Design Options That Make Fatigue Failure Extra Seemingly


Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>.

Kevin Cameron has been writing about bikes for practically 50 years, first for <em>Cycle journal</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Whereas studying concerning the historical past of supplies for jet engine shafts I got here throughout a pleasant sentence that recognized many frequent stress-raisers—options which by concentrating stress in small areas make fatigue failure extra prone to happen there.

It recognized “…keyways, steps, shoulders, collars, threads, holes, snap-ring grooves, and shaft floor injury.”

Many individuals have much more engine-build and servicing expertise than I’ve, however I’ve seen these traditional failures loads of instances.


Keyways are reduce into ignition/alternator shaft tapers as a way of conserving the rotor correctly timed to the shaft in order that ignition timing doesn’t range. Years in the past I used to be advised by old-timers that keys have been pointless offered the rotor and shaft have been lapped into “intimate contact” with abrasive powder. Simply place the rotor on the desired timing and “give ‘er a sensible rap with a gentle hammer.” My expertise has been in any other case, having seen such keyless mounting slip greater than as soon as.

However the means of chopping a keyway does focus stress at sharp angles, and I’ve seen the ends of ignition shafts break off with the failure originating on the keyway. A frequent response by producers has been to simply make the shaft a few millimeters greater to scale back general stress.


It was as soon as frequent within the design of pressed-together curler crankshafts to provide the crankpin two diameters—one for the big-end bearing’s rollers to bear upon, and barely smaller diameters the place the crankpin presses into the flywheels. This simplified meeting by permitting the flywheels to be pressed on till they made contact with the bigger diameter a part of the pin.

Sadly, until the step the place one diameter joins the opposite is fastidiously radiused and given a clean end, stress focus there causes a crack to kind over time, ensuing within the damaged crankpins I’ve seen. I’ve by no means, ever seen breakage in straight, stepless crankpins. I imagine Harley-Davidson adopted straight pins after a few years of creating pins with floor options akin to steps, keyways, threads, et al. Straight crankpins don’t have any stress-concentrating floor options!


The aim of a collar is to place both the shaft (by abutting in opposition to a bearing) or an element put in on it. They’re a particular case of a step, in that they’re typically designed with a small or nonexistent radius the place the collar joins the shaft. That’s the place hassle can dwell.


Many instances you will notice bolts or studs break on the root of the primary thread. That is the place the “strains of stress” that textbooks enjoyment of speaking about should funnel down from the total diameter of the fastener’s shank to the smaller thread root diameter. That concentrates the stress at that time. One frequent option to take away this focus is to scale back the diameter of the fastener’s shank to barely smaller than the thread root diameter. With my coronary heart in my mouth, I stood on the lathe doing this to the cylinder studs of my 1970 Kawasaki H1R race engine, hoping it will work. It did—we by no means had one other failure and Kawasaki later equipped alternative studs that regarded like mine—it had reached for a similar textbooks I had.

One other supply of hassle in threads is floor end (the smoother the higher—cracks like to kind at scratches and dings) and root radius (sharp corners should all the time be averted in high-stress elements).

Lastly, roll-formed threads are greatest at resisting fatigue as a result of the stress of the super-hard rolls that kind them place the fabric’s floor in compression. Pressure is required to supply cracking, however to create any rigidity in any respect, utilized stress should first overcome this floor compression.

There’s now a roll-forming course of for inside threads as effectively.


The crankshaft design staff is aware of that every one its good work in avoiding sharp edges (akin to offering easily radiused fillets the place crankpins flare out at their ends to change into the crank webs) may be for nothing if the holes supplying oil to the crankpins are wealthy in floor defects.

Wright Aero offered its giant plane radial piston engines with multipiece cast metal crankcases however when the corporate tried to up-rate the 18-cylinder engines on the B-29 bomber of World Conflict II, fatigue cracks unfold from stress equalization holes in fundamental bearing webs throughout take a look at operation at 2,600 take-off horsepower—a 25 p.c improve over customary. Fatigue loves holes and discontinuities! Why do you suppose tree trunks flare gracefully on the base to kind the foundation system? A billion years of trial and error.

Snap-ring Grooves

Snap rings are handy for finding gears on gearbox shafts, however the sharp corners of such grooves kind a dotted line saying “fail right here.” Normally, within the center parts of substances shafts, the grooves are reduce solely into the projecting shaft splines and don’t give hassle. However I’ve seen shafts break at such grooves. As with ignition tapers, the easy remedy is to extend shaft diameter a bit.


Splines themselves, if not fastidiously designed, may be carried out with sharp corners. A wide range of proprietary shapes exist for connecting elements with clean, steady shapes moderately than sharp-edged ones. Google “Curvic Couplings.”

Shaft Floor Harm

Once I tried to rebuild a buyer’s 50,000-mile H1 crank I practically pinned the gauge on my hydraulic press—the drive required to press mainshafts out of crank wheels was a lot larger than regular. As soon as the crank was aside I may see what had occurred. All these quick miles up and down the Maine Turnpike had induced ever-so-slight relative motions between mainshafts and webs, regionally creating welds that took loads of drive to interrupt. When the items did separate I may see they have been too broken for reuse.

In different cranks I noticed that course of—known as frettage—at an earlier stage, and in a single case a fatigue crack had unfold from the floor injury to lead to a mainshaft breakage.

Why not weld the elements collectively because the drag racers do? It just about makes your crank into what the late rider/engineer Hurley Wilvert known as “one-use sort”—not rebuildable.

Like many others who’ve accomplished this work, I’ve saved quite a lot of failed elements—a “museum of failure”—for the instruction and understanding they supply.


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