Broken crank and high compression

I wonder if balance factor plays a part in crank breakages ? I'll bet if you could strobe the middle of a standard crank while it was running at 7000 RPM in a motor, you would see it grow. If it is balanced to run smoothly at 7000 RPM when the motor is rigidly mounted, the result might be different.

jseng1 said:

I know of a machinist who had the journals of a nitrided Norton crank ground and he didn't bother to re-nitride it - the crank broke.

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We had a similar experience (perhaps you were referring to us) where the crank went out for a grind. It did not go back for nitriding and the filets were a bit smaller in radius. This was a 59.6mm crank that grenaded on the dyno while approaching 11,000 rpm - I guess it had enough.

As for burnishing the journal radius (I had heard it referred to as rolling), this places the surface in the immediate area in compression, thus increasing the resistance to "start" a crack. I seem to recall discussing this with Jim Comstock years ago and he stated it is common practice with diesel crankshafts. He would know as he was an owner of a diesel shop.

As I mentioned earlier in this thread, nitriding is a common practice to increase durability and wear resistance. Thinking about this philosophically, as an example, if one relies on say nitriding or burnishing a filet alone, then one is riding/racing right at the edge. You can get away with it in F1 where you have extreme QA/QC, a high level of design and analysis and engine controls but when racing an old stove, one missed shift and it will probably go up in a shower of shrapnel. For most, redundancy is wise (ex. larger filet radius, nitride, better materials...etc)

Dances with Shrapnel said:

...... but when racing an old stove, one missed shift and it will probably go up in a shower of shrapnel. For most, redundancy is wise (ex. larger filet radius, nitride, better materials...etc)

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Kinda gets us back to what Matchless/Martyn said earlier in this thread about only using Maney cranks in his Nortons.
Edit: Mr. Maney's crankshafts, being made out of better materials, seem to me to be the way to go on a rebuild, especially if building a race motor.

Shrapnel
I was actually referring to another nitrided crank that broke when it was ground and not re-nitrided (not yours) – so that’s at least two that we know of.

Martin/Martyn and Cjandme
The only Maney cranks I know of that failed were ground at the journals and not re-nitrided. When this happened, Maney sent out a mass email warning that his cranks should always be re-nitrided if the journals are ground. Should be common practice.

It takes tremendous pressure to burnish a radius (with a rolling wheel). BSA demonstrated the difference by subjecting a crank to some kind of massive vibration inducer which broke a crank in a few minutes. A burnished radius crank wouldn’t break. Shot peening is easier but who knows which is better?

Nitriding is only a surface effect. Are you saying that the minute hardened layer contributes to stress raising ? Perhaps lack of a decent radius after grinding might do more damage ?

acotrel said:

Nitriding is only a surface effect. Are you saying that the minute hardened layer contributes to stress raising ?

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Common sense would tell you that increasing stress raising (assuming you meant stress riser) would diminish durability of a component. I am neither saying that nitriding is nor am I saying it is not contributing to stress raising, whatever you might mean by stress raising. Perhaps you should do a search on the terms "nitriding" in conjunction with "durability" and zero in in the phenomena.

acotrel said:

Perhaps lack of a decent radius after grinding might do more damage ?

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Might I ask perchance, could you please qualify how much more?

I had this problem when I hard-chromed the journals of the billet crank in my Triumph engine. There was no radius at all except for the bare minimum in the corners. I couldn't think of a fix then and I cannot now. I'm sure for any serious racer, it is worth buying the billet crank, however where does it end. Do your race classes and the standard of racing justify the expense ? I am the wrong person to talk to about spending money on road racing motorcycles. In Australia, historic racing is like paddock-bashing with old motorbikes which would be better off being restored and used on public roads. So cheap as possible is the way to go. Of course with a billet crank, it might be 'quality does not cost, it pays' ?

I know I shouldn't contribute to the madness, but sometimes I just can't stop.

A "stress raiser" is the same thing as a "stress riser". Just depends on where you went to school, or maybe when. In any case it is generally used to refer to a mechanical feature of a design (like the sharp corner on the stock Commando crankshaft at the main bearing journal) that contributes to a higher stress concentration at that point than in the surrounding area. We worry about them because cracks are more likely to occur at such points.

Nitriding doesn't change the fact that a particular mechanical design acts as a stress riser/raiser. But it does reduce the effects of the stress riser. Cracks form when the stress in a material is greater than the strength of the material. Nitriding adds compressive strength of the material, thereby requiring higher stress to start cracks. The effect is the same for other surface treatments, like shot peening, that leave added residual compressive strength near the surface of the material.

As Dances said, plenty of info on all this sort of thing on the web nowdays. I kind of miss having to go to the library to dig through technical books for info, but not much. The wealth of engineering and science knowledge available on-line to anyone willing to look is incredible.

In the case of stock Commando crankshafts, you can improve their durability significantly by putting a proper radius on the output shaft, finished by abrasive polishing, and shot peening, if possible. Tuftriding after finishing the radius also adds to the cranks durability, and doesn't require finish machining afterwards, as nitriding usually does. On the output side big end journal, I don't know of much you can do to improve them besides tuftriding. The factory machining process in boring out the journal seems to create a built-in stress riser there.

Ken

' doesn't require finish machining afterwards, as nitriding usually does.'

I wasn't aware of that. I've never had a crank machined after having it gas nitrided, because there is supposed to be no change in dimension due to the process . In any case the hardened layer is extremely thin. Perhaps you were thinking about hardening the crank in a molten sodium nitrate salt bath ? I know what you mean about the boring inside the crank journals. The step does not have to be on the outside to create a stress raiser. If the internal bore was of smaller diameter, the crank might be stronger. However balancing it might then be a problem.

It is a long time since I was involved with this stuff. I only ever raced my 500cc short stroke Triumph for so many years because it had a billet crank. Because of the short stroke and high revs, wear was a problem. I hard chromed the journals in a bath normally used for plating the bores of naval guns. Had them ground to size without radius. Then used copper lead bearings from a Cooper Mini. When it was idling after a race, I could not push in the button on the relief valve, the pressure was so high. With a long stroke billet Commando crank, you might still have a problem if there is no radius on the journals and main shafts.

About the term 'stress riser' or 'stress raiser' - are we both talking about the situation where a scratch, change of dimension, or a step causes a concentration of the stresses causing metal fatigue at a particular point or area ?

not sure Al, I divorced my stress raiser after 2 years before I cracked.

@Madass: :lol:

acotrel said:

' doesn't require finish machining afterwards, as nitriding usually does.'

I wasn't aware of that. I've never had a crank machined after having it gas nitrided, because there is supposed to be no change in dimension due to the process . In any case the hardened layer is extremely thin.

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The depth of the hardened layer from typical crankshaft gas nitriding is .010" to .030", and it normally increases the journal diameter by .0005" or so. The longer the crankshaft is left in the nitrogen atmosphere in the furnace, the deeper the hardened layer, and the more the journal size grows. Gas nitriding also produces a thin, very hard "white layer" on the surface that is very brittle, with a strong tendency to develop cracks. Normal industry practice is to grind crankshafts to remove this layer after gas nitriding. In a crankshaft production process where nitirding is used, the final grinding both removes the white layer and finishes the journals to the correct size.

In the car world, the usual time for nitriding a used crankshaft is when you a are re-grinding it undersize. It's pretty straightforward to grind the journals .005" large of the desired size before nitriding, and finish grind after. That removes the white layer but still leaves plenty of depth in the hardened surface. Not sure how common it is to nitride a Commando crankshaft, but it would seem like a good thing to consider when having one re-ground. The only nitrided Commando crankshafts I've used came from Dave Nourish. Steve Maney has his crankshafts Tuftrided instead, and that process does not require any machining afterwards.

I've never had a Commando crankshaft nitrided, but way back when, I had a car crankshaft ground undersize and nitrided (Chrysler hemi engine) by a shop in socal ("The Crankshaft Company", I think), and they did the final grind after nitriding.

Ken

The old crank finally went at a local race meeting..

You're lucky it did not damage the cases!

In all seriousness, I hope you are ok and did not take anyone else out.

What are your plans?

I actually like the design of the Commando engine. However I always work on improving mid-rage power (torque) rather than top end. As the accelerations of internal components increase the forces follow a squared relationship. So high revs are really bad. I always try to keep the revs below 7000 RPM and keep my gear-changes very smooth. If you go top end , you need to make everything stronger, so it becomes much more expensive and the Law of Diminishing Returns applies. It is pretty obvious that if you can increase the revs, the motor will produce more power. However if you are racing, there is much more to winning than the blast down the straights. A good torquey motor in a frame which encourages you to get on the gas extremely early in corners, is very hard to beat. Especially if it also has the legs to get you to the ends of the straights fast enough to keep the other riders in sight.

When the engine blew it locked up instantly , highsided me down the rumble strips at around 80mph . I do not remember the accident apparently i cartwheeled , just bruises , concussion . It looks like my old Atlas motor will be reserected at some point ...thanks for asking .

I know that can happen and I simply hope it never happens to me. That crash of Doug Macrae at Daytona should be enough warning to stay away from big circuits. The trouble is that when the motor explodes there is often no warning, however a gearbox blow-up is probably worse because then everything locks up. You cannot even rip the clutch in and roll. These days I ride my Seeley like a two-stroke. I feed the throttle on, match the revs on all my gear-changes and I keep my fingers on the clutch lever.

Ashphalt Al said:

When the engine blew it locked up instantly , highsided me down the rumble strips at around 80mph . I do not remember the accident apparently i cartwheeled , just bruises , concussion . It looks like my old Atlas motor will be reserected at some point ...thanks for asking .

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Well, that's the sh*ts but at least you live to race another day. Looks like you had some decent PPE; oh yeah, and you still have all your fingers. Keep us posted when you have the time and inclination to do so.