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I am interested in documenting some baseline tach readings for my postwar engines. Toward that end I would like to purchase a reliable tachometer for measuring small motor RPMs.

A search of the forums came up with hits that refer to built in tach function in newer engines, no standalone devices that could be used on older engines. Perusing through Amazon it seems many of the "touchless" tachometers rely on reflective tape. Anyone here have any experience with this, or a favored/recommended tachometer?

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@S Rice posted:

... a reliable tachometer for measuring small motor RPMs.

It's the "small motor" that makes it challenging. The reasonably priced (say, $20) contactless HOBBY tachs I've seen (and used) detect optical reflections as GRJ points out.  It's amazing that they include a built-in laser to provide the optical source.   Can you find a surface that can be marked that is spinning/moving at the motor speed?

My question is are you measuring the RPM on a test-stand (rollers) so that you aren't trying to target the tach laser on a moving train?  Or are you dis-engaging the motor from the transmission/gears so as to measure the no-load RPM characteristics as might be found on a motor datasheet?

Depending on what you're trying to measure, there are alternatives.  For example if you determine the gear ratio and wheel diameter, you can measure track speed and back-out the engine RPM.  Track speed can be measured with a stopwatch and ruler...or with model train speedometers...or with a digital movie camera counting frames.

On the esoteric side, if you have access to electrical instrumentation like current probes and oscilloscopes you can measure electrical characteristics such as commutation discontinuities to determine motor RPM.  Yes, I've done this and while tricky it is exact if you can extract the signal-from-the-noise so to speak.

@stan2004 posted:

IMy question is are you measuring the RPM on a test-stand (rollers) so that you aren't trying to target the tach laser on a moving train?  Or are you dis-engaging the motor from the transmission/gears so as to measure the no-load RPM characteristics as might be found on a motor datasheet?

You pegged it.

To be clear, this isn’t a real problem I am trying to solve so much as a personal academic interest.

I intend to disconnect the motor to get a “no-load” reading. Then a load reading on rollers. And finally a live reading on the track.

The cheap Tachs come with reflective strips. My thought is to put a strip on the armature of a disconnected motor for no load baseline and then a strip on a wheel on rollers for load baseline. And last the wheel on track with just the engine, and maybe incrementally with cars added.

So for $20 its worth a whirl.

You can make an easy way to check these cheap $20 laser tachs.  On a piece of white matte board or anything fairly stiff draw a circle about 3" diameter then another circle inside the first about 1/4" smaller and another 1/4" smaller, and again.  you will end up with bands 1/4" wide.  On one band paint one 1/4" black square. On the next two  squares 180 degrees apart. The next paint four squares 90 degrees apart. Once you have enough bands cut your circle out.  You will need an AC induction motor with a marked known speed to mount this disk on.  Now you can test for multiples of that fixed speed. 1x, 2x, 4x.........  you can add more concentric circles and more black squares if you need to go higher. I am using a 3600 rpm synchronous motor from an old turntable.  This is a good indication of how much your cheap tach is deviating at that known speed.  If your motor is large enough you can make your wheel larger and add more bands or make them wider. Easier to keep the laser on the right circle.  My cheap tack is consistent not quite 3% fast up to 20k rpm.     j

Last edited by JohnActon

I'm having a hard time visualizing how you get RPM from this instrument.

Humboldt Vibration Indicator, Tachometer Type

System resonance.   I had one of these forty years ago, perhaps I still do,  they work well for model airplane engines. You slide the needle in and out to peak the vibration.  Or perhaps to null ?   Not sure if the amplitude of electric motors vibration would be enough. perhaps a little modeling clay on the flywheel would make it work till the clay flew across the room.  A bit related to why barrels climb a smooth rather steep incline in Lionel's barrel loader.  The cheap laser tachs work better and are more accurate !  j

To clarify, you place the front of the shaft on the item to be measured.  The wire is attached to a guide pointer that goes up and down the shaft.  The shaft is calibrated in RPM and you extend the wire out and in to find maximum vibration or maximum swing of the tip of the wire and then read the RPM where the pointer is pointing.  I agree it is not very accurate.

Yes, we used it on model airplane motors like Fox and McCoy 35s.  We were big on water skiing and U control air planes and model boats with tube radio control.  What fun living on a bay in south Texas.

Charlie

I found some videos demonstrating the use of the pen tachometer. Looks like it is best for combustion engines, and there were certain formulae to use based on 2 or 4 stroke. Very interesting from an historical perspective. While not very accurate today, I am always impressed at innovation from any era.

On another note, I have an old anemometer I may play around with to see if I can use it for RPMs.

No telling how many wild goose chases this list has prompted.  I went in the basement looking for my tach pen and found this variation on the tach pen.  Works about the same way but takes both hands to turn the inner and outer dial. You can use the tach pen with one hand. Though I think the scale on this tach is easier to read. While at it I took a shot of the cheap laser tach it is easier to use and more accurate than either resonance tachs. My greatest beef with the laser tach is there is no way to dump the memory and you do need the stripe or a painted disk (flywheel) for the thing to work. It does work well enough for our uses with toy trains.                         j

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