Bearings, Lubrication, and the Care of Running Gear

What the rule requires

The film is the bearing

Lubrication works by substituting fluid friction for sliding friction. When a journal starts to turn, oil adhering to the journal surface is carried into the clearance, the film thickens, and the shaft is lifted and supported on that wedge NAVEDTRA 14104 Ch. 10 — Functions of Lubrication. Stopped, the oil is squeezed out from between journal and shell. The worst moment in a bearing's life is therefore the start, and in large engines the oil passages hold enough volume that the time taken to fill them on starting is sufficient to damage bearings — which is why separately driven priming pumps circulate oil before the engine turns and are secured once prescribed pressure is obtained NAVEDTRA 14075 Ch. 3 — Journal Bearings. Fatigue failure of journal bearings, normally caused by cyclic peak loads, is accelerated by loose fit of the shell in its housing and by failure to prime before start NAVEDTRA 14075 Ch. 3 — Shafts and Bearings.

What each bearing carries

Journal (sleeve) bearings carry the radial weight of the shaft on a thin film of pressurized oil NAVEDTRA 14075 §4-1.
Thrust bearings take axial force. The propeller's thrust passes through the main thrust bearing into the ship's structure. Pivoted-segmental shoe (Kingsbury) bearings work on a wedge-shaped oil film: the collar drags oil into the space between collar and shoes, and the shoes tilt on their pivots to the angle suiting the load and the oil viscosity. Shoes are fitted on both sides of the collar because most marine installations must take thrust in two directions NAVEDTRA 14075 Ch. 4 — Reduction Gears ¶3.
Rolling-element (ball and roller) bearings are used where loads and speeds suit them, oil- or grease-lubricated .
Main line shaft (spring) bearings are ring-oiled, babbitt-faced, spherical-seat shell bearings that align themselves to support the weight of the shafting. Brass oiler rings hang loosely over the journal, are dragged around by shaft rotation, and carry oil from the reservoir at the bottom up to the top of the journal. Because the rings tend to slip at very low speed — jacking for 24 hours while turbines cool — some designs use oiler discs clamped to the shaft instead NAVEDTRA 14075 Ch. 4 — Main Line Shaft Bearings.
Stern tube and strut bearings are underwater, where oil or grease is impracticable. They use materials that become slippery when wet: natural or synthetic rubber, lignum vitae, or laminated phenolic. Rubber composition is the type most used in modern installations NAVEDTRA 14075 Ch. 4 — Strut Bearings.

Spring bearing temperatures and oil levels are checked hourly while underway, and at least once each year the bearings are opened, clearances taken, and defects corrected .

The gage tells you less than you think

Oil must reach the bearings at prescribed pressure and within temperature limits, indicated by a pressure gage in the feed line and a thermometer in the return. A decided drop in oil pressure means shut the equipment down immediately; even a moderate rise in oil temperature is investigated NAVEDTRA 14104 Ch. 10 — Lubricating Systems. On an internal-combustion engine the same standing order applies: any abnormal drop in pressure or rise in temperature is investigated at once, and it is advisable to secure the engine until the trouble is found NAVEDTRA 14104 Ch. 7 — Lubrication System.

A clogged passage is the case the gage misses. Stoppage occurring beyond the pressure regulating valve and the gage may cause very little pressure rise, if any. The reliable checks are made just after shutdown: inspect the bearing and look for plenty of oil in the vicinity of the parts being lubricated, and feel the bearing by hand — you should be able to keep your hand on it for at least a few seconds NAVEDTRA 14075 Ch. 3 — Oil Lines and Passages.

Oil grade, temperature, and condition

Propulsion turbines and reduction gears take 2190 TEP; the letters indicate additives that increase the oil's ability to displace water from steel and inhibit oxidation. Gasoline and diesel engines take symbol 9110, 9170, 9250 or 9500. Grease is used where retaining oil would be difficult — throttle links, pump bearings, small boat steering links — and such bearings are usually fitted with a zerk fitting or grease cups NAVEDTRA 14104 Ch. 10 — Lubricating Oils and Greases, .

Temperature is held high enough for satisfactory circulation and low enough to prevent excessive oxidation. Follow the manufacturer's manual; absent one, hold oil leaving the engine between 160°F and 200°F .

Water in reduction gear oil is extremely harmful, and even small amounts soon pit and corrode the teeth; saltwater contamination can also cause bearing burnout. Finding and sealing a leaking cooler is not enough — the contaminated oil must be removed from all steel parts, and gears, journals and couplings have been ruined by delays of a week or less. With continued use the oil grows acidic, free fatty acids form a mineral soap, and the oil emulsifies and loses its lubricating quality; at the first indication of an emulsion the plant is stopped and the oil renovated NAVEDTRA 14075 Ch. 4 — Effects of Water and Acid in. Saltwater is detected by a standard chloride test, and it is far more dangerous than an equal quantity of freshwater — particularly in units with oil-lubricated ball bearings. Lube oil samples are taken daily on operating auxiliaries, and where a unit has stood idle overnight, enough is drained from the lowest part of the sump to remove settled water NAVEDTRA 14075 Ch. 9 — Inspection and Maintenance.

A cooler leak has put saltwater in the main reduction gear sump. You isolate and seal the leak. Have you corrected the casualty?

No. Sealing the leak is only the first step; the contaminated oil must be removed from all steel parts. Postponing that treatment — even for a week or less — has left gears, journals and couplings so corroded and pitted that the gears had to be removed and the teeth and journals reconditioned. Saltwater in the oil can also cause bearing burnout.</details>

Weardown limits — 46 CFR §61.20-23

The regulation sorts the bearing first, then the machinery location, then the shaft diameter. For water lubricated bearings other than rubber, with propelling machinery amidship, the after stern tube bearing must be rebushed when worn down to 6.4 mm (0.25 in) clearance for shafts 229 mm (9 in) or less; 7.95 mm (0.3125 in) for shafts over 9 in but not over 305 mm (12 in); and 9.53 mm (0.375 in) for shafts over 12 in. Where the propelling machinery is located aft, the bearing must be rebushed when weardown is 1.6 mm (0.0625 in) less than the applicable amidship clearance. Water lubricated rubber bearings are judged on groove depth instead: rebush when any water groove is half the original depth. Oil lubricated bearings are rebushed when deemed necessary by the Officer in Charge, Marine Inspection, who must consider the manufacturer's recommendation 46 CFR §61.20-23.

Telling it apart

Damage on a bearing is identified by the mark it leaves and where that mark sits. Read the surface, and the cause names itself.

Brinelled (dented) races — smooth indentations pressed into the race, from excessive pressure during installation or removal, or from the peening action of the balls or rollers when vibration from other machinery reaches an idle bearing. Brinelled bearings must not be returned to service. The best insurance against vibration brinelling is to rotate idle shafts at regular intervals, at least once a day NAVEDTRA 14075 Ch. 3 — Frictionless Bearings.
Spalled or pitted rollers or races — first recognised by noisy operation, and still noisy when turned by hand after thorough cleaning. Inspect the inner surface of the inner race in particular, since most surface disintegration starts there, and treat any sign of rust on rollers or race contact surfaces as probable evidence the bearing is ruined, because pits may be hidden under it.
Cracked race — a definite thump or clicking during operation; cracks usually run parallel to the axis of the race and are confirmed by cleaning and inspection.
Abrasion on the external surface of a race — scoring, wiping or burnishing there means relative motion between race and housing or shaft. Slow creep of the push-fit race is desirable and its wear is negligible; abrasion from a locked bearing or improper fit of the races is the fault.
Excessive looseness with no surface disintegration — generally fine abrasives in the lubricant. Many of these bearings feel loose when new, so compare the suspect against a new bearing before condemning it.
A dirty bearing — detected by noise on rotation, difficulty turning, or visual inspection. Do not discard it until you have established that something besides dirt caused the trouble; proper cleaning may make it serviceable.

Journal bearings carry their own signatures NAVEDTRA 14075 Ch. 3 — Journal Bearings:

Very small pits covering the surface — corrosion from the chemical action of oxidized oil. The pits are so closely spaced they form channels, breaking the film continuity and cutting the load-carrying area below safe operation.
Small streaks of lead on the surface — the early stage of lead melting from high localized temperature, generally the result of very close oil clearances together with an oil of higher viscosity than recommended.
Bearing shell showing through the surface — inadequate bond between bearing metal and shell, from fatigue under cyclic loads or defective manufacture.
Gumlike varnish or lacquer deposit on the back of the shell — poor contact between shell and connecting rod, from foreign particles, excessive clearance or a rough surface.
Bearing material cracked at opposite ends of the upper and lower shells — misalignment between the connecting rod bore and the piston pin bushing bore.

A shaft on an idle auxiliary has stood untouched for a shipyard period alongside a running machine. On opening the bearing you find smooth, evenly spaced indentations in the race. What is it, and can the bearing go back in?

Brinelling from the peening action of the rollers or balls on the races under vibration while the bearing was inoperative. It cannot go back in — brinelled bearings must not be placed back in service. The preventive measure is rotating shafts carried on frictionless bearings at least once a day during periods of idleness.</details>

Working a question

You are standing by a tailshaft survey. The shaft is 11 inches in diameter, the propelling machinery is located aft, the after stern tube bearing is water lubricated bronze, and measured clearance is 0.24 in. Does it require rebushing?

Sort the bearing. Water lubricated and not rubber, so paragraph (a) governs. Had it been rubber, clearance would be the wrong measurement entirely and you would be looking at water groove depth; had it been oil lubricated, there is no numerical limit at all and the call belongs to the OCMI.
Take the amidship figure for the shaft diameter. 11 in exceeds 9 in and does not exceed 12 in, so the amidship clearance is 0.3125 in.
Apply the machinery location. Machinery aft, so the limit is 1.6 mm (0.0625 in) less than the amidship figure.
Compute. 0.3125 − 0.0625 = 0.25 in.
Compare and answer. Measured 0.24 in is below the 0.25 in limit, so rebushing is not yet required — but the margin is 0.01 in, which is a reporting matter, not a pass to forget it.

Step 3 is where the question is won. A candidate who adds the 0.0625 in gets 0.375 in, concludes there is 0.135 in of life left, and answers the opposite of the truth 46 CFR §61.20-23.

Where candidates lose the point

Treating "machinery located aft" as the more generous case. Aft installations get less clearance before rebushing, not more. The wording invites the error because the reduction is stated as a difference from the amidship figure rather than as its own table.
Applying the clearance figures to a rubber bearing. A rubber-lined bearing is condemned when any water groove is half the original depth. A candidate who has the clearance numbers in front of him will use them because they look like the answer.
Trusting a rising pressure gage to reveal a clogged passage. Stoppage beyond the pressure regulating valve and the gage may barely move the needle. The bearing is confirmed by inspection and by hand just after shutdown NAVEDTRA 14075 Ch. 3 — Oil Lines and Passages.
Condemning a bearing for dirt or noise alone. Dirty bearings may be made serviceable by proper cleaning, provided no other damage exists — and dirty frictionless bearings must be thoroughly cleaned before being rotated or inspected. Never spin a frictionless bearing with compressed air NAVEDTRA 14075 Ch. 3 — Frictionless Bearings.
Confusing overload with overspeed on main journal bearings. Overloading fails the lower halves; overspeeding may fail either the upper or the lower halves NAVEDTRA 14075 Ch. 3 — Shafts and Bearings.
Choosing a heavier oil as the safe answer. Viscosity higher than recommended, with close oil clearances, produces the high localized temperature that melts the lead and pits the bearing surface. The right answer is the specified grade NAVEDTRA 14075 Ch. 3 — Journal Bearings.
Wiping an engine down with cotton waste or paper towels. They leave lint and small bits that later collect in the oil lines. Filters are serviced at specified intervals, the case cleaned, and lines blown out with compressed air when removed .

Check yourself

A 14-inch tailshaft, water-lubricated bronze after stern tube bearing, propelling machinery located aft. At what clearance must it be rebushed?

0.3125 in. A shaft exceeding 12 in takes the 9.53 mm (0.375 in) amidship figure; machinery aft reduces that by 1.6 mm (0.0625 in), giving 7.95 mm (0.3125 in).</details>

The stern tube bearing is rubber. Clearance measures 0.30 in and the water grooves are worn to half their original depth. What do you report?

Rebushing is required. A water lubricated rubber bearing is judged when any water groove is worn to half the original depth; the clearance figures in paragraph (a) apply to water lubricated bearings other than rubber and are not the criterion here.</details>

Lube-oil pressure and temperature read normal on your rounds, but one main bearing has been suspect. How do you establish whether it is getting oil?

After shutdown, inspect it — there should be plenty of oil in the vicinity of the parts being lubricated — and feel it, holding your hand on it for at least a few seconds. Do not rely on the gage: a stoppage beyond the pressure regulating valve and gage may show little or no pressure change.</details>

You are told to top off a reduction gear with the drum of 9250 on hand because the 2190 TEP is stowed forward. What is wrong with that?

Propulsion turbines and reduction gears take 2190 TEP, whose additives increase the oil's ability to displace water from steel and inhibit oxidation. The 9000-series oils are for internal combustion engines. Use the specified lubricant for the unit.</details>

A main engine has been run over its rated speed for an extended period. What do you expect to find in the journal bearings and journals?

Increased inertia forces cause excessive wear of the journal bearings and other engine parts and uneven wear of the journals. On the halves: overspeeding may fail either the upper or the lower halves, whereas overloading fails the lower halves of main journal bearings.</details>

During reassembly a plain upper shell is fitted in place of the lower shell, which carries an oil groove. What happens?

Oil flow stops completely and the bearing fails early. The damage may extend beyond the bearing to the crankshaft, connecting rod, piston and wrist pin.</details>

Gear teeth in a reduction gear are found roughened where a foreign object passed through, and a second set of teeth is deeply pitted with no traceable cause. What is the correct action for each?

Rough surfaces from the passage of foreign objects are stoned smooth. Stoning removes a local hump or deformation only — it will not remove deep pitting or galling, and where the deterioration cannot be traced to a foreign object, give special attention to lubrication, to the condition of the bearings, and to whether a change in supporting structure has disturbed the parallelism of the rotors.</details>