Tdc

TDC

Hidden viscosity

Kevin Cameron

VISCOSITY-THE INTERNAL FRICTION OF fluids-is a very convenient phenomenon. With good alignment, smooth surfaces and adequate cooling, an oil-lubricated plain bearing can carry several thousand pounds per projected square inch. Here is how.

A typical crankshaft main bearing has a diametral running clearance of about .002-inch, but under load, the crank journal does not run centered in the bearing shell. Instead, the load presses the journal to one side, slightly off-center. This gives the oil space leading to the loaded region a wedgeshaped profile, tapering down smaller as it approaches the loaded area.

Oil is swept into this space by its own viscosity; it cannot slide frictionlessly over either itself or the surface of the crank journal, and the oil is thus pumped by friction/viscosity, down the narrowing wedge, into the loaded zone. The high pressure in the loaded zone constantly pushes oil out the sides of the bearing, but more is always being stuffed in to replace it.

On the far side of the bearing, where the clearance is large, the pressure is low and it is easy for the oiling system to push fresh oil into this region. Note that it is not oil-pump pressure that supports the load; pressure in the loaded oil film may be thousands of psi, while oil-system pressure is under 100 psi. It is the action of the bearing itself, combined with the oil’s viscosity, that drags oil into the loaded zone as fast as the load can squeeze it out.

Indeed, the loaded zone need not remain in one place, but may orbit the bearing as the direction of the load varies-as is certainly the case in a con-rod bearing. When the load orbits, lubrication is not continuous; instead, the low-pressure side of the oil clearance is refilled every time it orbits past the oil hole(s) in the crankpin, giving the bearing enough of a squirt of lube to carry it until it sweeps past the oil hole again on the next revolution, and so on.

Under very heavy load, a plain-bearing journal may operate on an oil film only .00006-inch thick, amounting to the thickness of about 4000 oil molecules. This makes it obvious why, for heavy service, a crank journal must be truly cylindrical and extremely smooth. Bearing makers warn against the procedure called “micro-polishing,” a kind of shoe-shine for crankshafts, in which a high-speed polishing belt is held against the rotating journal. Naturally, this rubs away soft material faster than hard. Since there are local variations in the crank’s surface hardness, the result is surface waves that can easily be taller than the minimum loaded oil film. Not all that glitters is truly cylindrical.

A bearing’s ability to carry load depends upon keeping oil between the surfaces. Any interruption in the surface, such as an oil-distribution groove, greatly increases the rate at which oil can leak away from the loaded zone and so reduces the bearing’s load-carrying ability-by far more than in simple proportion of areas. The farther the load must squeeze the oil before it can escape the bearing, the more load the bearing can carry. This is why there is new interest in end-fed cranks, as opposed to the time-honored system in which oil for the rods is fed into drillings in the crank through grooved main bearings, supplied from an external oil gallery. By supplying the rods with oil sent from the crank ends, grooved main shells can be done away with, with a consequent large increase in load capacity (or the possibility of using narrower, ungrooved shells of the same load capacity). In addition, there is always oil pressure in the crank-not just when the oil hole happens to be swinging through the grooved half of the bearing shell.

Now let’s consider much thinner oil films. Put a drop of oil on a metal plate, then tap it with a hammer. As compared with hitting the bare plate, the oil drop has a remarkable damping effect on the hammer blow, changing a clang into a much duller bump. Why the cushioning effect? Ordinary viscosity can’t explain this.

Some investigators looked into the matter more closely. They found that oil resisted being squeezed out from under the hammer face so tenaciously that the metal plate could be dented-but without any metal-tometal contact, as indicated by electrical measurement. Somehow, the oil resisted flowing out from under the hammer so strongly that the metal plate yielded under the resulting pressure. How could this be?

It’s known that the viscosity of oils increases under extreme pressures, as for example between heavily loaded gear teeth, or between a bearing roller and its raceway. This effect may be essential to the success and survival of lubrication under these extreme-pressure conditions, keeping the two metal surfaces apart so they don’t weld to each other locally, then pluck material from one another as they separate again. But why should viscosity increase?

Recent computer-modeling of oil at the molecular scale reveals a remarkable answer. Oil is normally a viscous liquid, a mass of writhing, wriggling, long molecules in every possible orientation. Under great pressure between hard surfaces, the thickness of the oil layer decreases to four or five molecular layers-and then the molecules in these layers begin to orient themselves like the logs in a raft. As the pressure increases further, the only way out for the remaining molecules is to wriggle out endwise, a motion the researchers call “reptation,” for its resemblance to a snake’s progress.

Because it’s hard for the molecules to escape in this fashion, the apparent viscosity of the squeezed oil is very high. Indeed, the researchers say it can rise by five orders of magnitude to a hundred thousand times its former value. Under sufficient pressure, all but one layer finally escape-but that layer remains, unbroken, bonded to the surfaces. Just the thing for gears and rollers.