Machined surfaces are not perfectly flat but contain microscopic high and low spots. When two surfaces are pressed together, such as a camshaft follower against the camshaft lobe, initial contact occurs only at the high spots. These high spots are crushed, deformed, and worn away until the load is spread across sufficient surface area to support the load (reduction of Rpk or the portion of the peaks that are worn away).
We usually use the term "Break In" to describe piston ring seating but there are many areas where break in occurs. For example:
- piston skirt and cylinder wall
- roller element bearings
- camshaft lobe and camshaft follower
- gear teeth
- rocker arm face and valve tip
- rocker arm socket and push rod ball
- threads, as in threaded fasteners
- sealing surfaces such as flared hose fittings.
What does break-in look like? The picture below shows a slightly worn camshaft lobe.
Now look at a close-up of the surface below. Notice the strips at the yellow arrows. These are called "shadow flats" . Metal-to-metal contact has occurred at the light areas and no contact in the dark area. The reason they are lines is that when the cam lobe was ground the grinding wheel leaves microscopic gouges where the abrasive particles ploughed a trough into the surface. The surrounding high spots are not large enough to support the load from the follower and wear down.
Can we avoid the break-in process by making parts with better surface finishes? There is a process that attempts to form a compatible surface finish during manufacturing, called "superfinish" but the process is both expensive and often severely abused by marketing huskers. You might conclude that a "superfinish" would be a perfectly smooth"mirror" finish as it provides the largest bearing area. In some areas, such as bearing journals, the smoother the surface, the thinner the lubricant can be before high spots on mating surfaces come into contact. However, it's hard to maintain a lubricant film on a perfectly smooth surface. In some areas, such as a cylinder wall, we require both a flat bearing area and surrounding low areas that provides a reservoir of oil to keep the surface lubricated. This is called a "plateau" finish. The picture below shows a honed finish on a Lycoming aircraft engine cylinder.
This is called a "cross-hatch" finish as the hone motion criss-crosses. The cross-cross pattern keeps the piston rings from spinning as might occur if the hone created a spiral pattern.
During break-in the high spots are ripped, torn, and abraded down until the surface area is of sufficient size to support the load from the piston rings and the piston skirt. (It's the piston skirt pressing against the cylinder wall that provides the torque that turns your propeller).
In normal break-in the process stops when the peaks are flattened down to a sufficient area to support the load. The load being the maximum load produced during the engine run-in. This is why the engine should be set to maximum loading (power) during the break-in process. Not immediately, as too violent grinding away produces heat and may start ripping away at the base metal, but gradually bring up the power over an hour or two.
The break-in stops when the flat surface, called a "plateau," is sufficiently large to support the pressure from the mating surface. The troughs or valleys left over from the honing remain and are reservoirs that keep the surface wetted with oil.
Enough on cylinder finish. Often neglected are other surfaces such as those listed above. As surfaces get to know each other they are unique onto themselves. Camshaft lobe to lifter, push rod ball to rocker arm socket, each one has formed compatible wear surfaces. If you go mixing them up then the break-in process is repeated to some extent. This is why you always put back camshaft followers back in the same spot as you removed them. You should do this for all wear or contact surfaces: keep pushrods identified to rocker arms so they go back into the same spot, which, by the way, mechanics seldom do but should.
You can read more on cylinder honing and aircraft engines from my book the Sky Ranch Engineering Manual.
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