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Tachometer Readings

I recently bought a Cen Tech digital photo sensor tachometer at Harbor Freight Tools and used it to measure the rpms of the drive wheels of some of my Marx locomotives - a Marx 999, a Marx 333, and a Marx 21. The device seems pretty easy to use, although it sometimes would display some spurious output that didn't make much sense. However, I managed to get the sensor pointed properly, and obtain some steady readings listed below, which seem realistic:

The Marx 333: I bench tested the motor at 13.5V AC output from the transformer and the drive wheel rpms was in the range of 805-820 rpm. The 333 has a gear ratio of 9.74 to 1, so I calculated a motor rpm in the range of of 7841 - 7987 rpm.

The Marx 999: At 14.0 VAC output from the transformer, the drive wheels were spinning at approximately 1900 rpm. Since the 999 has a 5.4 to 1 gear ratio, I calculated the motor would be spinning at 10,260 rpm.

The Marx 21: At 13.4 VAC, the drive wheels were spinning at 1400 rpm. Since the Marx 21 has a 4.3 to 1 gear ratio, I calculated the motor would be spinning at 6,020 rpm.

Does this seem realistic? I have no technical specifications for these electrical motors, and I have no way to directly measure the motor rpm. In regards to the gear ratios - I obtained them by just counting the teeth on each of the spur gears in the gear train and doing the math.

I am new to this forum, and this is my first post - so, if this is something that has already been discussed, please point me in the right direction.

Marx 999 Gear Train

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  • Marx 999 Gear Train
Last edited by Mossback Mike
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