Today's high output, close tolerance engines are more dependent than ever on quality remanufacturing procedures, durable parts and precise machining. One of the best ways to assure long bearing life in today's engines is to make sure the journals on the crankshaft are properly polished.
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.
According to Steve Bleggi, sales manager for Abrasive Accessories, Inc., Frisco, TX, a polishing belt with #320 or #400 grit abrasive is typically used depending on the surface finish requirements of the application. The most popular sizes are a 1" x 64" and 1" x 72" belt size.
Ian Bagnall, sales manager at RMC Rogers Machine Co., Bay City, MI, says most automotive crankshafts usually rotate clockwise in an engine. Some marine and industrial engines rotate counterclockwise, so the first thing you have to determine is which way the crank normally rotates before chucking it up in a grinder or polishing stand.
"Most crankshaft grinders and polishing stands rotate the crankshaft toward the operator (clockwise if viewed from the left end of the machine, counterclockwise if viewed from the right end)," says Bagnall.
"If the crank is mounted with the nose to the right, the crank will spin in a counterclockwise direction in the machine - which is opposite its normal direction of rotation in the engine. If the crankshaft is mounted in the grinder or polishing stand with the nose to the left, on the other hand, it will turn in the same direction it rotates in the engine."
Which way should the crank be mounted to achieve an unfavorable orientation when grinding and a favorable orientation when polishing? Bagnall says the grinding wheel on most crank grinders also rotates counterclockwise so the sparks and debris are thrown down as the journals are refinished.
This will leave ferrite burrs that are oriented in an unfavorable direction on the journals if a crank that normally rotates clockwise in an engine is mounted with the nose to the right. If the crank is mounted with the nose to the left, the grinding operation will leave the ferrite burrs with a favorable orientation and reduce the effectiveness of the polishing step.
To produce the best finish, the crankshaft must be turned around after it's been ground so the nose is to the left for polishing. This is necessary because the abrasive surface of the polishing belt that rides on the crank journal moves away from the operator and throws the dust and debris backward and out of the way.
However, if the crankshaft is mounted with the nose to the right and turns counterclockwise in the equipment, the belt will be polishing in the same direction the crank was ground. This will reduce the effectiveness of the polishing step and leave an unfavorable orientation on the remaining ferrite burrs. Turning the crank around so the nose is to the left for polishing will remove more of the burrs and leave a favorable orientation which is the best surface finish for the bearings.
Not everyone agrees with this recommendation. Some say they have achieved good results regardless of which way the crank is mounted, ground and polished. Some rebuilders say they've ground and polished crankshafts in both directions with no bad results. If the finish on the shaft is smooth enough, the rotation in which it is ground shouldn't matter. Even so, a microscopic examination of the surface finish will usually show the best finish on a cast iron crankshaft is achieved with an unfavorable grind and favorable polish.
Why not just mount the crank in the grinder with the nose to the left for both grinding and polishing? This approach saves time because you don't have to reposition the crank after grinding - but it leaves a favorable orientation of the ferrite burrs which will reduce the effectiveness of the polishing operation.
An alternative method is to use a two-step polishing procedure. Though all the bearing manufacturers do not agree on polishing procedures, Ron Thompson, a bearing engineer at Federal-Mogul Corp., Detroit, MI, says an improper crankshaft finish can be especially hard on bearings. When using belt-polishing equipment, he recommends polishing the journals in the unfavorable direction (opposite the direction of rotation) with a #280 grit belt, then finishing the journals in the favorable direction (same direction as rotation) with a #320 grit belt.
CRANKSHAFT POLISHING WITH TAPE
The tape makes contact at four points, which the suppliers of this type of equipment say produces a more even and consistent surface polish - though the appearance may be somewhat duller than what many people are used to seeing. The tape is then advanced about an inch for the next journal, and so on until all the journals have been polished. A lubricant is also used with the tape to help wash away debris.
Compared to belt polishing, which may remove .0002" to .0005" or more inches of metal from a journal depending on the belt grit, length of polish and pressure exerted by the operator, tape polishing removes almost no metal. The abrasive on the tape is very fine. A 15 micron tape abrasive is similar to a #600 belt grit. Polishing a cast iron crank with a ground finish of 12 RA for 15 seconds with the 15 micron tape, for example, can improve the finish to 7 RA or better.
One of the advantages claimed for tape polishing is that it reduces the risk of operator error. The pressure exerted by the tape on the crank is fixed and does not depend on how hard the operator is pushing down on a handle.
Another purported benefit is more consistent results. The cutting action of a polishing belt changes as it wears. A new belt cuts more aggressively than a used belt. Tape polishes the same way every time.
Tape also costs less over the long haul than belts. A roll of tape costs about $33 and typically does about 200 cranks if the tape is advanced about half an inch per journal. But the initial investment in tape polishing equipment is much higher than traditional belt polishing equipment.
Hand belt polishers typically cost $500 to $700 depending if the polisher is air or electric powered. Belt polishing stands typically sell for $2,000 up to $2,600 depending on the size of cranks the stand can accommodate.
Tape polishing equipment, by comparison, can cost from $13,000 for an aftermarket polisher up to $50,000 or more for an OEM type of unit. So a high volume of cranks is usually needed to justify the investment in a tape polisher.
Ken Barton of QPAC Corp., Lansing, MI, says his company supplies the original equipment vehicle manufacturers with tape crank polishers. "The OEMs use tape to polish virtually all crankshafts today," said Barton. "They typically grind a new crank to 25 to 30 RA, then polish it to finish specifications which may be 8 to 12 RA for an automotive crankshaft or 5 to 7 RA for a diesel crankshaft."
Barton says the best results with tape polishing are achieved when the crank is ground the same way it turns in the engine, then polished the same way. He says this gives longer tape life as well as a favorable finish.
Mark Jeltema, product specialist and provider of tech support at K-Line Industries, Holland, MI (K-line has an exclusive agreement with QPAC to supply the tape polishing technology to the aftermarket), also says the best results are achieved when the crank is turned in the same direction it normally rotates in the engine when it is polished with tape.
"Our machine is reversible, and the nose normally mounts to the left," explained Jeltema. "This will leave a favorable finish with a journal finish in the 3 to 6 micron range with a 30 second polish using 15 micron tape."
CRANKSHAFT POLISHING EXPERIENCES
Bagley says he uses the tape polishing equipment on both cast iron and steel cranks. "Compared to a belt polisher, it takes a little longer to clean up the journals with the tape machine, but we've very happy with the results," he said.
Bagley said he previously used a two-step polishing procedure with a belt polisher. "The tape machine is a one-step process, but we still put a different surface finish on journals that run against oil seals," he said. "We use a 40 micron tape that leaves a 14 to 18 RA finish. We feel this helps hold the oil better for a good seal than a highly polished surface."
Steve Schmidt at Jasper Engine and Transmission Exchange, Jasper, IN, says his crankshaft department has recently switched from belt polishing to tape polishing. "Tape can't do 80 crankshafts a day and is slower than belts, but we're consistently achieving journal finishes in the 7 to 8 RA range with a 3M 30 micron tape," Schmidt said."
PITFALLS
The amount of pressure that's exerted against the journal by a polishing belt will also affect the cutting action of the belt and the amount of material removed from the journal. A very light pressure is all that's needed, and for no more than a few seconds. Excessive pressure can change the geometry of the journal leading to clearance problems and increased oil leakage.
If a nitrited crankshaft has been ground to more than .010" undersize, the crank will have to be renitrited after grinding, then straightened prior to polishing.
Don't forget to polish the fillet radii and the seal surfaces as well as the journal bearing surfaces. Some crosshatch on rod and main journals is desirable, but seal and thrust surfaces should have a straight line polish.
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