How To Measure Diesel RPM

[Soundguy]

maybee a harmonic frequency?

soundguy

 

[Baby Grand]

maybee a harmonic frequency?

soundguy

maybee, but I sort of like harmonica frequency.
eg: James Cotton/The Creeper - talk about your side band frequencies!

YouTube - James Cotton - The Creeper

I guess I have to stop by HF now and get one of those tachs - the price is right and I can think of lots of uses. Like a dedicated PTO read out that doesn't keep resetting like the Intellipanel does. I hate when it does that!

-Jim

 

[SPYDERLK]

All the suggestion made are good ones but as far as safety is concerned nothing is better than strobe-light tachometer, although it is pricey. I bought one for work many years ago just under $1000. I can get close (safe distance) to a rotating mechanical equipment, shine the light on any rotating part on pumps, fans and motor and adjust the frequency in which the light strobes by a knob. Once the target is motion less then rotational speed is the same as readout on the device. Can read rpm on fan motor and fan pulley safely in few second.
JC,

You can get strobes for around $200. I have one that does 100 to 10,000 flashes per minute. Bought it for $175 in 1991. Still works. :thumbsup:
larry

 

[SPYDERLK]

something I would like to say about the strobe light. I worked on and rebuilt steam turbines. The strobe was very good when you had an idea were you were suppose to be but starting from scratch it was easy to find what they called a harmonica frequency. were it looked like everything was stopped. When sitting the trip on a turbine we also used a vibration tack. Once you brought the strobe in close to the vib tack you hat very good accuracy

You can get true rpm by finding adjacent harmonics. Suppose your strobe will do 500 flashes per minute and you get sync on the item youre measuring at 500. So 500rpm ... right? Well, it might be doing several revolutions for each flash and fool you. If you could flash faster you could check this by adjusting upward to see if there was a higher sync point. [There would not be if true rpm was 500 because it would quickly be obvious that you were flashing more than once per rev. To check by adjusting downward you carefully look for the very next sync below 500. Say you find one at 333. You can find the true rpm by multiplying the 2 together and then dividing that # by the difference. So ... 500x333/500-333. You have an item turning at 1000RPM. It initially had you fooled by doing 2rev per flash.
larry

 

[dex3361]

You can get true rpm by finding adjacent harmonics. Suppose your strobe will do 500 flashes per minute and you get sync on the item youre measuring at 500. So 500rpm ... right? Well, it might be doing several revolutions for each flash and fool you. If you could flash faster you could check this by adjusting upward to see if there was a higher sync point. [There would not be if true rpm was 500 because it would quickly be obvious that you were flashing more than once per rev. To check by adjusting downward you carefully look for the very next sync below 500. Say you find one at 333. You can find the true rpm by multiplying the 2 together and then dividing that # by the difference. So ... 500x333/500-333. You have an item turning at 1000RPM. It initially had you fooled by doing 2rev per flash.
larry

I usually mark shaft with 1 mark and start high with enough on the flashes of the strobe to see multiple marks (multiples of the true speed) then back off on the strobe flash frequency till I see only 1 mark and there is your true RPM. If you continue to slow down the frequency of your strobe flashes you will see 1 mark at fractions of the true speed so you have to watch for the first time you see only 1 mark as you are reducing the frequency of the strobe flash. It sounds more difficult than it is in practice.:thumbsup:

 

[SPYDERLK]

I usually mark shaft with 1 mark and start high with enough on the flashes of the strobe to see multiple marks (multiples of the true speed) then back off on the strobe flash frequency till I see only 1 mark and there is your true RPM. If you continue to slow down the frequency of your strobe flashes you will see 1 mark at fractions of the true speed so you have to watch for the first time you see only 1 mark as you are reducing the frequency of the strobe flash. It sounds more difficult than it is in practice.:thumbsup:

Starting hi is good. If your strobe has a high enuf flash rate it gives you that option. ... But if it doesnt you can still find the true rpm by the method I described.
larry

 

[irvingj]

Model airplane technology to the rescue... maybe.

I have an older (OK, MUCH older) version of this tool, and have used it for a number of RPM measurements. And yes, it will pick up the 60-hz strobing of a fluorescent light.

Cheap enough, but it does require a back-lit moving part to register.

Available from Tower Hobbies --or perhaps your local hobby shop-- for under $20.

 

[TractorRob]

...
I guess I have to stop by HF now and get one of those tachs - the price is right and I can think of lots of uses. Like a dedicated PTO read out that doesn't keep resetting like the Intellipanel does. I hate when it does that!

-Jim

Here is my dedicated PTO readout:

PTO RPM Indicator

 

[Baby Grand]

Here is my dedicated PTO readout:

PTO RPM Indicator

Tractor Rob,
Great idea, thanks!
-Jim

 

[ws105261]

Harbor freight has a digital contact tachometer; 29.99.
just press it against the end of the PTO shaft.
I used a similar tach to check pto speed on my tractor since the factory tach no longer worked.

Digital Contact Tachometer

Good Luck