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?
Vibration analysis on a pump shows a dominant peak at twice running speed with significant axial vibration. What fault does this indicate?
Oil drained from a pump bearing housing appears milky. What does this indicate and what should be done?
Why must a flexible coupling not be treated as a substitute for precision alignment?