Remotoring AHM/Rivarossi Big Boy

[gboule@wanadoo.fr]

Hi,

I have a bigboy from Rivarossi which the front gearbox had explosed, and I am interested by your proposal to change the brass gears by plastic gears with different ratio. 
Could you give me the reference of the plastic worm and gear for this replacement ?

Regards

gna,

one of the issues with old Rivarossi locos are their speedy gears.
It is just as you try to drove a fully loaded car equipped with manual transmission a very steep grade uphill in the highest gear. That does not work, you have to shift down to tackle the job or you will get stuck on the grade. So it is with the Rivarossi locos when the traction tires have still enough grip. Other case is when the traction tires are old an hardened, then the wheels will spun out. In both cases the loco will fail to pull the train uphill.
Your Big Boy is one of the older examples with the motor placed into the cab. From there a drive shaft will go to the first worm gear inside the boiler above the rear group of drivers and the to the second one above the first group of drivers. From this worm gear boxes there are vertical shafts leading to two more axle worm gear boxes each one inside the frames of rear and front engines.
You have in sum 4 gearboxes with an considerable demand of power and torque this is multiplicated by the low gar ratios.
In other words you have to install relatively huge motor with a lots of power and torque output. But this motor are of seizes which will not fit into the cab.


Not a Big Boy, but an other Rivarossi loco (B&O 2-10-2) and just the same gearboxes and ratio.
The photo shows in the middle right the gearbox inside the boiler. The one with the steel worm an brass gear is the original ratio of 1:11 and the old Rivarossi gears have a inappropiate friction factor too. In connection with the axle gearboxes with a ratio of 1:2 you have a overall ratio of 1:22.

In sum:
- low gear ratio -> speedy loco, needs power from the motor to pull
- disadvantageous friction factors -> needs more power from the motor, this extra power is completey devastated by internal friction
- a multiplication factor of 4 sepatarte gears
=> nearly all the power of the motor is consumed by the gears

In this 2-10-2 i replaced the original gears with Roco parts and got a ratio of 1:24 and in overall a gear ratio of 1:48. After that the running behavior of the loco has changed. Before it was not so fast, because Bernd (aka modelsof1900) has installed a slowly running but powerful motor to avoid hassle with the original Rivarossi motor and speeding. Also he filled the interior with heavy type metal.
At least i fitted a relative small Maxon coreless coil motor which left space for an boiler backhead too. The demand of amperes dropped dramatically after this conversion. In combination of new traction tires this loco now pulls out the hell.
That was at least done by changing the gear ratio.

And that is a way i recommend to you to change the gear ratios in the two gearboxes inside the boiler. Why, that i have described above.
Here i used gear with an module of 0.4, a one start worm and a spurgear with 24 teeth, both press fit onto 2mm shafts.

Also an posibility is to use NWSL gearboxes (1:28 type), there are also ones for 2mm axle diameter.

The option with change motor needs a very, very powerful motor. Look for one with the biggest power output you can purchase and will squeeze fit into the cab.

Cheers Lutz