6.5 Shaft alignment

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 heat and deformation caused by pipe stress and setting of frames and foundations, etc. In certain cases, such as electric motors with plain bearings, regard must be taken of the electric motor’s magnetic center. Alignment should be carried out at various stages during installation. When alignment is carried out at cold temperatures, it is necessary to make allowances in order to compensate for the thermal expansion caused by the difference in temperature to that of the normal operating temperature of a pump, pipeline and driving machine.

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 movements and deformations, can depend upon worn bearings and loose attachment bolts. An increase in vibration levels can often be caused by a change in alignment.

Within the petrochemical industry, refineries and other high end production facilities, reports are made with respect to alignment. The reports note the alignment prior to operation and after operation before removing the pump or dismantling for repairs. The same procedure is carried out to check shaft alignment of hot pumps after warm running. This is called Off Line To Running (OL2R) alignment.

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 pump 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 it can be mentioned that a final alignment check should not  produce greater parallel misalignment than 0.05-0.1 mm or an angular misalignment exceeding 0.05 to 0.1 mm per 100 mm measured length.

Alignment is adjusted by means of brass shims, or stainless steel in the case of process industries, usually placed beneath the machine supports. According to international standards the center line heights of driving machines (motors) are manufactured with minus tolerances, whereas the center line heights for driven machines (pumps) have positive tolerances. It thus follows that shims are usually required under the motor. Horizontal adjustment is performed by moving the machine sideways on its  mountings. Sometimes the pump and driving machine are fixed after final adjustment by means of cylindrical or conical location pins.

Various methods of alignment

In principle shaft 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 6.5c. Adjustment is continued until these deviations are zero, or nearly zero. Two shafts in a vertical plane, for example, can display two deviations from their common center line, namely parallel misalignment and angular misalignment. The amount of misalignment at the flexible section of the coupling is that which is of interest. It is therefore suitable to use a reference line which passes through the flexible section. Parallel and angular misalignment are then referred to this reference line, figure 6.5b. Notice in figure 6.5a that if the reference line were to be chosen at the intersection point of the two center 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.

Figure 6.5a Misalignment of two shafts in a common plane.

The position of the reference line depends upon the type of coupling and should naturally 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 6.5b. Unless otherwise stated by the coupling manufacturer the permitted misalignment is considered to be that which is measured from the reference line.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6.5b Location of reference lines for various types of coupling.

Shaft alignment procedure

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

1. Align the machines visually and check that the coupling is not “squeezed” in any way.
2. Attach the measuring device(s) and check that the dial indicator(s) moves freely within the area to be measured.
3. Check possible distortion of the motor attachment or base plate by tightening and loosening each attachment bolt individually. Shim the motor if distortion is present.
4. Set the dial indicator(s) to zero in the position shown in figure 6.5c (below).
5. Rotate both shafts simultaneously through 180° (half revolution) for methods II, Ill, and IV. By rotating them simultaneously, the influence of run-out between shaft bores and the outer diameter of a coupling half is eliminated. The coupling halves need not then be cylindrical. Determine the  measured values according to figure 6.5c. Note the measured values with plus or minus signs (see figure 6.5c for notation). Determine parallel and angular misalignment.
6. Determine shim thickness according to method described below Section Determine shim thickness or Graphical method of determining shim thickness and adjust.
7. Carry out checks according to stages 4 and 5.
8. Carry out sideways alignment in the same way as in the vertical plane.
9. Perform final alignment controls in both vertical and sideways directions and record for future reference remaining parallel or angular misalignment in both vertical and sideways directions. Also make note of operational conditions at the time of alignment, for example, cold motor with warm pump.

Choice of measuring method

Figure 6,5c 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 l). The difference in accuracy between method Ill 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 Ill 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.

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 a two flexible elements method V is suitable if the angular misalignment for each element is first calculated individually.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Determining shim thickness

Using the measured parallel and angular (inclination) misalignment, the necessary shim thickness can be calculated directly. The misalignment is expressed as positive or negative (+ or –) according to figure 6.5d, which shows positive misalignment.

Figure 6.5d Misalignment y and L with positive notation according to the figure to calculate shim thickness for proper shaft alignment

The shim thicknesses are calculated from the relationship:

U₁ = y + L * F₁/100     (Equ. 6.5a)

U₂ = y + L * F₂/100     (Equ.6.5b)

where

U₁ = shim thickness at foot 1 (mm)

U₂ = shim thickness at foot 2 (mm)

y = parallel misalignment with ± sign (mm)

L = inclination expressed as mm per 100 mm measured length (mm/100mm)

F₁ and F₂ = distance in mm from coupling reference line to each respective foot,, see figure 6.5d. The coupling reference line usually passes through the middle of the coupling

Example:

Indicator reading shows parallel misalignment y +0.28 and inclination L = -0.06mm/100mm. The distances to the feet are F1 = 300 mm and F2 = 500 mm. The shim thicknesses required are:

U₁ = +0.28 – 0.06 * 300/100 = 0.10 mm

U₂ = +0.28 – 0.06 * 500/100 = – 0.02 mm

Shims of thickness 0.1 mm are placed under foot 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 6.5a and 6.5b can also be combined so that paralIel and angular misalignment can be determined in cases where it is not possible to fit the calculated shim thickness, in which case:

y = (U₁ + U₂) / 2    (Equ. 6.5c)

L = (U₂ – U₁) / (F₂/100 – F₁/100)    (Equ. 6.5d)

where:

y and L are residual misalignment U₂ and U₁ respectively (with sign notation) shim thickness deviations.

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

y = (0 – 0.02) / 2 = -0.01 mm

L = (-0.02 – 0) / (500/100 – 300/100) = -0.01 mm / 100mm

Graphical method of determining shim thickness

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). The method is illustrated by the following example carried out according to measuring method IV or V (so-called periphery methods) with the various stages:

Figure 6.5e Shows the graphical alignment method notation for length measurements and location of reference line.

1. Fit the measuring device according to figure 6.5c method IV or V and take readings r_P and r_M, on the dial gauge.
   Example:
   dial reading at pump half gives r_P = -1.40 mm
   dial reading at motor half gives r_M = +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 6.5e.
   Example: measured results
   C = 180 mm
   F₁= 470 mm
   F₂= 890 mm
3. Draw up a diagram on squared paper as shown in figure 6.5f. Mark in the dimensions C, F₁ and F₂ on the horizontal scale.
4. Mark half the measured value at the pump half (=0.5 r_P) on the vertical axis furthest to the right. The positive sign for r_P means that the motor shaft lies above the pump shaft and is marked upwards, whilst a minus sign is marked downwards. The reading r_P = -1.4 mm should thus be marked as -0.7 mm (i.e. downwards).
5. Mark half the measured value at the motor half (0.5 * r_M ) at distance C. The reading’s positive value means that the motor shaft lies below the pump shaft and should be marked as a minus value and vice versa for negative readings. The reading r_M = +1.2 mm should thus be marked as -0.6 mm (i.e. downwards).
6. Join both the points and extend the line to the motor feet locations (F₁ 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 sideways alignment is carried out in the same manner.
7. The alignment can be checked simply by using the two measured values r_P and r_M 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 misalignment are experienced as angular misalignment by the in order to calculate angular misalignment, the parallel misalignment at the flexible element must be calculated first, i.e. calculated at both reference lines. These misalignment´s are:

h_P = (r_P /2) – (a * L /100)     (Equ. 6.5e)

h_M = (r_M /2) – (a * L /100)     (Equ. 6.5f)

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

∝_M = h_M / b (radians) = 57,3 * h_M / b (degrees)       (Equ. 6.5g)

∝_P = h_P / b (radians) = 57,3 * h_P / b (degrees)       (Equ. 6.5h)

The angular misalignment´s in the horizontal plane β_M and β_P are calculated in the same way. Thereafter, the total angular misalignment, θ , per flexible element is calculated from the relationship:

θ_M ² = ∝_M ² + β_M ²       (Equ. 6.5i)

and

θ_P ² = ∝_P ² + β_P ²       (Equ. 6.5j)

Figure 6.5f Graphical representation of method IV (scaled sketch of motor shaft location).