crankshaft journal surface finnish

crankshaft journal surface finnish

Postby grumpyvette » January 12th, 2010, 11:20 am

I must point out the links hold a great deal of useful info

federal mogal wrote:When refinished, the surface of a crankshaft will develop microscopic peaks which are “tipped” in the direction that the sparks spray during grinding (see the illustration above). If these peaks point toward the oil film area when the engine is running, lubrication is interrupted, and the bearing will show premature wear. It is important that the crankshaft be ground and final polished so that these peaks are tipped opposite the direction that the crank rotates when it is installed in the engine, this is referred to as the “favorable” direction. We recommend grinding the crank in the “favorable” direction, followed by a multi-step polishing process using progressively finer paper. The first polishing operation uses 280 grit paper with the shaft rotating in the reverse direction – this helps to “knock off” some of the raised material left over from grinding. The second polishing process uses 320 grit paper, and the crank should be rotating in the “favorable” direction. A third step polish with a very fine (400 grit) paper is optional, but should again be done in the “favorable” direction. If the thrust surface was contacted during the resizing operation it must also be polished.


care must be taken to ensure the journal does not get polished unevenly, tapered or egg shaped
journal surface must be polished so micro burrs face away from the direction of rotation on bearing surface for max durability on bearing surface, burrs far to small too see or feel still induce wear
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Crank And Cam Polishing: Are You Smooth Enough?

It is more important than ever for engine builders to be as perfect, or near perfect, as possible when it comes to surface finish requirements.

By Brendan Baker

Brendan Baker

Manufacturers are designing today's engines with tighter tolerances and less room for error. They make more power, live longer, produce less noise, vibration and friction, burn less fuel and produce lower emissions. So in light of all this, it is more important than ever for engine builders to be as perfect, or near perfect, as possible when it comes to surface finish requirements.

Crankshaft and camshaft finishes are no exception. In today's engines, rotating assemblies ride on a thin wedge of oil only .00005? thick in some cases. And to help reduce friction as much as possible, oil itself is much thinner today as well, so it is especially important to achieve the proper surface finish on all components in order to avoid problems down the road.

Engine builders big and small have the same need to produce a smooth surface on cranks and cams, but their respective budgets and business volumes may dictate what equipment is used to get the job done properly. Large production engine remanufacturers (PERs) can justify purchasing a micropolisher and an automated surface finish gauge while small custom engine rebuilders (CERs), by-and-large, feel they can't afford such luxuries. This is not to say that it's out of the realm of possibility for CERs to afford a micropolishing machine or a surface finish profilometer; but higher volume is generally what justifies making such a purchase.

Belt Polishing
Belt polishing is the traditional method, used by engine builders for many years, to polish crankshafts as well as camshafts. In the past, belt polishing worked well to easily produce surface finishes that were extremely close to OEM levels. Today, however, vehicle manufacturers have automated industrial polishing machines that cost many thousands of dollars and produce very smooth and consistent results, results that are increasingly more difficult for engine builders to reproduce with manual equipment. Difficult, perhaps, but not impossible.

"We achieve an extremely fine finish on the grind, before we polish." says Bob Heidbreder, Northampton Crankshaft in Cuyahoga Falls, OH. "Our method is to first dress the grinding wheel very fine and then use polishing belts for the final finish. We do all different styles of crankshafts this way and we've never had a failure or a comeback because of the finish."

Customarily with late-model crankshafts, says Heidbreder, rebuilders mic the journals and go through a two-step polishing process. "If the crankshaft is salvageable and you don't have to grind it, you polish it with a #400 grit aluminum oxide polishing belt. And then, if you need, you would have the option to micropolish the crank to achieve a finer Ra finish. Of course, this depends on the crankshaft you're working on but most want it as smooth as possible."

Aftermarket distributors recognize the demand for near-OE quality finishes and offer rebuilders a number of products. "We have a brand new belt that we introduced about a year ago," says Chris Jensen, Goodson Shop Supplies, Winona, MN. "It's a GSW micropolishing belt and it is the best thing we've found for final crank polishing. You put it on with a little crank polishing rouge and turn it approximately 10 rotations."

According to Jensen, these belts have even worked well enough for a number of NASCAR racing teams to give them a try. These teams now use them for final polishing the cranks in their race engines.

"What is so nice about this belt is that there's a serration on it so it can polish the large radius on these high performance cranks," says Jensen. "High performance cranks have a large radius for strength while many production engines have virtually no radius."

A number of other distributors offer micropolishing belts as well as portable belt polishers. Tom DeBlasis, K-Line Industries, Holland, MI, says portability brings polishing capability to the price range and user abilities of almost every rebuilder.

"We offer a couple of portable crankshaft polishers: one electric and one pneumatic, which can be used on either a crank grinding machine after you move the head away or on a rotating polishing stand," says DeBlasis. "Most guys start out with a #320 grit belt, then go to a #400 grit and then progress to a very fine micro-polishing belt for a few revolutions, depending on the application."

"A long time ago stones were used to polish cranks and cams but that technology has gone by the wayside now," says Ken Barton, QPAC, Lansing, MI. "Today, micropolishing is technically the most advanced way to achieve OEM-level surface finish on cranks and cams."

Barton says many engine builders believe in some polishing "myths," which can impact a shop's profitability. "Some rebuilders believe if you use very fine grit belts you will not remove any material. That's kind of a misconception. You always take off some metal when you polish," says Barton.

"When you are done grinding the piece with the grinder it looks like a mountain range," Barton continues. "If you put a micrometer on the piece you're going to measure from the highest peak to the highest peak. So when we take those peaks down and you remeasure it, you're going to get a different measurement. It might be just a small amount of material, but some material is being removed."

Typically that amount may be as small as .0002? according to Barton. One of the challenges is convincing shops that such a small amount actually makes a big difference. "Of course, you have to take material off to achieve the correct surface finish. With micropolishing you're taking off the peaks and getting down towards the valleys, and the more peaks you remove the more surface area you have," he says.

Another misconception that some users have is they think they can put a polishing belt on and go crazy with it. "Some people think making the part shiny is enough, but it probably has taper, crown and who knows what else," says Barton. "If it is within a couple of tenths the error is still there. That's why we back our machines up with a rigid shoe behind the tape. So now you have a rigid setup that won't taper or go out of round. That way we keep things round and flat. When you finish you index the tape approximately one inch and go on to the next journal."

Micropolishing machines use a polishing tape instead of a belt like a belt polisher does. The tape comes in approximately 150-ft. rolls, and when used on a micropolishing machine operators index each roll after each use. When a crank or cam is polished on one of these machines, it uses about one inch of tape per journal, so fresh abrasive is used for every polishing job, but because it's such a small amount, many users say they've saved money. "One PER reported a 50-75 percent savings in the cost of tape vs. belts," Barton says.

The type of abrasive tape you should use for grinding cranks and cams varies, according to Barton. "From our position it all depends on what is on the machine. Is it a steel, cast iron or nodular iron part? Is it a hardened steel or is it a forged part? Then once you get through the material, what kind of surface are you grinding? We've had surfaces as high as 45 Ra and as low as 15 Ra. All that determines what abrasive you use on the polishing tape. It could be 9 micron or it could be 50 micron (note: 20 micron is roughly equal to #600 grit). It just depends on how aggressive you need to be."

Barton understands that not every shop will purchase his equipment. "For shops to really see a payoff they need to rebuild about 15 cranks or engines a day. Some specialty and high performance shops that sell engines and cranks for much higher prices than average can also make their ROI."

Measuring the Finish
According to some of our experts there are still engine builders who refuse to use any measuring devices other than a fingernail to measure surface finish. Yet increasingly, it is more important than ever to know exactly what surface finish you have. Without a measuring system, however, you can't know exactly what you have when you finish polishing.

While you don't have to measure every piece, measuring Ra during a spot check is a good step toward safeguarding against problems you may not even know exist. There is hope that this trend is going to change as the industry becomes more aware of the need to measure surface finishes.

K-Line's DeBlasis says that a lot of engine builders are at least starting to look at portable profilometers. "Profilometers are not inexpensive and I think that's why some smaller shops are just looking right now, but we're starting to sell more of them," says DeBlasis. A portable profilometer can cost up to $2,000, according to DeBlasis.

There are three characteristics to understand about surface finish: profile, waviness and roughness. According to John Wilt who works with the American Society of Mechanical Engineers Board of Standards on surface finish specifications, a sand dune is a good example of all three. "The overall shape of the dune would be equivalent to the profile, and as you moved in closer to see the windswept ridges, that would be waviness, and the grains of sand would represent roughness," Wilt says. "Every crank and camshaft has these three features as well, and technically they all fall under the heading of 'surface finish.'"

However, just because they all represent an element of surface finish, it's important to approach each component separately.

"Profile is the size of something," says Wilt. "It could be the diameter. It could be the shape as far as being round or tapered or hourglass shaped. Profile can also be a length measurement. So the journals and the lobes all have to be the right shape and in the right location."

The second phase of surface finish is waviness or lobing. "If you go along the axis of a crankshaft journal or camshaft journal it would be considered waviness. If you go around the part it's now called lobing. And a very frequent slang term used for this is called chatter. If waviness is out of spec, that's where problems with noise and vibration come into play.

"Basically, if you go back to the sand dune analogy, if you were to walk a little closer to the dune and saw all the windswept ridges, there's something separate that caused that to happen than the profile itself," says Wilt.

The third phase of surface finish is roughness. "If you walk up and grab a handful of sand from the dune, you'll recognize that the grains represent roughness. Surface finish is kind of like grains of sand paper that over time creates wear. The valleys in the grains of sand actually are a major contributor to holding lubricant," says Wilt.

Wilt's company, Adcole, manufactures surface finish equipment designed to measure all three categories independently. It can measure a crankshaft or a camshaft, giving the manufacturer or rebuilder the ability to look at the shape, size and location first (profile) to ensure that it's correct. "If the profile is not right the rest doesn't matter. If it doesn't assemble or go together it's not going to work anyway," Wilt notes.

Roughness Average?
When you read the term "Ra" does it mean anything to you? "If you ask anybody in the industry, we've all seen a surface finish symbol: a checkmark with a number, or the roughness symbol," says Wilt. "The problem is that most people assume they know what it means."

While shops can often easily get an Ra reading using a handheld device, they may not really understand what it means. One reading may be different than another. According to Wilt most people think Ra is the peaks and the valleys - but it is only an area measurement.

Wilt continues. "If you say you have an Ra of one micro-inch it's hard for people to understand just what that means. It's like if I say I have one acre of land for sale for $5,000: would you want to buy it? What do you know about my acre of land? You know only one thing - it's an acre and it's got so many square feet. If you know the area is rectangular then you can describe it by its spacing and its peaks, but you really don't know anything about the area. Saying a surface has such an Ra is exactly the same thing."

"When you have your Ra you don't really know what that looks like," Wilt explains. "It could look like a saw tooth or it could look like a square wave, where it goes up and goes across a little and then it goes down or it could be much spikier or wavier. The net result is you have different capabilities for bearing load. In many cases, simply looking at the roughness average is not necessarily the best way to look at that surface. You want to make sure there's enough bearing area and you also want to make sure you don't gouge the bearing surface. So ultimately the part that's contacting the connecting rod bearing, for example, must have enough surface to bear the load. It's not just two contact points that will be crushed as soon as the engine rotates."

Favorable and Unfavorable Direction
One way to achieve the proper surface finish on cranks and cam journals is to grind them in the opposite direction they normally rotate in the engine. Most automotive cranks typically rotate clockwise, but some industrial and marine engines rotate counter clockwise. So you have to know which way the engine rotates before you mount it in the polishing stand.

Polishing the crank or cam in the opposite direction it was ground will also break off more of these ferrite burrs, leaving a cleaner smoother finish as well. Ferrite burrs, if not removed, can cause problems later on because they can wipe away the oil film and cause a bearing failure.

Not all engine builders agree this is necessary, but it should remove the sharp edges of the ferrite burrs leftover from the grinding process and leave what is called a "favorable" finish.

One rebuilder we spoke with says he grinds one direction and polishes the other to get as smooth of surface as possible. However, other engine builders we interviewed say they have not noticed any difference, no matter which way they polished the part.

The main goal in polishing any crankshaft or camshaft is to achieve the smoothest, flattest surface as possible. When cranks and cams are properly lubricated they turn and rotate very smoothly, which does two things: minimizes wear and more importantly it minimizes heat and fatigue. You want valleys for lubrication but also plateau peaks, not sharp peaks, to handle load.

So it is important to remember when you're polishing that although it looks like art when you are finished, it really comes down to a science.

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Re: crankshaft journal surface finnish

Postby grumpyvette » October 20th, 2010, 6:42 am ... ished.aspx

Crankshaft Polishing: Make Sure The Journals On The Crankshaft Are Properly Polished

By Larry Carley

Larry Carley

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.

Polishing techniques
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.

Polishing with tape
Another way to polish the crank journals after grinding is with equipment that uses microfinishing tape rather than an abrasive belt. This type of equipment works differently than a belt polisher. Instead of rubbing a rotating abrasive belt against a rotating journal, the abrasive tape remains stationary and is clamped against the journal as the crank turns.

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."

Polishing experiences
Tom Bagley at Grooms Engines, Parts, Machining, Inc., Nashville, TN, says his company recently acquired a used OEM tape polishing machine. "It's an oscillating type of machine that uses 30 micron tape," he explained. "We run the tape for 10 seconds on each journal with the crank turning first one way, then the other way, the last one being in the favorable direction. This gives us very consistent results in the 6 to 9 RA range."

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."

Polishing pitfalls
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.

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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