What's the RPM

Tony Wells said:
Calculate the circumference of the shaft. Put white marks (whiteout will do) spaced such that they are an even multiple of 60 Hz at either 1725 or 3450.
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Tony,
Okay, I like this kind of challenge. Give me a hint about the above quote. My old brain is spinning and smoking, but I'm lost on the spacing of the marks. I understand the methodology, just ain't sure about the spacing.
 
OK, imagine the circumference of the shaft flattened out. Now at 1725 RPM, how many feet per second (or inches, or MM or CM, as you wish) does it travel? Calculate how far apart the marks need to be to represent 1/60th of a second. Now convert it (mentally) back to a circumference, and you know how far apart to mark the shaft. Of course, there are limits to consider, a 1/4" shaft may not be "long" enough to mark, as the circumference may be shorter than the distance traveled in 1/60th of a second. I haven't done this kind of tach work in ages, but it does work. The 60Hz (or 50 in places) is generally very accurate. It's what keeps out plug-in clocks running
 
Tony Wells said:
OK, imagine the circumference of the shaft flattened out. Now at 1725 RPM, how many feet per second (or inches, or MM or CM, as you wish) does it travel? Calculate how far apart the marks need to be to represent 1/60th of a second. Now convert it (mentally) back to a circumference, and you know how far apart to mark the shaft. Of course, there are limits to consider, a 1/4" shaft may not be "long" enough to mark, as the circumference may be shorter than the distance traveled in 1/60th of a second. I haven't done this kind of tach work in ages, but it does work. The 60Hz (or 50 in places) is generally very accurate. It's what keeps out plug-in clocks running
Click to expand...

Thanks Tony. I just needed a kick start, but it makes sense to me now. I'll look for a strobe light (60Hz), however, it might be just as cost effective to get that laser tach from HF mentioned above.

Tim,,,
 
nctox said:
however, it might be just as cost effective to get that laser tach from HF mentioned above.

Tim,,,
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I think so, plus you will find lots of uses for the tach once you have it. I have used mine on my lathe. mill and drill press, as well as tuning up an old snowblower engine.
 
i have a rev counter that i bought 50 years ago and it still works as good as new but you require a watch with a second hand to use it.

BRIAN.

P1011361.jpg
 
What a great post... Just a comment on the little electronic RPM device. I bought one for around $15 and it came with a set of reflective stickers. Apply a small piece to a pulley, shaft or belt and the device gives you the RPM. If you run out of the stickers, you can apply black electrical tape to the pulley except for a small spot and the thing will pick up the rotating bright spot.

Rex

I have little reflective stickers on most rotating surfaces.
 
Hmmmm, how about setting your camera to 1/30 sec for 1725 (flash OFF)? 1725/60= approx 30 revs/sec 1/30 sec should freeze it, 1/60 for 3450 rpm. I too have that laser tach for < 10 from ebay...works great
 
The rev counter tool, of which Brian provided the photo, is very quick and easy to use. I somehow find opportunities to utilize this device at least once a month, if not more often. Feels better in the hand than some of that HF stuff, too.



Matt
 
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