hello and welcome to this quick

presentation on understanding unbalance

my name is Jason Tranter the founder and

Managing Director of Mobius Institute

Mobius Institute provides vibration

training and accredited certification so

unbalance exists when the mass center

line and the geometric centre line do

not coincide what does all that mean

so the geometric center line is the

center of geometry which for our fan

should be right through the center of

the shaft so if we draw a circle around

it and said where does it intersect well

it should if the machines designed

properly go straight through the center

of our shaft and that's where it also

wants to rotate so there's our geometric

centre line now our math centre line

should be in the same location if all

the mass is evenly distributed around

our machine then the center of mass

would be right through the center of the

rotor as well but when that is not the

case when the geometric centre and the

mass center are not equal then when we

spin the rotor it will wobble and that

is unbalance now I've got you know

offset to the right here but of course

it could be offset anywhere by any

amount so we need to reduce unbalanced

because unbalancing cause increases the

stress on the machine which reduces the

life of the machine and the structure it

will damage the structure it can cause

workers in the area to feel that high

vibration it can affect product quality

it can annoy neighbors I mean that's a

pretty extreme unbalance but in any case

it can certainly damage the bearings and

the seals and if there is any resonance

occurring then the more unbalanced

vibration we get more amplification and

therefore much higher vibration same as

if we have any looseness then

unbalanced drives those the ability for

it to be loose if you know him if it's

loose then we get more motion if there's

more unbalanced vibration okay so the

other key thing about unbalance is that

the forces generated are proportional to

the speed squared so if we double the

speed we get four times the amount of

force or four times the amount of motion

which this little animation is

attempting to show so therefore as we

increase the speed we will see higher

vibration which may not be something you

intend to do but what it really means is

that for a high speed machine unbalance

is extremely important it is important

to minimize it as much as possible so

why do we get unbalanced very commonly

in fans it's just a buildup of dirt it

might be a loss of material you know

through where or cavitation or erosion

or corrosion or something like that it

might be due to poor castings or

incorrect roundness for example a

centricity it might be yeah a loss of a

part of balanced weight might fall off a

fastener might come away might be loss

of part of the coupling it might be in

this case just the way it's been put

together which is not sensible of course

this is a little bit obvious but anyway

there's a lot of causes we have to you

know correct any mechanical problems

that might exist like missing blades and

so on but then we're going to go through

the balanced process so one way of

illustrating unbalanced if we have a

rotor here and we put an unbalanced

weight on then in a single plane

unbalanced case that we have here the

rotor will want to turn until that

unbalanced mass is at the bottom you

know thanks to gravity but if we let the

rotor spin then we're going to see a lot

of vibration as a result and of course

if we put even more weight causing even

more vibration more unbalance then we

get a lot more vibration so we need to

minimize

that unbalanced that exists and

therefore reduce the unbalanced forces

so the most common type of unbalanced

with narrow rotors like the one shown

here is called static unbalance that's

where all the unbalance is more or less

in one single plane we can see the

characteristic motion of an of static

unbalance in this case the center of

mass is moved away from the center of

geometry so there's our center of

geometry as the mass out of balance mass

is increased that I've tried to

illustrate here the center of mass is

pulled away from the center of geometry

now of course when we run the machine we

you know mounted in bearings and you

know design the machines are supported

but of course those bearings are now

trying to contain those circular forces

that we just saw a moment ago in a

classic case of couple unbalance we have

unbalanced mass that's 180 degrees

opposite each other and equal mass

that's a very specific case of unbalance

which you're unlikely to see in reality

but it's a term that you should

understand so in this case as these two

sources of unbalance mass increase it

draws the center of mass away at each

end of the rotor but in actual fact the

center still coincides with the center

of geometry so we see the the motion

that we see there but an actual fact if

we were to put this rotor on knife edge

it wouldn't roll because statically it's

balanced it's just got a couple

unbalance the more common situation that

we would see with a longer rotor like

this is what we call dynamic unbalance

where we can you know consider the

unbalance to be different masses at each

end at a different angle and it creates

this wobble and exactly how that mass is

built up just depends on the root cause

of the unbalance problem but

when we look at how the center of mass

is moved we get a static portion and a

couple portion so this dynamic is

therefore a combination of static and a

couple unbalanced but of course as with

the other situations our machine has to

and structure has to withhold those

centripetal forces and that's what

causes the damage in the case of and

overhung rotor we get the radial

vibration the radial motion that we've

seen so far which I'm not really showing

in this animation but we also get a

rocking motion which you can see there

and so in addition to those radial

forces which cause damage we also get

axial forces it's causing an actual

force along the length of the machine

and that causes its own form of damage

so in conclusion unbalance is common

more common in some machines than other

others and it is destructive to the

machine the forces associated with

unbalance are far greater with higher

speed rotors static unbalance is common

in narrow rotors where the unbalanced

forces are focused in a single plane

couple unbalance is a special case where

we have longer rotor but the unbalance

is equal and opposite the same weight

but opposite each other at each end of

the rotor that's the way we can model

the couple unbalanced situation but

dynamic unbalance is the most common

when we have longer rotors and that's

where you know if we were to sum up all

the unbalanced forces they would not be

equal at either end of the rotor and not

exactly opposite each other

so I hope this brief presentation has

helped clarify a few points associated

with unbalance thank you for viewing

this presentation and if you have any

questions please feel free to contact us