rpm counter

[peterd51]

Hi,

I've got one of these electonic 'laser' rpm counters to play with, off ebay, and just tried it on my pillar drill.

The drill was set up on the second pulley, out of four available, and the chuck was running at 1190 rpm. So I dropped it to its lowest pulley and it runs at 480 rpm. But how accurate will this be...

imagine the chuck is a couple of inches diameter, so about six inches circumference. The counter sees the refective strip whizz past at so many times a second. But if the chuck was four inches diameter, and 12 inches circumference, then would it read half the revs per second?

Sounds a reasonable assumption to me. There's nothing in the instructions about chuck sizes, etc. Chinese...

On the pillar drill the motor is a 1/3hp, marked up as 1450 rpm so do my readings for the two lowest pulleys sound about right?

I guess I could 'calibrate' the tester by using the car...stick a bit of strip on the bottom pulley at 1" circumference and run the engine at tick-over, I know that's 900rpm. Then try it again with the strip at 2" and again at 3", etc.

Or maybe try my B&D power drill as I'm sure that has the rpm marked on the case?

What do you reckon?

Regards
Peter

 

[chunkolini]

Surely RPM is a simple count of how many times the thingy is rotating per second/minute. Are you getting this mixed up with speed at the circumference? which would vary with the diameter of the spinny thingybob. Sorry to use technical jargon, but these things must be given their formal titles or we run the risk of being accused of dumbing down.

Chunko'.

 

[Benji]

I expect this post is a wind up (if you'll pardon the pun), but just in case it isn't, the linear speed of a point on the circumference of a disc increases with diameter, at a constant rpm at any diameter the point will pass a given datum a set no of times corresponding to the rpm. Hope this helps!!
Ben

Blimey Chunko, if I'd known you were a physicist, I would have left you to it; honest

 

[bigbadbugga]

RPM is revolutions per minute, it doesn't matter if the thing your measuring is 5cm diameter or 5metres diameter.

 

[Jaeger_S2k]

RPM is revolutions per minute, it doesn't matter if the thing your measuring is 5cm diameter or 5metres diameter.

But the RPM of each will be different.

 

[bigbadbugga]

But the RPM of each will be different.

errrr, no it won't

the speed of a single point at the circumference will be different, but the rpm will be the same.

(providing the subject of the measurement is not being driven as part of a gear system of course)

 

[bloobloo]

The way I understand it is simply this........... a 25mm shaft, spinning at lets say 1 RPM.... this is attatched to any size chuck / wheel or whatever, lets say a 100mm one for easy reckoning........ 100mm chuck still only spins at 1RPM as it is attached to a shaft spinning at 1RPM

 

[bigbadbugga]

The way I understand it is simply this........... a 25mm shaft, spinning at lets say 1 RPM.... this is attatched to any size chuck / wheel or whatever, lets say a 100mm one for easy reckoning........ 100mm chuck still only spins at 1RPM as it is attached to a shaft spinning at 1RPM

exactly.

RPM is the number of times the object revolves through 360 degrees in one minute. it is not a measure of speed.

 

[Jaeger_S2k]

The RPM is the same! You're all right I'm wrong. My bad.

Got mixed up with speed and distance travelled.

For me and the OP, the RPM is the same for any diameter point attached to the rotational shaft. The shaft rotates once and the chuck (say 50mm) rotates once, the chuck however travels a greater distance therefore travels faster but still only rotates once.

 

[peterd51]

Hi,

No it's not a wind up, I am interested in checking to see if this tester needs calibrating.

Some say there'll be a difference, some say not...

I'll have a play at the weekend and see if I can get a different reading.

Ah, I just had another thought...

I could place a disc in the drill, say a 6" diameter wheel, and place a strip on that, then I can take readings off each and see if it's the same.

Regards
Peter

 

[shenion]

Many optical RPM gauges can be calibrated by pointing them at a fluorescent light.

If it uses 1 mark per rev, then it will be 6000 RPM. 2-marks would be 3000. (50Hz)

at 60Hz, would be 7200/3600.

I test my RPM meter used in R/C engines that way.

 

[astro_al]

I'm not following why you think there might be an aliasing problem. I have on of these digital rpm jobbers. It works well.

Did you stick on a bit of the reflective tape that comes with it? I stuck a small square near the top of the chuck (well out the way of chuck keys etc). I get readings with 1 or 2 of the speed quoted on the plate on the drill (using it to check the reader device). It works fine. It doesn't know how big the object is its measuring, its just the number of pulses per second remember.

There is something in the small print about too high rpms not being correctly read, but I think its way out of range of tool rpm measurements etc. Presumably it has a minimum time between pulses - for a capacitor to charge / discharge or whatever.

It works. Probably. Whether the rpms sound right depends on your drill, but thats pretty fast for drilling steel etc with a 'fair' diameter drill bit.

Al.

 

[peterd51]

HI,

Sennion, interesting, I'll try that.

Al, I had no idea what speeds the drill was running at as it's not marked anywhere. I've used it on the 'second' speed for years and always thought it was around 500 rpm (I assumed the lowest was around 300 rpm). Now I've found out it's over 1000 I've dropped the belt to the lowest pulleys.

My thinking on the chuck size is that if the meter is counting how many times the strip passes each minutes, if the chuck's larger and the strip has further to travel then would it count less 'passes' in each minute and give an incorrect reading.

From comments above it could be that my thinking is way off and the shaft spins at so-many revs at a particular speed, but the outside of the chuck (no matter how big) will be travelling at a higher speed than the shaft (as it has further to travel). So the meter will give an accurate reading, but I want to be sure that it does, or have some idea of any 'fiddle factor' that I need to apply for different sizes of wheel.

Obviously it doesn't matter much for drills, or the lathe, etc, except now I've got my drill running at the speed that I always thought it was! I have an idea that over the next year or two I could build some motorised transport, go-karts, bikes, etc, with the grandsons and it'd be nice to have an idea of what sort of speed a creation will do before they jump in and press the 'go' lever.

Regards
Peter

 

[Jaeger_S2k]

Blimey Peter,
You're worse than me.

There was only you and I that thought it was different, I've admitted it was incorrect and was mixed up with speed/distance travelled. RPM is RPM irrespective of where on the disk/chuck/plate/roundabout/world/sun/pot you measure or mark.

You're on your own now. But here's a quick test for you to convince you.
Place a drill bit (the smallest you've got) in the chuck, now stick chalk mark on the side of the bit facing you, just needs a spot, now put another in the same plane on the chuck.
Disconnect the drive and spin the chuck by hand. The mark on the bit and the chuck will turn and return to sight at the same time. If it takes you 1 minute to rotate the drive, then the chuck and the bit are Revolving (R) once per minute 1 RPM, if it takes you 10 seconds then 6 RPM. The chuck will travel further and faster but it's mark will not pass you any more or less times than the bit.

 

[astro_al]

Peter, your thinking is just wrong. For each rpm the strip will pass the beam once. Thats it. The diameter is immaterial, it'll just mean the strip passes at a faster rate, but it'll still pass the same number of times per unit time. Unless the chuck is leaving bits of itself behind constantly, which could either get somewhat metaphysical, or mean you probably need a new chuck.

Its pretty simple. Disc spinning at a given rpm. Larger diameter disc = higher angular velocity (speed), but still the same rpm.

You don't need a fiddle factor. Its working.

Al.

 

[peterd51]

Hi,

OK, it's spinning faster at a greater diameter. That's what I wrote in my previous message.

I'll still check it though in case the tester I have doesn't recognise this.

Regards
Peter

 

[bill_gj]

You lot need a few drinks

 

[JimM]

Peter

Is your thinking that the 'reflective sticky' would be moving too fast for the sensor to detect it - thereby giving the wrong count in a given time and producing an RPM lower than actual ?? ie more a fault of the machine than in the maths

It's possible I suppose but looking at the spec of the items on Eb*y they are supposed to be able to measure up to 100,000RPM so would have thought that a few thousand rpm of a drill/lathe wouldn't pose a detection problem.

Give it a go though and let us know

Jim

 

[bill_gj]

As long as the marker you use on the rotating part is reasonably reflective and the ambient light falling onto the rotating part is reasonably bright, it all works fine.

Guaranteed.

One marker, made from masking tape or painted on with correction fluid or white paint pen, etc. will do the job. Just make it definite and, say, 1/8th of the circumference.

 

[peterd51]

Hi,

no, I was thinking that the time it takes for the strip to revolve will be increased as the diameter gets bigger, as it has to travel further. And if the meter was set up to measure a chuck with a diameter of 2", then a 4" chuck would give a different reading.

That's a reasonable assumption to make, on your own in the workshop one evening, with a new and untested piece of kit.

Now that several people have confirmed that I was wrong to think that, I can see that the outside edge of a larger chuck will be moving faster than the outside edge of a smaller chuck on the same motor shaft and would still do one rev in the same time...although the concept of two parts of a chuck moving at different speeds at the same time still seems odd.

However, I set up a test rig with a bolt through a six inch wheel and tried it, I get the same reading off the 2" chuck as I do off the 6" wheel.



So, now to check that the meter is actually accurate. Trying two different drills:

rpm marked observed
drill1 1200 1330
drill1 2000 3230

drill2 1100 1028

not impressive, massive differnces and not 'linear'. I assume these must be different as the drills were free running? I still fancy testing it on the car engine at the weekend as I'm sure that when it says 900 rpm on that it'll be more accurate.

Finally, back to the pillar drill. It states 1450 rpm on the motor, the smaller pulley on the motor is 57mm diameter, the larger pulley on the chuck shaft is 1200mm and the measured rpm on the chuck is 480. No matter how I try the maths I can't make this add up, it's 'out' by about 10x.

Any ideas?

Regards
Peter