Re: “Son, It was your idea to be a railroader…” (the inside the cab story of #168’s incident)

I'm not Dave, but here goes nothing.

The above drawing is basic Stephenson Valve Gear.

Essentially, there are four eccentrics (cams) mounted on the main driver axle. Two eccentrics control the valve motion on each side. One is known as the "forward eccentric" and the other is the "backward eccentric". Wrapping around the eccentrics is the eccentric strap. It is in two pieces so it can be removed and installed over the eccentric cams. The front of the eccentric straps are the eccentric rods (sometimes called "eccentric arms") that connect to the link. The link is suspended from the hanger. When the locomotive is moved from forward to reverse, the link is moved up, while the link block stays stationary. The block is connected through rocker arms to the valve stem. When the link is in the bottom position (and the block is in the top of the link), virtually all the movement done by the link is created by the eccentric rod, strap and cam attached to the top of the link. As this is the position for forward motion, this portion is called the "forward eccentric" parts. The eccentric cam, strap and rod attached to the bottom of the link provide the motion for backward movement, so they are the "backward eccentric" parts.

As mentioned above, the eccentric straps are in two pieces so that they can be installed around the cams, the two halfs are held together by fitted (tapered) bolts. The holes in the two halfs of the strap are also tapered to make sure the bolt has a good, tight fit. The bolt is double-nutted with a cotter pin through the bolt to keep everything in place.

Appearently, the cotter went missing, and the nuts came loose, the the strap began to open up. This induced lost motion into the valve timing events making 168 run lame ("out of time"), On the way back to Chama running light, the nuts came off completely, the strap opened up and it fell off the bottom of the cam. The back half of the strap, then turned upside down and caught on the ties, bending the lower bolt and generally making a mess of things.

Not mentioned, is when the reverse lever is moved back toward center, both forward and reverse eccentrics combine to make the motion for the valve timing, shortening the valve stroke and cutting off of the steam admission. Also when in full forward or reverse motion, the opposite direction eccentric does provide a small amount of motion to the valve events, which further complicates valve setting.

When I ran a vacation relief gig on the LK&P is Hawaii in the 1980's I had the eccentric strap come apart on one engine. I noticed it starting to go lame, then came to a down hill portion and let her drift. Coming out of the sag, I hooked up the Johnson Bar, opened the throttle and one side of the engine was completely dead. I quickly stopped and poked my head in between the frames and found one of the straps opened up, and the nuts missing. Luckilly, the valve gear was being held up by the brake rigging and nothing fell to the ground. Also, this managed to happen right next to the shop, where Rich Braden and this gang of worthies came to the rescue. A two new nuts and a cotter pin were installed and I was back underway in a few minutes.