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# Main/Rod bearing diameter and friction

## Main/Rod bearing diameter and friction

(OP)
Does anyone know of any good articles, studies or threads discussing main/rod bearing diameter and friction?
And or is there a formula to ballpark the friction related to RPM?

The next part of the question is around other variables. For example I'm thinking that if you had two rotating assemblies, otherwise identical in weight and dimensions with the same width bearings and one with a wider journal couldn't you run a little lower viscosity oil because of the increased surface area? How much effect would this have on the friction? Are there other variables that could allow a bigger crank reduce friction due to the added area?
And is the increase in friction related to RPM pretty much constantly linear or is there a point at which RPM or journal surface speed starts to have an increasing rate of friction?

### RE: Main/Rod bearing diameter and friction

i know ENGMOD4T has a tool to work out FMEP. im sure you could get some insights from it.

### RE: Main/Rod bearing diameter and friction

Your question regarding crankshaft journal bearing losses is very complicated and there is no simple answer. The bearing's hydrodynamic oil film supporting the journal load involves complex fluid mechanics and heat transfer processes. To give you a better appreciation of the principles involved I would recommend reading this free technical reference published by NASA.

### RE: Main/Rod bearing diameter and friction

Engine friction is a complex function with several major components that contribute to power loss at different rates. Simple friction from the rings rubbing on the cylinder walls is the major contributor at low and medium rpms with the relatively constant friction force of the oil ring being the main contributor. The top ring's friction follows cylinder pressure, but ring friction does not vary much with rpms. However the power loss from this source is proportional to rpm. It is responsible for a steady, linear rise in power loss (friction mean effective pressure). Other losses are mainly from viscous friction, oil shear and drag. This source increases with the square of rpm, so the power loss from this increases with the cube of rpm.
The total sum of friction power loss appears to show an almost linear rise with rpm at low rpms that begins to show an increasing slope in the upper rpm range.

### RE: Main/Rod bearing diameter and friction

Hi yoshimitsuspeed ,
You said "For example I'm thinking that if you had two rotating assemblies, otherwise identical in weight and dimensions with the same width bearings and one with a wider journal couldn't you run a little lower viscosity oil because of the increased surface area? "

Probably true in theory.
Also in theory wider journal bearings are more vulnerable to edge loading from reality, like manufacturing tolerances, and housing and shaft distortion from operating loads.

Supposedly Nascar engines are using pretty low viscosity synthetic lubricants.
I bet they have to control part geometry and lube temperature, and thus viscosity, way better to make that work, even if their synthetic oils have some desirable emergency features chemically "built in."

As an example, some high performance con rod bearings have different contours (eccentricity) for the upper and lower inserts trying to conform closely to the journal size like a small diametral clearance'e geometry provides for greater capacity, when firing, but greater clearance for cooling oil flow and surviving pinching when the cap deforms at TDC exhaust.
http://www.stealth316.com/misc/clevite-77-rod-main...

I think a little extra viscosity, superb journal and housing geometry, and a < 10uinch finish on the journals, polished in the correct direction are going to help ensure reliability, which wins more races than a few theoretical horsepower. And the money saved NOT reloading grenade engines is better spent on getting the engine tune real nice.

A famous drag race team from the early 60s was quite successful running nitro near 100%, when everybody else was struggling to run 80%.
"It you detonate it, youâ€™re going to push everything out on the ground. "
About 20% of the way down the page here -
http://www.hotrod.com/news/from-the-source-tom-job...

### RE: Main/Rod bearing diameter and friction

How bearing surface area and oil viscosity is related? :)

### RE: Main/Rod bearing diameter and friction

The are unrelated.

je suis charlie

### RE: Main/Rod bearing diameter and friction

yoshimitsuspeed-

Here is a graph that provides a simple illustration of the relationship between friction coefficient and oil viscosity, sliding velocity, load in a lubricated sliding contact. You'll note there is a narrow sweet spot within the EHD regime for minimum friction coefficient, but if the contact degrades slightly from that point things can get ugly in a hurry.

The contact regimes are mostly characterized by lambda ratio, which is the ratio of oil film thickness within the contact area to asperity height of the mating surfaces. Obviously, bearing and journal surface roughness/texture has an effect on lambda ratio. But oil viscosity within the fluid film is even more critical. The oil within the local fluid film experiences a rapid rise in temperature due to efficiency losses. This rapid rise in local oil film temperature, called flash temperature, can be quite substantial. The flash temperature rise will reduce oil viscosity within the local fluid film, which in turn will reduce fluid film thickness, and ultimately result in a lower lambda ratio. Efficient heat transfer between the fluid film and mating journal/bearing surfaces is also important for minimizing peak flash temperatures.

### RE: Main/Rod bearing diameter and friction

Hi tbuelna,

Does elastohydrodynamic really belong on that Stribeck curve when discussing journal bearings or sliding components?

regards,

Dan T

### RE: Main/Rod bearing diameter and friction

(OP)
I am trying to remember why I said wider when my question is more related to diameter. Maybe just using it as an example. Or maybe just mixed up for a moment.
The real world application at the moment is all in regards to diameter. Specifically the first generation 4AGE had 40mm rod bearings. Then they went to 42mm.
The 7AFE crank is often used for stroker builds and has 48mm bearings.

There is as always a lot of theory on the internet about efficiency. Some people claim that the 40mm will have a notable decrease in parasitic drag.
Seeing that Toyota was always pretty obsessed with efficiency I have a very hard time believing they would have gone to a bigger crank if it reduced efficiency unless there was an urgent reason to do so. Since people have used those cranks to spin at least 15% over stock redline and made probably almost double stock power NA and more than triple boosted I don't see any urgent reason to upsize bearings at the cost of efficiency. Which makes me wonder if there really is any cost of efficiency. Or if it would be possible to increase efficiency because the increased area allowed for bigger oil clearances, or lower viscosity oil or some other thing that kept similar or even improved efficiency even with the increased surface speed.
Looking at that graph it seems like it could be at least possible.

I have also been wondering how this ties into the 7A crank as we spin them much faster than stock. As a rule of thumb oil clearance gets increased a little with RPM but I have been wondering if with the added diameter of the 7A crank if it would be better to bump it even higher than what we would normally do with a 42mm crank and also if different ratios of oil clearance and viscosity would be better than others.

### RE: Main/Rod bearing diameter and friction

Tmoose, that's a valid point. The "elasto" in elastohydrodynamic refers to the fluid film pressure being high enough to produce sufficient local elastic deformation of the surfaces in contact to affect performance. This is more of a concern with largely non-conforming (point or line) surface contacts, such as those in rolling element bearings or gear teeth, where elastic deformation of the contact surfaces provides a significant expansion of the fluid film area supporting the load.

EHD is between full HD conditions where the fluid film is thick enough to prevent any asperity contacts, and mixed conditions where the fluid film is thin enough to permit significant asperity contacts. In reality there is also some elastic deformation of the contact surfaces in a journal bearing due to fluid film pressure, but not to the same degree as a rolling element bearing contact. It is true that journal bearings have sliding contact, but rolling element bearings and gear teeth also typically have some small amount of sliding/skidding in their contacts.

Maybe someone can provide a more explicit technical definition of elastohydrodynamic contact.

### RE: Main/Rod bearing diameter and friction

#### Quote:

Seeing that Toyota was always pretty obsessed with efficiency I have a very hard time believing they would have gone to a bigger crank if it reduced efficiency unless there was an urgent reason to do so. Since people have used those cranks to spin at least 15% over stock redline and made probably almost double stock power NA and more than triple boosted I don't see any urgent reason to upsize bearings at the cost of efficiency.

There's torque and there's time; most performance engines don't pull at max torque for any sustained duration.

It's a correct assumption Toyota engineers found it necessary to increase bearing diameter and crankshaft overlap, since their OEM engines have to survive sometimes days of max torque proving.

Bottom line, Toyota owners brag about the longevity of their engines; so a little too weak over a long enough duration equals failure. Alternately, top fuel dragsters make several thousand horsepower for fewer than 1,000 revolutions and less than four seconds and then are completely rebuilt.

jack vines

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