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tommylad

Making a rev counter - how hard can it be?!

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OK, got a challenge for all you bored people in lockdown atm!

I have this rev counter which is intended to be belt driven. I really like the idea of fitting new internals such that I can retain the clock face and needle, but drive it off the W post of my alternator.

 

I'm confident enough of being able to do the mechanical side of this job, but I would appreciate if anyone can advise if it's possible to buy a rev counter that can be 'calibrated' to display the correct engine speed on this face? My alternator will be doing 7 353 rpm when the engine is at 1 000 rpm.

 

Thanks!

DSC_0123.JPG.1b8339e0c84ea2de9c180a9872903de2.JPGDSC_0125.JPG.d088942d05d875b82ae747c4e882a691.JPG

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6 minutes ago, David Mack said:

Why not just fit a belt drive using a suitable pulley ratio to give the right reading?

I did wonder about this, but the pulley is the wrong orientation (the gauge would end up facing sideways!) and I'm not keen on a mechanical drive system which would need maintenance, a shroud for safety and also limit installation positions. As it happens, I'd also struggle to get a belt drive off my engine for it!

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12 minutes ago, David Mack said:

Why not just fit a belt drive using a suitable pulley ratio to give the right reading?

I suspect the OP wants to be able to see the counter from his conning / driving position...

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Try a ISSPRO R8585M programmable tachometer. Works on the alternator or magnet on flywheel.

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I assume you are going to fit the "guts" out of a modern electronic tacho into your housing?  That's pretty easy as long as you have enough depth. The angular range of a typical tacho is probably less than the range of your fascia so you might struggle to get to 1000, and starting at 250 will not help.

 

There must be a fair few electronics types on this forum who are currently at a loose end, maybe you could bribe one with beer.   One approach is to take the electronics out of the tacho and just use the meter movement (which is very linear) and then build your own electronics, and the easiest (🙂) approach is to do it mostly in software with an Arduino or similar.

 

................Dave

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5 hours ago, tommylad said:

I did wonder about this, but the pulley is the wrong orientation (the gauge would end up facing sideways!) 

 

Not if you use a quarter turn belt drive.

 

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I suspect that you have a mechanical drive tachometer in which rotation is transferred to the needle via a drag cup. See e.g. figure 10.54 I think that might be difficult to convert!

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3 hours ago, Onewheeler said:

I suspect that you have a mechanical drive tachometer in which rotation is transferred to the needle via a drag cup. See e.g. figure 10.54 I think that might be difficult to convert!

Like we all used to have as car speedometer.

The OP may be able to find a modern electronic tacho which fits within the housing, but you'd struggle to find one with a full scale deflection of 1000 rpm. No modern engine goes that slowly.

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Fix a speed-controllable electric motor to the tacho.  And control the speed by some clever sensor and electro-gubbins on the engine end.

 

Bizzard

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Lots to consider here. Yes, my thinking was to gut the existing tacho and basically only keep the housing, face and needle, which would be driven by the movement from an electronic tacho.

DMR - you've raised a very good point about starting from 250 - that may well be an issue. Perhaps the arduino idea could resolve this, although it sounds like a bit of a can of worms!

Tacet, a motor drive is an idea, although I'm not sure it would be any simpler, and I'm not convinced how linear such a system would be?

And jake_crew, I completely take your point about ruining an old instrument, however, I can't think of an application for this now? I assume it would've come off some form of stationary engine. To modify it for use on a boat surely can't be that bad?! (it beats sitting in a field at steam rallies :-P) That said, I think the modification may just prove to be too difficult...

 

I'm now moving towards perhaps still buying this https://www.interparts.uk/1224v-alternator-pick-up-tachometer---0-8000rpm-0-523-80-7314-p.asp?gclid=Cj0KCQjwjoH0BRD6ARIsAEWO9DtSmZFK_S817wbf8T0mHfa2QzVmw2uluzyn2P6K8YP2mDAjNQ4XPMgaAiTfEALw_wcB or the ISSPRO but asking a local instrument restorer to repaint the face such that it goes from 0-1200 rpm which is the correct range for my JP (although I never take it above 900 anyway). I'll then be able to calibrate it quite easily.

 

Many thanks for the tips!

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2 hours ago, tommylad said:

Lots to consider here. Yes, my thinking was to gut the existing tacho and basically only keep the housing, face and needle, which would be driven by the movement from an electronic tacho.

DMR - you've raised a very good point about starting from 250 - that may well be an issue. Perhaps the arduino idea could resolve this, although it sounds like a bit of a can of worms!

Tacet, a motor drive is an idea, although I'm not sure it would be any simpler, and I'm not convinced how linear such a system would be?

And jake_crew, I completely take your point about ruining an old instrument, however, I can't think of an application for this now? I assume it would've come off some form of stationary engine. To modify it for use on a boat surely can't be that bad?! (it beats sitting in a field at steam rallies :-P) That said, I think the modification may just prove to be too difficult...

 

I'm now moving towards perhaps still buying this https://www.interparts.uk/1224v-alternator-pick-up-tachometer---0-8000rpm-0-523-80-7314-p.asp?gclid=Cj0KCQjwjoH0BRD6ARIsAEWO9DtSmZFK_S817wbf8T0mHfa2QzVmw2uluzyn2P6K8YP2mDAjNQ4XPMgaAiTfEALw_wcB or the ISSPRO but asking a local instrument restorer to repaint the face such that it goes from 0-1200 rpm which is the correct range for my JP (although I never take it above 900 anyway). I'll then be able to calibrate it quite easily.

 

Many thanks for the tips!

 

It would be a little sad to redo the face as it will loose its oldness and character. If you only run to 1000 then its fine as it is.

First locate a tacho (or good photo of one) and check if its angular range is adequate for your needs (that is, that the 250 to 1000 angle on your meter is no greater than the 0 to max angular range of the tacho).  The electronics/Arduino approach is really not a big deal as long as you know someone who knows a bit about this. I speak here as someone who has actually done this, though in my case I converted to a brass pressure gauge to be an electric oil pressure gauge. I built the electronics from scratch but if I did it again I would use an Arduino. Maybe somebody on the forum would do the software side of things for you. Can you solder and do very basic electronics????

 

If the tacho is fully adjustable and has adequate angular range then you could use it as-is without any electronics building, but its zero will be at the zero position (not the 250) of your meter face which should not be a problem.

 

.................Dave

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13 minutes ago, dmr said:

 

It would be a little sad to redo the face as it will loose its oldness and character. If you only run to 1000 then its fine as it is.

First locate a tacho (or good photo of one) and check if its angular range is adequate for your needs (that is, that the 250 to 1000 angle on your meter is no greater than the 0 to max angular range of the tacho).  The electronics/Arduino approach is really not a big deal as long as you know someone who knows a bit about this. I speak here as someone who has actually done this, though in my case I converted to a brass pressure gauge to be an electric oil pressure gauge. I built the electronics from scratch but if I did it again I would use an Arduino. Maybe somebody on the forum would do the software side of things for you. Can you solder and do very basic electronics????

 

If the tacho is fully adjustable and has adequate angular range then you could use it as-is without any electronics building, but its zero will be at the zero position (not the 250) of your meter face which should not be a problem.

 

.................Dave

Dave,

 

Sorry, I wasn't very clear in what I said. What I was proposing was to give up on the old tacho, and simply buy a new one, repaint its face and re-calibrate. This has the benefit of being quite straightforward, and giving me the ideal rev range, however, loses the character of the old instrument.

 

I'm quite comfortable with programming though, I've done a bit in VBA and arduino code looks not too different to this, and I'm quite competent at basic soldering etc. I'm just not sure how big a project I want to make of this! I've managed for 30 years without a rev counter so it's definitely in the 'nice to have' category!

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6 hours ago, tommylad said:

 I assume it would've come off some form of stationary engine. 

The name 'Gilkes' suggests it was possibly once  part of a water turbine set up.

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If you just want to know the engine rpm then a bicycle computer sorts it. 

 

Not a very pretty solution but accurate and easy to mount near the steering position by simply extending the wire to the reed switch. 

 

 

 

 

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Just a couple of small points.

 

If the internals of the unit are indeed of the drag cup design then I suspect that Zero and 250, as far as the mechanism are concerned the same thing, that is to say that the magnetic drag exerted will be insufficient to move the needle until 250 rpm is achieved.

 

I note that just above the "RPM" legend there is printed the numbers  "4 : 3". This is most likely to be the pulley ratio required to read the revolutions correctly.

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