While 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 surface damage.”
Many folks have much more engine-build and servicing expertise than I’ve, however I’ve seen these traditional failures loads of occasions.
Keyways
Keyways are lower into ignition/alternator shaft tapers as a method of preserving 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 had been pointless offered the rotor and shaft had been lapped into “intimate contact” with abrasive powder. Just place the rotor on the desired timing and “give ‘er a smart rap with a soft hammer.” My experience has been otherwise, having seen such keyless mounting slip more than once.
But the process of cutting a keyway does concentrate stress at sharp angles, and I have seen the ends of ignition shafts break off with the failure originating at the keyway. A frequent response by manufacturers has been to just make the shaft a couple of millimeters bigger to reduce overall stress.
Steps
It was once common in the design of pressed-together roller crankshafts to give 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.
Unfortunately, except the step the place one diameter joins the opposite is fastidiously radiused and given a easy 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 resembling steps, keyways, threads, et al. Straight crankpins don’t have any stress-concentrating floor options!
Collars
The objective of a collar is to place both the shaft (by abutting in opposition to a bearing) or a component put in on it. They are a particular case of a step, in that they’re usually designed with a small or nonexistent radius the place the collar joins the shaft. That’s the place bother can dwell.
Threads
Many occasions you will notice bolts or studs break on the root of the primary thread. This is the place the “lines 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 widespread strategy 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 will work. It did—we by no means had one other failure and Kawasaki later provided substitute studs that regarded like mine—it had reached for a similar textbooks I had.
Another 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 averted in high-stress components).
Finally, roll-formed threads are greatest at resisting fatigue as a result of the strain of the super-hard rolls that type them place the fabric’s floor in compression. Tension is required to provide cracking, however to create any stress in any respect, utilized stress should first overcome this floor compression.
There is now a roll-forming course of for inside threads as effectively.
Holes
The crankshaft design crew is aware of that every one its good work in avoiding sharp edges (resembling offering easily radiused fillets the place crankpins flare out at their ends to turn out to be the crank webs) will 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 solid metal crankcases however when the corporate tried to up-rate the 18-cylinder engines on the B-29 bomber of World War II, fatigue cracks unfold from strain equalization holes in important bearing webs throughout check operation at 2,600 take-off horsepower—a 25 p.c improve over commonplace. 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 here.” Usually, within the center parts of substances shafts, the grooves are lower solely into the projecting shaft splines and don’t give bother. But I’ve seen shafts break at such grooves. As with ignition tapers, the straightforward remedy is to extend shaft diameter a bit.
Splines
Splines themselves, if not fastidiously designed, will be carried out with sharp corners. Quite a lot of proprietary shapes exist for connecting components with easy, steady shapes reasonably than sharp-edged ones. Google “Curvic Couplings.”
Shaft Surface Damage
When 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. Once the crank was aside I may see what had occurred. All these quick miles up and down the Maine Turnpike had precipitated ever-so-slight relative motions between mainshafts and webs, domestically creating welds that took quite a lot of drive to interrupt. When the items did separate I may see they had 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 harm to end in 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 type”—not rebuildable.
Like many others who’ve finished this work, I’ve saved quite a lot of failed components—a “museum of failure”—for the instruction and understanding they supply.
Source: www.cycleworld.com