Set Screw Advice

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Well-Known Member
HMEM Supporting Member
Dec 29, 2012
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Indianapolis, Indiana
I could use some advice on a set screw problem. I'm building a marine steam engine. When I put the eccentric valve assembly and tightening the set screw on the crankshaft (many times during the build to see movement) sometimes something binds a bit and the eccentric moves on the crank. This will cause some scoring on the crank and now my good fit won't allow the eccentric to come off. I'm able to get it off, take some light sandpaper and remove the scoring and everything fits smoothly. I can't put a flat or anything else on the crank as yet as I need to get to the finish of the engine to figure out where to adjust the valve eccentric for proper running. Any advice?
I put a small piece of SOFT aluminium wire in the hole first, this deforms under the pressure of the screw and does not mark the shaft. Depending on the engine I may leave it in or mark the eccentric's position so a flat can be machined once the best position has been found.
Most of my set screws are from McMaster in 18-8 stainless in a "non-marring oval tip". They usually hold well enough for prelim work, and might leave a dent, but they do not raise up a scratchy burr.
If you only need one, I bet you could carefully grind and buff the tip of one of the set screws you have on hand.
I know the problems you are having. Bin there. You cannot rely on a smooth surface friction fit with no marks in the shaft for anything but light fitting. Even light wrench torque on a good hard set screw will cause burrs. Relieve the shaft .010"-.020" as advised above. The standard cup point set screw is supposed to dig into the shaft to grip and hold. SS set screws are too soft. Use black steel alloy set screws from McMaster Carr. During fitting work don't tighten to the final fit torque. Use two set screws at 90 degrees apart on each item to be locked on a rotating shaft . On small set screws like #2's do the final tightening so you bend the long end of the allen wrench to about 30 degrees with a new or reground wrench. That is your final tightening torque indicator. Also, when you are all done fitting with no binding replace all your set screws with new unused set screws and tighten to final torque. Check them occasionally if you run your engine frequently.
My headstock has screw-on chucks that are secured with a setscrew. I have put a little brass tip at the end of the setscrew. It does the same work as the earlier mentioned aluminum or soft solder, but stands the test of time better as a chuck has to be changed rather often.
I have found that soft plugs or brass tipped set screws do not have sufficient "bite" to hold a cam or eccentric in place long enough for anything other than set -up. Under-cut shaft, 2 cup point set screws properly torqued ,90 degrees apart and the chance of slippage is minimized.
Hi Colin, sounds like you need a Cotter pin for that much torque? Grub screws are only for very low torque applications. On some models/applications, I replace them with "plumbing" olive or double cone compression fittings where I know there is some torque to be transmitted. e.g. a drive pulley, flywheel, etc.
Maybe not everyone appreciates how much torque oscillation occurs on reciprocation engines, so things like flywheels have to take the torque one way from the power stroke, then deliver it back to the shaft on the "between power" part of the cycle. So the compression fitting will see maximum torque applied and released every cycle. This oscillation will often exceed the durability of a grub-screw.... unless made into a shear pin of course, by some undercut.
Additionally, grub screws directly acting on a shaft have a point contact. Use of a brass insert or other makes a full surface contact which is better able to transmit torque without exceeding the coefficient of friction, due to the significantly larger area of contact pressure.
Output Pulleys, gears, etc. see the full power max. torque to "no power/torque" oscillations from the crank.
Cotter pins, shear pins, keys, splines, tapers, compression fittings, etc., are all designed to withstand more torque by higher friction of the connection, or metal in shear. Especially oscillating torque.
But a grub screw onto a flat on the shaft will probably operate an eccentric on many models as there is low friction, low acceleration (sinusoidal, not peaky) and low mass generating the oscillating torque, not the power of the engine. But you need to know exactly where to put the flat!
Hope this helps?
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Keyway and key does not tend to slip either ;)

In post #1 , dadviyat is looking for a method of holding the eccentric in place long enough to time the engine and be able to remove it or indeed turn it to the final position before finally fixing it in it's optimal position. Surely this obviates cotter pins ,taper pins or keyways in holding the eccentric in place while setting up. There also may not be room in his design for tapered collets or nut & olive retainers. Unless there is a great amount of torque required, 2 cup point screws of the correct size in a machined groove should be enough.
That's wht I suggested the soft metal option in post #2, it's worked for me on over 40 steam engine eccentrics and from the other posts similar works for a lot of others to.

As steamchick says there should be very little load on an eccentric so little chance of it slipping with the soft metal between shaft and screw

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