Monday, April 13, 2009

Alignment Handout

Factors Influencing Alignment procedure
1- Eccentricity (runout)
Check
· This might be done by a dial gauge

2- Baseplate of machines (soft-foot)
· Machines feet must be mounted perfectly horizontal with the baseplate.
· The contact between the baseplate and the feet can be checked with a set of shims or with feeler gages.
· During a new installation,
o it is essential to use accurate straight edges and levels to make sure that all feet of the machine are on the same plane.
· The accepted tolerance level for these planes is usually 0.1 mm.
· Simple tests for soft-foot are by setting up the dial gage (at fixed place) and place a shim under one front foot and the reading noted.
· It is then removed and placed under the next front foot. The reading should be the same.
· The same procedure must be repeated for the rear feet.

3- Axial position of machines
· The axial position of shaft ends is referred to as the distance between shaft ends (DBSE).
· Normally, most couplings allow a large tolerance in the axial position.
· However, for couplings like disk couplings, an error in the axial position result in;
o places the discs under stress and
o decreases their life.
o may generate axial thrusts, which ultimately add extra load to the machine’s thrust bearings.
· It is therefore necessary to take this aspect into consideration, especially when machines operate at high temperatures.

4- SAG
· For spacer couplings, a sag (deflection) check should be done on the indicator bracket to be used for the alignment.
· The DBSE in these couplings may be long, and when alignment brackets are clamped to one hub and extended to the other hub, there is a tendency for them to sag.
· This sag can alter the dial gage readings, leading to misinterpretation and errors.
· For bracket lengths larger than 25–30 cm, it is essential to provide additional stiffness to minimize sag.
· It is therefore necessary to perform a sag check of the bracket.
· A sag check is essential only for aligning horizontal machines, because the sag is caused by gravity due to the weight of the bracket.


Alignment techniques
· There are many methods to align a machine. The appropriate method is selected based on;
Ø the type of machine,
Ø rotational speed,
Ø the machine’s importance & production,
Ø the maintenance policy and
Ø alignment tolerances.

· Machines “I”
{Which are not fragile (breakable) in their construction}.
Ø rotating at less than 1500 rpm,
Ø lower horsepower range,
Use merely a straight edge to align machines.
Considering all aspects, it is acceptable to align them to the range of 0.3–0.8 mm.

Machines “II”
{Majority of machines}
Fragile (breakable) in their construction (mechanical seals and expansion bellows)
Machines operating at;
Ø speeds of 3000 rpm and higher,
Ø in the medium power range of 20 kW–1 MW
should be aligned within 0.1 mm.
· This requirement necessitates the use of comparators like dial gages, and methods with minimum residual errors.

Alignment conventions using a dial indicator
· The dial gage is the most common comparator used during alignment.
· The dial gage functions based on the rack-and-pinion principle. The conventions that are followed are shown in Figure 6.9.
· When the spring is compressed, the dial pointer is pressed inward and the clock needle moves clockwise, indicating a positive reading.
· When the pointer moves outwards, the clock needle moves counter clockwise, indicating a negative reading.
· It is recommended to jog the pointer from the top to ensure that it is not stuck.


The dial gage functions based on the rack-and-pinion principle. The conventions that are followed are shown in Figure 6.9.

Figure 6.9 Dial indicator


Another convention for alignment readings in the horizontal plane is shown in Figure 6.10.


Figure 6.10 Alignment readings in the horizontal plane


Thus, the convention maintains left and right when standing behind the driver, facing the driver.
Left and right readings on the dial gage are recorded accordingly.


Shaft setup for alignment
· The connection to the shaft must be simple and rigid.
· The clamp shown in Figure 6.11 is a good example. Magnetic clamps must be avoided, because their attachments are not reliable.

Figure 6.11 Shaft setup for alignment

· There are many types of alignment brackets available in the market, and a typical one is shown in Figure 6.12.

· The guiding principle for the selection of brackets is that they should be rigid with minimal sag (see the rod diameters).

Figure 6.12 Alignment brackets

Types of misalignment
Misalignment in machines is due to;
· angularity and
· offset,
· but in almost all cases the misalignment of machines is a combination of both.

i- Angularity
· is the difference between the values on the comparator (dial gage) for a half revolution”180o” (because for one complete revolution we return to the original position).
· For a given angular misalignment, angularity depends on the diameter described by the dial gage.
· It can be seen that when d1 increases to d2, p1 increases with the same ratio to p2. This value must be fixed when a certain tolerance is given (Figure 6.13).


Figure 6.13 Angularity (parallelism)

Angle of misalignment:


Where p1, p2 = dial gage reading when rotated by 180°; d1, d2 = diameters described by the dial gage.

ii- Offset (concentricity),
· The offset is the radius of rotation for the dial gage, as indicated in Figure 6.14.

Concentricity =1/2 dial gage reading


· The dial gage readings would indicate the diameter, and hence should be reduced by half to obtain the true offset reading.


Figure 6.14 Radial misalignment (concentricity)


However, as mentioned before, in practice misalignment of machines is due to a combination of both factors, as depicted in Figure 6.15.

Figure 6.15 Misalignment of shafts with angularity and offset

Two dial method of alignment
The necessary steps to align a machine are:
1. The first step is to loosen the coupling bolts so there is no restriction during the measurement of angularity of the existing misalignment.
2. A feeler gage is then run through the coupling hubs to ensure that the hubs are not touching.
The necessary steps to align a machine are:
i- The radial test (R) to measure the OFFSET ;
· The dial gage is attached as shown in Figure 6.16.
· The test done in the vertical and horizontal planes.
· To obtain the offsets in both planes, four readings will be required.
1. Top, bottom, left and right
2. Clock positions – 12 o’clock, 3 o’clock, 6 o’clock and 9 o’clock positions.
Clamping
· The dial gage here generally placed on the top (12 o’clock) position, and the zero on the scale is turned to coincide with the needle.
· The pointer must be jogged to ensure that it is free and that the readings are repeatable.


Figure 6.16 Dial gage setup at top position. The difference in readings after 180

indicates offset in vertical or horizontal planes


· Shafts are turned manually through one complete revolution, and readings at every quadrant (quarter) are noted.
· The readings recorded at the four locations are written down in the format shown below (fig. 6.17).
· The ‘R’ in Figure 6.17 indicates that these are radial readings, meant for offset corrections.

Figure 6.17Readings in mils

ii- The Facial reading to measure the ANGULARITY;
· The clamp is re-adjusted with the dial gage pointer now set to measure the angularity, as shown in Figure 6.18.
· The pointer (as shown in the figure) is now parallel to the axes of the shafts.
· Just like the offset, the angularity must be measured in horizontal and vertical planes as well.
· The dial gage is rotated through one complete revolution and stopped at every quadrant to make a note of the readings.

Figure 6.18 Dial gage setup at top position. The difference of readings
after 180° indicates angularity in vertical or horizontal planes


· The ‘F’ in Figure 6.19 indicates that these are facial readings, meant for angularity corrections.


Figure 6.19 The ‘F’ indicates facial readings
(note the diameter described by the dial gage)

iii-Steps to fix the alignment
· The next step is to convert these values of (R) and (F) to appropriate shim thickness that should be added or removed to fix the alignment.
To proceed to the next step, additional information about the location of the front and the rear feet from the dial gage pointer is required.

*In Figure 6.20
· The pump is the fixed machine (FM) and the motor is the machine to be shimmed (MTBS).
· This implies that all the corrections will be done by adding and removing shims under the motor feet. The pump will not be disturbed from its position.
· The distance from the pointer of the dial gage to the front foot (FF) of the motor is designated as ‘A’.
· The distance of the rear foot (RF) to the dial gage pointer is designated as ‘B’.

Figure 6.20 Shimming Calculation


· Two sets of calculations are required. One set for the vertical plane and the other for the horizontal plane.

1. Calculations for the vertical plane
*Offset correction
· Let us say the offset readings for the top and bottom positions are 0 and -5 mils, respectively.
· If the dial gage pointer is on the motor (MTBS) and the dial gage is rotating, hence the –ve and +ve signs are as shown in (figure 6.9).
· The negative sign indicates that the motor shaft is higher than the pump shaft.
· It is higher by half the final reading minus the initial readings. Thus:


Hence, shims of 2.5 mils should be removed from the front and rear feet of the MOTOR.

*Angularity correction
· Let us say the angularity readings for the top and bottom readings were 0 and - 2 mils, respectively.
· If the dial gage pointer is touching the rear face of the motor coupling hub see (figure 6.9).
· The negative sign indicates that the coupling has a narrower gap at the bottom than at the top.
The dial measures at (scribes) a circle of 5 in.


The angle


Because the angle is very small, the tan inverse function can be neglected:
Hence, P1=.002 in.

(The formula would reverse if the pointer is touching the front face of the coupling hub, which is normally the case when there is a long spacer between the couplings.)
= 0.0004 radian
=0.4 milli-radians = 0.0004´ (180/p) = (0.023o)

· This angle “θ” is also the angle of inclination of the motor axis w.r.t. the pump axis.

· Line AB is the existing axis inclination of the motor (Figure 6.21).
· It must be lifted by amount x at the FF (front foot) location and by y at the RF (rear foot) location.


Figure 6.21 Calculating X and Y values

· The x and y values are calculated as follows;
x and y are approximated as arcs and the following formula can be used:
S = r × θ

Where;
S = arc length;
r = radius;
θ = included angle in milli-radians.


Final- Vertical

The final results should include corrections for both the offset and the angular corrections.
At point A{Front Foot FF}:
Offset results – remove shims of 2.5 mils
Angularity results – add shims of 3.2 mils
Thus, insert shims of 0.7 mils under the front foot of the motor.

At point B{Rear Foot RF}:
Offset results – remove shims of 2.5 mils
Angularity results – add shims of 7.2 mils
Thus, insert shims of 4.7 mils under the rear feet of the motor.

Calculations for the horizontal plane
The dial gauge is: from behind the motor, left is the initial reading and right is the final reading.
*Offset calculations:
Left reading: +1 mils
Right reading: - 6 mils
Pointer on left;
+1 means the measured point on motor shaft is to the left by “1”
Pointer on the right;
- 6 means the measured point on motor shaft is to the left by “6”
{To imagine this, just draw the dial gage and its direction}
· Because the dial pointer is on the motor shaft, a negative reading indicates that the motor shaft axis is to the left of the pump shaft axis.

Offset = (Final reading – Initial reading) /2


Move points A and B of the motor to the right by 3.5 mils.

*Angular calculations:
Left reading: + 4 mils
Right reading: - 6 mils
+ 4 means that the left point of the motor hub is away to the pump hub by “4”.
- 6 means that the right point of the motor hub is close from the pump hub by “6”.
{To imagine this, just draw the dial gage and its direction}

As the dial pointer touches the rear face of the motor coupling hub, the shaft axis resembles what is shown in Figure 6.22.
In this case:
mils = 0.01 inch.
Thus:



= 0.002 radians (0.114o)
= 2 milli-radians

Hence:
x = 2 ´ 8 = 16 mils – move to the left;
y = 2 ´18 = 36 mils – move to the left.


Figure 6.22

Final- Horizontal

At point A{Front Foot FF}:
Offset results – move 3.5 mils to the right
Angularity results – move 16 mils to the left
Thus, move to the left by 12.5 mils.

At point B {Rear Foot RF}:
Offset results – move 3.5 mils to the right
Angularity results – move 36 mils to the left
Thus, move to the left by 32.5 mils.


The procedure would be;
· The vertical shim corrections should always be done prior to the horizontal shifts.

· Once the vertical shims are adjusted, the bolts should be tightened and a quick test of the vertical plane reading should be made to confirm the accuracy.

· If the accuracy is satisfactory, the bolts can be loosened and the horizontal alignment should be done with jack bolts (if provided).





The limitations of this method are:
· Calculations are necessary, which may be difficult to do in the field.
· It is beneficial to be able to visualize the shaft orientation from the dial gage readings but this requires practice.
· Inexperienced technicians can find this confusing.
· Errors in calculations may occur if there is bracket sag and/or error in the dial gage readings.
· If the shaft of one or both the machines has substantial axial floats, the angular readings can be erroneous.


Laser alignment
Alignment with comparators such as dial gages characterized by;
· a fair degree of precision,
· demand skill,
· demand training and
· Require experience.
Consequently, these methods are;
· tend to provide errors and
· can take a considerable amount of time.

The method of alignment using LASERS (Figure 6.34);
· overcomes the disadvantages listed above and
· it is gradually becoming the preferred method of alignment for most machines.
· Data collection and calculations have become;
o fast and
o accurate
· Some laser systems need less than a quarter turn of the shaft to produce very good shim correction data.
· They have built-in alignment tolerances, and hence there is no need for an expert to judge on the quality of the residual misalignment.
· Laser beams can travel over long distances, and alignment can be done very accurately with relative ease (comfort).
· Laser beams do not bend over great distances and for this reason the sag effect is entirely eliminated.


Figure 6.34 Laser alignment

· The laser alignment system (Figure 6.35) comprises;
Ø an analyzer and
Ø two laser heads.
· The laser heads are attached to the two shafts
· The laser heads must face each other, and each head has a laser emitter and receiver.
· When the shafts are turned, the receivers trace the movement of the laser beams.
· These values are communicated to the analyzer.
· Machinery data and the required distances are initially entered into the analyzer.
· The data from;
o the laser heads and
o the given machinery data
are used to accurately determine the shim corrections for the machine.
· Once the laser head and the reflector are installed, the shafts must be rotated.

Figure 6.35 Laser alignment system comprising of laser head,
reflector and analyzer (Prueftechnik – Optalign Plus system)

One emitter and one receiver system;
· Some entry-level laser alignment systems only have one laser emitter head and a reflecting prism on the other.
· These systems are ideal for general purpose machines.
· They eliminate the dial gages and provide an alignment calculator.
· The methodology with these systems is same as the previous one.
· At every quarter revolution, the analyzer must be activated to acquire the reading.
· After this, the analyzer provides the alignment correction information.

Some advanced LASER systems
Some systems include additional features that make alignment of machines an easy task.
These features are:
· Complex trains comprising of as many as five machines can be handled.
· Communications that eliminate cables between the laser heads and the analyzer.
· Errors due to vibrations from other machines can also be eliminated through averaging.
· Uncoupled and non-rotating machines can also be aligned.
· Less than a quarter rotation may be sufficient to obtain misalignment data.
· It is possible to do live horizontal alignment. This means that there is no need to take a reading and transfer it to the analyzer for calculation. The instant communication of the heads and analyzer accomplishes this automatically.
· One or two soft foot conditions can be identified.
· Once a machine is aligned, its history and data can be stored.
· They provide built-in misalignment tolerances.


Alignment tolerances
In practice, it is almost impossible to obtain;
· a zero offset
· and zero angularity,
and thus machines have to be left with a certain residual misalignment.
This residual misalignment has little or no detrimental effect on the operation of machines.

· The above values are assumed to be pure offset or pure angle.
· In practice, a combination of the two is more common and tolerances should account for this combination.
For example, a machine is running at 3000 rpm and the residual misalignment data is:
· offset: 2.6 mils
· angularity: 0.25 mil/in.
In pure terms, these values would be acceptable.
Nonetheless, let us see if the combination of the two is acceptable. To achieve this, a XY graph is made as shown in Figure 6.36.
If an offset of 2.6 with an angularity of 0.25 mils/in. is plotted, it could be beyond the acceptable range.

Figure 6.36 Alignment tolerances


ROTALIGN PRO
(Laser shaft alignment system)

1. Lock-out the machine.
2. Mount the chain brackets to the shafts.
3. Mount the emitter [waterproof, dust proof] on one bracket.
4. Mount the receiver [waterproof, dust proof] on the other bracket.
5. Turn on the emitter.
6. You have only one laser to adjust.
7. Only one cable is needed to connect the laser head to the hand-held computer.
8. Three short steps and you have your alignment data;
§ Dimension
§ Measure
§ Results
9. DIM
§ Press the DIM key your screen displays to you some data to put in.
§ Then determine the dimension of your machine and put them into the computer.
10. Measure (M)
§ Press measure key then you are ready to begin.
§ Turn the shafts for 1/4 (quarter) turn or less and forget about the clock position
11. Results
§ Press the result key, the screen displays as found to scale the alignment conditions AND the corrections needed.
12. Do physically the corrections required for the machine and repeat your job UNTIL the computer tells you that you have a good alignment.
(This is appeared with the sign on the screen).
13. Some benefits of this laser system are;
§ Determine the soft foot condition and analyze it and give the suggestions.
§ It can provide alignment for non-rotating shafts.
§ It can automatically the thermal expansion and gives the corrections.
§ It is capable of aligning the vertical machines.
§ The software can be updated from the website.




































8 التعليقات

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fred

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srinivasa rao

Like your way of seeing things! Still you may do some things to expand on it. Thanks for sharing with us!

HIMT

how to do alignment of single bearing alternator using laser alignment tool