Matching Loco Speeds (Question)

You could install a DCC decoder in each unit and fine tune the speed and curve of speed for each unit. That is how I do it.

If you are running with pure DC, try reducing the voltage of the B unit (the faster one) by installing two diodes in parallel (and back to back) in series with the engine. It will give you a drop of about 0.7 volts per diode set and will force the Pittman motor to turn slower. If it is not enough, use another set of diodes in parallel(back-to-back) to reduce by another 0.7 volts. It is nice to have the B unit running slightly faster than the A unit, as it avoids putting too much stress on the A unit coupler when you pull long trains.

Yves

Below is a crude drawing of two diodes in parallel and back to back.
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I don't believe slight mismatches in locomotive speed are anything to worry about. Here is my thinking. Say you have two locomotives pulling thirty cars. By themselves running light one locomotive runs a little faster than the other. When coupled together, they are forced to operate at the same speed. What happens? One just pulls a little more than the other. How much more?

Here is how you could find out: Take the two locomotives, put them on parallel tracks. Split the thirty cars, fifteen to one locomotive, fifteen to the other. One runs faster. Move one car from the slow one and put it behind the fast one. Long one slows down, short one speeds up. Continue moving cars until the speeds match. That is the load-share they will have when coupled together with all thirty cars. Impractical to do for most of us, but the point is you wouldn't worry about either locomotive if one was pulling seventeen and the other was pulling thirteen by themselves, so no need to worry about it when they are coupled together.

Further, I would not worry about which one as fast or slow. The rear coupler of the second locomotive pulls all thirty cars no matter what. If that coupler can take the load, then all is good.

Having said that, I am assuming that the mismatch is not severe. If it is then one locomotive is either pushing or dragging the other. However, this is happening only if the train runs faster with the slow locomotive removed. As long as the train runs faster with both, then there is some amount of load sharing.

Finally, if the mismatch is somewhere between mild and severe, then you could have one locomotive carrying most of the load and the other not doing much work at all. The only worry here is the long-term effect on the running gear of the heavily loaded engine.

For those interested in physics and engineering, I believe a free-body diagram of the forces on both locomotives, combined with an analysis of the torque-speed curves of permanent-magnet DC motors, will support my arguments here. The balance is a little more sensitive for wound-field motors, but the basic arguments still hold.

Doorstop Jim

From my "analog" perspective, if you're worried about the differential running speeds; an external jumper plug set between the two unit's power harnesses would likely "calm down" the performance delta , minimize the "push-pull" issue as well as smooth out the running across gaps, switches etc. I don't know nor care that much about digital control systems, but if the control circuit can handle the dual amperage I have to assume that one receiver would handle both? But then you'd have to treat like one unit at the pit...

I am with JJ. I routinely double-shot steam models with different driver diameters, and have one Mallet with two different gear ratios. I never have any trouble. On the other hand, I am not doing complicated switching maneuvers - mine go round and round.

Yves:

Perfect.

I tried paralleling the two loco's together. Evidently my two are too different, as the B unit Pittman is twice as long as the dual shafted A unit Pittman. Still too large a speed difference.

I added one IN914/IN4148 diode in series and measured the voltage drop, then added the second one for the reverse current. At a reasonably high speed, the B unit is ever so slightly faster than the A unit, thus better protecting the shared couplers.

Now to the assembly and paint shop.

Thank you....