Minitrains Brigadelok valve gear problem

[tedp]

My new Minitrains Brigadelok, when on a trial run on Rae Bridge, suddenly locked up solid. The cause was the return crank of the valve gear on one side, which had worked loose. The return crank slipped and the valve rod, due to the increased throw, slipped out of the hole in the cylinder block and locked the mechanism.

On investigation I found the pin which secures the return crank in the driving crank, had come loose - not only in the driving crank, but also in the return crank itself.

I am told this problem has cropped up in several other cases. Here is a solution.

In a prototype Stephenson valve gear, there are two eccentric rods making a scissors motion. The Minitrains valve gear is simplified. There should be two eccentrics on the return crank, located centrally in line with the axle, which is nearly impossible on a H0e locomotive. If the gear on the Minitrains loco moves slightly it imparts an impression of motion, that is all, but it really should be stationary.



This is what I did to fix the problem. First, here is the chassis with the offending valve gear. The return crank is on the crankpin of the drive crank, and it should be a tight fit. If it isn't, it will slip during running, causing the valve rod to drop out of the cylinder block (at the extreme right in the photo).



Here are the procedure steps I took to fix the return crank.



I had a slight problem as superglue found its way to the axle where it comes through the dummy outside frame behind the drive crank, but I managed to unstick it. The procedure carries some risk, so use a very small amount of superglue. Adding a tiny amount of oil to moving links which shouldn't become stuck may help.

I had to do some further work on the valve gear, as I found the crosshead on the side that had come loose was fouling the valve rod. With the cranks at 12 o'clock, there was no play between the crosshead, the rods and the slidebar. The loco had a slight binding tendency at that point.

So I carefully pushed sideways against the slidebar with a small screwdriver, so it got a slight curve towards the outside frame on that side. Now the rods and crosshead could move slightly from side to side. Note that the valve gear parts seem to be made of something very tough and springy - I think they are steel. So do take care when attempting what I did.

Here are a prototype photo and a drawing of the Stephenson gear. It is evident that the representation on the model is wrong.


 
Stephenson valve gear: Only the expansion link moves up and down as opposed to the Allan/Trick gear where the radius rod moves in the opposite direction.



image sources: Wikipedia.

[tedp]

No sooner had I written my last post when the loco stopped. When I picked it up... predictably, the valve gear on the other side had come off!    >:(



I gave this the same treatment. First I fixed the crankpin in the return crank, just to make sure that wouldn't slip again. Then I spent a fiddly 10 minutes trying to get the crankpin through the connecting rod which kept flitting away, then into the drive crank. You need an angelic patience which I don't have. Not always, that is.


Then I pressed home the crankpin in the drive crank by using a strong set of tweezers and nip it in.


Finally, I applied the superglue to the rear face of the drive crank to fit it in. I hope it stays stuck now as well as the other does... up to now, at least...

To be fair to Minitrains, it is almost impossible to make a working double eccentric valve gear in H0e. I know Fleischmann did so in their 0e Magic Trains loco, but in H0e it would amount to watchmaking.

Since the gear has to be stationary anyway, I wonder why they hung it from a return crank. I think the easiest method would have been a stationary representation of a correct Stephenson valve gear. Perhaps replacing it with an etch, fitted to the chassis with the double eccentrics just outside a normal crankpin, would be the way to go.

[tedp]

Some hours later: I have had the Brigadelok running circuits on Rae Bridge for at least an hour now, both ways, jogging along at scale speed with a short train of Egger stock in tow, and I'm happy to say the problems are over.

[tedp]

Just to make up for the problems I wrote about, here is a video showing the Brigadelok on a trial run over the C&DR. The valve gear problem has been solved - not a lot of trouble if you know how to do it, in the end it was a minor irritation.

The slow running of this mechanism is excellent. So here is the video I made just a moment ago. The squeaking is one of the Egger coaches.

https://www.youtube.com/watch?v=mYMKQMuBkP8