12.3 Bearings, Seals, Couplings & Lubrication
Key Takeaways
Misalignment is the leading cause of premature coupling, bearing, and seal failure, and it must be corrected in both angular and parallel directions.
Over-greasing a bearing is as damaging as under-greasing because excess grease churns, overheats, and blows out the seals.
Mechanical seals should not leak visibly, while packing must leak a controlled amount to lubricate and cool the shaft sleeve.
Vibration analysis identifies the fault by frequency, with imbalance showing at running speed and misalignment showing at twice running speed.
Never mix incompatible grease types, since combining thickeners can break down the grease structure and destroy the bearing.
12.3 Bearings, Seals, Couplings & Lubrication
When a pump fails, it almost never fails in the impeller. It fails at a bearing or a seal, and the root cause is almost always misalignment, lubrication, or contamination. This section is where an operator saves their plant the most money.
1. Bearings
| Type | Description | Application |
|---|---|---|
| Ball (antifriction) | Rolling elements between races | Most motors and pumps; moderate loads, high speed |
| Roller | Cylindrical or tapered elements | Higher radial loads |
| Thrust | Handles axial load along the shaft | Vertical turbine pumps, where the entire column weight and hydraulic thrust bear down |
| Sleeve (journal / babbitt) | Shaft rides on a lubricant film in a plain bore | Large, slow, heavy machines; very long life if the oil film is maintained |
Why bearings fail
| Cause | Share of failures | Indication |
|---|---|---|
| Improper lubrication — wrong type, too much, too little, contaminated | The largest single cause | Heat, noise, discolored grease |
| Contamination — dirt, water, process fluid | Very large | Gritty feel, rust, dark grease |
| Misalignment | Large | Heat on one bearing, coupling wear, vibration at 2× running speed |
| Overload / improper fit | Moderate | Brinelling, spalling |
| Electrical shaft currents (VFDs) | Growing | Fluting — evenly spaced washboard marks in the race |
Temperature rule of thumb: a bearing running above roughly 180 °F, or more than about 40–50 °F above ambient, warrants investigation. Also watch for a rising trend — a bearing that has climbed 15 °F over a month is failing even if it is still within limits.
2. Lubrication
The over-greasing trap
More grease is not better. A bearing cavity should be roughly one-third to one-half full. Excess grease has nowhere to go, so it churns, generating heat that thins it further, and it blows out the seals — which then admits contamination. Over-greasing is one of the most common causes of premature bearing failure at treatment plants, and it is entirely self-inflicted.
Proper procedure:
- Clean the grease fitting before applying the gun. A dirty fitting injects contamination directly into the bearing.
- Use the specified grease in the specified quantity at the specified interval.
- Where a relief plug exists, remove it before greasing and run the motor briefly afterward so excess purges, then replace it.
- Grease while the equipment is running where it is safe to do so, so the grease distributes.
Never mix incompatible greases
Greases are built on different thickeners — lithium, lithium complex, polyurea, calcium sulfonate, aluminum complex, clay. Mixing incompatible thickeners can cause the grease to soften into a liquid and run out, or harden into a block that stops flowing. Either way the bearing loses lubrication. When changing grease types, the bearing must be purged or cleaned, not simply topped off.
Oil lubrication
| Element | Practice |
|---|---|
| Level | Maintain at the sight glass mark; a constant-level oiler holds it automatically |
| Viscosity | Use the specified grade; higher ambient temperature generally calls for higher viscosity |
| Condition | Milky or cloudy oil means water contamination — find the source, do not just change the oil |
| Change interval | By hours or by oil analysis; oil analysis also detects wear metals before failure |
| Breathers | Desiccant breathers prevent moisture being drawn in as the housing heats and cools |
3. Shaft Sealing
| Packing (stuffing box) | Mechanical seal | |
|---|---|---|
| How it seals | Compressible rings compressed by a gland follower | Two lapped faces, one rotating and one stationary, running together |
| Expected leakage | Must drip — roughly 20 to 60 drops per minute | None visible |
| Adjustment | Gland tightened gradually, never fully | None; not adjustable |
| Cost / complexity | Low cost, simple, operator-serviceable | Higher cost, requires disassembly |
| Failure mode | Gradual increase in leakage; scored sleeve if over-tightened | Sudden — goes from dry to gushing |
| Best for | Abrasive service, low pressure, where some leakage is acceptable | Hazardous or clean service, where no leakage is permitted |
Rules that get tested
- A lantern ring in the middle of the packing set distributes seal water to lubricate and to prevent air being drawn in on a suction-lift pump. Getting the lantern ring in the wrong position — not aligned with the seal water port — starves the packing.
- Never over-tighten packing. Tighten in small increments, allowing the pump to run between adjustments, and stop when leakage is controlled to a drip.
- Mechanical seals must never run dry. Even a few seconds without liquid destroys the faces. This is why dry-run protection matters on chemical and sludge pumps.
- Replace all packing rings as a set, staggering the ring joints roughly 90° apart.
4. Couplings and Alignment
| Coupling | Character |
|---|---|
| Rigid | No misalignment tolerance; requires near-perfect alignment |
| Flexible (elastomeric, grid, gear, disc) | Accommodates small misalignment and damps shock |
| Spacer | Allows seal or bearing service without moving the driver |
The misconception to kill: a flexible coupling does not excuse misalignment. It tolerates a small residual amount. Every thousandth of an inch of misalignment beyond that is transmitted into the bearings and seals as a cyclic load, and it is the leading cause of premature rotating equipment failure.
The two kinds of misalignment
- Angular — shaft centerlines meet at an angle; the coupling faces are not parallel.
- Parallel (offset) — shaft centerlines are parallel but not collinear.
Most real misalignment is a combination of both, in both the horizontal and vertical planes.
Alignment practice
- Lock out and tag out first.
- Check and correct soft foot — a machine foot that does not sit flat, which distorts the casing when bolted down and makes alignment impossible to hold.
- Align with a dial indicator or, preferably, a laser alignment tool.
- Correct vertical misalignment with shims, then horizontal by jacking.
- Account for thermal growth where a machine runs hot — align cold to a calculated offset so it is aligned at operating temperature.
- Recheck after grouting, after piping is connected, and after the first run. Pipe strain is a frequent hidden cause: piping that must be forced into place pulls the pump out of alignment every time it is bolted up.
5. Belt Drives
- Tension matters in both directions. Too loose slips, glazes, and overheats; too tight overloads the bearings on both shafts.
- Sheaves must be aligned — a straightedge across both sheave faces is the minimum check.
- Replace V-belts in matched sets. Mixing an old belt with a new one loads the new belt almost entirely.
- Belt dust, glazing, and cracking indicate slipping and imminent failure.
6. Vibration and Condition Monitoring
Vibration analysis identifies the fault by the frequency at which the energy appears:
| Fault | Characteristic frequency |
|---|---|
| Imbalance | 1× running speed, primarily radial |
| Misalignment | 2× running speed, often with significant axial vibration |
| Looseness | Multiple harmonics — 2×, 3×, 4× and beyond |
| Bearing defects | High frequency, non-synchronous, with characteristic defect frequencies |
| Cavitation | Broadband random noise, no discrete peak |
| Vane pass / hydraulic | Number of impeller vanes × running speed |
| Belt problems | Belt frequency and its harmonics |
Other condition monitoring tools
- Infrared thermography — finds hot bearings, loose electrical connections, and overloaded motors without contact.
- Oil analysis — wear metals, viscosity change, water, and particle count identify problems long before failure.
- Ultrasonic — detects early bearing distress, leaks, and electrical arcing.
- Motor current signature analysis — detects rotor bar and mechanical faults from the current waveform.
A predictive maintenance program built on these tools consistently outperforms both run-to-failure and fixed-interval preventive maintenance, because it schedules work when the equipment actually needs it.
An operator greases a motor bearing until grease is packed completely full, believing more is better. What is the likely result?
Extended bearing life due to improved lubrication
Churning that generates heat and blows out the seals, allowing contamination and causing premature failure
No effect, since excess grease simply purges harmlessly
Reduced motor current draw from lower friction
Vibration analysis on a pump shows a dominant peak at twice running speed with significant axial vibration. What fault does this indicate?
Imbalance
Misalignment
Bearing defect
Cavitation
Oil drained from a pump bearing housing appears milky. What does this indicate and what should be done?
Normal aeration; simply refill to the sight glass
Water contamination; find and correct the source rather than only changing the oil
Excessive viscosity; switch to a lighter grade
Grease and oil have been mixed; flush with solvent and return to service
Why must a flexible coupling not be treated as a substitute for precision alignment?
Flexible couplings cannot transmit full torque when misaligned
Misalignment causes the coupling elastomer to expand and lock the shafts together
Flexible couplings require more frequent lubrication when misaligned
They tolerate only small residual misalignment, and anything beyond that is transmitted into bearings and seals as cyclic load
Sections you finish are checked off in the contents.