Guys I need some advise on bearings..

user-9274568 · Post by **user-9274568** » Tue Nov 07, 2006 8:43 pm

I've seen motors that were run tight, at .0025, running 0000 oil with 15-18" vacuum. When the crank came back from the grinder, a lesson was learned. I'm not sure why I was even considering it? I know, I just wanted to get it done!!

How much vacuum does a typical cup motor have? You got to remember, some drag motors NEVER see 140 degrees. I make a run with about 100 degrees of heat in the motor, sometimes less.

This is what I think is going on. When I was at MGP, the PS guys were having pin problems. This was around the time high vacuum became popular. One of the things that fixed it was a Amco 45 bushing. The reason, it was honed to a courser finish. This allowed oil to be held in the pin area. It took lots of time before you could get those guys to open up the pin clearance.

OK, this is why I think the clearance opened up, works. The light oil has more room to protect the bearing. The opposite is happening due to the high crankcase vacuum. So, opening it up allows more area for the oil to stay. Not sure if it's logical, but it's worth a thought.

MadBill · Post by **MadBill** » Tue Nov 07, 2006 9:01 pm

Ed-vancedEngines wrote:..But a What If. What is the hole was not in a circular part? In other words what if the hole were in a solid flat piece of thick steel or aluminum? Isn't that what an aluminum rod is? Isn't it basically a flat piece of thick aluminum plate before maching? Not talking about the Forged Aluminum Rods but am talking about those that are machine cut from a flat sheet of raw material. ..Ed

The principle remains, regardless of shape. A infinite number of infinitely thin elements which, when they expand, grow larger uniformly. Another perspective could be to visualize two reference points anywhere on the material's surface. When the object expands, the points get further apart, whether they are separated by solid material, air or a combination of the two.

Post by **Baprace** » Wed Nov 08, 2006 1:11 am

cspeier, now that you have told us what the temperature of the engine is going to be at the start of full throttle power , your bearing clearance now makes perfect sense at .003/.0035 minimum, where .0035 is probably the prefered minimum. I was wondering what in the heck you were building this engine for, the SB2 head should have tipped me off but I'm thick in the head if you know what I mean. You did the correct thing sending that crank back. good luck to ya.

ChrisU · Post by **ChrisU** » Wed Nov 08, 2006 7:13 am

Wow I never run these kind of clearances being discussed. I look at it in terms of a percentage though too.

On a Honda Journal I run around .002-.0022 but on a BB Chevy Rod Journal I might run around .003.

Everyone has their thing, and obviously based on such a wide varience of clearances shown on this thread, it must not be too important!!!!

Cobra · Post by **Cobra** » Wed Nov 08, 2006 10:32 am

The eccentric shape inherent in a bearing assembly allows for an oil wedge to lubricate. Excess clearance can reduce oil film area. Bearings should be checked periodically for proper wear pattern. Journal size must be considered in the clearance equation. Bearing clearance is unique to the application, and more is not always better. Bearings should be select fitted to each individual journal. Coated bearings may be the best thing since sliced bread!

miniv8 · Post by **miniv8** » Wed Nov 08, 2006 2:45 pm

Ed,

When putting together a larger than life timing gear in a marine diesel engine I quickly found out that by heating the "hole" or the center surface of a sprocket, the hole would in fact get smaller.
I think that when heating the close surroundings of a hole in a large object the material tries to expand but has in fact nowhere to go but inside the hole.

devilbrad · Post by **devilbrad** » Thu Nov 09, 2006 1:59 am

miniv8 wrote:Ed,

When putting together a larger than life timing gear in a marine diesel engine I quickly found out that by heating the "hole" or the center surface of a sprocket, the hole would in fact get smaller.
I think that when heating the close surroundings of a hole in a large object the material tries to expand but has in fact nowhere to go but inside the hole.

Due to heat dissipation? I'm sure if the whole timing gear where heated equally across its surface in an oven, you may find a different outcome. Someone above said that if you take two points on the surface of an object, they will grow apart as the object is heated, but what if the heat is centralized to one area. Measure from the heated area to a non heated area and I imagine the measurement would shrink. I'm with Ed on this one, that is, wondering WHY the hole gets bigger when everything is expanding at the same rate. I'd think that the molecules closest to the bearing would say "Hey, look, its easier to go this way then against all the other molecules." On the other hand, I also agree with others experiences with the hole getting bigger, as I've experienced this myself. Just one of those things that make you think.

ChrisU · Post by **ChrisU** » Thu Nov 09, 2006 7:13 am

Well, is someone going to heat one up and tell us if it grows or shrinks in the hole? couldn't be that hard to figure out.

miniv8 · Post by **miniv8** » Thu Nov 09, 2006 2:24 pm

Due to heat dissipation? I'm sure if the whole timing gear where heated equally across its surface in an oven, you may find a different outcome.

yeah I know.... And thats the way it should be done,,,, We were trying to save some time and BTU's (wich we didnt) by heating only the core of the sprocket..

it's really a matter of % growth per heat input I belive, so things do get wider, longer and thicker all the same percentage given the same heat input

rooster · Post by **rooster** » Thu Nov 09, 2006 9:51 pm

it's always amusing to hear the differences in preferred bearing clearances between drag vs oval engine builders. Oval track guys are as scared of "big" clearance as the drag guys are of "small" clearance.
My personal success with oval track engines has been with approximately one thou per inch of diameter, i.e. 2" rod journals at .0018-.0022 and 2.25" mains at .0022-.0025. These are dry sump engines. RP41 oil 75 psi @ 240 degrees, water under 190 degrees, 12-15" vacuum, methanol injection, 8800 rpm, aluminum or steel rods. 800 laps (1/3-5/8 mi tracks) and the bearings will just begin to wear through the coating (calico).
My suspicion is that .0035+ clearances in an 8500 rpm oval track application may not last long, not to mention the increased windage/ oil control issues.

Post by **Baprace** » Thu Nov 09, 2006 11:50 pm

ChrisU and rooster, I agree with both of you on your recommended bearing clearances for big and small journals, but in cspeier's engine it runs at wide open with a lot of horsepower at a very cool engine temperature ( 100* ) and I just don't believe oil will flow very well in a high RPM engine with no warm up, so I agree with his wide clearances ONLY because of the temperatures he is running with. He seems to have no trouble with that clearance so I say leave it be, plus he never indicated rod side clearance either

. NOW in a stock car at 7500 rpm's , I hope you have a big oil pan and not many laps with .0035 clearances because it will drink the oil , not counting the windage loss.

needforspeed66gt · Post by **needforspeed66gt** » Fri Nov 10, 2006 1:19 am

I set things up just like rooster said, and like him our stuff is mostly oval track.

Cobra · Post by **Cobra** » Fri Nov 10, 2006 11:48 am

An engineer tends to view a small amount of additional bearing clearance as insurance for severe conditions. Bearing design allows for lubrication and proper bearing performance. Arbitrarily assigning excess additional clearance to an engine is often more habit than necessity, and may be a source of reduced bearing performance. If you can not find the science, maybe it is not there!

Justin Jones · Post by **Justin Jones** » Fri Nov 10, 2006 8:37 pm

guys what effect do you think running a vac. pump or dry sump has on bearing clearance?

Post by **Kevin Johnson** » Sat Nov 11, 2006 5:39 am

devilbrad wrote:
miniv8 wrote:Ed,

When putting together a larger than life timing gear in a marine diesel engine I quickly found out that by heating the "hole" or the center surface of a sprocket, the hole would in fact get smaller.
I think that when heating the close surroundings of a hole in a large object the material tries to expand but has in fact nowhere to go but inside the hole.
Due to heat dissipation? I'm sure if the whole timing gear where heated equally across its surface in an oven, you may find a different outcome. Someone above said that if you take two points on the surface of an object, they will grow apart as the object is heated, but what if the heat is centralized to one area. Measure from the heated area to a non heated area and I imagine the measurement would shrink. I'm with Ed on this one, that is, wondering WHY the hole gets bigger when everything is expanding at the same rate. I'd think that the molecules closest to the bearing would say "Hey, look, its easier to go this way then against all the other molecules." On the other hand, I also agree with others experiences with the hole getting bigger, as I've experienced this myself. Just one of those things that make you think.

Yes, it is a good thought experiment. It seems because of changing temperature gradients in real parts that some relative shrinking of the hole is also occuring. A dance. Assuming uniform temperatures across a part in a complex system is a bit like assuming perfectly rigid bodies.

Related order of assembly caveat:

I was sitting in a class and the professor related a failure problem he was researching. An assembly with interference fits was being supercooled for yet another interference fit. Not surprisingly the resultant assembly was failing in service. The answer popped into my head but this was a big publically funded research project for him.

The relative forces underlying the interference fit don't disappear when the assembly is supercooled but the material strength takes a hit and so micro-cracks develop and subsequently express themselves in service. Something to remember if you are considering cryogenically treating parts.