Thomas B. Davis (Tom) is a graduate mechanical engineer who owns Maintenance Troubleshooting, a consulting firm specializing in assistance for corrective repair of rotating equipment. He can be reached at email@example.com or 302-690-0871.
Place the shaft bearing seats on the V-blocks and position the indicator at the center of the shaft. Turn the shaft slowly while watching the indicator hand. On a 24-inch pump shaft (or smaller), the indicator should not deflect more than 0.002 inch. In the 24-inch to 60-inch range, 0.003 inch is the limit. For 60-inch to 120-inch pump shafts, the deflection can be to 0.006 inch at the midpoint. If a shaft is bowed in places, an unnecessary push on the bearings and seal faces occurs with each revolution. Seal life is reduced to months instead of years.
Carefully Heat the Bearing for Shaft Installation
Almost all pump bearings have an interference fit between the ID of the bearing (the bore of the inner race) and the shaft seat (the place where the bearing sits on the shaft). The bearing bore is smaller than the pump shaft and must be pressed on or heated to expand the bore before assembly. An anti-friction bearing is a great example of metallurgy. The bearing companies use excellent quality control to produce a bearing that is hard, but not too hard, to provide a long service life. If the bearing is overheated, it becomes annealed and will not last for its intended life.
Modern shops use induction heaters or cone heaters to rapidly heat the inner race to allow shaft assembly. However, the temperature-sensing mechanism on the heater can often be out of calibration or non-existent. In that case, the mechanic must use an infrared thermometer or temperature sensitive crayon that melts at the correct temperature value to make sure that the bearing is not overheated.
The magic number to avoid is more than 250 F. Most good pump shops never heat them to more than 230 F to avoid the possibility that they will overheat the bearings. Overheating a bearing during assembly removes years from its life, and plant management may never know the true reason for the shortened life cycle.
Square the Bearing to the Shaft Shoulder
Improper squareness causes frequent problems. All pump shafts have a shoulder that determines the stopping point for a bearing on the shaft. The face of the inner race of a bearing should meet this shoulder all around the shaft—it makes the bearing square to the shaft (at a perfect right angle).
Pump manuals caution, “Make sure the bearing is square.” However, many do not indicate how. The check is an easy one. A feeler gauge of 0.001 inch to 0.002 inch is used to see if any gap exists between the face of the inner race and the shaft shoulder at the 12, 3, 6 and 9 o’clock positions.
When using a press to install a bearing, a gap is usually not present, or less chance exists of one. If thermal means are used to expand the inner race (no more than 230 F), the bearing must be held against the shaft shoulder so it does not shrink away as it cools. Most mechanics may think that holding the bearing in place for a minute or two will be enough to avoid a gap. This line of thinking is incorrect. The bearing should be held in position for 3 to 5 minutes. This simple step, if not performed, leads to cocked bearings and rapid bearing wear following installation.
If the bearing is not square to the shaft shoulder, bearing misalignment occurs. Checking squareness is vital to ensure that a pump spins freely.
Rebalance a Trimmed Impeller
With emphasis on energy savings and a desire to operate a pump closer to its best efficiency point (BEP), the impeller’s diameter is often trimmed to ensure that the pump more closely fits the system’s requirements. The affinity laws are used to calculate that, for instance, a 10-inch diameter impeller should be machined down to 9½ inches to better marry the pump curve to the piping. If an end user orders a 9½-inch diameter impeller from the factory, it is dynamically balanced.
However, if a machine shop trims the impeller in a lathe, the impeller is unbalanced. It is a casting. The removal of as little as ¼ inch from the diameter can result in massive unbalance when the pump spins at 1,750 rpm or worse at 3,550 rpm.
Asking the machine shop to send the impeller out for dynamic balancing is no trouble and costs little, certainly less than the failed bearings and unplanned downtime that can occur as the pump vibrates because of unbalance. When the pump is assembled, that is not the time to think about balance.
Balancing must take place during disassembly. The components can be placed on a balancing machine and corrected before reassembly. In some cases, the shaft and impeller should be assembled together and balanced as a unit to ensure against excessive vibration forces. P&S