aligned shafts will do more to increase bearing, seal, and rotor
life than any other single thing you can do after lubrication.
Unfortunately, many maintenance departments in smaller plants
still think that alignment is only needed for large, high-speed
shafts on somebody else’s equipment. Many have no idea how
to align two shafts beyond using a straight edge to get them close.
Besides, the guy who sells the couplings says that the coupling
can take up to one degree of misalignment and not hurt anything.
That is a
pretty gross figure. They are correct, though. They design couplings
that will not wear out with that much misalignment. The life of
the coupling, though, is not controlling here. Badly aligned shafts,
and by that I mean much less than the one degree of misalignment
the coupling manufacturers use, will ruin the bearings on the
equipment in short order. All shafts, even low speed ones, must
be aligned to within a few thousands of an inch TIR (Total Indicator
Runout) if the bearings are to last for their full expected life.
Typical Types of Misalignment in Shafts
some tremendous systems on the market for alignment. They use
lasers, computers, and proximity sensors. They will practically
move the equipment and install the shims. They do no good at all,
though, if they are not used by trained, qualified technicians
who understand what the systems are telling them.
does not purport to explain shaft alignment. The presumption is
that the reader already has some idea of how to align shafts using
a dial-indicator set. What we want to do in this paper is to point
out five common errors made while aligning and what to do about
have two shafts that are perfectly aligned but one has a coupling
improperly mounted on a shaft so that there is some error in colinearity
of the axes. As we approach the maching the error is only in the
horizontal direction. Let’s set up the dial indicator so
that it reads the outside of the coupling then rotate only the
shaft on which the indicator is mounted, leaving the coupling
still. When we get to position 1 the dial indicator will be extended
to, say, -0.010 inches due to the error. Turning to position 2
will bring the dial indicator back to zero. At position 3 the
indicator is compressed due the error and reads +0.010. If we
believe that the coupling is square on the shaft and the axes
are collinear we will believe that there is a TIR or 0.020 inches
in the horizontal direction. Acting on that information will cause
us to misalign the shafts by 0.010 inches – introducing vibration and potential bearing damage.
How can we
eliminate that error? By rotating both shafts together. The roundness
of the coupling, the roughness of its surface, and the poor mounting
will be eliminated from the readings (actually, all of those things
will cause compensating errors so that only the actual misalignment
of the two shafts will be indicated by the dials.
A better way
is to eliminate reading the coupling at all by using a target
for your indicator that is securely mounted on the shaft. This
will necessitate rotating both shafts but eliminates the need
to break the coupling, even to do rim-face readings.
almost intuitive – at least if you have been in the field
for any time. Yet, I repeatedly have had clients who inexplicably
believed it was easier to move the driven machine. I will not
argue that there is a time when the driven should be aligned to
the driver. In all of industry, there are undoubtedly situations
where this is better. As a general rule, though, it is better
to move the driver that is not connected to your process than
to move the driven that is.
suppose we are aligning a pump and motor. The pump is connected
to the process by an inlet pipe and an outlet pipe. There are
some applications where the piping is connected through flex-joints
but I haven’t found very many of them. Most of the time,
the pump is hard-piped into the system. I will make an assumption
that you have corrected any pipe strain in the system. If not,
then do that before trying to align the pump. You are just kidding
Once the pump
has been connected to the system, any attempt to move the pump
to align it to the motor will induce pipe strain; whereas, moving
the motor strains nothing but the flextite on the electrical connection
– and it was made to be strained. If you move the pump and
introduce pipe strain, you will end up with a misaligned pump
as soon as you start the system up. Hot fluid, cold fluid or just
the movement of the fluid will begin to flex the piping system
which will, in turn, move your pump, which will in turn ruin that
careful alignment you just accomplished.
don’t increase Strength
At one client’s
plant I gave a four-hour class on shaft alignment. At the end
of the class, we went to the field to practice on a 50 Hp pump.
This was a pump that had been “aligned” by that crew
the week before. The plant had been in service for about two years
and the bearings had been failing on this pump every six months.
In the middle of the class, there had been some sheepish looks
and, when I questioned them, they admitted that they had never
heard of angular misalignment and had only been correcting offset
from the first pass of readings, the computer required a movement
of the back end of the motor which was 0.010 to 0.015 more than
the holes in the motor mount would allow. After some cursing,
the crew began to disassemble the coupling and pull all the mounting
bolts. I stopped them and asked what they were planning. “We
have to take the motor to the shop and enlarge the holes in the
feet,” was the answer.
times when the two devices may be so misaligned that that kind
of action may be necessary. Needing less than 0.015 inches is
not that kind of misalignment. When you run across this situation,
do this: Pull the offending bolt, take it to the shop and turn
or grind the threads off in the area that is in contact with the
motor foot. You can grind all the threads down to the root diameter
and not effect the strength in tension at all. On a 5/8 –11
bolt the OD at the threads is 0.614 inches. The root is 0.515
inches. So, you can pick up 0.050 inches of movement with no decrease
in strength by grinding the threads.
have seen many alignments where the computer moved the offending
foot right back after the second pass. Enlarging the hole in the
foot would have been an extreme waste of time.
Planes are Better Than 1
I have seen
alignment techniques that suggest that the technician align first
the horizontal then the vertical. I disagree with that. I believe
you should be making complete passes – all four positions,
and calculating both your vertical and horizontal correction at
the same time. Here’s why:
If the alignment
is far out, it is better to correct both planes (horizontal and
vertical) at the same time. Otherwise the requirement for positioning
the indicators becomes too precise. Error will be introduced and
you will end up bouncing around the solution for any one plane.
are nearly perfect in the vertical direction yet still grossly
misaligned in the horizontal as illustrated in the above figure.
If you do not place your dial indicator exactly vertically, part
of the gross horizontal misalignment will be read as vertical
misalignment. This will cause you to bounce around the solutions
with each pass.
It is better
to remove the error from both planes together so that this situation
does not arise. With the availability of computers, laser systems,
etc. there is not reason not correct both planes simultaneously.
mechanics and engineers will shrug at that statement. Of course,
reverse indicator systems work. My experience as a teacher and
as a consultant belies that complacency. I have found experienced
crews who knew nothing but rim-face techniques or who knew of
RI but did not believe it worked. I had one very experienced technician
tell me that RI only worked on very large shafts.
this thinking, I utilize a very old technique popular and needed
prior to the advent of hand held calculators and computers: the
plotting board technique.
In it, the distance between in the indicators is plotted as X.
The distance from the B indicator and the front foot of the moveable
machine is plotted as Y. The distance between the feet of the
machine is plotted as Z. So, the horizontal axis is normally in
inches. The vertical axis, though, is not in inches but in thousands
of an inch.
On the A line
is plotted the TIR of the A indicator in the plane being corrected
(I know, I just told you to correct both at the same time –
only, we didn’t have the calculating technology back then.)
The B indicator TIR is plotted on the B line. A straight line
is drawn through them and extended all the way to the Rear line.
The needed correction at each foot is read off the vertical axis
where the plotted line intersects the “foot” lines.
In the example
shown the A TIR was +0.003 inches. The B TIR was +0.002 inches.
The plotted line crossed the Centerline very near the Front line
and crossed the Rear line at approximately –0.0015 inches.
If this was the vertical plane being measured, then the front
legs would need no correction and the rear legs would need a 0.0015
shim added to bring it up to the centerline.
when this technique is shown to the technicians, a light bulb
goes on and they are able to grasp and appreciate the RI technique.
RI is not
right in every situation. It is just another tool the alignment
technician has for getting the job done. Its biggest advantage
is ease of set up and a balanced rotor compared to Rim-Face (RF).
The RI set up is balanced and will not rotate unless made to.
The RF is unbalanced and will rotate to the bottom position unless
held in place.
These five errors, although simple and basic to the experienced
mechanic, are ones that I have found many clients making –
and making without even realizing there was an error involved.
Teaching these things to an inexperienced crew or to your new
craftspeople will improve your alignments and make them more time