Click on a thumbnail to see the full-size image
Crank And Cam Polishing: Are You Smooth Enough?
Page 2 of 2
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.
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.
More Most Read Articles...
Page 2 of 2