Rob Roy valve gear alignment

[chris organParticipant @chrisorgan39405]

Hi, next step on my Rob Roy 3 1/2 inch build is to machine the cylinders so I thought I'd better first clue up on the oft mentioned alignment error in the drawings. Having checked a few times, the drawings I have at least are consistent in having a 1/2 inch centre line through the cylinder, suspension bearing and eccentric, all relative to the frame. The key dimension is the valve centre which is 5/8" from the cylinder mounting flange. Less the frame thickness that's 1/2" again. What am I missing ?

I can see the utility in shifting the whole centre line across to provide more clearance from the horns and axlebox and if the castings allow I will do that.

[chris organParticipant @chrisorgan39405]

[Phil H1Participant @philh196021]

Chris,

The early drawings and one of my books has an error for the steam chest. It gives a dimension of 5/16" from the port face to the valve centre which should be at least 7/16". I think this is the main problem. However, please check where the 'meat' is on your castings before machining them. You might find that there isn't enough metal to machine the steam chest to give the correct dimensions.

I say at least because as you may have noted, if the valve rod is made to the drawings with a round head rivet form to its fixing pivots, they would be very close to the inside face of the leading axleboxes.

I made the mistake of working to the old version of the steam chest drawings – so mine are beautifully machined to the wrong dimension of 5/16". I will probably use the stepped valve cross head solution when I continue with my build.

Phil H

[chris organParticipant @chrisorgan39405]

Many thanks Phil, I see it now.

The cylinder diagrams (with steam chest attached) works out correctly at a 7/16 centre from the port face but like yours my book has a 5/16 centre on the later diagram for the steam chest. I'll see how far I can push that centre over with the castings I have, otherwise will step the cross head.

Thanks again. Chris.

[Phil H1Participant @philh196021]

Chris,

Another consideration that has just occurred to me…. if you do manage to get enough meat out of your steam chest casting, it might be worth double checking your slide valve and slide valve nut sizes too.

Phil H

[chris organParticipant @chrisorgan39405]

You are right, if I follow the plan to take the centre line to 7/16 then the slide valve would need to be thicker and possibly (depending on how the valve slot is machined) also the nut. There is enough on the cast steam chests to follow this plan.

I'm still pondering the relative merit of doing this (Vs cranked cross head) as it pushes the spindle significantly away from the working surface i.e. the port face. I note that the valve is required to have at least a few thou play from the port face (Evans says MOST important) but my instincts tell me a slide valve works best when the spindle is close to the working surface. Probably a false worry but I could go either way still !

[Phil H1Participant @philh196021]

Chris,

I am forced to go for the cranked cross head but I can also see the advantage of leaving a little bit more space between the steam chests. I can imagine that the extra space might allow for easier valve setting and tightening of the steam chest screws/ studs?

Have you also noted that some Rob Roy builders have moved the steam inlet to the front end of the steam chest rather than penetrating the steam chest covers?

Anther modification is to add a round frame stretcher between two of the cylinder/ frame bolts to stop the frames from flexing close to the cylinders

Regarding the slide valve, I believe that having slide valve clearance is a good plan. Others might be in a better position to comment but my understanding is that there is a slight steam pressure difference between the steam chest inlet and the steam in the ports. This difference tends to press the slide valve onto the port face during its operation.

Phil H

[Rex HanmanParticipant @rexhanman57403]

There was enough meat on the steam chest castings for me to avoid the off set crosshead. This does mean that the covers are nearer to each other making it even more difficult to get the steam pipe in. For this reason I moved the steam pipe to the front ends of the steam chests.

Very worthwhile as it makes valve setting/adjusting easier as you can remove the covers without having to remove the steam pipe, which is a right PITA.

I can't remember if I modified the valve nut or not. A few thou clearance is advisable.

[chris organParticipant @chrisorgan39405]

Thanks Rex, that sounds like a worthwhile modification. I'll follow that up. If you were able to ping me a snap or sketch of the layout you used that would greatly appreciated ! A couple of elbows then some different bends in the steam and lubricator pipes I'm thinking.

I notice now that the drawings I have (as opposed to the book and drawings within) detail the larger steam chest and valve to match so I'm good with that aspect.

[Phil H1Participant @philh196021]

Rex,

I would also appreciate the sketch of the steam inlet please.

Phil H

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