Shaft alignment

Flexible shaft couplings are normally used to transfer torque between rotating shafts where the shafts are not necessarily in perfect alignment. It should be noted that a flexible coupling is not an excuse for poor alignment. Careful alignment is impor­tant for the purpose of achieving maximum operational reliabil­ity whilst reducing service and maintenance.

When carrying out alignment, consideration must be given to relative movements of the respective machines due to thermal expansion and deformation caused by pipe forces/moments and setting of baseplates on foundations, etc. In certain cases, such as electric motors with plain bearings, notice must be taken of the electric motor’s magnetic centre. Alignment should be carried out at various stages during installation.

When alignment is carried out at cold temperatures, it is neces­sary to make allowances to compensate for the thermal expan­sion caused by the difference in temperature to that of the nor­mal operating temperature of a fan and driver. When possible, a final check should be made at operating conditions after a few weeks in service. Alignment checks should then be carried out at regular intervals. Misalignment, apart from being caused by any of the previously mentioned loads and deformations, can depend upon worn bearings and loose holding down bolts. An increase in vibration levels can often be caused by a change in alignment.

Within the petrochemical industry and refineries, reports are frequently made with respect to alignment. The reports note the alignment prior to and after operation, before removing the fan or dismantling for repairs. The same procedure is carried out to check alignment of hot gas fans after warm running.

Correct alignment can be achieved in many ways depending upon the type of equipment and degree of accuracy required. Information regarding alignment requirements is usually to be found in the fan manufacturer’s instructions.

Never use the limiting values for the coupling as given by the coupling manufacturer since they greatly exceed the values for machines if smooth running and long service life are to be achieved.

As a guide, a final alignment check should not produce greater parallel misalignment than 0.05 to 0.1 mm or an angular mis­alignment exceeding 0.05 to 0.1 mm per 100 mm measured length. For the definition of misalignment see Section 12.11.2.

Alignment is adjusted by means of brass or stainless steel shims, usually placed beneath the machine supports. Baseplates are generally machined so that a minimum number of shims are always required under the motor. Horizontal ad­justment is performed by moving the machine sideways on its mountings. Large machines must have horizontal jacking screws fitted. Sometimes the fan and driver are fixed after final adjustment by means of parallel or tapered dowels.

Methods of alignment

In principle, alignment is based upon the determination of the position of two shafts at two points. Measurement or assess­ment can be made by straight edges, feeler gauges and dial in­dicators for the various radial and axial distances or run-out, see Figure 12.12. Adjustment is continued until these devia­tions are zero, or nearly zero.

Misalignment and reference lines

Two shafts in a vertical plane, for example, can display two de­viations from their common centreline, namely parallel mis­alignment and angular misalignment, see Figure 12.13. The amount of misalignment at the flexible section of the coupling is that which is of interest. It is therefore appropriate to use a refer­ence line which passes through the flexible section. Parallel and angular misalignments are then referred to this reference line, Figure 12.14.

Inclination as mm

Per 100 mm measured length

Reference line

Shaft alignment

Figure 12.13 Misalignment of two shafts in a common plane

Parallel

Misalignment, mm

In Figure 12.13 it is important to note that if the reference line were to be chosen at the intersection point of the two centre lines of the shafts, point A, then only angular misalignment would exist. From a practical point of view angular misalign­ment is best measured as an inclination expressed as mm per 100 mm measured length rather than as an angular measurement in degrees.

Reference Kn« tor on* flexible element

Shaft alignment

Element and eohd-coupled ipacer

Shaft alignment

Reference line* for two flexible elements

The position of the reference line depends upon the type of cou­pling and naturally should always be located in relation to the flexible section of the coupling. For couplings with spacers and one or two flexible elements the position of the reference line is shown in Figure 12.14. Unless otherwise stated by the coupling manufacturer the permitted misalignment is considered to be that which is measured from the reference line.

Alignment procedure

In the case of a horizontal unit, alignment is best carried out by first aligning in the vertical plane, followed by transverse align­ment. For vertical units alignment is measured in two directions at 90° to each other. For a horizontal unit, alignment is carried out in the following steps:

1. Align the machines visually and check that the coupling is not crushed in any way.

2. Attach the measuring device(s) and check that the dial in­dicators) moves freely within the area to be measured.

3. Check possible distortion of the motor mounting or base­plate by tightening and loosening each, holding down bolt individually. Shim the motor feet if distortion is present.

4. Set the dial indicator(s) to zero in the position shown in Figure 12.12.

5.

U,=y+L-^ 1 100

подпись: u,=y+l-^ 1 100For methods II, III, and IV in Figure 12.12, rotate both shafts simultaneously through 180°, half revolution, thus eliminating the influence of run-out between shaft bores and the outer diameter of a coupling half. The coupling halves need not then be cylindrical. Determine the mea­sured values according to Figure 12.12. Note the mea­sured values with plus or minus signs, see Figure 12.12 for notation. Determine parallel and angular misalignments.

6. Determine shim thickness according to Section 12.11.3 or

And adjust.

7. Carry out checks according to steps 4 and 5.

8. Carry out transverse alignment in the same manner as in the vertical plane.

9. Perform final alignment checks in both vertical and trans­verse directions and record for future reference remaining parallel or angular misalignments in both vertical and transverse directions. Also make note of operational con­ditions at the time of alignment, for example, cold motor with warm fan.

Choice of measuring method

Figure 12.12 shows the five most common measuring meth­ods. From the point of view of accuracy it is difficult to compen­sate for manufacturing tolerances between the two halves of the coupling by using a straight edge and feeler gauge, method

I.

500

100

подпись: 500
100
The difference in accuracy between method III and method IV is determined by the differences in the dimensions D and C re­spectively. Accuracy increases in both cases as each respec­tive dimension increases, whereby method III is chosen if D is larger than C and method IV or V is chosen if C is larger than D. The choice of method is also determined, apart from accuracy, by the available measuring surface and by attachment facilities and space requirements of the measuring devices.

U, =+0.28-0.06-

подпись: u, =+0.28-0.06-The difference between methods IV and V lies in the location of the reference lines. Method IV is universally applicable and suitable for smooth shafts or where it is sufficient to measure the total parallel misalignment and inclination. In the case of a coupling with two flexible elements, method V is suitable if the angular misalignment for each element is first calculated individually.

Optical methods are also available. Light sources and mirrors are attached to each coupling half. The units are connected to a small dedicated portable computer which, when supplied with information regarding the feet position, will calculate the re­spective feet adjustments. Similar optical devices can be at­tached to machine casings to detect differential expansion when warming up.

Determination of shim thickness

Using the measured parallel and angular misalignment, the necessary shim thickness can be calculated directly. The mis­alignment is expressed as positive or negative, + or -, according to Figure 12.15, which shows positive misalignment.

Shaft alignment

Figure 12.15 Positive misalignments y and L

The shim thicknesses are calculated from the simple relation­ship:

Equ 12.3 Equ 12.4

U2 = y + L

100

Where:

Ui = shim thickness at foot 1 (mm)

U2 = shim thickness at foot 2 (mm)

Y = signed parallel misalignment (mm)

L = inclination expressed as mm per 100 mm mea­

Sured length

Fi distance in mm from coupling reference line to

& F2 = each respective foot, see Figure 12.15.

The coupling reference line usually passes through the middle of the coupling.

Example:

Indicator reading shows parallel misalignment y = +0.28 mm and inclination L = -0.06 mm/100 mm.

The distances to the feet are F! =300 mm and F2 =500 mm.

The shim thicknesses required are:

U, = 0.28 = -0.06 —=0.28-0.18=0.10 mm

1 100

= 0.28-0.30=0.02 mm

Shims of thickness 0.1 mm are placed underfoot 1. The calcu­lated value of U2 = -0.02 mm means that 0.02 mm should be re­moved from foot 2, but can probably be accepted as permissi­ble misalignment.

Equations 12.3 and 12.4 can also be combined so that parallel and angular misalignments can be determined in cases where it is not possible to fit the calculated shim thickness. In which case:

Equ 12.5 Equ 12.6

U1+U2

2

U2-U,

подпись: u1+u2
2
u2-u,

5.

подпись: 5.Y =

L =

_ Д 100 100

Where:

6.

подпись: 6.Y and Lare residual misalignments

Ui and U2 respectively (with sign notation) are shim thick­ness deviations.

For the previous example, when the proposed correction has been carried out, the residual misalignment is:

0-0.02 … y = : = -0.01 mm

2

: -0.01 mm /100 mm

подпись: : -0.01 mm /100 mm

L =

подпись: l =-0.02-0 500 300 100 100

Graphical method of determining shim thickness

A-L

Too

подпись: a-l
too
The required shim thickness can also be determined graphi­cally by drawing the position of the shaft in respect of the mea­sured values using a greatly enlarged vertical scale, 100:1 for example, and a reduced horizontal scale, 1:5 or 1:10 for example.

Equ 12.9

подпись: equ 12.9The method is illustrated by the following example carried out according to measuring method IV or V in Figure 12.12 with the various stages:

1. Fit the measuring device according to method IV or V and take readings rP and rM on the dial gauge.

Equ 12.10

подпись: equ 12.10Example:

Dial reading at fan half gives rP = -1.40 mm dial reading at motor half gives rM = +1.20 mm

2. Determine the dimensions C, F1 and F2. Note that the ref­erence line in this example has been chosen to pass through the measuring pointer as shown in Figure 12.16.

Shaft alignment

Figure 12.16 Length measurements and location of reference line

Example:

Measured results

C = 180 mm

F-i = 470 mm

F2 = 890 mm

3. Draw up a diagram on squared paper as shown in Figure 12.17. Mark in the dimensions C, Fi and F2 on the horizon­tal scale.

4. Mark half the measured value at the fan half, 0.5 rP, on the vertical axis furthest to the right. The positive sign for rP means that the motor shaft lies above the fan shaft and is marked upwards, whilst a minus sign is marked down­wards. The reading rP = -1.4 mm should thus be marked as -0.7 mm, i. e. downwards.

Mark half the measured value at the motor half, 0.5 rM, at distance C. The reading’s positive value means that the motor shaft lies below the fan shaft and should be marked as a minus value and vice versa for negative readings. The reading rM = + 1.2 mm should thus be marked as — 0.6 mm, i. e. downwards.

Join both points and extend the line to the motor feet loca­tions Ft and F2 respectively. The motor shaft shown in the example lies 0.44 and 0.21 mm too low at the respective foot locations and should be raised by shims of corre­sponding thickness, after which transverse alignment is carried out in the same manner.

The alignment can be checked simply by using the two measured values and rM and the distance “b” between the two flexible elements. In the case of couplings with two flexible elements, only the total angular misalignment of each element should be calculated. Parallel misalign­ments are experienced as angular misalignments by the coupling.

To calculate angular misalignment, the parallel misalignment at the flexible element must be calculated first, i. e. calculated at both reference lines. These misalignments are:

7.

Equ 12.7

Equ 12.8

The angular misalignment in the vertical plane is then deter­mined from the relationship:

OcM= ^(radians) =57.3—(degrees)

%=^ (radians) = 57.3 — (degrees)

The angular misalignments in the horizontal plane pM and pP are calculated in the same way.

Dial gauge Dial gauge

Shaft alignment

Figure 12.17 Graphical representation of method IV of Figure 12.12, scaled sketch of motor shaft position

Thereafter, the total angular misalignment, 0, per flexible ele­ment is calculated from the relationships:

^2=«m2+Pm2 Equ 12.11

E^v’+pp2 Equ 12.12

Optical alignment

Recent advances in micro-electronics and laser technology have allowed optical alignment techniques to become portable and cost effective. A laser source is mounted on one shaft and a mirror is mounted on the other. The source module includes a detector which measures the position of the returned beam. The shafts are rotated incrementally through 90 and readings stored. A small control unit, sometimes small enough to be hand held, which is programmed with the drive geometry calcu­lates the shim adjustment necessary to achieve good align­ment. Figure 12.18 shows a typical set up for a small cast iron fan. Laser alignment can be used for shafts which are 10 m apart.

Shaft alignment

Figure 12.18 Atypical laser alignment set up Courtesy of Pruftechnic Ltd

Similar equipment can be attached to fan casings, gearboxes, motor stators or baseplate pads to monitor movement or de­flection under operating conditions.

Posted in Fans Ventilation A Practical Guide


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