Notes on using a stroboscope to help
balance fan blades: (By John McComas)
I would suggest you get a roll or two of white vinyl tape.
(like electrical tape but white)
(white tape with black lettering is easier to see when using a stroboscope)
A small magic marker like a Sharpee. (for writing on tape)
A pair of sharp scissors. (to cut tape into pieces)
Cut tape pieces into size of ~ 1/4" squares.
Write the numbers 1, 2, 3, 4 etc. on them with the marker.
You will be sticking the pieces of tape on the blade wings.
The first thing you want to do is find the fundamental frequency or
motor speed RPM. FPM (flashes per minute equates to RPM)
(FPM is sometimes the designation used on stroboscopes)
The easy way to do this is to take a small white tape strip and stick it so that
it starts at the hub center and goes outward (a radial)
This is used to determine the fundamental, or actual RPM.
Adjust the stroboscope until you see TWO white strips then adjust stroboscope
to half that speed.
You adjust flash rate so that you see this mark stop (and only one mark)
(you could also be at half or quarter speed, that's why until you get used
to finding the fundamental, crank the rate up so you see two marks,
then slow flash rate until you get one mark.)
Flash rates or RPM vary based on how much the blade "loads down" the motor,
and how much "slip" it was designed for. Standard motor load range is
typically 75 to 90% of synchronous speed: 120 times Frequency divided by Poles.
(3600 2 pole, 1800 4 pole , 1200 6 pole)
Normal flash rates for properly loaded motors:
2 pole motor ~ 2800 - 3200 Small fans, drugstore fans, Bersteads, Caframo, etc.
4 pole ~ 1400-1650 normal fans
6 pole ~ 900-1150 6 wings, or (deep pitch blades)
OK, now you got the cage off the fan and you're in a dimly lit area,
you're flashing your stroboscope at the fan and it's got 4 wings,
the oscillator is turned off (stationery) and you determine the fan speed
to be 1600 RPM.
In order to view the blade symmetry, you want to see all the wings at the
same time overlapping each other. (Four flashes per shaft revolution)
(The persistence of vision of your eyes helps accomplish this.)
You want the flash rate to be the actual motor RPM times the number or wings.
For this example then: 4 wings times 1600 RPM = 6400 FPM stroboscope setting.
How will you know which wing is out since you can't adjust it with fan running??
I usually take a good look at the blade before I start sticking tape markers on
the wings. See where the wings are not in alignment, then stop fan and stick
markers in that area of each wing.
That's where the little white pieces of tape with numbers come into play.
Stick the tape pieces with the numbers on the wings so you will know which one!!
The first wing I put a piece of tape on it with the number 1 marked on it.
The next adjacent wing I put the number 2 on it.
The next adjacent wing I put the number 3 on it.
The next adjacent wing I put the number 4 on it.
You need numbered stickers in all the places that need adjustment and they need to
be close to the same position on each wing of the blade.
Adjust stroboscope to 6400 and you will see all four numbers on each wing.
Take the stroboscope and move it around the wings looking directly in front of blade
and it will show you if the blade is in clock.... (Meaning wings 1 & 3 are directly across
from each other, and 2 & 4 are directly across from each other.... Or, with wing one
at 12:00 wing two is exactly at 3:00, wing 3 at 6:00 and wing 4 at 9:00)
If that symmetry is not exact, then you would consider the blade "out of clock"
Lets assume that all wings are in clock, if not, they need to be bent back.
Now move the stroboscope all around the blade...
(be careful working around an open blade)
Take a look at the spider and see if the fingers are all in alignment.
If they are not, stop fan and bend spider, then re-strobe until they are the same.
Important operational note:
Start getting the blade bent back into symmetry by starting from the center, hub,
and working your way to the wing tips.....
If the clock looks good, the spider looks good, then start bending the wings back into
ADJUSTING WING PITCH:
Now would be a good time to decide how much pitch the wings should have.
(Pitch is the "bite" of air the wing takes when it spins. More pitch =slower speed)
Based on High Speed setting only:
4 pole AC motor @ 60 Hz = about 1780 RPM unloaded speed.
From what I've seen of motor data, it looks like maximum torque is in the neighborhood
of 77 to 85% of 1800 RPM ...or 1400 to 1550 RPM. (I would shoot for 1550 RPM)
If your fan does not seem to push much air, and the RPM is over 1600 RPM, you might
want to consider bending the wings for more pitch.
More pitch and lower RPM equals more airflow and less blade noise.
I've seen this on several large and small fans where they were noisy, they did not put
out much air, and the motor was hot, and the amps/watts were normal.
I bent each wing to increase pitch, then measured the RPM and motor power.
I kept increasing the pitch until I either got to around 1550 RPM or motor power just
started to increase. The result was that adding more pitch made a BIG difference
in the air flow and the motor ran cooler without using more power!
This may all sound like gibberish now, but after you get the stroboscope, fire it up,
start putting a bunch of numbered stickers on the wings, I think this will start
making sense to you....
The importance of correct "Clock"
The more of these blades I work on, the more things I find out.
At Geoff Dunaway's Pickin 2002, I came to the revelation that "clock"
(the equidistant spacing of the wings like the numbers on a clock)
is REALLY critical in order to smooth out the running of a blade.
This seems to be really important with large Vortalex and
overlapping wing Emerson's. 1/8" error measured near the end
of a wing makes the difference between a shaker and a smoothy.
(I had not realized before how such a small amount of clock error
affected blade balance)
Another note is to make sure that ends of the spider are bent
so they are all the same. (I saw some blades where the ends of
the wings were "right on" but the spider was twisted and caused problems.)
This is what I think will help you strobing guys to make them shakers into smoothies:
1st: Bend them back to get the "clock" corrected.
2nd: Bend them back to get the spider ends corrected.
3rd: Bend the wings to get them back in alignment which
consists of front to back AND curvature from leading to trailing edge.
I suggest you use white vinyl electrical tape and dice it up
to about 1/4" squares and label with a permanent black marker
numbers that coincide with the number of wings on the blade.
Make about 4 or 5 sets of these markers and stick them on your
strobe case until you need them.
Then like on an overlapping Emerson blade (elephant ears):
I place a set of numbers on the spider ends on the back of the blade.
I place a set on the leading edge, a set on the middle, and a set on the trailing edge.
Then with all these little numbered stickers in all these critical spots,
you tell exactly which wing is out of whack and where and how much.
Make sure you use the same number for all the stickers you put on each wing.
Normally I just number the wings sequentially in the rotation direction the blade
turns because you will be starting and stopping the fan several times, and this
makes it easier to go to the wing you want to tweak faster.
I hope this helps you guys since this information about obtaining exact clock
seems to have GREATLY helped me get these blades "dialed in"
John McComas 5/30/05