The Home Machinist!

I am working on the design of a 4-4-0 based on Bill Van Brocklin's No 28. It will use Walschaert valve gear. I plan on keying the eccentric to the crank pin.

My question is as follows. Is it common to also key the crank pin to the wheel to prevent the pin from rotating and throwing the valve timing off?

Thanks,
Daris

I would. I’ve seen the main pin work loose and begin rotating on a friends LE Atlantic

dnevil wrote: ↑Wed Aug 21, 2019 9:05 am My question is as follows. Is it common to also key the crank pin to the wheel to prevent the pin from rotating and throwing the valve timing off?
Thanks,
Daris

I would be say YES.
this is more important than keying the eccentric crank to the pin,

My drivers are keyed, the eccentric is not.

is this new, being assembled..or already partially together? I am guessing that it is new based on your description.

I ask because of a recent experience with a BvB chassis and things that should have been tight press fits that were not..and basically the chassis had to be disassembled and new axles and crank pins fitted...since there is no way that it would have stayed together.

I have seen valve gear run practically forever with no keys holding the crank pin to the wheel...and others that didn't last 3 years...

The only realistic answer is that if it was fit properly, it is most likely OK if it is already together..however if it is not a good, tight fit...it may well come loose..

Keys are nice... but small square keys on 3/4" loco crank pins are tough to get to fit well.

a recommended thought would be press fit round 'key' pin in from the back...put in the crank pin, then drill / ream / press fit a small 'pin key', then assemble the driver to the axle

Taper pins work nicely too. Whatever you do, don't drive a pin into a blind hole. I had to fix that for someone that did and it was a real pain to get it apart.
Fred

In 2016, a "dutchman" on the Rutland main crank pin fell out while on a road trip. Was hell trying to move the loco up a long grade back to the yard. When the loco was up on a steaming bay, the main and side rods were removed. Had a crank pin in my hand...and a hole in the driver...
Where did the dutchman go? DOWN...into the ballast...never to be found again! Had no measuring tools to determine the pin size. Made a trip to the corner hardware store to get several drill bits in the 1/8" diameter +/- range and some Permatex thread locker products. Ground off the butt end of an 1/8" drill bit and made a temporary repair for the loose crankpin. Enough or a repair to finish the weekend running no-load.

Back home, the loco was blocked up. All the drivers were dropped. The crank pins pushed out. A light bore cut was made in the drivers for each of the crank pins. New crank pins and hub cabs were machined, heat treated and ground to size.
The original eccentric was "locked" in place with a set screw. Was convinced this was not necessary with a proper fit.
The new crank pins were pressed into their respective drivers. Don't believe the eccentric was bored. There was just enough play on the crank pin for a snug fit to allow adjustment before tightening the bolt.
End result...no more set screws, dutchmen or other means of locking the eccentric in place were harmed in this procedure. The eccentric has held tight for several years without slipping. Just saying... Carl B.

Thanks to all who replied.

Bill Shields, this design is based on Billy Van Brocklin's drawings I obtained from John Kurdzionak of Friends Models. Billy's drawings do not specify keys. However, I'm machining the parts on a Haas VF-0E CNC mill, and want to use the accuracy of the mill to place the keys so that there is no guesswork when pressing the crankpin and fitting the eccentric. Keying the eccentric is suggested by Joe Nelson in his book "So You Want To Build A Live Steam Locomotive." I read somewhere else (perhaps in Nelson's book) that crank pins can rotate after time as well.

Carl B, how did you properly adjust the eccentric location without the aid of keys?

Did I mention this is a 1.5 inch scale model? Here are photo's of Billy's version:

http://ibls.org/mediawiki/index.php?tit ... in#Loco_28

Regards,
Daris

Just because BvB did not use keys does not mean that they are not needed.

Like many people, I have locos that have run for years without them and have fixed a dozen or so for people that should have had them. I even have a 1-1/2" loco with the drive wheels glued on to the axles...no keys...works a charm...others say that I am crazy and should do things the 'better way' and press fit them...

irrespective of how anyone mills parts, I would recommend they check everything after assembly prior to fitting keys...

Please don't get me started on JFN's book...

there are a lot of good ideas in there...and there are some really not-so-good ideas.

A book is a lot like today's internet...

Just because it is in a book does not mean it is necessarily a good idea.

However...the idea of using keys is difficult to argue with.

The Brits had an interesting idea where they made the end of the crankpin 'square' and broached a square hole in the eccentric crank..so it could only go on one way..

of course, if the pin was not true to the driver...then there you are.

I fixed one like this a few years ago where I put the driver assembly in the mill and RECUT the square so that it was truly at the correct position WRT to the axle and remade the eccentric crank...all this because the pin was 'pinned' to the driver with a hardened roll pin that was a real bear to consider removing....

the person who had built it had never considered the need for 'future maintanance'...

dnevil wrote: ↑Thu Aug 29, 2019 4:28 pm
Carl B, how did you properly adjust the eccentric location without the aid of keys?

Well Daris...don't burn me at the stake for this (or anybody else for that matter). Having photos of the eccentric locations before the drivers were dropped and the old crank pins pushed out provided a starting reference point. Once the new crank pins were pressed in place, no longer had the old set screw "key" hole to use for a reference.

Used a Sharpie to draw a line on the end of the crank pin to coincide with the eccentric slit at the approximate setting. This was the starting point. Based on the locomotive performance and sound, would loosen the clamping screw, shift the eccentric forward or back in small increments and re-tighten the clamping screw. Continued to do this until the locomotive "sounded" best. Wasn't any more qualitative than that.

So far...so good! Carl B.