Calculating journal bearing oiling requirements

[Rick!]

GPM = 29.3 x 10^-9(L + .0043(W/D))mD^2N

D = journal diameter (in inches)
L = bearing width (in inches)
m = bearing clearance factor
N = rotational speed
W = bearing load (in pounds)

This will get you your minimum. However, as has been stated, you can't really have too much. There's no benefit to restricting oil flow to a bearing. The viscosity of the oil is the bigger factor as you need a minimum oil film thickness to withstand the bearing load. The wider the clearance, the more viscous the oil needs to be.

Maybe somebody smarter than me can do some dimensional analysis on that equation and make some sense out of it. Who has the long version of the calculation?

[dannobee]

The old myth about connecting rod side clearance just will not die, or go away it seems.

Some people need to see a Cup engine's lower end with the pan off and see the piston guided rods before the epiphany happens.

[Stan Weiss]

Try #2

► 29.3 * 10^-9 * ((.5 + .0043) * (1000 / 2.2)) * ((2 * .002/2.2) * 1000) * 2.2^2 * 9000 = 0.53193564

Stan

[fabr]

Will someone please explain to me why viscosity is not part of the equation? Am I wrong in thinking it is a factor?

[RDY4WAR]

Will someone please explain to me why viscosity is not part of the equation? Am I wrong in thinking it is a factor?

This is only looking at the oil flow required to maintain sufficient oil volume in the bearing and avoid bearing cavitation. The viscosity is a factor for the minimum oil film thickness necessary to maintain hydrodynamic lubrication once the oil is in the bearing. That deals with the Reynolds equation.

[Walter R. Malik]

The old myth about connecting rod side clearance just will not die, or go away it seems.

Actually, side clearance CAN diminish oil flow if it is tight and NOT ENOUGH. To much side clearance will make no difference.

[Walter R. Malik]

too much oil causes excessive friction (but not wear).
and I'm not thinking of reducing oil flow to the mains, just to equalize them. the reason I'm asking for a formula to get the minimum required oil flow to a journal bearing is because in my engine design, if the oil pump drive shaft journals wear even a little bit, oil pressure suffers a lot, (dropping to around 0.4bar pressure at idle, around 2bar at redline)
this is because the supply to this journal bearing is straight from the oil filter and trough a 4mm hole as a restriction, then the oil needs to do a 90 degree turn to get to the main oil gallery
to fix this I will be tapping a carburetor fuel jet in this 4mm hole, but I want some guideline to what size of jet I will put in it, I don't want too little to cause premature wear, nor do I want too big which then defeats the purpose.

If you already have all the answers about this, then why are you asking this question ...?
Probably because you are trying to find justification for what you think ...?
Sorry, I rained on your parade.

what?
so because I ask, that means I know the answer already?

NO, not at all ... because you attempt to argue with an answer shows this to be. If you don't like someone's answer, just forget it.

[racear2865]

The old myth about connecting rod side clearance just will not die, or go away it seems.

Actually, side clearance CAN diminish oil flow if it is tight and NOT ENOUGH. To much side clearance will make no difference.

Walter
I went thru an argument with 3 people on another forum and they specified that side clearance does not impede flow. I told them that it does if too Tight. They told me I was voicing old wives tale and I was tooooo old to verify. Several years ago, I had a customer bring a engine in to dyno and this thing was on the unloader spring with a 3/8 drill. I asked about clearances and he went thru that he had difficulty installing the rods because there was less than .002 side clearance. I told him that was to tight and it would hold old in the bearing too long and would overheat the oil. He said "Ive built several that tight and it would be just great". We proceeded to fire and eventually oil temp got to heck yeah hot. He carried back home and disassembled and files the side down on the rods. I did not want to see that but he brought back to run and all was well on dyno. I asked no question and finished it up and went on my way.
reed

[Rick!]

Here's a more comprehensive calculator that uses viscosity and all the right dimensionals.
https://www.engineersedge.com/calculato ... _15314.htm

[pamotorman]

i rebuilt a Jr johnson built BBC that had .060 rod side clearance but the rods were located by the pistons on the small ends. doing it this way means the oil flow which cools the bearings passing thru rod bearing is consent

[modok]

Shot in the dark here
If this driveshaft beating flows too much oil can you relocate and or resize the oil feed hole in that bearing?

[Tom Walker]

I do agree that not enough side clearance will bring trouble into your day, but doesn’t that same philosophy about clearances in an engine apply many times over?
A little too much we can usually deal with, a little on the tight side, we need a broom and a mop!

[Tom Walker]

It would for curiosities sake be interesting to find out when rod side clearance begins to hinder rod bearing oil flow.
I feel it would obviously differ with so many variables such as shaft diameter, oil viscosity, rod material, R.P.M., load factor and many others.
Sorry to get off track from the threads original direction, I have a curious mind.

[Walter R. Malik]

It would for curiosities sake be interesting to find out when rod side clearance begins to hinder rod bearing oil flow.
I feel it would obviously differ with so many variables such as shaft diameter, oil viscosity, rod material, R.P.M., load factor and many others.
Sorry to get off track from the threads original direction, I have a curious mind.

Side clearance on a V8 with 2 rods per journal ... Twice the bearing clearance starts to show extra heating of the oil at the journal.
I forgot the number of the SAE report which told about this; it was over 30 years ago and it was strictly about oil heating at the bearing surface using 30 viscosity oil maintaining 20 pounds of idling oil pressure. Not about bearing wear or anything else.

I also remember that they concluded that a side clearance of 4 times the bearing clearance showed absolutely no additional heating of the oil.
They tested vertical bearing clearances from .0020" to .0035" with bearings having only a thou of ellipse on a 2.125" crank pin journal so, I am sure it differs with different diameter journals and bearings.

[Tom Walker]

Appreciate the information and comments Walter, definitely good stuff, thanks for feeding a curious mind!