A proper rebuild is a key contributing factor in the longevity of a pump, but rebuilding a pump to last requires skill and attention to detail. In the first article in our “Pump Rebuild Tips” series, we discussed checking for piping strain and inspecting bearing fits. Today, we will explore pump alignment and heating the bearing for shaft installation.
Ensuring Shaft Straightness
Pump shafts are subjected to unbalanced impellers, worn bearing fits, impeller rubs and other mechanical strains that can cause them to bow. With the pump shaft removed from the pump during the rebuild and all other components removed, end users should take the opportunity to check the shaft for straightness.
A pool player takes the cue, lays it on the pool table and rolls it back and forth. If it bumps as it rolls, it is not straight. A similar check on a shaft can be completed on a shop bench with a dial indicator and two V-blocks.
Place the shaft bearing seats on the V-blocks and position the indicator at the center of the shaft. Tum 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 Heating 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.
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.
In our next and final article in this series, we’ll look at rebalancing a trimmed impeller and squaring the bearing to the shaft shoulder.