Gearing problem

Question

I have a technic MOC with lots of gears which runs fine when motorized with the Control+ L motor, the motor itself attached to the "dumb" battery box that would have been included with the Osprey but which you now can buy separately.

The Philo home page https://www.philohome.com/motors/motorcomp.htm says a loaded Control+ motor has the following characteristics:

8,81 N.cm torque, 198 rpm

I then wanted more power/speed so I tried to replace the Control+ L motor with the race buggy motor and battery box as in set 8287 "motor box"... The motor stalled immediately.

The Philo home page says the race buggy motor has the following characteristics

5.7 N.cm torque, 780 rpm

This is for the outer output, the inner output which I'm using is geared up by a factor 23/17 so I expect the characteristics to be:

4.2 N.cm torque, 1055 rpm...

So understandable, higher speed but less torque -> motors might stall. I have in my technic MOC 2 pairs of gears gearing up by a factor 25 (8 tooth gear and 40 tooth gear, two pairs) I tried to replace the gears by 2 pairs of bevel gears, the 12 tooth gear and 36 tooth gear. 2 such pairs giving a speedup of factor 9.

So taking this modification in mind, I expect the torque to increase by a factor of 25/9 and the speed to decrease by a factor 9/25 Giving me

11,6 N.cm and 379 rpm

Which is clearly a greater torque than the Control+ motor, so I expected it to work. Unfortunately it didn't; the contraption moves slowly for about 1 second and then stalls...

Am I making a mistake in my reasoning? Are there other factors to consider? Or do I just have bad luck with my buggy motor (or good luck with my Control+ L motor) ? Perhaps the friction of bevel gears is significantly higher than the older 8 and 40 tooth gears ?

I used in both cases new AA alkaline batteries, all from the same brand.

Answer 1

I think the issue here is with your power unit - battery box. It sounds like your motor is consuming more current than battery box can provide.

RC motor (5292) is known to be power hungry and as far as I know the only LEGO control unit that can provide enough of it is Electric RC Race Buggy Battery / Receiver Unit with Auxiliary Output (6272c01):

Electric RC Race Buggy Battery / Receiver Unit without Auxiliary Output (6293c01) maybe as well.

Any other LEGO battery box will have issues with its current/thermal protection kicking in sooner than RC motor can provide its full potential.

This is issue was one of the reasons why 3rd party controllers/power units became available.