Hi all,I have a Key brass steam loco with a problem.The screw that holds one of the rods to a wheel unthreads when the train moves forward. See the attached photo.I can rethread it if I run the wheels backward. It seems to me that this is because the head of the screw is soldered a short arm that goes around the screw as the wheel turns in one direction or the other. I'd blame the soldering job, but the other side seems similar, and yet the screw is not being turned in that case.I don't want to trash it, and I'm handy with soldering (if that's what's needed), so I'd like to try and fix it.Help!!Thanks in advance,Scott G
A small bit of white glue on the screw threads will act as a thread lock. carefully done, superglue will do the same. I dont recommend actual thread lock since that will hold to well. if you nee to take the piece off, the threat lock will cause tyou to strip the head. Beeswax is another common thread lock for something like that.
Shane
A pessimist sees a dark tunnel
An optimist sees the light at the end of the tunnel
A realist sees a frieght train
An engineer sees three idiots standing on the tracks stairing blankly in space
Scott,
Thanks for including the picture, it helps greatly. The partial fix is indeed to secure the screw/post to the driver. As mentioned, Loctite (or its commercial equivalent) is probably overkill and might might make future maintenance more difficult. Same for superglue (AC). White glue, maybe. Maybe even a tiny (single?) wind of teflon pipe tape would snug the screw in place. To me, you'd do better to snug it than to chemically bond it in place - it has to remain purposefully removable for maintenance.
But don't forget to address the gorilla in the room: something is gripping this post to rotate it out of its socket. That's your first issue to address. And it's something that does not exist on the other side. And it's also something that's creating unnecessary friction, and therefore, drag.
There are four things in contact with this screw/post if I'm seeing it in the picture(s): the driver, the tie rod, that other (black) rod, and the outer connecting arm that's the end of the rocker assembly. Toss out the driver as it's not supposed to rotate against the screw anyway. As for the tie rod, it looks even sloppier than it should be as the diameter of the hole is much larger than the diameter of the post's shaft (should there be a tiny bushing there to eliminate all the slop? What's the other side look like?). So that's not grabbing it.
That leaves the outer two items as where the binding is occurring. Fundamentally, that's where your problem probably lies. Loctite, white glue or pipe tape is only a fix (Band-Aid) that doesn't address the real issue at either of these two locations.
Some very careful comparisons with the relative movements of other side's gear will assist in resolving this permanently.
Attuvian John
I think the large hole in the side rod is because it's a one-piece rod. To 'bush' that larger hole, he'd need to ensure all his drivers were in consistent quarter.
I'd bet that the issue is primarily binding in the valve gear, and although I only see picture #2, I would speculate that a major part of the problem is a bent eccentric crank. The part that the screw head seats against has to be both flat and true to allow proper tightening, and also has to be parallel to where the rivet attaches to the eccentric rod. Any binding will exert a lever arm effect on the screwhead, which is probably why running in reverse tightens it up again.
But this still leaves the possibility of distorted threads or other issue in the driver center, keeping the screw from being tight enough to 'resist'. That is where the 'glue-threadlocking' solution would come in, and you could test by temporarily swapping the screws on both sides to confirm the threads in the driver center are the issue.
There is an amusing potential 'solution' -- it's an awful kludge, but I have to mention it. Normally you can't 'key' the eccentric crank directly to the driver, because the side rod bearings are in the way. But with that laaaarge clearance hole in the single-piece 'siderod', you could use a short piece of relatively thin-wall tubing to make a 'bush' around the screw with a little roughness or even 'teeth' on both ends, that would lock the crank directly to the driver center when the screw is tightened down...
Jim
Soo Line fan Scott, The reason the screw is loosening on the left side and not the right is the direction of the wheels. The right-side drivers turn clockwise going forward and the left turns counter clockwise. And we all know that most screws are right hand threads which are loosed by turning counter clockwise. Laptop screws have Loctite preapplied and they always come apart. I would apply a small amount of purple Loctite and let it dry on the screw and then install it. This way it’s not really bonding the 2 threads together. BTW, if you ever run into an instance where the wrong type of Loctite is used and you need to take something apart, use a pencil type soldering iron and heat the screw head.
Thanks for the added perspective, Jim.
And Merry Christmas, all.
John