How To Measure Diesel RPM

[flusher]

hey.. that's like an adult male toy store.

I know there is a good deal of junk there.. but on the other hand.. there truly are somegood deals especially for a tool you may only use once.. or for the hobbiest who needs a set of tools for tractor maintenance vs making a living off them.


soundguy

Yep, you have to be selective at HF.
I bought a $149 compound miter saw with laser at HF. Used it 5 years. Retired it when the plastic safety shield cracked up badly around the pivot screw. The motor and bearings (Chinese of course) were still going fine.
Also got 5 years out of a $89 pancake compressor at HF.
I'm not complaining.

 

[Soundguy]

Yep, you have to be selective at HF.
I bought a $149 compound miter saw with laser at HF. Used it 5 years. Retired it when the plastic safety shield cracked up badly around the pivot screw. The motor and bearings (Chinese of course) were still going fine.
Also got 5 years out of a $89 pancake compressor at HF.
I'm not complaining.

Funny you mention a pancake compressor from HF. i have one I got back in 97.. so it's now 13ys old. I actually used it and a 15 g spare air tank to paint my first 2 tractors.. so that poor thing ran continously.

I've changed the oil in it 3 times.

about 3ys ago.. it stopped pumping up past 20 psi. I was about to chuck it when i decided to open it up.. turns out the gasket betweent he head and cyl was bad.. melted and gummed up.. reeds were fine. Knowing i likely couldn't get oem parts for it, I simply grabbed some appropriate looking gasket paper fromt he sheld and an exacto knife and cutout what all looked like it needed cutting out on the 3 gaskets it needed.. one on either side of a spacer.. and one less critical looking one on a i/o manifold... to my surprise.. it worked. and in fact.. worked better than when new.. IE.. built air pressure faster.. methinks the oem gasket leaked a bit when new.

lil thing shows no signs of stopping.. I keep it inthe house garage for airing tires now.. with a newer better upright 30g/5hp unit out in the shop.

I paid 89$ as well for that compressor.. not bad for 6.80$ per year of use.. with the first 3 years being 'abuse'...

soundguy

 

[20 20]

Back to topic, if I'm not mistakin Northern tool also carrys some sort of tach's.

 

[JC-jetro]

Back to topic, if I'm not mistakin Northern tool also carrys some sort of tach's.

can't beat $20.99 and free shipping.

JC,

Digital Laser Photo Tachometer Non Contact RPM Tach : GadgetTown.com

 

[RobertN]

No doubt, very viable. Just pricey.

One thing I like about the current laser tach I use, is it is light, small. Our old strobe tachs were big ole honkers. I did like the fact e didn't have to monkey with reflective tape with the strobe-tach.

Used plenty of lockouts/tag-out. Mostly on 115VAC/20A or 230VAC 30A circuits. Mostly working on componenets of photoniic emission microscopes. Failure Analysis stuff for microprocessors and flash memory.

There is a ton of good information that is being dispersed here every day. I was just merely pointing out one option not talked about that is very viable.

For whom who have not heard of Lock out/Tag out.

 

[zzvyb6]

Follow up:

Got the HF Photo Tach. Put the batteries in, removed the display protector, applied 3/4" of tape to the crank pulley, started her up:

1100 rpm at idle (low should be 1450).

3280 rpm at fast idle (low, should be 3640).

The low idle made me suspicious of a bad engine mount or weak cylinder (she's a shaker). The low high-end made me believe the mower spindles were going a tad slow (not that 350 rpm is a huge change).

Interesting that both readings are low by about 350 rpm. I wonder if that drop is caused by wear/hours or a problem with the last dealer tuneup (flawed tach) or the HF tach is off. I'll check it against an AC motor just for reference.

This instrument came with an appropriate, protective case and a set of instructions in only one language so I didn't have to paw through sections for French, Islandic, Antarctic, or Bronx translations.

 

[JC-jetro]

Follow up:

Got the HF Photo Tach. Put the batteries in, removed the display protector, applied 3/4" of tape to the crank pulley, started her up:

1100 rpm at idle (low should be 1450).

3280 rpm at fast idle (low, should be 3640).

The low idle made me suspicious of a bad engine mount or weak cylinder (she's a shaker). The low high-end made me believe the mower spindles were going a tad slow (not that 350 rpm is a huge change).

Interesting that both readings are low by about 350 rpm. I wonder if that drop is caused by wear/hours or a problem with the last dealer tuneup (flawed tach) or the HF tach is off. I'll check it against an AC motor just for reference.

This instrument came with an appropriate, protective case and a set of instructions in only one language so I didn't have to paw through sections for French, Islandic, Antarctic, or Bronx translations.

350 rpm discrepancy/ inaccuracy for the range from 1000 to 2000 rpm is too much. Under 20-40 rpm one either low or high is tolerable. I'd suggest you to compare and contrast ( calibrate) with something that you know the rpm of. Your furnace motor rpm is most likely 1150 or on a single phase motor. The way motor leads are landed also make a difference on RPM. rpm information should be affixed inside your furnace sheet metal access door. Check your meter against that to see how off if any you are. if you have any other stuff around the hose such as a box fan can be used (direct drive) as long as the rated rpm is stamped on the device. Utility voltage fluctuation if any can make insignificant change in the reading.

JC<

did you measure your final pto shaft rpm?

 

[Soundguy]

could check against a vehicle with a tach as well.. like a car engine.. etc..

soundguy

 

[Treemonkey1000]

I have the same tester from Harbor as you do. I just haven't gotten it over to my tractor to try it out or bring it into work to see how accurate it is. You may aim the tach tester at a incandescent or flourescent lamp. Those are strobing at 60 hertz. Our eyes don't see it. Your tach tester might pick up the strobing. If not try a pair of polarized sunglasses in front of the tester lamp.. It might work.

 

[CFHartman]

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

 

[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