Making a Stephenson’s Link for a triple expansion steam engine
by John
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 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.
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?
johnsmachines 
Hiya, First time to this site, you certainly know your stuff! Question, why is the HP eccentric set to 30 degrees offset when the IP and LP are set to 15 degree offset, it does make it tricky to time the valves for fwd and rev, when ones good, rev or fwd, the other direction is off time (ie, when fwd selected, the valve block position to all ports is good, but when rev selected the valve HP block opens the ports at the wrong time. Thanks and looking forward to your response.
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Hi Chris, and welcome to the site. I am not really understanding your question. Which post refers to the 15º and 30º offsets? I am sure that the offsets are the same for each cylinder, and for FWD and REV for each cylinder. And the engine runs well in each direction. Sounds like there is a misunderstanding of my explanation. John
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Hiya John, Firstly, many thanks for your speedy reply. The blog I found for timing is at your site ://johnsmachines.com/category/triple-expansion-miniature-steam-engine/ dated 12 July 2018 where the timing for Stuart Turner triple exp engine is explained. This is where the 15, 15 and 30 degrees is mentioned.
I’ve tried to insert a drawing i have of the eccentrics showing the degrees offset, but I’ll be damned if I can figure out how to do that – any ideas?
Thanks for your time
Chris
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Hi Chris, the instructions in that 12/7/18 post refer to the Stuart triple expansion engine. I cannot vouch for the Stuart instructions, except to say that Stuart are usually very reliable. I used a different method for the Bolton triple, which I described in other posts. Essentially, each cylinder is timed independently, starting with the HP in forward direction, then the IP in forward, then the LP in forward. Then the Stephensons links are placed in reverse, and the timing is done for HP, IP, LP. Opening of the valve port for each direction was determined with a sliver of copy paper, and valve opening was positioned so the piston was just past TDC. The 360º protractor was quite helpful. My posts detailed the method. John
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Thanks John, found your post re timing, should have found it sooner then I wouldn’t have needed to bother you. I will let you know how it all goes.
Regards, Chris
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