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 widespread stress-raisers—options which by concentrating stress in small areas make fatigue failure extra more likely to happen there.

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

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


Keyways are minimize into ignition/alternator shaft tapers as a method of maintaining the rotor correctly timed to the shaft in order that ignition timing doesn’t differ. Years in the past I used to be instructed by old-timers that keys have been pointless supplied 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 wise rap with a comfortable hammer.” My expertise has been in any other case, having seen such keyless mounting slip greater than as soon as.

However the strategy 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 larger to cut back general stress.


It was as soon as widespread within the design of pressed-together curler crankshafts to present 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, except the step the place one diameter joins the opposite is fastidiously radiused and given a clean end, stress focus there causes a crack to type 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 constructing pins with floor options corresponding 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 a component 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 bother can dwell.


Many occasions you will note bolts or studs break on the root of the primary thread. That is the place the “traces of stress” that textbooks enjoyment of speaking about should funnel down from the complete diameter of the fastener’s shank to the smaller thread root diameter. That concentrates the stress at that time. One widespread technique to take away this focus is to cut 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 might work. It did—we by no means had one other failure and Kawasaki later equipped alternative studs that seemed like mine—it had reached for a similar textbooks I had.

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

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

There’s now a roll-forming course of for inner threads as nicely.


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

Wright Aero supplied its giant plane radial piston engines with multipiece solid metal crankcases however when the corporate tried to up-rate the 18-cylinder engines on the B-29 bomber of World Battle II, fatigue cracks unfold from strain equalization holes in fundamental bearing webs throughout take a look at operation at 2,600 take-off horsepower—a 25 % enhance over normal. Fatigue loves holes and discontinuities! Why do you suppose tree trunks flare gracefully on the base to type 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 type a dotted line saying “fail right here.” Normally, within the center parts of drugs shafts, the grooves are minimize solely into the projecting shaft splines and don’t give bother. 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 applied with sharp corners. A wide range of proprietary shapes exist for connecting components with clean, steady shapes slightly 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 greater 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, domestically creating welds that took a whole lot 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 components 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 completed this work, I’ve stored a wide range of failed components—a “museum of failure”—for the instruction and understanding they supply.