How To Measure Diesel RPM

[zzvyb6]

I have a 3 cyl JD diesel mower with a Yanmar motor. Engine (hence blade) speed seems a little low. There is no tachometer on this machine. So how would I go about measing the engine rpm at full throttle? I see there is a governor adjustment screw on the pump/throttle body, but I don't want to mess with it unless it needs some adjustment. There are no direct electrical signals coming off the crank or camshaft. Here's what I've considered:

My Dad had a gizmo that was some type of shaft rpm indicator (maybe for a lathe or drill press). Its just a huge worm gear reduction set. You push the bit into the shaft center and count the wheel turns per time interval. Not sure I want to put my fingers that close considering the tight space.

Clip a playing card onto a bracket and let it rub on the crank pulley with a notch filed into it. Record the noise with Windows Sound Recorder and analyze the wave file for dominant frequency.

Park the mower near my JD tractor with a 4 cylinder motor (which has an accurate tach) and see if they hook up into a Dopplar frequency tone. This probably isn't accurate because the 4 cyl firing frequency is not the same as the 3 cyl firing frequency. The 4 cyl with tach is accurate because when the rpm is set to 540 and my pto alternator is hooked up, I get a measured 60 Hz line frequency off my circuit analyzer (measures Watts, Hz. and Voltage).

Another deviation from #3 is to use the playing card trick on both motors and listening for when they both hook up.


Is there a tool I could buy or rent to give me this answer? It's for setting idle AND top speed rpms. Once it's done I won't need it again so buying a tachometer is a hard investment to justify.

Just wondering out loud today....

 

[CharlieS]

Do a web search for laser tachometer, or non-contact tachometer. lots of hits.

Google is your friend

 

[Craig Clayton]

To zzvyb6
I used a non contact tachometer that I borrowed from work for over night. I put the sticky reflective paper on the air shroud of my 19.5 HP JD. It worked great.
Craig Clayton

 

[Treemonkey1000]

I just bought one of those no contact tachs at Harbor freight for about $40. It has reflective tape that you stick on the item you want to measure. Start the machine up and squeeze the trigger on the tach tester. I am going to use it for tuning up my chainsaws and checking the 540/1000 pto on my Kubota.

 

[RobertN]

Laser tach. Use them all the time at work. As others mentioned, put a small piece of reflective tape on the pulley, start the motor, and shine the laser at it.

Ours are fancier; I think they cost a couple hundred bucks. We require a calibrated unit with high accuracy. Should be able to find a suitable one for cheaper, to check engine RPM.

Or just add a tach...

this is the one I use
Monarch Instrument Pocket Laser Tach 200 Tachometer - instrumart.com

 

[dex3361]

I agree with Treemonkey1000. I started a thread a while back on this very issue so here is a link. Tractor RPM

 

[Soundguy]

you can get a vibration tack for a diesel that feels the pulse of the injector line.. big diesel shop might let you rent one.

that or the contact tach you havem just pressed against the PTO shaft.. that way you can measure direct pto rpm

if you like math and have a rolling wheel measuring device you can measure the circumference of the pto shaft, and rolling wheel circumference. spin it for a set time, then do the math on what the wheel counts and figure it out..

soundguy

 

[Treemonkey1000]

And of course keep your cotton pickin hands, clothes, long hair, hippy beads or what ever away from those rotating parts!

 

[travis potter]

I bought a tack from HF and use it by Wifes Dr. Offices to set their centrifuges It passes state lab inspection . I also have a vibro tac that I bought 40 years ago you plase it on any a part that dosen't turn and slide a wire in or out till it viberates the most then read a scale that gives you the rpm. I use it on mowers when I tune them up.

 

[kennyd]

You could add this to your machine: Welcome To Tiny-Tach. Small and economical tachometers for gas and diesel engines.

 

[zzvyb6]

Thanks for the advice. I went to the Harbour Freight website for the photo tack and almost had it ordered (+9.99 shipping). I was curious about where a store was and low and behold, there's one 28 minutes from here. Set the TomTom for cruisin' and headed out. They had the Photo Tach in stock (I called first to confirm).

I know there's a thread here for HF Tools that Don't pass muster, but I have to admit I stayed there 2 hours longer than I planned. It wasn't just for the blond in the short shorts, either....

 

[dave1949]

, but I have to admit I stayed there 2 hours longer than I planned. It wasn't just for the blond in the short shorts, either....

Ain't Summer a great time of year? :laughing:
Dave.

 

[Soundguy]

Thanks for the advice. I went to the Harbour Freight website for the photo tack and almost had it ordered (+9.99 shipping). I was curious about where a store was and low and behold, there's one 28 minutes from here. Set the TomTom for cruisin' and headed out. They had the Photo Tach in stock (I called first to confirm).

I know there's a thread here for HF Tools that Don't pass muster, but I have to admit I stayed there 2 hours longer than I planned. It wasn't just for the blond in the short shorts, either....

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.

i bout a decnt 3/4 set of breaker bar and ratchet, with 3 and 6" extension, plus sockets from 7/8 thru 2.5" for a whopping 28$ a few ys ago. couldn't touch that anywhere else for that price. metal ont he BB and ratchet is 1" pared down to 3/4. I've had an 8' pipe on them with me and a buddy pulling on it to do stuff and the pipe flexed but not the BB.

soundguy

 

[JC-jetro]

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,

just one for reference,

Strobelight Tachometer. 1542-B Strobe Tachometer. RPM Measurement.

 

[Soundguy]

at 1000$ it pretty much prices it out of this world for the hobbiest who doesn't have a business of reading moving parts speeds...

hopefully the OP will be safe in whatever manner he chooses to test his rpm.

soundguy

 

[JC-jetro]

at 1000$ it pretty much prices it out of this world for the hobbiest who doesn't have a business of reading moving parts speeds...

hopefully the OP will be safe in whatever manner he chooses to test his rpm.

soundguy

Absolutely correct. I could not justify it for home use as well. I'd go to HF as well. I'm very much in to enforcing lock out/ tag out in my projects. I have used the optical drive myself too but I do want to make sure that I throw the disconnect off and have it locked out so I can safely put the reflective tape on motpr shaft, etc. One of our maintenance guys at work got so close to getting his finger stuck in motor sheave when the motor started running while he was trying to put the tape on..:shocked: he came pretty close.

I'd suggest to OP to make sure do a dry run with the engine off , figure out where is the best place to put the reflective tape on, clean the surface so the tape would not come off.

JC,

 

[RobertN]

I test fan heatsinks for PC's fairly often at work. Although most are four wire now, with their own tach output, we still verify with second source. We used to use strobe-light tachs. Too expensive though, and a hassle to use. At least compared to a laser tach.

Our little Monarch laser tachs are cheap(relatively) at less than $250. And, are calibrated(we keep ongoing yearly calibrations too).

The laser tach is instantaneous too. Well, you do have to put a piece of tape on whatever you are measuring.

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.

 

[Treemonkey1000]

I am fortunate enough to work in a calibration lab that we certify mechanical and optical tachs along with a ton of other electronic equipment items.. Even a few percent error is no big deal for the tractor stuff or other rotating items. The Lock out tag out might not be a known factor to a lot of folks.. I have to recertify on lock out tag out each year. Some of the high voltage power systems I work on will run small factories. :confused2:

 

[JC-jetro]

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.

JC,


For whom who have not heard of Lock out/Tag out. in order to avoid live circuit injuries or death during repair or modification one would throw out the electrical disconnect to off position or turn off a circuit breaker, use a special locking device including pad lock to lock and would leave an identification tag as to who had done it. The person who is doing the work holds on to the key for obvious reason.

 

[Soundguy]

we have a similar program at work. if we have equipment in the shop that is down for repair, and it has guards on rotating parts that are removed, the machine needs an out of service tag.. otherwise if osha inspected and saw a machine with an open belt, etc.. we could be fined. didn't matter if it had all the pistons out and the oil pan dropped..

soundguy