Today’s high output, close tolerance enginesare more dependent than ever on quality remanufacturing procedures,durable parts and precise machining. One of the best ways to assurelong bearing life in today’s engines is to make sure the journalson the crankshaft are properly polished.
The oil film between the journals on the crankshaftand the loaded portion of the main and rod bearings is only about.00005" thick when the engine is running. If the journalsare too rough or have burrs, particles or other debris that sticksup above the surface, it can abrade the bearings and increasebearing wear and the risk of bearing seizure.
Cast iron cranks typically contain about 4%carbon. The carbon forms little nodules of graphite surroundedby a relatively soft form of iron called "ferrite."When the crankshaft journals are ground and polished, the ferritearound the graphite nodules forms little burrs or jagged flapsthat protrude above the surface. The height of these burrs canbe as much as .00035", which is more than enough to cut acrossthe oil film and dig into the bearings.
Ferrite burrs create a sawtooth-like finishon the surface that is directional, usually facing away from thedirection the journal was ground or polished. If the sharp edgesface away from the direction that the crankshaft normally rotates,it is said to be a "favorable" orientation because theburrs 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 morelikely to cause problems.
The trick, of course, is figuring out whichway is which – that is, which way to grind the crank and whichway to polish it to achieve the proper orientation of the ferriteburrs.
The ultimate goal when polishing crankshaftjournals is to achieve a relatively flat and smooth surface finish(an average roughness of 10 microinches or less) with plenty ofbearing surface to support the oil film. But it is also importantto orient the remaining ferrite burrs in a favorable directionso they will have less of an abrasive effect on the bearings.
With forged steel cranks, there are no graphitenodules or ferrite to worry about, so it isn’t necessary to grindthe crank in one direction then polish it in the opposite direction.Even so, for best results, the recommendation is to polish a steelcrank in the same direction it rotates.
One way to achieve an optimum surface finishon the journals of a cast iron crank is to grind the crankshaftjournals in the opposite direction it normally rotates in theengine, then polish it in the same direction it rotates in theengine. This will leave a favorable finish with the sharp edgesof the ferrite burrs facing backward. Polishing the crank in theopposite direction it was ground will also break off more of theferrite burrs leaving a cleaner, smoother finish.
According to Steve Bleggi, sales manager forAbrasive Accessories, Inc., Frisco, TX, a polishing belt with#320 or #400 grit abrasive is typically used depending on thesurface finish requirements of the application. The most popularsizes are a 1" x 64" and 1" x 72" belt size.
Ian Bagnall, sales manager at RMC Rogers MachineCo., Bay City, MI, says most automotive crankshafts usually rotateclockwise in an engine. Some marine and industrial engines rotatecounterclockwise, so the first thing you have to determine iswhich way the crank normally rotates before chucking it up ina grinder or polishing stand.
"Most crankshaft grinders and polishingstands rotate the crankshaft toward the operator (clockwise ifviewed from the left end of the machine, counterclockwise if viewedfrom the right end)," says Bagnall.
"If the crank is mounted with the noseto the right, the crank will spin in a counterclockwise directionin the machine – which is opposite its normal direction of rotationin the engine. If the crankshaft is mounted in the grinder orpolishing 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 achievean unfavorable orientation when grinding and a favorable orientationwhen polishing? Bagnall says the grinding wheel on most crankgrinders also rotates counterclockwise so the sparks and debrisare thrown down as the journals are refinished.
This will leave ferrite burrs that are orientedin an unfavorable direction on the journals if a crank that normallyrotates clockwise in an engine is mounted with the nose to theright. If the crank is mounted with the nose to the left, thegrinding operation will leave the ferrite burrs with a favorableorientation and reduce the effectiveness of the polishing step.
To produce the best finish, the crankshaftmust be turned around after it’s been ground so the nose is tothe left for polishing. This is necessary because the abrasivesurface of the polishing belt that rides on the crank journalmoves away from the operator and throws the dust and debris backwardand out of the way.
However, if the crankshaft is mounted withthe nose to the right and turns counterclockwise in the equipment,the belt will be polishing in the same direction the crank wasground. This will reduce the effectiveness of the polishing stepand leave an unfavorable orientation on the remaining ferriteburrs. Turning the crank around so the nose is to the left forpolishing will remove more of the burrs and leave a favorableorientation 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 waythe crank is mounted, ground and polished. Some rebuilders saythey’ve ground and polished crankshafts in both directions withno bad results. If the finish on the shaft is smooth enough, therotation in which it is ground shouldn’t matter. Even so, a microscopicexamination of the surface finish will usually show the best finishon a cast iron crankshaft is achieved with an unfavorable grindand favorable polish.
Why not just mount the crank in the grinderwith the nose to the left for both grinding and polishing? Thisapproach saves time because you don’t have to reposition the crankafter grinding – but it leaves a favorable orientation of theferrite burrs which will reduce the effectiveness of the polishingoperation.
An alternative method is to use a two-steppolishing procedure. Though all the bearing manufacturers do notagree on polishing procedures, Ron Thompson, a bearing engineerat Federal-Mogul Corp., Detroit, MI, says an improper crankshaftfinish can be especially hard on bearings. When using belt-polishingequipment, he recommends polishing the journals in the unfavorabledirection (opposite the direction of rotation) with a #280 gritbelt, then finishing the journals in the favorable direction (samedirection as rotation) with a #320 grit belt.
Polishing with tape
Another way to polish the crank journals aftergrinding is with equipment that uses microfinishing tape ratherthan an abrasive belt. This type of equipment works differentlythan a belt polisher. Instead of rubbing a rotating abrasive beltagainst a rotating journal, the abrasive tape remains stationaryand is clamped against the journal as the crank turns.
The tape makes contact at four points, whichthe suppliers of this type of equipment say produces a more evenand consistent surface polish – though the appearance may be somewhatduller than what many people are used to seeing. The tape is thenadvanced about an inch for the next journal, and so on until allthe journals have been polished. A lubricant is also used withthe tape to help wash away debris.
Compared to belt polishing, which may remove.0002" to .0005" or more inches of metal from a journaldepending on the belt grit, length of polish and pressure exertedby the operator, tape polishing removes almost no metal. The abrasiveon the tape is very fine. A 15 micron tape abrasive is similarto a #600 belt grit. Polishing a cast iron crank with a groundfinish 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 polishingis that it reduces the risk of operator error. The pressure exertedby the tape on the crank is fixed and does not depend on how hardthe operator is pushing down on a handle.
Another purported benefit is more consistentresults. The cutting action of a polishing belt changes as itwears. A new belt cuts more aggressively than a used belt. Tapepolishes the same way every time.
Tape also costs less over the long haul thanbelts. A roll of tape costs about $33 and typically does about200 cranks if the tape is advanced about half an inch per journal.But the initial investment in tape polishing equipment is muchhigher than traditional belt polishing equipment.
Hand belt polishers typically cost $500 to$700 depending if the polisher is air or electric powered. Beltpolishing stands typically sell for $2,000 up to $2,600 dependingon the size of cranks the stand can accommodate.
Tape polishing equipment, by comparison, cancost from $13,000 for an aftermarket polisher up to $50,000 ormore for an OEM type of unit. So a high volume of cranks is usuallyneeded to justify the investment in a tape polisher.
Ken Barton of QPAC Corp., Lansing, MI, sayshis company supplies the original equipment vehicle manufacturerswith tape crank polishers. "The OEMs use tape to polish virtuallyall crankshafts today," said Barton. "They typicallygrind a new crank to 25 to 30 RA, then polish it to finish specificationswhich may be 8 to 12 RA for an automotive crankshaft or 5 to 7RA for a diesel crankshaft."
Barton says the best results with tape polishingare achieved when the crank is ground the same way it turns inthe engine, then polished the same way. He says this gives longertape life as well as a favorable finish.
Mark Jeltema, product specialist and providerof tech support at K-Line Industries, Holland, MI (K-line hasan exclusive agreement with QPAC to supply the tape polishingtechnology to the aftermarket), also says the best results areachieved when the crank is turned in the same direction it normallyrotates in the engine when it is polished with tape.
"Our machine is reversible, and the nosenormally mounts to the left," explained Jeltema. "Thiswill leave a favorable finish with a journal finish in the 3 to6 micron range with a 30 second polish using 15 micron tape."
Tom Bagley at Grooms Engines, Parts, Machining,Inc., Nashville, TN, says his company recently acquired a usedOEM tape polishing machine. "It’s an oscillating type ofmachine that uses 30 micron tape," he explained. "Werun the tape for 10 seconds on each journal with the crank turningfirst one way, then the other way, the last one being in the favorabledirection. This gives us very consistent results in the 6 to 9RA range."
Bagley says he uses the tape polishing equipmenton both cast iron and steel cranks. "Compared to a belt polisher,it takes a little longer to clean up the journals with the tapemachine, but we’ve very happy with the results," he said.
Bagley said he previously used a two-step polishingprocedure with a belt polisher. "The tape machine is a one-stepprocess, but we still put a different surface finish on journalsthat run against oil seals," he said. "We use a 40 microntape that leaves a 14 to 18 RA finish. We feel this helps holdthe oil better for a good seal than a highly polished surface."
Steve Schmidt at Jasper Engine and TransmissionExchange, Jasper, IN, says his crankshaft department has recentlyswitched from belt polishing to tape polishing. "Tape can’tdo 80 crankshafts a day and is slower than belts, but we’re consistentlyachieving journal finishes in the 7 to 8 RA range with a 3M 30micron tape," Schmidt said."
One mistake that’s sometimes made when beltpolishing a crankshaft is overpolishing the journals. Whetherthe operator is trying to achieve a bright, chrome-like appearanceor is trying to clean up excessive roughness left by the grindingoperation, excessive polishing can create a "halo effect"around the oil holes. The depressions created will reduce thebearing area and strength of the oil film which may lead to prematurebearing failure.
The amount of pressure that’s exerted againstthe journal by a polishing belt will also affect the cutting actionof the belt and the amount of material removed from the journal.A very light pressure is all that’s needed, and for no more thana few seconds. Excessive pressure can change the geometry of thejournal leading to clearance problems and increased oil leakage.
If a nitrited crankshaft has been ground tomore than .010" undersize, the crank will have to be renitritedafter grinding, then straightened prior to polishing.
Don’t forget to polish the fillet radii andthe seal surfaces as well as the journal bearing surfaces. Somecrosshatch on rod and main journals is desirable, but seal andthrust surfaces should have a straight line polish.