another crank.

Option 2 - Machine the Flywheel
I can have the flywheel recessed .005" and run three shims.

Option 3 - User for Shims
This is an easy option, low cost and doesn't require a trip to the machine shop.

I realize that option 2 is most likely the best solution for the given crank. However, option 3 sure would be quick and simple. So my question to the mechanical engineers in the group is what are the pros and cons of three versus four shims?

HB

[neil68]

Not trying to second guess you, but is it still at 0.007" with the gland nut fully torqued to 350 lb? I had the same situation but then got mine down to 0.006" with the final torque, which is fine for a hi-po engine...
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Neil.

Der Kleiner Rennwagens
‘68 Beetle 2332 cc, 204 WHP
12.5 seconds @ 108 mph
Dynojet Test: https://www.youtube.com/watch?v=M9B_H3eklAo

[SRP1]

[HBRag]

It's a Berg glad nut, and I torqued it to 400 ft lbs, and the .007" is with three .015" shims. The easy answer would be four shims and a final assembly. I just always regret the short cuts, so I'm double thinking this one.

At least the flywheel will still be usable for a different crank in the future.

[BugUser]

From the engineer's stand point I would also consider the use of four shims...

As a benefit you have two more friction surfaces between crank and bearing to compensate the relative speed.

BugUser

[Jimmy111]

Three shims guys.
If you add another shim, you need to also add another .003 for the oil space. And that just doesent work well. The shims get stuck together then you have .01" clearance....

[SRP1]

Jimmy
How do you come up with the addition of .003" for one shim?
I have had to use the forth shim a number of times, and have never seen any negative recourse when using an end play setting of .005"

[Jimmy111]

Its like this
You have one shim next to the thrust bearing. This shim you want to stick to the bearing but also be able to rotate under pressure. then you will have a coating of oil between this shim and the center shim. this is about .003" Then you have the center shim. Then another oil space of .003 for the oil so the center shim can rotate freely. then you have the shim next to the flywheel. This shim like the shim next to the bearing should not rotate unless there is a lot of pressure on the shim. These 3 shims are what make up a viscous axial bearing. Very common in turbines and machinery that has high axial load.

Now if you add a 4th shim. you need to also add the oil space for the oil. If you dont add the oil space, you will have no room for the oil to flow between the shims and they will stick and rub and create heat. But what usually happens if you add a 4th shim is that the 4th shim will stich to one of the outer shims.

Remember that the one on the flywheel is roataing but not slipping. the one near the bearring is not roatating but not slipping either. it is the center shim that is floating and taking stress. when you add the 4th shim it will stick somewhere and disrupt the whole setup.

[HBRag]

Thanks for the explination Jimmy, all makes good sense to me. I guess, I'll just machine the flywheel and run three shims. It's only a two day delay, so no big deal.

Thanks for pulling me back from my moment of weakness. Now to go do it right...

[BugUser]

[SRP1]

[Jimmy111]

Got pics of that 6 shim pack?
This is a good discussion. I have some studies somewhere about this and I will find them and post them later. Not mine, someone elses.

You need to remember that the shims are not pressure fed. They sit in a oil bath and pick up the oil from the bath. It is the centrifugal force that keeps oil flowing thru the shims. Lets say you go with typical beaing clearances of .0015 in a typical crank. of course the crank has much more force on it but it is also pressure fed so the small clearance is fine. You dont want the shims that are next to the flywheel or bearing to rotate. It they do rotate they will wear the flywheel and the bearing quickly everytime the crank is forced in a axial direction. They are hardened just for that reason

So lets go by The .003" -.006" endplay. I used the large clearance because like you said when the clearances are too close the shims catch and burn. but .001 between each shim is a very small space for oil to flow. A typical piece of printer paper is almost 4 times that thickness for example. If there is oil between the shim and the bearing or flywheel, the shim will rotate. This is not good because it will wear the bearing every time there is pressure applied to it. you want the hardened shim to rotate against the hardened shim. Not the soft aluminum bearing.

Ill make a drawing and show you what I mean. Kind of hard to explain in writing.

[SRP1]

I wish I had taken a pic, all I can post up now is a pic of the whole engine minus the head at this point.
I think to say that the shims are oil bath feed is a little obscure, the oil from the main bearing is pushed under pressure out of the side of the main bearing and up threw the shims, the shims are an oil feed part of the engine and not only bath oiled as you make it sound. After the oil passes threw the shims it then fills the cavity around the shims bathing them if you will, but once the oil passes the shims it never re-enters.
I'll prove my point, hypothetically speaking you stated the shim on the main bearing would be stationary and in place, while the center shim rotated as a bearing. If that where true then your saying all the oil exiting bearing would be doing so between the bearing and the shim next to it, thats not so. The whole entire pack rotates and move as oil flows threw all shims because the oil is feed into the pack from the bearing regardless of the number of shims.
Granted in theory if the shims where to be pushed all the way against the flywheel surface the oil would escape between the bearing and first shim leaving the rest to lube in bath oil only, but the crankshaft never sits loaded in one position so oil flows from the front to back of the shim stack as the engine runs. The stack is even under full oil feed pressure when the the clutch is engaged as the only other outlet is the #3 connecting rod, my point is things are not a constant here.

I think this throws off your needed oil clearance theory.

[Jimmy111]

Its a bath. Take a good look.

the oil is under pressure when it is forced around the shim next to the bearing. But there are scallops cut into the bearing to ensure that the shim does not lift from the bearing. After the oil enters the bath, there is no longer any pressure. it flows out the oil relief hole that is located at the 7 oclock position while running then out the small drain (if equipped depending on case) when the motor isnt running.
Ill take photos.

[SRP1]

Ok its a bath, whatever the oil gets to the shims bottom line

Jimmy with all do respect, have you ever tried, tested, and run various VW engine combos with the four shim stack and pulled them back apart for inspection, or are you findings in this topic book based?
Be honest please. I say this because I have done this many times and it has worked just fine. Not to say VW or anyone else is wrong this is just a special circumstance.

[Jimmy111]

With VW's, no I have never tried to use 4 or more shims.

But I have repaired motors that someone had installed either 2 or 4 shims. and they were always motors with worn thrust bearings and low oil pressure problems.

However, I have done a lot of Large Refrigeration compressors which use a very simmilar thrust package as the VW. The same sort of 3 shim system was used and worked quiet well. But as the compressors wore and cranks were remanuractured, the clearances grew and adding another shim seemed like a easy way to solve the problem. (cranks were $2-3000 dollars) But compressors that were in operation for 10 years with little trouble were now comming back for new bearings after only 2 or 3 years after we added another shim.

Anyways. Dont you think that people should fix their motors correctly? not just use a band aid?

[shok]

if the band-aid doesn't have a negative effect on service life, is it a band-aid?
I haven't tried, I'm just watching this thread to hopefully get the correct info.
_________________
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1971 Squareback parts car

[BugUser]

Hello again,

I just fired up Paint and did a quick'n'dirty sketch of how I understand the lubrication of the axial shims...



First a picture of a quite beat-up old main bearing:

Next a sketch of how I understand the situation:

Note the tilted left shoulder of this sketched cross section (I tried to depict the scallop). The scallops don't touch the bearing's outer edges, therefore oil pressure can't escape that way. The oil pressure should be maintained in the whole area between crank and shims and is sealed to the gland nut and its threads by the flywheel seal. The fact that this O-Ring is loaded with oil pressure explains why the seal is so important and any damage will lead to an oil leak towards the gland nut.

It should as well be noted that pumping effects of the bearing can contribute to pressure on the "secondary" side (i.e. the shims and FW seal). I have seen engines where bearings started severe pumping due to crankshaft harmonics....


BugUser

[Jimmy111]

Hey great! another with visual aids.

Well not quite right. How do you explain how the oil becomes presurized after it exits between the bearing and the shim?
Note the 5mm drain hole at 7 oclock.

[SRP1]

Ok
Jimmy this is where this application is different from refrigeration compressors that are splash and bath oiled. Notice the bearing, on the outer edge of the bearing there is a .050" flat on the outside of the scallop, the shims ride on this surface, that will force the oil threw the shims under pressure just as BugUser's sketch shows. Having said that now tell me how the oil is not pressure feed to the shims?
The sketch BugUser posted is spot on and thats how this system is feed oil, the oil only pools around the shims once it has passed threw them. Note; the oil has to pass threw the shims, thats the key here. After that the oil then flows threw the drain back hole and back to the sump. IMO the hole is located in that position to pool oil behind the seal and keep it cool and lubricated.

Whatcha think.

BugUser thank for taking the time to sketch this out.