Eccentric Rods, Expansion Links, and Rocker Shaft

Eccentric Rods

The Shay uses an eccentric-driven Stephenson’s valve gear arrangement allowing engine reversing as well as permitting manual cutoff of steam admission to utilize the natural expansion of the steam, using its own energy rather than continuing to draw from the boiler.

Each eccentric strap is attached to the expansion link by a flat stainless steel rod.

Drilling eccentric strap mounting holes in 1/8″ thick stainless steel flat stock

The final widths of the upper and lower ends were machined and then the centers were tapered to meet. The lower ends were also shaped to fit the eccentric straps.

Test fitting the rods to the eccentric straps.

In order for the eccentric rods to line up with the expansion links above them, slight bends were put in the lower end.

Using a bender to accurately place bends in all four eccentric rods.

Completed bends. The upper expansion link holes were eventually drilled but I forgot to photograph that process.

Rods attached to the eccentric straps showing how the slight bends help to line up the upper portions to the expansion link.

Expansion Link Assembly

The curved and slotted expansion link pivots on the valve stem cross head via two eccentric eccentric rods. There is one expansion link assembly for each cylinder and both must be identical.

First came the pivot brackets…

Slots were milled in two pieces of steel at the same time.

Completed expansion link pivot brackets.

Next, came the fabrication of the slotted expansion links. Because the radius of the expansion link curve is greater than my 10″ diameter rotary table, I affixed a 3/4″ thick piece of aluminum to the rotary table and indicated it off the center with scribed lines and spot drill marks in case I need to remount it in the future.

Scribed arc indicating the center of the expansion link. It’s out there!

To ensure both expansion links are exact duplicates, I stacked two steel plates in position over the scribed arc. A couple of homemade angle clamps did the trick for one end.

I then drilled and tapped holes for bolts to provide additional hold down capabilities. These bolts will also secure everything in place as the outside shape is milled away and the angle clamps become separated.

Once the two plates were sufficiently mounted, I commenced with milling the curved slot.

I placed some high visibility duct tape on the rotary table degree marks to highlight the 12 degrees of rotary movement needed. This made it easy to repeatedly hit the ends of the milling path as the cutter went deeper.

Next came the outside shape of the expansion links…

This is where the additional bolts came into play, holding down the pieces as the bottom of the links were shaped.

Upper Rod Ends

The eccentric rods are attached to the expansion links via stainless steel rod ends. These were ganged together whenever possible to ensure they were machined identically.

Eccentric rod ends test fitted to their expansion link assemblies (two of them actually will get flipped over).

Expansion Link Block

A bronze link block rides in the curved slot of each expansion link. These blocks need to be machined with the same arc properties of the link, so the aluminum block fixture on my rotary table was put to use again.