Rob Roy Build/ Rally

Thanks Rex. Painfully slow though. But I now have a boiler, super heater and regulator from Bromsgrove. Just need to get it tested.

Hi Phil,

Although the original valve gear design isn't awful it can be improved a lot by increasing the offset of the suspension pin on the expansion link from 0.0625" to 0.160". If you haven't already made the suspension brackets for the expansion link then I would recommend making the alteration. It looks as though you have already made a start on the lifting arms but it also helps to reduce the length of those from 1.25" to 1.0625".

John

John,

Thanks very much for those tips. Exactly the kind of stuff I am looking for.

I have just (10 minutes ago) machined the short sections of angle for the expansion link but I haven't drilled the holes yet. So the first change is quite easy if I have understood correctly. Just to clarify (please), the little press fit pin in the angle is moved closer to the eccentrics (away from the valve chest) by 0.0975". Is that correct?

Again, if I have understood you correctly, reducing the length of the lifting arms is a pain (specially the slot milling) but not a real problem. A shortening of 0.1875" according to your recommendation.

That would suggest to me that on the original valve gear drawing, at the central expansion link position, the lifting link looks vertical. But with the changes, it will tilt slightly like a forward slash ..../.... because the lifting arm is shorter than the change on the expansion link.

I will sketch it and post here.

John,

Have I got you right? I thought a sketch to clarify might be a good plan before I hack metal. Thanks very much

in advance. Phil H

Hi Phil,

Yes, you have got that correct

I did some work on a Rob Roy for a chap at our club and out of interest I had a look at the valve gear as very few designers seem to understand Stephensons correctly.

I put all the figures into the Don Ashton spreadsheet and calculated what the correct offset should be to equalise the events over the complete range of cut offs. I then used the Alan Wallace valve gear simulator to check that the figures were correct.

The reason for also shortening the lifting arm is to compensate for moving the suspension pin on the expansion link so that the suspension link is vertical when the piston is at mid stroke and the reverser is in mid gear. In Martin's design the lifting link does slope backwards slightly, hence the modification being larger than the increase in the offset.

I have recently inherited my late Brother's Rob Roy which was built to the original drawings so I must make the modifications to this as well

John

Edited By John Baguley on 05/04/2022 23:47:33

John,

Excellent. Thanks very much for that, you caught me just in time. I can easily make those modifications now.

Phil H

John,

Perhaps you could modify your late brothers engine and bring it along to the Rob Roy Rally day in September?

Phil H

Another question for experienced locomotive and or any other stationary engine builders please.

I have now completed most of the valve gear parts and Mr Evans suggests that the various rod ends (including their nicely reamed 1/8" diameter holes) should be case hardened.

Is this a step that I really should go through with or will the plain mild steel parts last a reasonable time without case hardening? Am I asking for trouble if I don't case harden?

Thanks,

Phil H

Never case hardened mine and it still runs just fine after about 30 years. Some things are decidedly slack now but so what? No harm in case hardening, Just a matter of choice.

Rex, Thanks for the prompt answer. If yours has lasted 30 years, that is good enough for me. I'll skip the case hardening.

I can fix some of the parts. For example, the expansion links are gauge plate and were noticeably hard. I heated up to bright red and cooled as slowly as possible before drilling the various holes in them. They are both now harder than the other mild steel parts but I suspect less brittle than they were. I will leave them in that condition and use gunmetal or bronze for the die blocks rather then the suggested mild steel.

Hmm - not that far from a running chassis excluding the pumps.

Phil H

I have a question about the Rob Roy ashpan. I bought, the stainless steel, laser cut ashpan from ME laser. The drawing has a dump pin to allow the ashpan to drop but it looks like it will be a very tight awkward fit - specially around the rear axle. So my question is really how easily the pan drops out and if I fit the boiler, can the ashpan really be slotted up into position whilst the rear axle is in place?

Thanks.

The short answer is it's a pain! With the pin pulled out I have to poke and prod it with the pin to get it to drop.

That's the easy bit, getting it back in is worse. I have found the easiest way to get it back in is to lay the engine on its side on a towel. Insert the firebars and then wiggle the ash pan back in until the pin can be inserted.

I can't see an easy way to improve things. You will probably need to cut lots of holes in the side and front of the ashpan to improve the air flow.

Thanks Rex, sounds like fun. Ian did warn me about Rob Roy maintenance issues at the rally but I bet that most of the other engine designs are a bag of maintenance delights also.

Phil H