Mod-01 Lec-28 Ball and Journal Bearings
Mod-01 Lec-28 Ball and Journal Bearings


In this class we are going to discuss about
ball and journal bearings as you know by now that everything depends on the bearings we
put our transducer at the bearings okay but the bearing themselves could be having fault
so far we have discussed about faults in the rotating systems bear the shaft misalignment
shaft having unbalance shafts having cracks them but eventually all the measurements are
done at the bearing locations and we in the earlier cases we have assumed that the bearing
were okay but that is not true bearing themselves have so many rotating elements that themselves
could be defective so we in this class we are going to focus on this two important classes
of bearings the ball bearings the anti-friction bearings and the journal bearings in our class
in roto dynamics we have seen the importance of journal bearings because of the fact of
journal bearings can take huge amount of static loads on to them and they are practically
there the shaft is actually supported on a fluid film so there is no contact between
the shaft and the journal there is no metal to metal contact and then can take heavy loads
however in the journal bearings there are issue of stability in particularly in flexible
systems the stability in journal bearings is of critical concern which is not apparently
show in the case of antifriction bearings okay but in last gas turbines steam turbines
where we have journal bearings stability is very very important so all film thickness
the clearance between the journal and the shaft all this load on the shaft of the rotating
speed all these control the stability of the system and that is very very important which
we will not discuss in this class we have discussed i think the class roto-dynamics
but in this class we are going to focus on what happens or what indication we get when
there is defect in the ball bearings and defects in journal bearings okay so why do we have
bearings we have bearings to reduce friction to moving parts and the bearings support this
rotating shafts okay say if i have a shaft which is rotating shaft is carrying lot of
heat so they have to be supported on bearings and i have to give very very less friction
to the rotating unit that is my requirement less friction and support the load so one
way to reduce less friction is to have them finely polished surfaces hot surfaces which
will not wear off and then they will support load another is pertain all fail when we all
film here in general bearing we have eccentricity okay because of the converging diverging room
there will slit pressure there is fluid here and because of the fluid pressure this shaft
is going to get lifted off the journal and then the shaft will support so we have the
case of the journal bearings and of course antifriction bearing
but in the journal bearing particularly there is a layer of oil or lubricant or it is actually
this oil so this lubricating oil can give different amounts of friction depending on
the z and by p where z is the viscocity and p is the pressure of this oil and is the speed
rpm and this is the petros law and then you will see that
okay am sorry some extent this will go up okay and this is the case of the boundaries
are almost approaching mixed layer and then the hydrodynamic okay and we are talking about
bearing in this hydrodynamic region where the friction is less
so this antifriction bearing could be of many types ball bearings roller bearings basically
an antifriction bearing has an outer race and inner race and this could be the this is the outer race this is the inner race
these are the rolling elements which are usually balls rollers or even sometimes tapered rollers
and then there is a cage or a retainer which ensures that no to rolling elements come together
in a thin metals there actually reverted here and this is the cage or retainer at sometimes
the inner race rotates with the shafts typical cases or sometime the outer race rotates in
the case of ceiling fan bearing the hub is actually stationary hub is attached to the
ceiling the shaft coming ceiling fans outer race rotates in typical other cases all the
electrical motors etc is the inner race rotating sometime we call this as race or ring okay
so the idea behind this bearings antifriction bearings they have to in any bearing they
have to less friction so to give less friction they have to be moving very moving without
any obstruction so these elements are manufactured to be very fine surface hard surface and they
are lubricated once they come under the fact lubricated at the factory and seal for every
thin layer of lubrication and this is the first class first type that the antifriction
bearing of the ball bearing of the roller bearing this is what looks like and the hydro
dynamic bearings basically we have a journals where is pad and then where there are roof
for the oil to come in and so on which is to be discuss in the case of rotodynamics
i will not to journal bearings right now we will focus more on the antifriction bearings
and because most so the cases small machines are they only have such antifriction bearings
which could be self-aligning also okay so this is how what are the main components of
rolling element which we just saw the inner race outer race the rolling elements could
be ball roller cylinder tapered roller cage separator retainer the same thing okay now
why do vibration signals from the bearings occur that is the characteristics of vibration
signals from the bearings because once bearings are put an housings i have bearing here some
shaft transducer here this is my transducer so these shafts are carrying loads okay and
sometimes these loads are direction speed may be fluctuating so the load coming to the
system or the excitation coming to the bearings is varying with load varying with speed so
these are responsible for this vibration signal which is coming out of the bearing to modulate
them by modulate them i mean certain the amplitude will increase decrease so actually so this
sorry these are the amplitude modulation which are occurring which are the load and speed
variations okay on top of it these all these rolling elements have different relative emotion
one is the balls are for example the balls are spinning about their run axis and also
going about the circumference these is if this outer race is fixed okay this inner rate
is rotating at an rpm so i can assume that this is rotating at n / 2 the train the fundamental
train of this rolling elements the moving elements at an speed of an by 2 okay this
is fair option okay this is 2 this is an n / 2 linear okay you see all these individual
components cage the inner race once is outer race non-stationary either inner race is stationary
in case of the ceiling final like i told you or the out race stationary in the case of
an electric motor so these elements are all rotating at different frequency and that is
what gives rise to the high frequencies in the even in the good bearing even in a good
bearing you will see the frequency is coming up in the vibration spectrum of the vibration
which we measure the best part about the bearing vibrations is suppose example i have a shaft
which are supported on bearing these are my bearing and i have put my transducer somewhere
here transducer will record 1x vibration because of the additional speed but they will also
rotate and find out the bearing frequencies plus bearing frequencies but you will see
the bearing frequencies you will calculate at any time their actually some fraction either
point 6 of x or 12 of x and they depend on the bearing geometric by bearing geometric
i mean the pitch diameter the ball diameter of the rolling element diameter the number
of balls small and the rotational speed the contact angle so these frequencies are no
way to related to 1x 2x 3x which we saw in the case of rotating shafts and that is again
you will wondering well always i am putting my transducers on the bearing mount what if
the bearing frequencies are contaminating or mixing with my actual shaft frequencies
of misalignment crack unbalance that will not happen because they are at different frequencies
the bearing frequencies themselves are at different values compared to the shaft cases
and that is actually helping us unlike i told you in this spectrum analysis every peak in
the vibration spectrum corresponds to a particular defect in a particular mechanical element
it could a bearing fault it could be a shaft fault it could be a unbalance it could be
a gear box fault they are all different frequencies and that is really they are really saved the
work fast okay but another thing also happens in the bearing is because there is a defect
in a bearing a lot of high frequencies is generally a lot of come to that just in a
little while but a what are the source of bearing vibration okay once we manufacture
this bearing when you say this outer race to be a perfect circular ring
we manufacture through a grinding process through a polishing process through a buffing
process to make a perfect circular ring but after the ring as been manufactured but if
you take a dial gauge and measure the ovality of this ring it will not be circular it will
be surprise to know that it will be somewhere this is a not a shaft peak here but these
variation the ring a circle we should have been in green circle actually looks like this
and these could be order of two microns very very small but these is there in a new bearing
also and this is known as the waviness of the ring or what is known as the out of roundness
through a precision of measurement of the wave of the surface profile we will see that
it is not a perfect it is something like this so bearing manufacturers when they manufacture
bearing the ensure that they had a good amount of finishing operation in the buffing final
in the sense so that this happens and on top of it you will see that we have this rolling
elements actually riding on these waves okay so if you put it flat on the surface
so we have wavy surface on which this rolling elements are moving this train is moving at
an high speed so this give rise to the vibration response in vibration we have studied when
you go wavy surface what kind of amplitudes we get all of you must have experienced going
a motor bike we have experienced those amplitudes coming up same happens in a bearing so this
is why even in a good bearing we have vibrations because of the outer frame of the while manufacture
it okay on top of it if you look at another important component that is the surface roughness
of components this surface it was any of the surface are suppose to be manufacture all
this surfaces where supposed to be manufactured as nice flat surface which in ra of the center
line average or the surface roughness very very low but if you look under the microscope
there will be lot of values and pits and this is because of the values and peaks and they
are not smooth they have an surface roughness so imagine a rough surface moving over another
rough surface this rough surface could be that of the ball or a rolling element or a
roller riding on a inner or outer which also as surface roughness and which is wavy and
this is what is actually there in a bearing in microscopic level so all this wavy surfaces
they are having emotion and that is why this vibration come even if you have a good bearing
now to reduce this manufacturers what they do is they ensure that they have good amount
of manufacturing of operating in terms of polishing operations on top of it that the
factors of the bearings are manufactured they put a layer of lubricant right once the bearings
can manufacture it so this lubricant will go into pits and values and try to make it
smooth surface so if you hold a new bearing and if you just play around with a bearing
you will not feel any roughness to your hand because of this lubrications to remove the
lubrications if you take this new bearing and wash it with kerosene and remove the lubricant
you can all do that and feel it you will feel very huge amount of roughness in your feel
because of vibration have increased we have removed the lubricant by washing with kerosene
okay so that is the reason why new bearings also have vibrations it is not true that new
bearings not have vibrations new bearing have vibrations because of these two reasons waviness
and surface roughness on top of it becomes more complicated in the sense let us going
to this case of the presence of dirt now imagine on a surface smooth you suddenly come across
a bump or a path hole so these and then you have a rolling element so these bumps or path
holes the bumps could be presence of a dirt on the surface of the inner race and outer
race very hard silicon particles sand particles a very hard they will not disappear they are
going to scratch scrap the surfaceness they are going to crawl there are certain scratch
mark like path holes so imagine when you go on a bicycle of a path hole you will get a
impulse so these rolling elements are subjected to a force of this nature and impulse force
in the time domain if you look at the force they are subjected to a pulse like this or
this is an impact so the rolling element which are rotating
at the inner or outer rage or subjected to such impact forces because they came in contact
with a hard dirt particle which is like a bump on a road or there are some pits on the
inner race because of some corrosion something as happened a pitting mark some scratch marks
so these impact basically this forces impact the bearings elastic structure bearing as
mass bearing as elasticity so bearing also have natural frequency of its own bearing
as a natural frequency k / m it as a mass it as a stiffness so it has natural frequency
now if you look at the frequency response of an impact force is basically a random force
in the frequency domain an impact is look like this force impulse short time force would
have lot of frequency lot of forces in the frequency domain or white marks now imagine
is the bearings equation if i write some force this force happens to f sin omega t where
it is omega=1 infinite for that matter because this goes all the way up so that means these
bearing is getting excited at all frequencies agree why did they excited to all the frequencies
because of this impact force are impulse force so the right hand side is equation as got
all the frequencies importance now it may happen so these are the forcing frequency
it may also happen that the forcing frequency is equal to the natural frequency of the rolling
element bearing as been designed so and we always design a bearing to ensure that its
natural frequency of a bearing of the bearing is away from the operating frequency by the
operating frequency means i mean the shaft rotating speed at few harmonic so it should
not interfere in operating frequency usually a bearing of natural frequency are in the
order of 20 to 30 kilohertz because we never operate at this frequencies so the bearing
would never undergo rather this conditions but the problem is because i have excited
it here by an impulse force which as all the frequencies the natural frequencies get excited
so i will have a large response at its high frequencies okay so resonance of components
occur that means in bearing have excited at natural frequencies so if i see a high frequencies
vibration in the order of 20 to 30 kilohertz in ultrasonic range if i see high increase
in vibration level i can say for surely a bearing as under gone effort okay and that
is what actually people use in an instrument called as a shock pulse meter say shock pulse
meter is actually nothing but a hand held instrument for bearing fault detection it
is nothing but an high frequency bearing equation measurement so basically essentially a high
frequency 20 to 30 kilohertz vibration measurement or monitoring device and because high frequency
occurs in defective bearing because the resonance frequency of a defect is race because of a
pit scratch dirt gets excited and you know the physics why it is got excited because
i excite at right hand side of a equation by a forcing vibration which happens to have
the natural frequency of the system obviously the resonance happen and then high frequency
vibrations so bearing vibrations a bad bearing vibration would have the frequencies of this
inner race outer race cage retainer etc plus a sure case to say that the bearing as scaled
is when you see a high frequency vibration in the 20 to 30 kilohertz range okay so to
under this we did a small experiment in the lab basically what we have here this is a
motor having a 6203 bearing okay and all we have to do is put an accelerometer and put
a filter i have an data acquisition card and we take all the data to the computer and then
we have a proto-electric probe to measure the rotational speed because as you see every
all the bearing frequency is dependent on the rotational speed of the this shaft and
this frequency can also be calculated and infact the rolling element bearing defects
frequencies we can call them as defect frequencies we can call them as bearing frequencies also
and they are given by this equation the outer race defect frequency is given by n / 2 times
rpm / 60 into 1 – ball diameter / pitch diameter cos sign of the contact angle beta
okay and 1 is – 1 is + and then the ball defect frequencies where small n is the number
of balls rpm is the relative rotational speed between the inner and outer race bd is the
ball diameter pd is the pitch diameter and beta is the contact angle so this can be calculated
from a the bearing manufacture where all the bearing manufacturer give this data some of
them give only the ball diameter not the pitch diameter some time they do not give the in
the catalogue in the number of ball but you can always find them in the internet or measure
them top of the manufacturer so here what you have done this experiment is we took a
6203 bearing okay which had a ball diameter of 6747 millimeters pitch diameter of 287
millimeter then number of balls in the rolling element bearing is 8 contact angle as 0 sin
beta is 1 speed of motor is running at 1800 rpm is 30 hertz okay this motor was running
at 1800 rpm we have a bearing here and then this bearing actually we had a series of 6203
bearings manufactured by a company who wherein we could artificially create defect in the
race by putting an electric scratch mark put on electric itching pen we could create scratch
marks in the rolling elements we could create scratch marks in the outer race so then we
one by one we introduce we manufacture bearings defected bearing and put them in (()) (32:08)
and then we measure the vibrations so a theoretically calculated values for a rotational speed of
30 hertz the fundamental electric frequencies is 114 hertz ball spin frequency is 603 outer
race frequency is 918 inner race is 1482 you will see here nowhere the bearing frequencies
of multiple of 30 hertz they are some decimal fractions of the inner race outer race other
rotational speeds so that is a good indicator now this one here just to give you a feel
of the bearing vibrations how they look like when there is no defect and there is lot of
defect so when there is no defect the amplitude level the vibration of very very low if you
compare to the second one which is case of a defective bearing and if you look at the
time is there almost periodic okay and then the very small peaks but the magnitude is
very very low 01 but in this case we have bearings which where there were lot of defects
and you will see this impact nature okay this is because of the rattles of the dirt’s
so it is ringing okay so if you just look at the time domain signal of good vibration
from a bearing and a bad vibrations when you can right away say looking at it these is
a bad bearing okay so you will see time domain analysis itself is very very helpful to identify
bearing faults and if you do the feature calculation from these signals like its mean rms you will
see obviously the rms value of this is much higher than this but there are factors like
this kurtosis of the bearing signal if i measure the kurtosis you will see the kurtosis value
of the bad bearings are predictable pretty much high more than about 4 or 5 and then
you can say for sure that this a bad bearing and this is how this spectrum looks like the
earlier example was for the case of the time domain okay and then these are both time is
the same now as been converted to the frequency domain and from 0 to 500 you will see for
a good bearing the amplitudes are low and you see the fundamental frequency and the
shaft frequencies is going up nothing else but once we have a defective bearing first
of all the amplitude are high and almost in this case there was a in organized defect
the ball defect out race defect all this frequencies are showing up okay this are sure cases and
in this bearing we are not even monitoring the high frequency vibrations are told you
about 20 to 30 kilohertz okay the reason i told you to monitor the high frequency vibration
from 20 t0 30 kilohertz is for the fact that bearing are actually put in systems there
could be frequencies of faults from other defect like thin shaft misalignment unbalance
etc so they will smear the spectrum okay it is very difficult for you to distinguish many
frequencies other defect frequencies okay unless we have a very very fine resolution
or very very high for it okay so a sure test is that high frequency monitoring to a short
measurement okay and this is how this waviness on out of round is looks like so this is the
good bearing acceleration signal okay this is the bearing signal with outer race defect
okay you can measure okay you see 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 02 /18
or that would be approximately how much about 03 is it on the time period is 02 seconds
per 200 / 18 001 second okay this is 1 / 30 is actually 3 is not it 03 okay you can actually
looking at this you can say whether the more what is these special spacing of the defects
whether two defects are there or one defects are there in fact am not sure the multiples
come up is not only the single frequency by also there are multiples yes two outer is
less defects 180 degree apart in this case what we did as we created one defect here
and then the defect here okay and this is for the less defect some time you will see
the shaft frequency is showing up because inner race is shaft and here when we have
a ball defects and the defect in the ball frequency will show up and how are the frequencies
is calculated this is inner race outer race all three were there okay what i am going
to show you is this equations so these equations are there in the books this equation you can
know only thing is that you have to know the ball diameter pitch diameter and contact and
find out this and then you can find out okay so a this is usually what you will see when
you measure the vibrations out of a bearings in case of the defect free and in case of
defect okay and this kurtosis value gives you can indication as to why this fault whether
as occurred in the bearing okay you blindly pass it through a signal processor where in
you just calculate the kurtosis of the tanned signal and usually kurtosis gives the
peakiness of the signal okay and usually kurtosis value is greater than more than 5 in case
of a defective and less than 3 is normal okay because if this bearing faults go unnoticed
for example a crack in a bearing outer range if it goes unnoticed we are going to have
problems the bearing will fail okay and many time i will tell you other case study in a
many times when in a particularly in a electrical motor when there is a electrical conductance
occurring through a bearings okay this shafts are carrying through rotors okay and there
is lot of magnetic flux and this flux gives rise to small ground voltages okay and this
at the bearings because the all film that is the sparking very significance parking
occurs so this grounding does not done properly this sparking is going to occur and this sparking
is giving rise to pits so if a bearing goes dry not lubricated not properly careful pitting
marks will occur pitting occurs because of electrical sparking so this is going to give
rise to the initiation of the defects okay as a good bearing as to lubricated made kept
dirt free environment and ensure that pitting dirt’s do not enter and then you also do
not overload the bearing more than it was designed so bearing if they go unnoticed eventually
the bearing machine as failed the shaft is going to come down rolls are going to come
down everything is going to fail misalignment in the shaft is going give rise to high axial
force sometimes in the journal bearings spiral bearings you will see lot of ununiformed wear
up ununiformed wearing again gives rise to ununiformed clearances and then forces are
going to vary support forces are going to vary again this is going to rise to fatigue
loading so it is actually a domains effect one leads to another okay so we have to careful
whether you know who came first so i will give you as closer i will talk about a small
case in a particularly in a paper mill wherein we have to be careful about the supporting
the bearing which are there in the number of roles which are there in a bearing so this
is the flow chart of a paper mill basically you will have the this area where the pulse
etc come okay the paper part etc pump and then they are faded next with the casting
so that clean bleached impurities are removed and this pulp paper pulp is actually fed through
a wire press wherein the wire is going in and then basically their pressed here the
first stage are the water is squeeze out from the pulp okay and this pulp is made through
pass roller wherein we do lot of drain so initially once you dry this wet pulp okay
immediately then slowly this paper is going to get formed and paper will be pass through
about the dryer big drying drums and each of them supported on roller on bearings and
bearings are and they are stinted dry there are steam went to the drums and eventually
and they are you know hot pressed after the dryer is called cal ending operation and finally
they are put it into roll and then the rolls go to the mark so if you see in the typical
paper mill there are about 200 to 300 roles just roles rotating at typical speed about
1000 meters per minute that speed a 1000 meters per minute this paper come about 1000 meters
per minute and then roll typical production about 80 tons per day in a 80 tons of paper
per day particular the new spread okay and this bearing one bearing fails because this
is the operation in serious everything as to be shutdown so this bearings have to be
monitored around the clock this is one view of the press section near the washer wherein
the the pulp has been fed this is the wire on which they take of roll this is a very
big role you can see is standing there is about 1 meter to 2 meter in diameter okay
and then going through this rolls and then this is the pressing section there are few
presses here and other side you will see this is the rolls coming in here and this particular
plant and here then you will lot of drums a big drums and in one drum one side of the
drum where actually steam going in heating the drum so drums are very hot okay because
the water as to be dried after you pressed and removed the water from the paper pulp
it has to be dried over series of rolls okay and then these are the bearings another view
every roll there is a bearing here and you will see in such a plant the bearings are
monitored by vibration you see this white lines here this is a bearing transducer here
another bearing transducer here another bearing transducer here so round the clock vibration
monitoring of this rolling bearings and then there are diagnostic algorithm wherein a people
can e know which vibration signal is coming from which bearing what is its kurtosis value
what is the spectrum value okay and if you because this are very very critical operations
because imagine if one roll breaks and falls it is going to damage few other rolls it is
going to damage few other bearings and to put everything together you can imagine the
problem one as to face okay so everywhere depending on the criticality of the equipment
of the process plant monitoring of rolling element bearing is essential okay and particularly
in paper mills etc this monitoring of bearing is done sometimes continuously online through
these and now a days this state of the hot is write at the bearings they are having transmitters
wireless transmitters so that in an complicated plant we need not have all the cabling coming
to the junction rather you have over the wireless and with the proper routers you have signals
you know which are coming in and and then sitting somewhere remotely in the control
room we can know which bearing signal is where and people are now you know in fact monitoring
them over the internet okay sitting in kharaghpur you can be monitoring plant in malaysia okay
it is possible it is no longer you can be having your paper mill in spain and you will
monitoring bearing at kharaghpur okay and then we can be giving them a diagnostic measures
so they have to take okay thank you

2 thoughts on “Mod-01 Lec-28 Ball and Journal Bearings”

  1. Arbaj Sayyad says:

    awsome sir

  2. umut ünlü says:

    is it has to be every video maded by some indian guy which spokes to shitty english? wtf man

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