The fillet determines the strength of the crankshaft
The fillet is a concave easing of an interior corner of a part used to reduce stress concentration (red arrow in picture). A fillet is where stress is concentrated and where one should examine carefully for any cracks or damage.
Examination of fillets:
- Any damage to the fillet such as tooling marks, corrosion pitting, "circumferential scoring",
- Fatigue cracks,
- Correctly formed radius (especially important after machining.)
Generally, the more rounded the fillet (greater radius) the lower the stress concentration. Most fillets, like the picture above, use a circular profile and can be measured using a radius gauge. A circular radius fillet is the most common but there are other fillet shapes that border on the radical.
Fillet radius is often limited by space considerations or nearby parts. For this reason high fillet stresses are tolerated and special techniques used so that the metal can tolerate the stress concentrations. Cold forming, shot peening, nitride hardening are some of the techniques used to introduce benificial compression stress into the metal. Grinding or removing metal from the fillet, as one might do during crankshaft journal grinding or polishing, acts to reduce the benificial compression stress and weaken the part.
Inspect fillets carefully for any corrosion pits and tool marks (scratches). The mechanic (and more so the owner of the part) often judges the severity of corrosion pits and scratches based on their size. This is incorrect; increases in local stress depends solely uon the SHAPE of the hole and has nothing to do with the size. The weakening effect of the scratch or pit has very little to do with the amount of material removed. The mere existance of a pit or scratch in a critical fillet is enough to weaken the part. The part should then be removed from service until and if an engineering determination is made that might allow the part in the weakened condition to continue to perform safely.
The crankshaft might operate fine for a period of time in this weakened condition but because of a lowered fatigue strength might fail at some later date in service.
Red arrow points to the fillet
Yellow arrow shows fatigue "beach marks"
Fillet Shape Optimization
The circular or elliptical profiles for fillets are easy to machine but are not the optimum shape to reduce stress concentration. There are other shapes that appear quite radical to the untrained eye, such as undercutting the fillet. Shape optimization can significantly reduce the peak stress, thus increasing the fatigue life of the part. Expect to see more of these in the future.
If you don't respect the radius, the tube will break when the nut cuts into the tube.
Fatigue crack at fillet radius
Checking radius of a tube flare with a comparator
Stop drilling is an example of reducing stress concentration by changing shape.
But if your drill leaves even the slightest scratch in the hole then you haven't reduced the stress! Ream the hole or "coin" it by plugging it with an expansion rivet.
"...fatigue cracking in the No.4 main/No. 4 connecting rod crankshaft web initiated as a result of damage created in the fillet of the No.4 main bearing journal. This damage comprised circumferential scoring created through contact between the No.4 main bearing insert and the crankshaft during engine operation." ATSB TRANSPORT SAFETY INVESTIGATION REPORT Technical Analysis - 200502231
Most shafts have fillets. This magneto shaft is broken at the fillet.
