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.

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Last edited by Mossback Mike
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Marx and many locos of that era run fast at relatively low voltage. Easily can run fast enough to fly off the rails.

So yeah- perfectly normal. In fact, typically running voltage starts off around 8 volts from many postwar transformers as your minimum speed.

Last edited by Vernon Barry

You are right about that. This morning I was performing some time trials with a 999 locomotive - without any cars, and then with 5 cars.

Without any cars, the 999 wouldn't budge unless I applied at least 5.5 VAC to the track, and I couldn't apply anymore than 7 VAC - I was afraid it would derail.

With a tender, gondola, tank car, box car and caboose the range was even smaller. I needed at least 7 VAC to get the train moving, and at 8 VAC it was going so fast I was afraid it would derail.

The time trials I did yesterday with my Marx 999 locomotive produced some interesting results. First let me describe the test rig. I have a loop of 0-31 gauge tubular track, and the total center-rail length is 13.4434'. I have a 275 watt Lionel ZW transformer, and I have power applied in two separate equidistant points along the line. I have a digital multimeter to monitor the track voltage. I used a lock-on to connect the red and black probes. I have an inexpensive clamp-style amp meter to monitor the amperage from the hot wire between the transformer and the track. I also have a 10 turn loop in this hot wire to improve the accuracy of the amp readings. For the time trial, I used the stopwatch feature of my Android phone, and timed the train over 5 laps. I tested just the locomotive at 5.5 VAC, 6 VAC, 6.5 VAC, and 7 VAC. Outside of this narrow band, the locomotive either would not move or would move so fast I was afraid that I'd derail and damage the 80+ year old antique. I then connected a tender, three freight cars, and a caboose and repeated the time trials at 7 VAC, 7.5 VAC, and 8 VAC. This band is even narrower, and again the train would not move at less than 7 VAC, and would run too fast beyond 8 VAC. Below is the velocity calculations and amp draw:

Locomotive Only:

 TRACK VOLTAGE Elapsed Time MIN Speed FPM Motor RPM Speed MPH Scale MPH Amp Readings Power Watts 5.5 1.1422 58.8508 880.3 0.67 42.8 1.68 9.24 6 0.7260 92.5854 1384.9 1.05 67.3 1.71 10.26 6.5 0.5912 113.7036 1700.8 1.29 82.7 1.77 11.505 7 0.4160 161.5793 2417.0 1.84 117.5 1.86 13.02

Locomotive pulling five cars:

 TRACK VOLTAGE Elapsed Time MIN Speed FPM Motor RPM Speed MPH Scale MPH Amp Readings Power Watts 7 1.0458 64.2733 961.4 0.73 46.7 2.1 14.7 7.5 0.5832 115.2634 1724.2 1.31 83.8 2.12 15.9 8 0.4375 153.6389 2298.2 1.75 111.7 2.2 17.6

The most interesting finding is at 7 VAC, the locomotive alone was running at 117.5 MPH (scale), drawing 1.86 amps, 13.02 watts. When I attached the cars to the locomotive and ran it at 7 VAC, it slowed down to 46.7 MPH (scale) drawing 2.1 amps and 14.7 watts. One thing I should point out is that my amp readings would usually spike when the train was on the curve sections, so my amp readings are sort of a composite. It seems as though the majority of the power was consumed just moving the locomotive. The locomotive weighs 1 lb and 12.1 ounces, and the mechanical efficiency of the gear train is typical - i.e. not so good.

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Last edited by Mossback Mike

The ratio of your Scale MPH to Speed MPH is 64. That is the correct ratio for S gauge and S scale. If you are  running on O-31, why aren't you using a factor of 48 to calculate the Scale MPH? I realize the model may be compressed to S scale dimensions but just thought I would ask.

MELGAR

Last edited by MELGAR

MELGAR Thanks for showing an interest in this, and reviewing the numbers so thoroughly. The Marx train scales seem enigmatic to me. They are billed as "3/16th scale" - an architectural scale i.e. 3/16th of an inch is equal to one foot. This translates to 1/64th. However, these trains run on 0 gauge track. On paper it seems strange, but the old tinplate Marx trains have always seemed so nicely proportioned and attractive to me. So I used the 1/64th scale factor for my testing results. Later on, I am going to use the same test rig for some of my Lionel trains, and I will use the 1/48th scale factor for those. I have an old Pennsylvania GG-1 electric, a S-2 steam turbine loco, and a New York Central F-3 diesel.

As a follow-up, I bench tested the 999 Marx locomotive at 7 VAC and I am summarizing the performance as follows:

On the bench, with 7 VAC applied to the pickup, the motor's amp draw was 1.7A, and power was 11.9 watts.

On the track, without any cars, and with 7 VAC applied, the amp draw was 1.86A, and power was 13.02 watts.

On the track, with 5 cars, and with 7 VAC applied, the amp draw was 2.1A, and power was 14.7 watts.

Most of the power consumption seems to be attributable to the cumulative friction of the locomotive's drivetrain.

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Last edited by Mossback Mike

I did some bench testing and conducted some time trials with my Marx 333 Hudson-type 4-6-2 locomotive. The 333 is probably the finest locomotive that Marx produced. It weighs 2 lbs and 6.4 ounces.

My test rig, instrumentation and methods are described above.

I first bench tested the locomotive at 7 VAC, and the motor's amp draw was 1.14A, 7.98 Watts. Bench testing like this allows me to get a good idea of how much power it takes just to overcome the combined friction loss of the drivetrain.

I ran the locomotive only on the track described above, and measured the elapsed time to cover 5 laps, while also measuring the amperage, at 5, 6, 7, and 8 VAC. I did not go outside this range because the locomotive wouldn't run consistently at voltages less than 5, and would run dangerously fast at voltages greater than 8. My results are:

Without tender or passenger cars

 TRACK VOLTAGE Elapsed Time MIN Speed FPM Speed MPH Scale MPH Motor RPM Amps Power Watts 5 1.8376 36.5787 0.42 26.6 1227.1 1.32 6.6 6 0.7698 87.3141 0.99 63.5 2929.1 1.5 9 7 0.5892 114.0896 1.30 83.0 3827.3 1.53 10.71 8 0.4853 138.4975 1.57 100.7 4646.1 1.6 12.8

I then ran the locomotive with its diecast tender and three lighted passenger cars on the track described above, and measured the elapsed time to cover 5 laps, while also measuring the amperage, at 7, 8, 9, 10, 11, and 12 VAC. I did not go outside this range because the locomotive wouldn't run consistently at voltages less than 7, and would run dangerously fast at voltages greater than 12. My results are:

With tender a three passenger cars:

 TRACK VOLTAGE Elapsed Time MIN Speed FPM Speed MPH Scale MPH Motor RPM Amps Power Watts 7 1.1013 61.0342 0.69 44.4 2047.5 2.1 14.7 8 0.7482 89.8431 1.02 65.3 3013.9 2.2 17.6 9 0.6316 106.4234 1.21 77.4 3570.1 2.35 21.15 10 0.5602 119.9961 1.36 87.3 4025.5 2.45 24.5 11 0.4920 136.6199 1.55 99.4 4583.1 2.5 27.5 12 0.4488 149.7705 1.70 108.9 5024.3 2.63 31.56

It is interesting to compare and contrast the results when the 333 locomotive was operated at 7 VAC. On the bench, the power requirement was 7.98 watts. On the track without anything in tow, the power requirement was 10.71 watts. On the track with a tender and three passenger cars in tow, the power requirement jumped up to 14.7 watts.

The 333 runs slower than the 999, it is more powerful, and operates within a wider range of voltages. The 333 however, does not run very well on 0-27 tracks because the turns are too tight and the drive wheels get hung up, loose adhesion, and start spinning. Other differences between the 333 and the 999 - the 333 has smaller diameter drive wheels, so at comparable rpms, the 333 will run slower than the 999. The 333 has a gear ratio of 9.74 to 1. while the 999 has a gear ratio of 5.4 to 1. The 999's motor runs at much higher rpms than the 333. See my tachometer readings above.

One other observation, it takes a good deal of power to pull the lighted passenger cars - because of the rolling resistance of the wheels, and because of the resistive friction of the lighting pickups. I did some interesting testing of the rolling resistance of my train cars and I produced some interesting data which I will provide in a follow-on posting.

One final note on my 333 - it does not have an e-unit ( I physically removed it) and the headlight is an E10, 24V cold-white LED screw-type bulb. So my locomotive is running a little more efficiently than a stock 333.

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Last edited by Mossback Mike

I did some bench testing and conducted some time trials with my Marx 21 Diesel F-3 locomotive. The Marx 21 is a large, good-looking, tinplate locomotive - closer in scale to 1/48th rather than most of the other Marx tinplate trains which are at 1/64th scale. It weighs 2 lbs and 10.6 ounces.

My test rig, instrumentation and methods are described above.

I first bench tested the locomotive at 9 VAC, and the motor's amp draw was 1.85A, 16.65 Watts. Bench testing like this allows me to get a good idea of how much power it takes just to overcome the combined friction loss of the drivetrain.

I ran the locomotive only on the track described above, and measured the elapsed time to cover 5 laps, while also measuring the amperage, at 7, 8, 9, and 10 VAC. I did not go outside this range because the locomotive wouldn't run consistently at voltages less than 7, and would run dangerously fast at voltages greater than 10. It's worth noting that the Marx 21 operates in a narrow band of voltages like to 999, but the band of voltages is higher. My results are:

Without a dummy A-unit or passenger cars

 TRACK VOLTAGE Elapsed Time MIN Speed FPM Motor RPM Speed MPH Scale MPH Amp Readings Power Watts 7 1.5223 44.1549 824.4 0.50 24.1 2.2 15.4 8 0.4855 138.4490 2585.0 1.57 75.5 2.25 18 9 0.4003 167.9166 3135.2 1.91 91.6 2.35 21.15 10 0.3700 181.6676 3392.0 2.06 99.1 2.4 24

I then ran the locomotive with its dummy unit and three unlighted passenger cars on the track described above, and measured the elapsed time to cover 5 laps, while also measuring the amperage, at 8, 9, and 10 VAC. I did not go outside this range because the locomotive wouldn't run consistently at voltages less than 8, and would run dangerously fast at voltages greater than 10. My results are:

With dummy A-unit and three unlighted passenger cars:

 TRACK VOLTAGE Elapsed Time MIN Speed FPM Motor RPM Speed MPH Scale MPH Amp Readings Power Watts 8 1.5672 42.8899 800.8 0.49 23.4 2.52 20.16 9 0.5538 121.3741 2266.2 1.38 66.2 2.6 23.4 10 0.4445 151.2193 2823.5 1.72 82.5 2.65 26.5

It is interesting to compare and contrast the results when the 21 locomotive was operated at 9 VAC. On the bench, the power requirement was 16.65 watts. On the track without anything in tow, the power requirement was 21.15 watts. On the track with a dummy A-unit and three passenger cars in tow, the power requirement jumped up to 23.4 watts.

The operation of the Marx 21 is very fast. It's almost too fast, with a tendency to derail on curves at voltages above 10 VAC.

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