Rod positions on Articulated/Mallet Engines

[Fergmiester]

Are the rods of the forward and rear engines suppose to be offset from each other? If yes by how much? How critical is this for smooth operation?

I have scanned several books to find an answer to this question but with little success.

I am asking this question as I am peicing my Rivarossi Big Boy back together with a new motor and, the "Beast" though running, is slow in the forward direction but fast in the reverse direction. I have operated the Loco off load with no friction and she seems to be running without difficulty, just a bit more clatter in the forward direction.

Your assistance as always is appreciated [:D]

[Sperandeo]

The two engines of Mallet articulateds (and of simple articulateds, for that matter) were not connected mechanically and could run in and out of phase. This was especially true if one engine slipped at all. There was a tendency for the two engines of a Mallet to get "in step" as they ran because of back pressure between the high and low pressure cylinders, but even a momentary slip could put them out of step again. Models generally don't simulate this very well, although a few people have built articulated drives using separate motors for the front and rear engines. It's an interesting challenge if you want to take on building your own drives, but personally I'm more bothered by the prevalence of models with unrealistic articualtion – rear engines not rigid under the boiler and both engine pivoted near the center like trucks.

So long,

Andy

[Eriediamond]

I'll try to answer this for you. I don't understand what you mean by off set, but assume you mean the relationship between the two engines in the position of the drive rods. there is none. the two engines are seperate engines under one boiler. On the full size locomotives wheel slip would determine the position of those rods in relationship to each other. Just for conversation sake, lets say the connecting rods on the front engine are position at six o'clock on a parked loco, and the rear engine has the rods positioned at 12 o'clock. The trains starts and the front engine breaks traction. When it regains traction those rods would no longer be in the same relationship as they were before the wheel slip occured. Also curves have the same effect. With this in mind, our locomotive is drive by an electric motor geared to both sets of engines so if one slips the both slip so both engines are always in "time" with each other. Only thing I would consider when reassembly is what looks best to you as far as that timing is concerned. Hope this helps you, Ken

[Fergmiester]

Thank You Gentlemen

That answers part of my question but I'm still having issues with the engine stalling. The wheels have been cleaned, wires checked and replaced as required but there seems to be a lot of "chatter". No indications of shorting.

Any suggestions?

[ndbprr]

Model steam engines are even more cantankerous then the real ones and there are a number of issues to check. In a model the drive USUALLY drives both sets of drivers which sets up some thing happening that don't in the real engines. Here is a list to check:
1. Does the engine stall in the same place all the time? if so maybe it is a trackwork problem or the daius is too tight for the engine.
2. Does the mechanism have a bind in it? Lift the engine off the track and apply power through leads so that the engine barely turns the wheels. If there is a bind it will stall the engine in the same place every time.
3. If #2 is happening you need to disect the mechanism to determine the problem. A fast visual check of the side rods may indicate one is bent causing it to bind against another. If that is true just bend it away slightly
4. Disconnect the motor and see if it runs freely. the start adding drive trains to it until you determine where the problem is. It may just be lack of lubrication or a piece of flash on a gear.
5. Did you take the wheels out of the frame? On my Rivarossi 2-8-8-2 the outside two wheel sets in each set have lateral motion ability to help on tight curves. You may have put an outside set in the middle on reassembly or tightened something too tight so there is a bind.

As to real engines - each engine and set of drivers is balanced within itself since all the forces acting on it are contained within the unit. The only external fact is the length of the frame and the ability to unweight one set of drivers causing it to spin instead of pull. PRR T-1's were notorious for this with their rigid 4-4-4-4 wheel arrangement.

[Eriediamond]

Ok, know keep in mind I'm not familiar with the Rivarossi mallet, so these are only hints on what to look for. Does the new motor installed have the same dimensions as the old one. Are the drive shafts replaced as exactly before, sometimes drive shafts may look the same but have different lengths so small that not readily seen. Is the stalling do to electrical loss or mechanical binding. Was the loco doing this before new motor installation. are all gears intact, no broken or worn teeth and properly lubed. Are the drivers properly quartered. So many questions to give one answer. You may have to use the proccess of ilimination here . Have patience. Wish I was able to look at it myself but hope we can give you help here on the forum.

[Anonymous]

Since you are replacing the old motor I am guessing that the engine has some miles on it. If this is so I'd bet you have some slop in the gears.

When you run the engine forward it is lifting the gear in the gear box up and binding just a little. When you run the engine in reverse it is forcing the gear down where it belongs and the engine runs correctly.

Now with that said I have no idea how you would fix this short of replacing the gear boxes.