# Rpm indicator



## Brian Rupnow (Jan 17, 2015)

This morning I woke up with an idea. About 2 years ago I had to purchase a Neodymium magnet for a project. I had to buy a pack of 10, so had 9 left over. I was thinking of how old mechanical speedometers worked, whereby a magnet which was spun by the speedometer cable which acted on the speedometer needle to make it move through an arc, and the faster the car went, the faster the cable spun the magnet, and the more speed the speedometer registered.--I thought "Hmmm---That same system could work to register rpm of an engine---I think." If a pulley was attached to the lower yellow shaft, and it was driven by the same size pulley on an engine for a 1:1 ratio, the grey aluminum disc would spin at the same RPM as the engine. If I Loctited my 9 remaining Neodymium magnets into  the grey aluminum disc----. The green pointer would be made from steel, and pivot at the upper yellow shaft. It's weight would normally make it hang with the pointed end straight up. As the disc spun, the magnetic field would pull the large end of the green pointer farther and farther to the direction of spin. The pointed end would swing in the other direction, and register on a scale. Makes you think, doesn't it.


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## MachineTom (Jan 17, 2015)

A couple thoughts on your design, The nature of the magnetics will make for a jumpy needle because the magnetic field will be weak/strong/weak strong... On the tach's and speedo's I've seen, there was a very simple bar magnet that spun inside or outside a cup shaped disk, to which was attached the needle and a fine clock wound spring, like the kind used on the balance wheel of a watch. 

I had an old tachometer late 1800's, that was a nice 6" diameter. Its mechanism was a shaft, with a round disk, mounted on pivots that spun with the shaft, and was angled about 45° to the shaft axis. There were springs to counter the force of the disk, and linkage to move the needle.

As the disk spun, it would want to move from the 45° angle to a point of 90° to the shaft axis by centrifugal  force, it was opposed by the spring force. This was  a low speed tach top rpm was 400rpm IIRC, that may be better for a slow engine.


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## Blogwitch (Jan 17, 2015)

Rather than wasting time and messing about on something that might never work, just go onto ebay and search for 'Digital Tachometer'.
You should find something like this that will take an hour to fit and cost you less than ten bucks.

http://www.ebay.co.uk/itm/Red-LED-4..._Measurement_Equipment_ET&hash=item487237eb76

On the bench, mine works perfectly, and it even came with a neodyn magnet. I am pulling my lathe out from in front of the wall soon, and it will be going on then.

John


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## RichD (Jan 17, 2015)

On your old school idea...why not just use an old speedometer or tach with a cable then renumber the dial? Although John's idea is turn key and makes sense, I think it's always more fun to try out your ideas. The whole point of being a tinkerer I guess.

I made a mechanical X-axis readout for my mill table and it works fine for my needs. Is it as good as a digital scale?...no, but I still chuckle every time I use it since I only spent $9.00 for the parts.

Another oddball project for a direct diameter readout...funky but it works fine for me.

So I say go for it and have fun!

Rich


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## goldstar31 (Jan 17, 2015)

Somewhere I have a hand held thing that flashes against a bit of reflective paper that revolves.- and so on. Of course, my lathe has one of those 3 phase things which tells me- if my eyesight can stretch across the lathe to the wall.

However, this AC electricity thing will give a stroboscopic reading on speeds if a proper disc is fitted.

Ironically, someone wrote it up in Model Engineer or MEW with fancy disks shown.

Anyone point to it, please? 

Norman


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## tornitore45 (Jan 17, 2015)

I think Brian is more interested in experimenting than solving an immediate need.
Would be nice to have a design within the typical HSM capability to add to our engines.
That said, I do not think the proposes design would work.  I fail to see the physic principle.

The average attractive force is zero, as soon as the two magnets reach symmetry with the pointer the pull reverses; the approaching magnet is getting closer and the receding magnet is getting farther.   I may be wrong.

What I know about speedometer is that they work as a stalled induction motor thereby  generating torque only, without revolving the shaft (needle, in this case).

The torque is proportional to the induced current in the copper disk (replacing the squirrel cage in a motor); the current is proportional to the induced voltage which is proportional to the frequency (RPM).  The torque is contrasted by a spring, thus the higher the torque the higher is the spring elongation.

You can still use your magnets, possibly in even number, alternated N, S, N, S facing a copper disk or cup.


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## Herbiev (Jan 17, 2015)

Just a thought, but if the disc of magnets were to spin by a small relay coil the induced voltage could be read on a voltmeter that could be calibrated for rpm.


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## Brian Rupnow (Jan 17, 2015)

Well, one thing for sure, even if it never gets built, it has certainly stirred up a few comments. I guess my understanding of how speedometers worked was a bit off-----. Sometimes when I am just about asleep, or just newly wakened, I think of the strangest things. Most of the time they evaporate before I am full awake, but I remembered this one long enough to make a quick model of it before it got away on me.---Brian


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## Brian Rupnow (Jan 17, 2015)

Check this out--Tubal Cain is rambling on about "Lenz's Law. Apparently when a high strength magnet passes in close proximity to a non ferrous metal, it sets up eddy currents which oppose the magnetic field. 
[ame]https://www.youtube.com/watch?v=RU5RVQotz-o&src_vid=U3Dw_4GUYyk&feature=iv&annotation_id=annotation_2105765177[/ame]

Wonder what would happen if the green pointer was made from aluminum or copper and overlapped the magnets like this.


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## bazmak (Jan 17, 2015)

Your a man after my own heart Brian.Brain never seems to stop thinking
about things.Some work, some dont and some develope,it would be a miserable world if we all stopped.I am not technical enough but have thought
i would like a tacho for the lathe.Ebay seems my best option


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## tornitore45 (Jan 17, 2015)

> Check this out--Tubal Cain is rambling on about "Lenz's Law.



An easy experiment:   Take a 1/2 inch copper pipe 3 or 4 feet long.  Drop a nut and it will fall fast.   Drop a magnet and it will take about 3 seconds.
As the magnet move down it induce a current in the tube that creates a magnetic field repelling the magnet braking the free fall.

Lenz's law at its best.


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## Brian Rupnow (Jan 17, 2015)

Tornitore--Yes, that shows up in part #1 of the video I posted a link to.---Brian


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## Cogsy (Jan 17, 2015)

I have to agree there is a flaw in your design Brian. Almost half the time the force on the pointer will be in one direction, for exactly the same amount of time it will be in the other direction and for the rest of the time the force will be equal in both directions.

I assume you will get some oscillation of the pointer when the forces are not in equilibrium, but I can't see how the pointer would substantially deflect in one direction and it wouldn't provide meaningful information.

Don't stop thinking this stuff up though. We need people thinking outside the box.


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## ruzzie (Jan 18, 2015)

I found my old tacho in the shed and opened it up. It would need a gear reduction to get the correct rpm as I can turn it by hand and spin it up to 3000 rpm.


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## tornitore45 (Jan 18, 2015)

I shudda have watched the video before posting redundant material.
Anyway the magnet drop in the tube made me win a bet with the grandson, then he said grandpa` is weird.


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## gus (Jan 18, 2015)

Blogwitch said:


> Rather than wasting time and messing about on something that might never work, just go onto ebay and search for 'Digital Tachometer'.
> You should find something like this that will take an hour to fit and cost you less than ten bucks.
> 
> http://www.ebay.co.uk/itm/Red-LED-4..._Measurement_Equipment_ET&hash=item487237eb76
> ...



Hi John,
Thanks for vendor info . ''Born Loser Gus'' bought two pcs  10  mins ago..One more for backup. Been looking for Tacho for long time. May need help to get it working. At long last ,I have a tacho to read speed of my DIY Nemett-Lynx Engine.


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## Brian Rupnow (Jan 18, 2015)

A quick bit of morning work before wife and I are off to see "The Imitation Game" movie.---Old farts like to catch the afternoon matinee!!


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## Cogsy (Jan 18, 2015)

Theory is one thing but it will be very interesting to see what actually happens! Thanks for continuing Brian.


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## bazmak (Jan 18, 2015)

Hi Brian,this old fart also took the missus to the same movie.Well worth it.My wife and i go to the movies weekly,and that was the best one for a while


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## /// (Jan 19, 2015)

I also took the missus to that movie and we both enjoyed it muchly.
Unfortunately, as is typical with Hollywood, they have taken liberties but still a good story.

I found it rather sad though that we watched the movie on its opening night and we were only one of maybe 4 couples in the cinema and it would be a very safe bet we were the youngest couple in there (36 and 39)
Seems people would rather watch robots transforming into cars and vice versa, that cinema appeared to be full.

Apologies for going OT, watching this thread with interest.


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## Brian Rupnow (Jan 19, 2015)

This thing is evolving as it develops. I have decided that to really work in a balanced manner, I need the pointer plates to extend down on both sides of the magnet disc. I am going to call around today and see about getting the two strange shaped outer pointer plates water jet cut. I'm not going to break the bank, but they are rather miserable shapes to cut.


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## ShopShoe (Jan 19, 2015)

Love it. I'm following Brian. 

I like old-school mechanical instruments because they quiver and make everything say: "I'm alive!" Just another reason why I also like old cars and whatever else has an engine or lots of visible mechanical parts.

(In an attempt at humor, I say So I can see your little power plant with the tach and a voltmeter and whatever else you can cook up with a few late-night inspirations and only a few days to build.....

Keep 'Em Coming,

--ShopShoe


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## Brian Rupnow (Jan 19, 2015)

So, we are all ready for a test drive except for the pointer. I called the local water jet guy, and he said there was a minimum $150 set up charge. I explained that I would have to sell my youngest grand child to come up with that kind of money for a something that was basically a toy. When he asked what I meant by "a toy", he seemed very interested and told me to "Send a .dxf file over anyways, we MIGHT have a run of 3/16" plate going through today on a big job". Wink Wink/nudge-nudge. So---I sent him of a dxf file of the pointer. Who Knows??


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## Brian Rupnow (Jan 19, 2015)

First try----fail
[ame]https://www.youtube.com/watch?v=nUv1HGaz44E&feature=youtu.be[/ame]


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## canadianhorsepower (Jan 19, 2015)

Brian Rupnow said:


> First try----fail
> https://www.youtube.com/watch?v=nUv1HGaz44E&feature=youtu.be


 

Hi Brian, interesting
I think that if all your magnets have the same polarity
"all north on the same side and south on the other side."
and that in your pivot and magnet in the opposite direction 
so they will push agains each other. It would work but you might have
to use a spring to bring it back to Zero

good luck


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## tornitore45 (Jan 19, 2015)

Let's call the force or better the torque pulling the pendulum to the left positive, and the torque pulling to the right negative.

If you could visualize and plot the torque versus time it would look like a sinusoid with equal positive and negative peaks, equal positive and negative areas under the curve and therefore zero average value.

Turning slowly the pendulum will have time to follow the attraction left and the attraction to the right. As the speed goes up, the inertia of the pendulum "filters" out the oscillations, it is sensitive only to the average torque and stands still.


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## tornitore45 (Jan 19, 2015)

> It would work but you might have
> to use a spring to bring it back to Zero



Once Brian gets it working by using a different operating principle, he can use the weight of the arm instead of a spring, although far from linear.
Instrument like ammeters and speedometer really need a spring to be independent on gravity for obvious reasons but in this case the "instrument" orientation is fixed and is not called to be shaken like a speedometer on a bumpy road.


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## Brian Rupnow (Jan 19, 2015)

As a point of interest---I suspended a length of 3/4" square aluminum bar in much the same position as the steel bar with the slot in it. It didn't do anything at all.--Made no effort to move. If I made the hanging weight a disc of either steel or brass, the steel disc would probably counter rotate somewhat similar to a gear drive, but that wouldn't be of any use to determine rpm. Likewise, if the "Lenz" effect caused the brass disc to rotate, it couldn't serve the purpose of measuring rpm. Not all of my "Eureka" moments bear fruit.


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## canadianhorsepower (Jan 19, 2015)

Brian Rupnow said:


> As a point of interest---I suspended a length of 3/4" square aluminum bar in much the same position as the steel bar with the slot in it. It didn't do anything at all.--Made no effort to move. If I made the hanging weight a disc of either steel or brass, the steel disc would probably counter rotate somewhat similar to a gear drive, but that wouldn't be of any use to determine rpm. Likewise, if the "Lenz" effect caused the brass disc to rotate, it couldn't serve the purpose of measuring rpm. Not all of my "Eureka" moments bear fruit.


 
hi Brian,
I think your not getting the picture of what we are trying to explain
simply look at it a a centrifugal clutch. the more magnet you have spinning agains each other the harder it's going to be for your lever to go back to 0
then set it at "500 RPM" with your Tach and make your scale from there 

cheers

Luc


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## Cogsy (Jan 19, 2015)

I don't want to say I told you so (I'm not like that) but your experiment did confirm what I thought would happen.



Cogsy said:


> I assume you will get some oscillation of the pointer when the forces are not in equilibrium, but I can't see how the pointer would substantially deflect in one direction


 
But all is not lost, you just have to work out how to fix it.


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## RichD (Jan 20, 2015)

Brian,
What if you use the magnets to generate current via a small coil, then process the output voltage via a large capacitor and a diode. Buy or make a simple amp meter to measure the current flowing to ground through a resistor. Make your own scale in cad for the amp meter to indicate rpm.
Rich


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## Blogwitch (Jan 21, 2015)

gus said:


> Hi John,
> Thanks for vendor info . ''Born Loser Gus'' bought two pcs  10  mins ago..One more for backup. Been looking for Tacho for long time. May need help to get it working. At long last ,I have a tacho to read speed of my DIY Nemett-Lynx Engine.


Gus,
The sensor with those units does have a fairly large surface area on the face, it is about 10mm diameter, so not tiny, but you could just stick a small neo magnet onto the flywheel and bring the sensor into fairly close contact and it will read the RPM.
That is why they are such a boon for machinery, very robust and on machines there is usually not a problem mounting the sensor somewhere.
BTW, the leads should have the correct plugs on the end so that it all just plugs together, and I used a cheapo voltage switchable wall wart to give me the input voltage. Just mount the bits into a small plastic case with the sensor nose sticking out of it and you will have a portable (almost) unit.

BTW, the only reason I am fitting one to my geared head lathe (with fixed speed ranges) is that soon I will be fitting a variable drive to it, and I will need to know what the speed of the spindle is.

Hope this helps

John


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## tornitore45 (Jan 21, 2015)

There are many way to skin a cat, making an "alternator" and then measuring the voltage with a voltmeter is one way but I think Brian was after a "direct" method like is used in the old car speedometer.

To that end I suggest studying the principle and copying the design in modified form still keeping the same operating principle.

Magnet moving next to a conductor are "dragged" back by what appears to be friction, but in reality is a magnetic effect due to induced currents, as we have seen in the video.

Basically build a wrapped up equivalent of the magnet flying next to a conductor (like the video), Instead of a single event like the fall on a linear bar the phenomenon is restarted by making the end return to the beginning by developing the device in circular form.

In other words think of a rotating motor as a linear (mag levitation style) motor wrapped around.  (Usually the thinking goes the opposite way: we are explained how a linear motor is nothing but an electric motor unwrapped,)

If the conductor movement was only restrained by friction, then it would move following the magnet (dragged) but at a slower speed until an equilibrium between the pulling force proportional to the relative speed magnet-conductor and the friction force proportional to the conductor speed in a viscous medium.

By restraining the conductor with a spring the conductor disk will rotate until the spring torque contrasts the attraction.   What we got is a conducting disk angle of rotation proportional to angular velocity of the spinning magnet.  Add a hand pointer and a calibrated scale and you got an RPM meter with no wires, coils or fancy electronic.

Construction note:
A cylindrical magnet generates a flux exiting from the N and returning to the S trough the air.   The permeability of air is thousands of times smaller that Iron, therefore by placing the magnet in a closed circuit made of Iron, leaving only a small gap of air will increase the flux intensity 100 folds.  That is the idea behind forming magnets into a horseshoe shape.


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## bazmak (Jan 21, 2015)

And when you have made all these gizmos ask Gus to make you a box to house it all safely


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## Brian Rupnow (Jan 21, 2015)

I am not going to pursue this idea any farther. I have decided that if I truly wanted a mechanical rpm indicator, it could be easily accomplished with my 3 ball governor and a pointer attached to the lever arm. Depending on centrifugal force and the strength of the return spring it would be quite repeatable after initial calibration. I am semi busy with some engineering design work at the moment, so I probably will not do any more work on this thread.---Brian


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## canadianhorsepower (Jan 21, 2015)

Brian Rupnow said:


> I am not going to pursue this idea any farther. I have decided that if I truly wanted a mechanical rpm indicator, it could be easily accomplished with my 3 ball governor and a pointer attached to the lever arm. Depending on centrifugal force and the strength of the return spring it would be quite repeatable after initial calibration. I am semi busy with some engineering design work at the moment, so I probably will not do any more work on this thread.---Brian


 
so,it's a fail ??


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## gus (Jan 21, 2015)

Blogwitch said:


> Gus,
> The sensor with those units does have a fairly large surface area on the face, it is about 10mm diameter, so not tiny, but you could just stick a small neo magnet onto the flywheel and bring the sensor into fairly close contact and it will read the RPM.
> That is why they are such a boon for machinery, very robust and on machines there is usually not a problem mounting the sensor somewhere.
> BTW, the leads should have the correct plugs on the end so that it all just plugs together, and I used a cheapo voltage switchable wall wart to give me the input voltage. Just mount the bits into a small plastic case with the sensor nose sticking out of it and you will have a portable (almost) unit.
> ...



Hi John,

Thanks.With your expert advice Gus don't have to run around chasing his tail or re-invent the wheel. Will be fun getting it going and using it to read speed on my DIY Engines. Will DIY a wooden case to house new DIY Toy. Now waiting for kit to arrive from HongKong. Ordered place with UK and shipped from HK.
Back in the 60s ,it will take at least 3 months to receive it into my hot little hands.


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## Brian Rupnow (Jan 22, 2015)

Yes Luc--In it's current form, it is indeed a "fail".


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## canadianhorsepower (Jan 22, 2015)

Brian Rupnow said:


> Yes Luc--In it's current form, it is indeed a "fail".


 
I was wondering if you will go back to it one day

or simply said "the heck with it"

cheers


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## Brian Rupnow (Jan 22, 2015)

Luc--I never stop thinking of mechanical things. I don't do it intentionally. My mind just always goes there, always has, if I don`t have something else to be thinking about. I am flooded by ideas for mechanical things, especially just before I drift off to sleep or when I have just woke up. Most of the things I think of work. Some work better than others. Some work immediately. Some work after I have spent time tweaking them. And some---well, some I try and if they don`t work immediately, I`m not really interested enough in them to put very much work into them. This is one of those things. It doesn`t really interest me enough to put any more effort into it. If somebody else wanted to take the idea and run with it, that would be great, but I won`t spend any more time on this one.---I think.


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## canadianhorsepower (Jan 23, 2015)

Brian Rupnow said:


> , but I won`t spend any more time on this one.---I think.


 

hi here is some gas for you

[ame]https://www.youtube.com/watch?v=3_vfyt6-2Ic[/ame]


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## RichD (Jan 23, 2015)

Well, at least "Now" I understand how a speedo works.
Thanks,
Rich


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## bmac2 (Jan 23, 2015)

Thanks Luc
Not the best animation Ive seen on YouTube but it certainly does a good job of explaining things


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