Keep going Bob, I will chuck-in ideas as I think of them, but you have made what seems to be a very good con-rod!
- Just odd ideas (from what I remember from when I worked on Engine design years ago!):
The con-rod-bolts. What material are they? They "look OK", but as an engineer, that is where I always see a big "CAUTION" sign! The tensile strength of the con-rod bolts are severely stressed, and fatigued, so need to be a VERY robust design (usually) to cope with the stresses and resist fatigue. Typically, the endurance limit is less than half the tensile strength of a bolt (the single cycle failure point). EVERY corner of the bolts is usually radiused with quite a large radius, unlike simple "bought" bolts. Never having design the bolts, I cannot advise much, but on car engines it is common for the bolts to be 120ton steel, often forgings, heat treated, and very complex shapes. At 2000rpm (a pretty slow model?) the rods perform 120,000 cycles in 1 hour of running, or 1 million cycles in little more than 8 hours running. Of course a car engine/industrial engine can be expected to perform thousands of hours running, and typically up to 6000rpm or more. (Motorcycle engine s easily double that). So lifetimes are designed for hundreds of millions of cycles.... for reliability. Can you get some high tensile hex. socket cap screws with solid shanks through the most part of the rod? (I can only see the heads, not shanks, of your bolts) - preferably installed in "sized" holes (reamed?). (I guess you have done so?). And another idea... the washers under the bolt heads are normally precision ground so they do not create a skew face for the bolt heads to sit upon. The stress raiser of even the tiniest "out-of-square" between the axis of the bolt and the face under the bolt-head can dramatically reduce the life of con-rod bolts, and is OFTEN a cause of failure of the bolt-heads. (Ask guys that race engines!). Many engine makers specify replacing big-end bolts with brand-new parts EVERY time that the rod-end is dismantled, and re-assembled. It is that critical. Models mostly survive because they have very short running periods, at slow speeds, and low loads. Except aero-engines.
I guess there will be some simple maths somewhere on the web to determine bolt sizes against your design? I'll have a look when I get some time. (DIY at the Daughter's house today!). AND often the threads are special rolled threads, as machined threads are simply too prone to early life failure at the "pre-engineered cracks" in machined thread roots. Nuts also need to be as good as the bolts.
Bolted Joint_R1.doc (live.com)
- may be of interest/relevance?
Also, based on root diameter of thread, the #6 x 40 is about 13% "stronger" in tension than the comparable #5 x 40. (88% of tensile stress), because of the increased thread root area... Bolts threads should be as "perfect" as you can get them, lubricated and torqued accurately to the pre-load so they are never slack in the dynamic loading on the rod. (allowing of the coldest start-up - differential expansion). Best Way to Improve Fatigue Resistance of a Bolted Joint - Nord-Lock Group
may be of interest?
Well done so far with the rod! I'm sure your expertise (and CAD-CAM) has set datums that work, (as otherwise you would have a lot of scrap!).
Most interesting from my chair.
Thanks for posting your process and thinking for making the rod.