21 Aug 13 12:32
"If the pin is very loose in the hole, then elongation would take place."
I know standard calculations proceed along those lines, but the most ideal doweled joint I can think of existed in aircooled VW and Porsche 4 cylinder engines from yesteryear, and it is far from immune to hole elongation when asked to endure unintended dynamic loads. VW used 4 hardened 8 mm dowels in the joint between the (much better then mild steel) flywheel and crankshaft. Porsche used 8 dowels. I think the diameter the the dowels were installed on varied.
On Porsche 912 it is about 1.5 inch/38 mm "bolt circle". The dowels were an interference fit in the crank, and essentially line-to-line in the flywheel. The entire flywheel/crank assembly was clamped by a central large "bolt" (m 28×1.5 ?) torqued to 350 lb-ft or more.
A Porsche 912 engine is rated around 120 lb-ft of torque. There is likely almost 4X greater peak torque, and ~100% torque reversal at the crank flange in operation.
A quick calculation suggests the nominal mean loading on each dowel in that very carefully fitted joint would be ~ 240 lbs, which over the dowel's 0.247" "shear" area would result in less than 1000 psi loading. During each cycle there will be about 4000 psi peak loading. The carefully located and fitted diameters should result in the best attainable load sharing amongst the dowels, and the deeply embedded dowels with very small chamfers on the holes should create about the best commercially attainable purely shear loading.
The attached picture shows the typical damage that will result not many miles down the road when the gland nut is inadequately torqued after clutch service. A brand new dowel is inserted in the now wallowed out hole. By hand the dowel can lean and wobble, and several thousandths of an inch gap is visible between the dowel and hole entrance. This one was caught fairly early. I do not know the condition of the dowels.
I'm sure the failing process is complicated, probably starting with micromotion between flywheel and crank, then micro wiggling of the dowels creating clearance leading to impact loading. With thousands of engine revolutions per mile and 2 power strokes per rev it would only take several hundred miles to accumulate a million cycles, although not necessarily many at full load.