Rotary Valve Gear Crab

[45609]

Here are a few pictures from my workbench. Apologies to those that may have already seen these on RMweb.

Although progress has been slow, most of this work was done over a year ago, the locomotive will eventually become 42825 as it would have appeared circa 1954 with original Lentz valve gear and sporting BR mixed traffic livery. The basis is a Bachmann body, Comet chassis and Brassmasters detailing kit.









Driving wheels are from Alan Gibson






Gearbox is a High Level unit driven by a Mashima 1420 mounted vertically into loco firebox. A torque reaction tab can be seen to the right of the motor poking up through a slot in the frame spacer.






The tender uses a Masokits sprung underframe running on Ultrascale wheels. Most of the Brassmasters detailing kit parts have been added to the tender.

Will post a few more pictures as and when I do some more.

Cheers....Morgan

[HowardGWR]

Very nice Morgan. What is a 'torque reaction tab' please?

[45609]

Thanks Howard. Just hoping posting it on here will encourage me to make some progress with it. Has been in this state for nearly a year....

Below are a couple of pictures that hopefully explain what a torque reaction tab is. I suppose it could also be called a torque reaction bar but in this case it is an L shaped piece of metal strip soldered to the opposite side of the gearbox motor mounting face. When the wheelset, with the gearbox, is assembled to the frames the short piece of the tab that is parallel to the motor shaft pokes through a slot in the frame spacer. It has a fairly slack transverse fit in the slot but a good sliding fit in the longitudinal direction. The purpose is to allow the springing on the driven axle to work correctly by letting the hornblocks to rise and fall without restriction but prevent uneccessary rotation of the gearbox about the axle when power is applied. If you are not familiar with the principles they are explained far better than I can in Digest 41.0 on page 15.

Cheers....Morgan

[Tim V]

Looking good.

Can't see any pickups Morgan, though I can see some PCB is that part of it?

[45609]

Thanks Tim. The pickups will be part of a removable busbar/axle keep arrangement. This has been fabricated but is not shown in any of the photos that I have posted so far. As you noticed I have used some thick copper clad for some of the frame spacers. The only reason for this is that I have a load of it precut to P4 frame spacer width and it is thick enough for tapping holes into for various component retaining screws etc....

Cheers....Morgan

[modelmaker87]

Morgan,

Very nice workmanship, keep it up. I really do hope that you will now have the encouragement to finish it off. Should look good nicely weathered in front of a train pulling somehintg. Looking forward to seeing it done.

Cheers, Tony S

[davebooth]

That's looking excellent Morgan.
I think that I see springy beam suspension on the tender; whats happening about suspension of the loco?
Have you used springy beam before and can you recomend it as a simple system or does it end up more complex?
I'm considering it for a Doncaster built O6 or an LNER 2-8-0 as per Stanier's 8F.

[45609]

Tony, Dave,

Thanks. I should have said encouragement and spare time. A looming exhibition deadline for Brinkley usually gets me motivated. We have one coming up in October so watch this space.

The Crab tender is running on a Masokits universal springy beam underframe. The loco will be suspended on coil sprung hornblocks from Brassmasters as it seems a shame to not use the ready etched spring tabs that Comet have provided. I first used the BM hornblocks on their Super D kit and have been very happy with the running qualities. IMHO it is far superior to the ocassionaly clunky ride you get from beam compensated locos. The tender of the same Super D was my first use of CSBs and I would recommend it without hesitation. The complexity, if you can call it that, is in the planning and setting out of the horizontal and vertical position of the spring fulcrums whilst avoiding all of the other stuff between the frames. I'm in the process of doing this for my first loco with CSBs, a Black 5. Russ Elliot very kindly helped me out by posting the horizontal fulcrum info on the CLAG website.

http://www.clag.org.uk/beam-annex3.html

Come to think of it you can also find here the basics of what you need to do a Stanier 8F with CSBs. Incidentally I have built an 8F using the Society (Bradwell design) individual wire sprung hornblocks. I personally wouldn't do a loco with these again because I found them a real fiddle to fit and set up the 10BA adjusting screws. I would say that this was partly due to my positioning of the springs above the axles which made access very difficult. It was a learning experience.

Cheers....Morgan

[Ashley]

That is a very nice piece of work you've done, and I'm looking forward to seeing you post further progress in due course. I am at this point in time knuckling down to building track, and everything else is on hold until the track is done. So I appreciate seeing locomotives and wagons being made, as it reminds me why I'm building track.

[Steve4RoseGrove]

Very nice, as always from you Morgan. I'm interested in the torque reaction idea. I would be interested on how this works, as I was wondering if, by using a fairly large contact area (the strip/slot), friction under heavy torque would restrict the axle movement. I have seen another idea which was a piece of thick straight wire (circa 0.9 m.m.) soldered to the gearbox and passing through a larger hole in a tab on the chassis. This would give a degree of freedom in all directions but with a small contact area/low friction.
Steve

[45609]

Hi Steve,

It is an interesting point you make and I'm not sure whether friction will ultimately be a problem. It seemed to work OK on a couple of previous locos. Although it may look like it is a close fit in the slot the tab off the gearbox has a few thou clearance to rock back and forth a little. The 0.9mm wire idea is certainly one I have seen before and I imagine that works pretty well too.

Cheers....Morgan

[Dave Holt]

Hi Morgan,

I'm not sure i understand how your torque reaction tab works. The tab itself lies more or less tangential to the axis of rotation of the gear box, so in one direction the reaction pushes the tab further into its slot, but in the other it would tend to slide the tab out of the slot. Is there some sort of retaining pin through the tab to hold it in the slot? Also, to allow for twists in the track, the tab must allow some sideways movement; otherwise the driven axle will not be able to accommodate this movement.
A better arrangement might be to have a straight torque link, prevented from rotating by a screw through the frame stretcher which is not tightened fully home or has a spacer collar on it to allow angular movement of the link.

Dave.

[Will L]

Sorry guys, but I have doubts about the torque reaction tab you describe here. Rather than hide what could be an interesting discussion at the end of a non obvious thread I have started a new topic on the subject.

Link here Torque Reaction Links - Suspending The Driven Axel

[Dave Holt]

Morgan,

Just for comparison, here's my part completed Walschaert's valve gear version - also based on a Comet chassis with Bachman body. Still lots to do! I was going to show it without the body to illustrate the motor torque reaction link, but found to my shame that the only thing preventing counter rotation of the gear box-motor unit is the wires to the motor - Oh dear!! [Seems to work, though.]

Crab_001.JPG

Dave.