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Hi Chris, I did a quick calculation for shear force to strip #1-72 brass screws. The shear /strip force could be 25.2 to 62.9 lb depending on hardness of the brass and the brass grade. Take these figures with a grain of salt though - they are based on the theoretical full thread area, but in real life the thread area is probably not 100% of theoretical perfect, nor will the tapped hole be. Even at the low number 25 .2 lb per screw I doubt that the screws in your eccentrics would ever see 25.2 lb load each. The friction load on your small area valve in the engine is probably only a few pounds and the eccentric being a cam will ease the load application on the screws. They aren't loaded like say conrod bolts would be.The 932 bronze will be considerably higher strength than the brass screws, so the screws would likely strip (if anything is going to) before the bronze strap thread. You could use replacement mild steel or stainless screws if you ever did strip the brass ones. Just food for thought. I think the brass #1-72 screws will be fine, and if not, you could substitute mild steel or stainless ones and they would be more than enough. Don't overtighten the screws though. Snug and loctite blue will do.
Hi Chris, I calculate the area of the thread itself for a given thread (OD area - core dia area) and multiply by the yield strength of the material to get the shear strength prediction. I find the most accurate strength data comes from matweb.com but you can usually find it for various materials by googling, if you are not registered on matweb. I use the lowest spec'd yield strength in the range rather than the ultimate tensile strength because when screws strip, the material is at yield not at UTS, and by using the lowest spec'd number the result is not optimistic, but practical. The thread area of the screw is the part under shear load, not the core of the screw, so strength before strip figures are at the lower end of the range for best certainty they will be strong enough in the worst case. If the actual screws are much stronger, so be it, no harm done.For brass in particular, there are so many variations in the quality and hardness nowadays it is hard to predict strength, so I use figures from a well known threaded insert and hardware supplier for the range of yield strengths for the brass they use. They state the yield strength of their brass at 18,000 to 45,000 psi. For 932 bronze (sae 660) I usually use 20,000 psi yield for calcs, and for steel 50,000 psi yield (in case the screws are Chinesium). (Good quality name brand steel screws can be up to 180,000 psi yield just FYI)Hope this info helps.
Just FYI Chris, I don't like brass chips in my smoked meat on rye sandwiches, Only mentioning it as I see you found the deli slicer. "Morty - two sliced extra thick, no pickle, Loewensenf on both!" Oh sorry, wrong deli......
Hi Chris, the expanding mandrel will work well, but if you wanted to avoid the work of making a special one for the straps, there's another alternate method I have used in similar circumstances. If you clean the straps well and superglue them to a plywood circle held in the chuck, they can be faced with very light cuts then gently pried off with a sharp wide blade. Two sided tape can be used too. Also heard of laquer being used in a similar way to glue with a metal disk by clockmakers. I've also soft soldered parts to plates or disks for further work, then melted the parts off, but this requires solder cleanup. If the soldered faces are not visible afterward that might be fine, but if the faces show, the glue / tape/ laquer methods might be better. Just food for thought. Lots of ways to do things!
Hi Chris good to see the engine coming together Willy
Hi Chris, nice progress, waiting for the next engine parts.