Rpm indicator

[Brian Rupnow]

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.

 

[MachineTom]

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.

 

[Blogwitch]

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

 

[RichD]

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

 

[goldstar31]

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

 

[tornitore45]

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.

 

[Herbiev]

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.

 

[Brian Rupnow]

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

 

[Brian Rupnow]

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.

 

[bazmak]

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

 

[tornitore45]

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.

 

[Brian Rupnow]

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

 

[Cogsy]

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.

 

[ruzzie]

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.

 

[tornitore45]

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.

 

[gus]

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

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.

 

[Brian Rupnow]

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!!

 

[Cogsy]

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

 

[bazmak]

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

 

[///]

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.