Analouge tachometer

[Old SchoolParticipant @oldschool]

I race tether cars and I want to do some engine development in my workshop. The engines and tuned pipes are going to be run in a bench mounted test rig using model aircraft propellers as the load. I need to measure the rpm of the propeller or from the reduction gear box. I want to build the tachometer into the test rig and have a display I can easily read like a a full size car tacho. The tachometer needs to be capable of working to 30,000rpm. Where can I get one of these or can someone point to ready made modules that I can asessmble. Thanks for any suggestions.

[Old SchoolParticipant @oldschool]

[Brian WoodParticipant @brianwood45127]

Hello Old School.

​One suggestion for a simple rig might be to drive a permanent magnet DC motor and measure the voltage that is produced.

I take it the actual numbers of rpm are not in themselves important, you are more likely to be interested in orders rather than precise numbers and with imagination you could establish some calibration points to draw a graph for the system.

A fancy non contact method would be to use a strobe light shining on a segmented disc for 'stopped motion' calibrations if the graph is not sufficiently linear; it might even be possible to hire something for that stage.

Smiths Instruments made rev counters in the past, hand held devices that had different operating ranges which went up to the speeds you are expecting, but finding one of those today is a bit of a lottery.

Regards

Brian

 

 

Edited By Brian Wood on 03/09/2016 10:40:03

[Les Jones 1Participant @lesjones1]

You could use a normal electronic car tachometer such as this one. A six cylinder 4 stroke engine will give 3 pulses per rev from the contact breaker (Or electronic equivalent.) so if you arranged to have one reflective marker on your propeller then it would read one third of the actual speed. 30000 RPM would display as 10000 RPM. You could either make a new scale for the meter or mentally multiply the reading by 3. It would be much easier to get a digital tachometer.

Les.

[Geoff TheasbyParticipant @geofftheasby]

Have you a Smartphone? There are tachometer apps available.

Geoff

[Paul LousickParticipant @paullousick59116]

Why not a digital laser tachometer.

Available at electronic stores or on line for ~$25. Some measure up to 99,999 rpm.

[Neil LickfoldParticipant @neillickfold44316]

There are optical tacho's available that go off the light from the prop and will go to 50k or so rpm.

Mine is a futaba and is about 34 yrs old. I think place like hobby king sell them and maybe model shops inthe UK might also sell them.

Neil

[Old SchoolParticipant @oldschool]

I like your idea Les Jones1. The engines that I am testing are 2.5cc two strokes glow plug ignition with tuned pipes rpm is around 42,000 the life time of a liner and piston is measured in minutes. I will be making changes to the pipe length and fuel supply whilst the engine is running what I am looking for is the change in rpm up or down due to these changes. It's so much easier to read a pointer rather than scrolling digital numbers. Les how do I generate the pulses for the tacho. Thanks for the help.

[Les Jones 1Participant @lesjones1]

You could put a reflective marker on one blade of the prop. (So as to get one pulse per revolution.) It may make it easier to put a divide by 2 circuit between the sensor and the tachometer so that you could just use the reflection from the blades of the prop. (Dependingon your setup it may be easier to let the prop break a light beam.) If the prop was white in colour it would make sensing it easier. Having a cheap laser pointer pointing at the prop would make it easier to sense the reflected light pulse than if you just used an LED as a light source. Let me know which sensor option would fit in with your test rig best and I will have a look for suitable photo sensors. (A picture of your test rig might halp with suggestions.)

Les.

Edited By Les Jones 1 on 03/09/2016 12:43:34

[Greg HParticipant @gregh]

I use one of these on my model aeroplanes and they work well. It's hand held, but you could mount it in a frame pointing at the prop.

**LINK**

[Gordon Brown 1Participant @gordonbrown1]

I have in my workshop a mint condition Smith's analogue tacho that would be perfect as you can drive it from the prop spinner by using the included rubber cup drive, but it only goes up to 10,000 rpm I'm afraid.

Gordon

[Neil WyattModerator @neilwyatt]

Posted by Brian Wood on 03/09/2016 10:39:01:

​One suggestion for a simple rig might be to drive a permanent magnet DC motor and measure the voltage that is produced.

This works rather well. My dad made an anemometer for our scout troop when I was kid, using a big old voltmeter and a simple amplifier and calibrated it by driving back and forth on a disused aerodrome

He also used a huge length of multicore cable and a big handful of reed relays to make a weather vane with an LED display (this was in the early 80s so there weren't many arduinos about).

Both dials were nicely finished and mounted in a slab of hardwood. As I recall it was going for the best part of 20 years and may still be working.

Neil

[Old SchoolParticipant @oldschool]

It will need to be a good motor to be capable of running up to 30,000 rpm. I prefer something that is non contact.

[Brian WoodParticipant @brianwood45127]

Old School,

​Non contact is to be preferred, certainly at those speeds, I threw it is as a potential and fairly cheap solution. It is perhaps more a workshop solution to machine tools spindle speed measurement; ultra high speed will be tough on the bearings of any contact method.

Regards

Brian

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