This third part of our installation series covers the basis of coupling alignment and the factors affecting misalignment.
COUPLING ALIGNMENT:
General
After the pumpset has been grouted in, the foundation bolts have been tightened, and the suction and delivery pipework have been fully installed, the alignment of all couplings on the pumpset must be checked and, if necessary, corrected.
N.B., The manufacturer aligns the pumpset before dispatch, but transportation and installation can cause the pumpset’s alignment to change.
Parallel or Offset Misalignment – Fig 1.1
Angular Misalignment – Fig 1.2
Correct Alignment – Fig 1.3
If the driver has been supplied separately from the rest of the pumpset, the driver is aligned after the baseplate is grouted.
It is essential to note that flexible couplings allow for the axial movement of both the pump and driver shafts, and also compensate for thermal expansion.
Before beginning coupling alignment, ensure that the power supply is isolated from the motor.
When aligning pumpsets, it is normal for the pump not to be moved; any adjustment is made to the driver.
For outdoor installations, make sure extreme temperatures do not adversely affect the alignment.
Typical Couplings
Fig 2.1
Notes:
- Normally, couplings do not need to be balanced for speeds up to 3600 rpm; however, care must be taken to ensure correct installation. When running at high speed, i.e., beyond 3600 rpm, balancing is recommended.
- Pin and bush couplings are designed to reduce shock transmission to bearings and other components, but do not compensate for misalignment very well; therefore, they should be aligned to the same accuracy as solid couplings.
When checking a coupling’s alignmen, the following errors can be expected:
Errors in angularity, Fig 2.2, the axis of the shafts are not collinear and intersect in the plane of the coupling faces.
Fig 2.2
Errors in eccentricity, Fig 2.3, the shafts are parallel but not concentric.
A combination of the above, Fig 2.4 under these circumstances the axis need not be parallel, and they may or may not intersect. By separating the errors of angularity and eccentricity and measuring them independently, the extent of the combination of errors can be determined and corrected. The axial movement of the pump and drive shafts should be measured. Appropriate allowance can be made for the clearance between the coupling faces.
Other Factors Affecting Misalignment:
Apart from the actual misalignment factors, there are three external factors which need to be considered when aligning couplings and these are:
Pipe Strain:
Pipe strain occurs when the pipework connected to the pump places an unnecessary load on the pump flanges such that it affects the alignment of the pumpset. This can occur if the suction and discharge piping flanges connecting to the pump are not correctly aligned with the appropriate pump flanges and the suction and discharge pipes and valves are not adequately supported adjacent to the pump flanges.
In addition, pipe strain can also cause deflection of the baseplate particularly when unrestrained expansion joints are used.
It is recommended that prior to checking the pumpset alignment, after the coupling has been disconnected, that the pump flange bolts be removed. There will be no strain on the pump if the flange bolts slide easily in and out of the bolt holes, otherwise, it will be necessary to fix the piping either by moving it or providing better support.
After the pipe strain has been eliminated it would be expected that when the alignment is checked there will be some misalignment.
Soft Foot:
The external force most commonly found during shaft alignment is soft foot. Soft foot, e.g., a rocking four legged table has a soft foot, is the improper contact between the foot of a pump or motor and the baseplate which supports it. It may be angular or parallel, but usually a combination of both, as shown below.
The main causes of soft foot are:
- The surfaces connecting the mounting feet and baseplate are not in the same plane.
- Deformed mounting feet.
- External forces as applied by pipework and/or brackets.
- Improper shimming or dirty feet.
- To many shims under the feet, a maximum of five should be used.
Soft foot should be checked for and corrected before starting the alignment of a pumpset.
Feet are shown in black where significant movement has been noticed.
| Type | Caused by | Correction | |
 |
1 foot |
Angular foot |
Shim |
 |
Opposite corners |
Rocking soft foot |
Shim one or both |
 |
Same side |
External forces |
Correct source |
 |
3 foot |
Bent or angular foot |
Shim or remachine feet |
Fig 3.2
16.8.3.3 Thermal Expansion:
Another factor that causes equipment to move once aligned is thermal expansion.
In the case of pumps handling hot liquids, this will be a factor as the pumpset is normally aligned when the pump is cold, which means that when the pump warms to its operating temperature, the alignment will change.
For example – A pump operating at 150 ⁰C with a cast iron casing, a distance of 250 mm from the pump centreline to the pump feet and an ambient temperature of 20 ⁰C. Basis a coefficient of thermal expansion of 0.000010 mm/mm/⁰C, for cast iron, the thermal expansion will be:
= T (Change in temperature) x L (Length of material) x C (Coefficient of linear expansion)
= (150-20) x250 x 0.000010
= 0.325 mm
As can be seen from this example the alignment of the pumpset will be out 0.325 mm when the pump warms to its operating temperature. It is therefore essential that:
- Allowance for thermal expansion must be made when aligning the pumpset when cold.
- Ensure the coupling connecting the pump to the driver can accept the change in alignment, normally a spacer type coupling is the most suitable.
It should be noted that if a pump has centreline mount feet then correction for thermal expansion is not required.
This article has described the need for coupling alignment and the factors affecting coupling misalignment. Our final chapter in this series will cover the actual process of aligning couplings.
