Making a Stephenson’s Link for a triple expansion steam engine

Progress on the triple has slowed lately, partly because I am spending spare time on the Colchester lathe commissioning, but mainly because the plans for the Bolton 9 triple expansion steam engine are fairly vague and hard to interpret with respect to the Stephenson’s link reversing mechanism.  I think that I have finally got my brain around the workings of the mechanism, partly thanks to the many Youtube demonstrations, but mainly thanks to a series of articles in “Model Engineer” in 1985 -6, to which a colleague directed me. (thanks David).

The author of those articles has taken the trouble to document improvements to the original OB Bolton plans, and the improvements are much more comprehensible. (unlike this blog.)

My uncertainty was compounded by finding castings missing from the kit of parts which I had purchased.  I had taken the precaution of taking photographs of all of the castings when they were originally unwrapped, so I know that they were never there.  The supplier was not interested in rectifying the problem, so I am making the parts out of brass bar stock.

The following photos are the situation to date.

The eccentrics.  These are all split, and joined with M2 bolts.

The eccentrics. These are all split, and joined with M2 bolts.

The components of each eccentric.  The brass "halves", the bolts and the grub screw.

The components of each eccentric. The brass “halves”, the bolts and the grub screw.

The eccentric straps, also made in 2 pieces, joined with M2 bolts.  A groove is turned in each circle, and a corresponding ridge is turned in each eccentric.  All very precise and fiddly.

The eccentric straps, also made in 2 pieces, joined with M2 bolts. A groove is turned in each circle, and a corresponding ridge is turned in each eccentric. All very precise and fiddly.

Six valve rod "yokes" need to be made, but there was only one casting, so I have decided to make them all from bar stock.  The dimensioned bar stock (10x16x55mm) is seen here, with the "Model Engineer" article on the subject underneath.

Six valve rod “yokes” need to be made, but there was only one casting, so I have decided to make them all from bar stock. The dimensioned bar stock (10x16x55mm) is seen here, with the “Model Engineer” article on the subject underneath.

I will machine the yokes next week some time.   Space ships found in the Kazakhstan desert much more interesting, no?