Shot peened crankshafts ?

[otrcman]

Has anybody ever explored shot peening of Model T crankshafts to reduce the likelihood of breakage ?

For those not familiar, shot peening looks a lot like sandblasting, but it serves a different purpose altogether. Shot peening became common in the late 1940's on highly stressed aircraft parts where reciprocating loads and metal fatigue were a problem. Today, it's not uncommon to see OEM automotive parts getting the same treatment. Among modern car parts often peened during manufacture are crankshafts, rods, cam shafts, and valve springs.

The way shot peening works is that the part is bombarded with tiny metallic or ceramic particles, usually rounded in shape. If you could watch the process with a microscope, it would look like somebody has been pounding on the entire surface with the round end of a ball peen hammer. The result is that the part becomes far more resistant to fatigue cracking. The peening creates a surface layer that is in compression. Remember, cracks almost always start on the surface of a part, and only begin from tension loads, not compression.

The benefit of shot peening is that the fatigue life of the part can be greatly improved. According to Wikipedia, "Depending on the part geometry, part material, shot material, shot quality, shot intensity, and shot coverage, shot peening can increase fatigue life up to 1000%."

Surely somebody smarter than me has already thought of this for our T cranks. It sounds like it could reduce the breakage problem significantly.

[Jerry VanOoteghem]

Might have been beneficial before T cranks commenced with 100 years of fatigue life. If you did it on the ground journals, won't you just have to regrind them to get back the surface finish, thereby removing the compression layer?

[otrcman]

Jerry,

I wondered about the journals too. Did some searching on the internet and didn't find anything on whether they masked the journals to avoid peening or actually did all the surfaces. I didn't find anything, but called a friend who does antique airplane engines. He said he sends his original cranks out for magnaflux and peening and they mask the journals so that area isn't peened. But they take great care to peen the radii as close to the bearing areas as they can since that's a primary breakage area.

By the way, one thing I forgot to mention is that both new and in-service parts can be peened. Either way, the fatigue life is greatly increased. My first experience with peening was fulfilling a service bulletin on a Lockheed JetStar wing spar. The airplane had about 5,000 hours service time already and the Bulletin called for peening some spots that had been found to be cracked on higher time ships.

[Art M]

I believe that shot peening would be beneficial at the journal corners, but a regrind might remove the compressed surface. I did some research on shot peening about 55 years ago. Depth as I recall depends on hardness. I was working with rc 63 hardness. The technology has most likely improved since then. For the Montana 500 folks it would be worth investigating if it hasn't already been done.

[kmatt2]

I believe that after shot peening is done on the crankshaft journals some type of micro polishing is done on the journals. The resulting surface on a microscopic level is like hand polishing a hammered copper surface with a buffing wheel.

[Bud Delong]

Way back at the Chevy plant i started many raw castings were shot blasted in machined named WHEELABRATOR. I think this was only done to remove scale and rust to give a clean casting to machine. The shot used [steel] is very small?? Bud.

[Bill Dizer]

Has anyone had a T crank nitrided? Where the crank is treated with nitrogen in one of various methods, to strengthen and harden the surface down about .005. From what I googled, it is fairly reasonable price wise. It is very common for aircraft crankshafts and is quoted at about $70.00.

[otrcman]

No nitriding experience with a car motor, but with light airplane motors it's very common as you say. On a crank or a cam, nitriding makes the parts wear darn near forever. All you do is just keep putting in new bearings shells as required.

That's why I was asking about shot peening, which has a different purpose than nitriding. Peening doesn't imrove wear resistance; peening is to prevent cracking. And it makes an amazing difference on parts that are prone to cracking. Peening is used not only on new parts, but also to extend the life of used parts. Peening is no more expensive than nitriding.

[Playswithbrass]

Shot peening is/was best used on con rods. However the new game for a while has been cryogenic freezing do a google search. It will give you lots of good info. Cheers pete. P.s. Use cryogenic freezing car parts. That will keep you away from the human body stuff

[otrcman]

I did a bit more research on shot peening of crankshafts. Of course, original cranks wouldn't have been peened because the technology didn't exist at that time. But the T crank is something that would have benefitted greatly if it could have been done "back then". We probably would be seeing far fewer breaks today if the process had been available when the cars were new.

I talked to Scat to learn about the T cranks that they make. Turns out that Scat shot peens their T cranks as well as nitriding the journals. Peening improves crack resistance, while nitriding improves wear resistance. So it looks like Scat takes advantage of all the modern technology with their crank.

Then I called Metal Improvement Co., which is probably the #1 player in shot peening. Their quality manager tells me that a used T crank can be peened providing it is magnafluxed in advance to ensure that there are no pre-existing cracks. He said that even an old crank gains significant resistance to cracking if it has been peened.

[Jerry VanOoteghem]

I did a bit more research on shot peening of crankshafts. Of course, original cranks wouldn't have been peened because the technology didn't exist at that time. But the T crank is something that would have benefitted greatly if it could have been done "back then". We probably would be seeing far fewer breaks today if the process had been available when the cars were new.

I talked to Scat to learn about the T cranks that they make. Turns out that Scat shot peens their T cranks as well as nitriding the journals. Peening improves crack resistance, while nitriding improves wear resistance. So it looks like Scat takes advantage of all the modern technology with their crank.

Then I called Metal Improvement Co., which is probably the #1 player in shot peening. Their quality manager tells me that a used T crank can be peened providing it is magnafluxed in advance to ensure that there are no pre-existing cracks. He said that even an old crank gains significant resistance to cracking if it has been peened.

Metal Improvement in Detroit?

[otrcman]

Jerry,

The facility that I talked to was in Lynwood, CA. The parent company is Curtiss-Wright Surface Technologies, which has facilities world wide. When I was working for NASA back in the 1960's we used the Los Angeles area facility, so that's the one I was familar with.

By the way, I asked about whether they peen the journals also and then re-polish. He said they normally mask the journals and don't peen them. However, he said that if the customer really wants the journals done on a new part they would peen that area with the part slightly oversize and then grind/polish to final dimension. The peening effect goes plenty deep to allow for surface refinishing. But it costs more that way and isn't normally done since most cracks begin in the corner radii and not out on the journal proper.

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