As a follow-up to yesterday's post on crankshaft polishing, Bill Bradley sent me the explanation below and the photo that describes/illustrates what can occur when a cast crank is polished in the wrong direction:
When metal freezes from the liquid state to the solid state, grains form and grow until they encounter an adjacent grain and then they stop growing. In a casting, this results in a large grain structure that has less tensile strength and is less ductile than the much smaller grain structure of a forged item. Boulders vs. sand, so to speak.
In the close up you can see several areas after polishing where the nodule structure is visible and in one area to the left, there appears to be a sliver of a grain, or flap, that looks to be a sharp edge that may spall off the surface or dig into the bearing material.
If you enlarge the pic of the crank pin journal, you will also see the graphite nodules on the right side below the oil gallery bore and also in the fillet radius on the right side.
The oil film between the journals on the crankshaft and the loaded portion of the main and rod bearings is only about .00005" thick when the engine is running. If the journals are too rough or have burrs, particles or other debris that sticks up above the surface, it can abrade the bearings and increase bearing wear and the risk of bearing seizure.
Cast iron cranks typically contain about 4% carbon. The carbon forms little nodules of graphite surrounded by a relatively soft form of iron called "ferrite." When the crankshaft journals are ground and polished, the ferrite around the graphite nodules forms little burrs or jagged flaps that protrude above the surface. The height of these burrs can be as much as .00035", which is more than enough to cut across the oil film and dig into the bearings.
Ferrite burrs create a sawtooth-like finish on the surface that is directional, usually facing away from the direction the journal was ground or polished. If the sharp edges face away from the direction that the crankshaft normally rotates, it is said to be a "favorable" orientation because the burrs are less likely to dig into the bearings. On the other hand, if the sharp edges are towards the same direction of rotation, it is an "unfavorable" orientation and is much more likely to cause problems.
The trick, of course, is figuring out which way is which - that is, which way to grind the crank and which way to polish it to achieve the proper orientation of the ferrite burrs.
The ultimate goal when polishing crankshaft journals is to achieve a relatively flat and smooth surface finish (an average roughness of 10 microinches or less) with plenty of bearing surface to support the oil film. But it is also important to orient the remaining ferrite burrs in a favorable direction so they will have less of an abrasive effect on the bearings.
With forged steel cranks, there are no graphite nodules or ferrite to worry about, so it isn't necessary to grind the crank in one direction then polish it in the opposite direction. Even so, for best results, the recommendation is to polish a steel crank in the same direction it rotates.
One way to achieve an optimum surface finish on the journals of a cast iron crank is to grind the crankshaft journals in the opposite direction it normally rotates in the engine, then polish it in the same direction it rotates in the engine. This will leave a favorable finish with the sharp edges of the ferrite burrs facing backward. Polishing the crank in the opposite direction it was ground will also break off more of the ferrite burrs leaving a cleaner, smoother finish.
One mistake that's sometimes made when belt polishing a crankshaft is overpolishing the journals. Whether the operator is trying to achieve a bright, chrome-like appearance or is trying to clean up excessive roughness left by the grinding operation, excessive polishing can create a "halo effect" around the oil holes. The depressions created will reduce the bearing area and strength of the oil film which may lead to premature bearing failure.
When metal freezes from the liquid state to the solid state, grains form and grow until they encounter an adjacent grain and then they stop growing. In a casting, this results in a large grain structure that has less tensile strength and is less ductile than the much smaller grain structure of a forged item. Boulders vs. sand, so to speak.
In the close up you can see several areas after polishing where the nodule structure is visible and in one area to the left, there appears to be a sliver of a grain, or flap, that looks to be a sharp edge that may spall off the surface or dig into the bearing material.
If you enlarge the pic of the crank pin journal, you will also see the graphite nodules on the right side below the oil gallery bore and also in the fillet radius on the right side.
The oil film between the journals on the crankshaft and the loaded portion of the main and rod bearings is only about .00005" thick when the engine is running. If the journals are too rough or have burrs, particles or other debris that sticks up above the surface, it can abrade the bearings and increase bearing wear and the risk of bearing seizure.
Cast iron cranks typically contain about 4% carbon. The carbon forms little nodules of graphite surrounded by a relatively soft form of iron called "ferrite." When the crankshaft journals are ground and polished, the ferrite around the graphite nodules forms little burrs or jagged flaps that protrude above the surface. The height of these burrs can be as much as .00035", which is more than enough to cut across the oil film and dig into the bearings.
Ferrite burrs create a sawtooth-like finish on the surface that is directional, usually facing away from the direction the journal was ground or polished. If the sharp edges face away from the direction that the crankshaft normally rotates, it is said to be a "favorable" orientation because the burrs are less likely to dig into the bearings. On the other hand, if the sharp edges are towards the same direction of rotation, it is an "unfavorable" orientation and is much more likely to cause problems.
The trick, of course, is figuring out which way is which - that is, which way to grind the crank and which way to polish it to achieve the proper orientation of the ferrite burrs.
The ultimate goal when polishing crankshaft journals is to achieve a relatively flat and smooth surface finish (an average roughness of 10 microinches or less) with plenty of bearing surface to support the oil film. But it is also important to orient the remaining ferrite burrs in a favorable direction so they will have less of an abrasive effect on the bearings.
With forged steel cranks, there are no graphite nodules or ferrite to worry about, so it isn't necessary to grind the crank in one direction then polish it in the opposite direction. Even so, for best results, the recommendation is to polish a steel crank in the same direction it rotates.
One way to achieve an optimum surface finish on the journals of a cast iron crank is to grind the crankshaft journals in the opposite direction it normally rotates in the engine, then polish it in the same direction it rotates in the engine. This will leave a favorable finish with the sharp edges of the ferrite burrs facing backward. Polishing the crank in the opposite direction it was ground will also break off more of the ferrite burrs leaving a cleaner, smoother finish.
One mistake that's sometimes made when belt polishing a crankshaft is overpolishing the journals. Whether the operator is trying to achieve a bright, chrome-like appearance or is trying to clean up excessive roughness left by the grinding operation, excessive polishing can create a "halo effect" around the oil holes. The depressions created will reduce the bearing area and strength of the oil film which may lead to premature bearing failure.
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