How do you tell the difference (and deal with) between a scratch and a crack?
The transmissions in our performance cars are special in that we will rarely ever see their inner working, so long as they continue to operate properly. Usually it’s failure that leads us to open the case of a transmission, or, in many cases, simply replace it altogether.
But there’s a case to be made for staving off those failures altogether and spending a little effort up front to prevent a lot of effort and heartache on the back end.
That’s where crack checking comes in. Both the constant and instant load stresses in the shafts and gears of a transmission make the inner working particularly prone to crack failures, and, as Jeff Horton of GearFX Driveline puts it: “These are not issues that will ever get any better, or even stop getting worse. Once cracks start to develop in a stressed part, it’s basically the beginning of the end. The only question is when.”
Worse yet, a part failure inside a transmission is rarely going to be isolated to the failed part itself. Think of all those spinning, meshing and sliding bits inside a transmission. What do you think is going to happen when you introduce tiny, razor-sharp shards of hardened metal to that mix? Basically the same thing that happens when you put your sick cat in your car to take it to the vet and stuff starts spraying out of both ends of him half way there. Nothing survives.
So, the best way to prevent these failures is to preemptively replace parts that are on their way to failure. To do that, we need to identify parts with any crack, signaling impending doom, and that’s where GearFX comes in.
Scattered (actually, neatly organized and cataloged) through the GearFX facility are transmissions, rear ends and drivetrain parts from NASCAR and IMSA teams, vintage racers, road racers, any vehicle that places lots of stress on rotating and meshing parts. GearFX specializes in prepping these parts to perform at their optimum, and “not exploding” is a key mission in delivering that performance.
Definitely not exploding was certainly on our list of things we wanted for the transmission in our C5 Z06 project, so while it was out of the car we took it to GearFX to work their magic. After disassembly, the inspection process is fairly straightforward, and wet magnetic particle crack checking is included in the cost of a transmission rebuild from GearFX. If you just have some loose parts you need checked, the service is available ala carte for $95 an hour—assuming the parts are clean and ready to go.
“First we just look at everything. If you can see defects with the naked eye, you know right off that the part is no good and needs to be replaced. The kind of damage can also lead you to other aspects that may warrant a further inspection. Worn engagement rings can be a sign of worn synchros, damaged gear faces will warrant a close look at the shafts and bearings holding them, and so on,” says Horton.
After the visual, it’s time to get a little more high-tech with a wet magnetic particle inspection. This process is commonly referred to as “Magnafluxing,” which is actually a trade name for a specific brand of magnetic particle inspection gear, but has become ubiquitous in use, even among the industry. The Kleenex and Band-Aid people feel their pain. Despite the sci-fi name, Magnafluxing is a pretty straightforward process that yields excellent results when it comes to finding cracks not visible to the naked eye.
Each part to be inspected is magnetized by passing a magnetic field through it. The part is then drenched in a fluid that suspends microscopic magnetic particles, which are pulled in to the cracks, and fluoresce dramatically when hit with UV light. The process takes only a few minutes, but can prevent a world of hurt at the worst possible time. For non-magnetic parts, like aluminum or magnesium, a fluorescent penetrating dye can be applied which draws into cracks via capillary action.
“We Magnaflux practically everything that comes through the door,” says Horton. “Obviously any parts that are coming off an existing assembly that we’re going to reuse, but new stuff as well. It’s just too easy to check everything to make sure something wasn’t damaged in shipping or has the rare manufacturing defect that would mean catastrophic failure down the road.”
After the check of our transmission, everything looked good. “There were a few burrs here and there that we could deal with on a case by case basis,” Horton continues, “but nothing showed up during Magnafluxing that worried us from a crack perspective.” He noted that regular fluid changes and proper technique—not putting any more stress on the trans during up and down shifts than it was designed for—actually goes along way toward preventing long-term damage.
How do you tell the difference (and deal with) between a scratch and a crack?
fearlesfil said:How do you tell the difference (and deal with) between a scratch and a crack?
a) Experience. A skilled tech can tell how much dye a crack will hold and how it will react to being simply wiped off as compared to a surface imperfection.
b) X ray/MRI. Same way you tell if a bone is broken, you can use imaging technology to find interior structural imprefections in a solid piece.
As for dealing with it, that's going to come down to the individual part. If the scratch is removed during polishing, it's probably nothing to worry about. If the scratch needs major refinishing to remove, it's probably a stress-riser anyway and it's best to just replace the whole part. At some point the thorough investigation is going to cost more than a pre-emptive replacement.
Well it should be noted that this is one of the main methods of industry on may new products going to assembly or out the door! While defects are not common in many forged, machined, heat treated parts, in new parts you almost never get a bad part out of the box with this type of defect that fails the moment you put the part in service or very shortly thereafter.
This is the exact reason to check USED parts! The longer you run them the closer they are to death! In fact they are dying a slow ever present fatigue death with each use!
Transmission parts, gears and shafts epically prone to fatigue failures, ring & pinions, spider gears, CV shafts and just about every other part you can think of is slowly failing sooner or later! Its the best case of pay me now or pay me 10X more later I can think of!
It is one of the most widely used methods for part inspection I can think of and is extremely well used in all Aviation inspections for life cycle inspections and crack detection prior to failure and on THOUSANDS of parts on a single plane! I really like reliability in my racer and not to mention safety!
We are mostly all making big power and power increases fatigue substantially and checking is always better than replacing the entire kit of transmission, rear end, loss of the car as a result of a failure.
Not to be an alarmist just food for thought!
Scott
With either method, how do you tell the difference between a scratch and a crack? Visual only or do they react differently?
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