Set Screw Advice

[Steamchick]

All I am really asking is "why" does the grub-screw not hold the eccentric in place while the initial running and adjustment is taking place? - If because there is is too much torque required, or the oscillating torque is fatiguing the interface between grub screw and shaft, then no amount of fettling will satisfactorily acquire enough torque to overcome the failure of the contact point under stress. BUT changing to a large surface contact in whatever form should provide more torque to be transmitted.
Maybe it is Mass, Acceleration, or simply Friction that is generating torque high enough to cause the slippage/contact point failure... in which case reduce what is possible (Friction?) or increase the surface contact area between screw and shaft - with something that will conform to the shaft surface?
So while many have used grub screws and had no problem, and many have done other things to increase the torque capability of the joint, Davidyat has a problem, and I am proposing there is a root cause that simple grub screws are not resolving. So as adding a "fixed point" on the shaft is not what he needs during the initial tuning phase, that is the final fix after the tuning that he is struggling with.
K2

 

[oldengineguy]

I can't imagine why you would want to mess around with blobs of solder or tiny brass balls especially with #2 screws when it so easy to machine a groove when making the shaft. You can use the proper screws for set-up then just tighten them for the finished job. If you ever need to take it apart later there will not be a burr on the shaft to prevent the pulley/ cam/ eccentric from sliding off easily.
Colin

 

[Steamchick]

Hi Colin, unless I am mistaken, Grasshopper said the eccentric moves using the current grub-screw. A shaft with flat ended screw tightening onto the shaft has a line contact. This is not good for transmitting torque. A sharp pointed grub screw is also likely to damage the shaft, as Grasshopper has experienced, making difficulties adjusting or removing the eccentric.
He also has a problem that he wants to be able to fine tune the valve timing, by moving the eccentric, before doing anything like machining a flat or drilling the shaft to locate the eccentric at its final position.
A soft piece interposed between the grub screw and shaft will deform to engage the shaft with a much larger surface area than a line, thus being capable of a significantly increased torque transmission from shaft to eccentric, without slippage. I have read this in books, and others have experienced it, so I hope this helps you understand why we are offering these various solutions to Grasshopper. One solution I found successful was a small circular piece cut from a section of O-ring. It made tuning the eccentric position much easier than just the grub-screw on the shaft. - I think the O-ring was 1/8" diameter material, cut to give a disc 1/8" diameter x 1/16" thick, beneath a 4BA grub screw, onto a 5/16" shaft? (But that was 20 years ago).
As the application of concern is a marine engine - for a boat? - with probably a reversing gear, the friction or other loading on the eccentric may be higher than you have experienced in your models. I am sure Grasshopper will work out a solution from the various options proposed.
Cheers,
K2

 

[Rustkolector]

Set screws do have limitations. Binding can easily exceed the limits on non marring small set screws. Other methods such as Dykem should be used first to detect the source of binding. Never try to "run-in" any engine with serious binding issues. Always finish up with new set screws once everything is free rotating and proven. This website was my set screw application reference

https://www.atlanticfasteners.com/tech-tips/set-screw-point-styles-and-their-use

 

[Bentwings]

not a bad idea I’m goingvto apply it shortly as I’m working with eccentrics on my little steamer
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

 

[davidyat]

I want to thank everyone for their advice. This engine will not power anything. I just like to build them, take a video of it running and move on to my next project. Sometimes I forget something, "When all else fails, read the instructions". I've been engrossed in making parts. So, I looked at the assembly instructions on the timing sheet. You can see that they tell me where to align the 2 eccentrics, at 9:45 and 2:15. OK, take out the dividing head and machine a couple of flats on the crankshaft at those locations. The air valves have a threaded hole on the bottom and a screwdriver slot (which I can access) to raise or lower the valve (it does have a provision for a jam nut to hold it in place). Here is a scan of the instructions:

 

[Bentwings]

Regarding set crews / grub screws I’ve had nothing but trouble with them right from my beginning engineering career. . Early on there was machine in the shop that was full of them . Not a small motor driven thing either it was a big vibrating thing with heavy pounding sounds . I was given Theo task of maintaining the thing and supervising the crew. The very first day I saw all the set screws. My report said there was going to be issues with slipping pulleys and the eccentrics s. It was poo po Ed by the chief then shortly after a big flywheel came loose the set screw tore up the shaft very badly ultimately the big thing came to a screeching halt . I YHINK nearly every set screw either slipped or had been loose the main drive shaft was trashed a couple eccentrics had worked loose and slipped . The whole thing was in big trouble . I had predicted this there were no flats or landing spots for the set screws. Turned out this was not the first time the cash had happened. So my job was to “ fix” the ****** thing . So I set about changing all the parts on the shaft to be s to QD hubs with jkeyways some had to be custom made “ beginning cad” some I used split double clamp modified shaft collars one of the big flywheels was almost totaled out but I had a custom collar made that clamped both the shaft and flywheel hub the shaft was scrapped snd re made. Once it was all done everything was positively located and mounted plus easy to service. It had a multi vee belt drive that I converted to tooth belt drive there was a shaft coupler that I converted to a gear coupling . It started and ran smoothly with almost no real vibration . I was there for almost 10 years andvitvwascstill running when I left . I can’t stand set screws . I already had issues with my own steamer that I’m still working on Ive had just endless interruptions so progress has been slow . We have had almost record snowfall and now cold weather my son finally got the lathe back together so I’ll be able to have him make a few items . It’s a long ways to his shop so communication is hard I send stuff to be made then he sends parts back. I finally got my laptop connected to the RV so I can see a larger screen but with lower resolution. As I’m getting back to cad now too . It’s been a long time so I’ve forgotten things so there is scre learning curve .