The Thrust Block and Thrust Shaft — Clearance, Pads and Support

1. Propulsion Shafting Layout and the Thrust Shaft

Operating rule: The thrust block clearance is always measured by jacking the shaft axially and gauging with a dial indicator, never by forcing feeler gauges into the pad/collar gap. Total axial float is approximately 1.0 mm. Forcing a feeler gauge between the thrust collar and the white-metal pad damages the working face and produces misleading readings.

The ship engine transmits rotational power to the propeller through a series of connected shafts forming the propulsion shaft line:

• Thrust Shaft: Coupled directly to the engine crankshaft or reduction gearbox output flange. It passes through the thrust block, carrying the integral thrust collar that transfers axial propulsion forces to the ship's hull.
• Intermediate Shaft(s): Supported at intervals by journal tunnel bearings (plummer blocks) along the shaft tunnel, transmitting torque aft toward the sternframe.
• Tail Shaft (Propeller Shaft): Passes through the stern tube seals and bushes to carry the propeller outside the hull.

Thrust Shaft Material and Construction:

The thrust shaft is manufactured from solid forged ingot steel (often normalized low-carbon or alloy steel). An integral forged collar is machined directly on the shaft body with finely ground, polished thrust faces perpendicular to the axis of rotation.

Auxiliary Shaft Equipment Siting:

• Shaft Brake: Situated between the thrust bearing and the propulsion engine or motor to arrest shaft rotation during manoeuvring or emergency stops.
• Shaft Locking Device / Turning Gear Interlock: Located aft of the thrust bearing to secure the shaft safely during maintenance, drydocking, or single-screw immobilisation.

2. Working Principle — How Axial Thrust is Transmitted to the Hull

The purpose of a marine thrust block is to transmit the massive axial thrust generated by the rotating propeller blades into the ship's hull structure, while preventing that axial thrust from entering and damaging the engine crankshaft.

The Physics of Thrust Generation:

1. The engine delivers torque and rotational power along the shaft line.
2. The rotating propeller blades exert dynamic force on the surrounding water, driving water astern.
3. In equal and opposite reaction, the water drives the propeller blades forward with immense axial force.
4. This axial load pushes the propeller inward, transmitting axial compressive thrust along the tailshaft, through the intermediate shafts, to the thrust shaft.
5. Without a thrust block: The axial force would push the shaft directly forward into the engine bedplate, deflecting the crankwebs, wiping main bearings, and destroying the crankshaft within minutes.

Transmission Path to the Hull:

When running ahead, the forward face of the rotating collar bears against the ahead thrust pads. When running astern, the after face of the collar bears against the astern thrust pads. The axial force is transferred through the oil film into the pads, into the heavy thrust block housing, and down through fitted holding-down bolts, side chocks, and taper wedges directly into reinforced double-bottom hull girders.

3. Thrust Pads and the Hydrodynamic Oil Wedge (Michell Principle)

Marine thrust blocks operate on the Michell principle of hydrodynamic lubrication. Instead of flat face-to-face contact (which would squeeze out oil and cause instantaneous seizure), the bearing consists of a ring of loose pads free to tilt slightly about a pivot.

Formation of the Hydrodynamic Oil Wedge:

• As the thrust collar rotates, surface adhesion drags a film of lubricating oil into the entrance between the collar face and the tilting pad.
• Because the pad is supported on an eccentric pivot line or pivot pin, the entering oil tilts the pad slightly, creating a converging wedge-shaped clearance.
• As viscous oil is forced into this continuously narrowing gap, hydrodynamic pressure builds rapidly — reaching local peak pressures of several hundred bars.
• This fluid pressure completely lifts the thrust collar off the pad surface, ensuring continuous fluid-film lubrication with zero metal-to-metal contact during normal operation.

Pad Materials and Geometry:

• Pad Facing: Traditionally high-grade tin-based white metal (Babbitt) cast onto steel or bronze backing plates. Modern high-load installations increasingly adopt PTFE-faced (polytetrafluoroethylene) pads, which tolerate higher specific bearing pressures, resist wiping during low-speed barring, and lower start-up friction.
• Kidney vs Circular Pads: Kidney-shaped pads were the traditional standard. Modern designs often employ circular pads because the resulting pressure contours are more concentric with the pad area, yielding higher load-carrying efficiency per square centimetre.

Engine-Integrated Bedplate Thrust Bearings:

In large two-stroke crosshead diesels, the thrust bearing is frequently incorporated directly into the after end of the engine bedplate rather than built as an independent stand-alone block:

4. Axial Clearance — Why It Is Vital

Total axial clearance (end float) is the total free axial travel of the thrust collar between its ahead pads and its astern pads. A controlled clearance is indispensable for two physical reasons:

1. Formation of the Hydrodynamic Oil Wedge: The tilting pads must have physical room to rock and establish the converging oil wedge. If axial clearance is zero, the collar clamps the pads tight, oil cannot enter, the hydrodynamic film collapses, and boundary friction melts the white metal.
2. Thermal Expansion: As the engine, shafting, and lubricating oil warm from ambient temperature up to operating temperatures (typically 45°C to 65°C), the collar, casing, and pads expand axially. Clearance accommodates this thermal growth without binding.

Nominal Clearance Values:

For a typical 500 mm diameter shaft, total axial clearance is approximately 0.5 mm to 1.0 mm (nominal maker specification ~1.0 mm). The maximum allowable service clearance before re-shimming or pad renewal is typically 2.0 mm.

Asymmetric Consequences of Clearance Errors:

• If axial clearance is too small: The oil wedge cannot form adequately. Viscous shear produces extreme heat, oil film breakdown occurs, and the bearing fails catastrophically through white-metal wipe within minutes. A hot thrust bearing is always treated as an immediate emergency.
• If axial clearance is too large: The thrust pads themselves remain undamaged because hydrodynamic lubrication continues normally. However, excessive axial float allows the shaft line to surge fore and aft during sea conditions and engine manoeuvring. This places dangerous axial bending loads on engine crankwebs, threatens flexible couplings, and disrupts gearbox tooth meshing.

5. Measuring, Calculating, and Adjusting Clearance

Accurate clearance measurement is performed with dial test indicators (DTI) or hydraulic jacking gear. Feeler gauges should never be pushed between the pad and collar faces.

Step-by-Step Jacking and Dial Gauge Procedure:

1. Remove the top inspection cover of the thrust block.
2. Place a hydraulic ram or mechanical screw jack between the after end of the casing and the shaft coupling flange.
3. Jack the shaft firmly forward until the collar is seated hard against the ahead pads.
4. Check that the shaft remains concentric within the journal bearings and that liners bear evenly on the casing.
5. Mount a dial test indicator on a rigid magnetic base attached to the casing, with its measuring plunger resting perpendicular against the shaft coupling face. Zero the dial indicator.
6. Relocate the jack to the forward side and force the shaft firmly aft until the collar seats hard against the astern pads.
7. Read the dial gauge. The total registered deflection is the total axial clearance.

Clearance Calculation on Engine-Integrated Thrust Bearings:

On slow-speed engines where the thrust collar is located immediately aft of the aftermost crank throw, axial clearances are calculated by turning the crankpin to bottom dead centre (BDC) and measuring crankshaft thrust clearances:

Crucial maintenance safety precaution: Whenever removing thrust pads from the housing for survey or overhaul, ALWAYS insert dummy wooden pads of the exact same dimensions into the vacant slot. This prevents any axial or angular shifting of the shaft line, protecting seals, couplings, and journal bearings from dangerous cocking and misalignment.

Adjusting the Clearance:

If the measured axial float falls outside maker tolerances, clearance is adjusted by removing or inserting precision-ground shims behind the pad carrier distance pieces, or by machining the back-liner faces.

6. Siting and Structural Foundation Support

The main thrust block is located immediately aft of the main propulsion machinery. There are three sound engineering reasons for this forward siting:

1. Minimizing Differential Thermal Expansion: Steel propulsion shafts expand and contract with temperature variations in the machinery spaces and sea. On large midship-engined refrigerated cargo ships, hull contraction relative to the shaft reached up to 20 mm. Siting the thrust block directly adjacent to the main engine anchors the shaft line at the machinery end, ensuring that thermal growth expands aft toward the stern tube rather than distorting the engine crankshaft couplings.
2. Isolating the Engine from Stern Tube Deflections: The after end of the shaft line is subjected to tailshaft whirl, propeller weight droop, and hull flexing in heavy seaways. Locating the thrust block forward creates a stable reference anchor at the rigid machinery foundation.
3. Foundation Rigidity: The heavy double bottom structure directly below the engine room provides the stiffest structural girder in the ship.

Hull Stiffening and Chocking:

Because the thrust load acts above the tank top level at the shaft centreline, it generates an overturning moment that tends to tilt the thrust block forward during ahead propulsion:

• Preventing Forward Tilt: If the thrust stool lacks adequate stiffness, the block tilts forward under load, lifting the after journal bearing off the shaft and causing severe shaft misalignment.
• Internal Tank Reinforcement: The double bottom beneath the thrust block is reinforced with additional intercostal girders, closely pitched floor plates, and heavy I-beams extending across multiple frames.
• Fitted Bolts and Taper Wedges: The thrust block is bedded on cast iron or resin chocks. In addition to standard holding-down bolts, fitted bolts and taper wedges (having a slow taper of approximately 20 mm/m) are installed. These positive mechanical stops take the full shear load, relieving the holding-down bolts from shear strain.
• Axial Vibration Damping: Cyclic variations in propeller torque and diesel engine crankweb splay excite axial vibration. The oil film and massive foundation of the thrust block serve as a primary axial damper, suppressing tank-top panting and structural resonance.

7. Detailed Mechanical Features of the Michell Block

A self-contained independent Michell thrust block incorporates specialized lubrication and support components:

• Peripheral Oil Scraper: Rides lightly on the outer circumference of the rotating thrust collar. It collects oil carried up from the lower sump and diverts it into an overhead distribution channel, cascading a continuous flood of cool oil across the pad faces and journal bearings.
• Horseshoe Pad Carriers: The tilting thrust pads fit radially into upper and lower half horseshoe carriers, secured circumferentially against rotation by heavy stop blocks.
• Liners and Distance Pieces: The pad carriers bear axially against precision-ground steel liners fitted into the casing recesses. Adjusting the thickness of these liners sets the fore-and-aft axial float.
• Oil Sump and Seawater Cooling Coil: The lower casing acts as an independent oil reservoir equipped with a level gauge glass, temperature sensor, and a copper-nickel or titanium cooling coil through which cooling water is circulated.
• Deflectors and Oil Seals: Labyrinth seals and oil throwers fitted at the forward and after shaft exits prevent lubricant leakage and protect the sump from external bilge water and dirt contamination.

8. The Michell Thrust Indicator

For sea trials, research vessels, and specialized propulsion monitoring, a thrust block can be equipped with a hydraulic thrust meter to measure true delivered shaft thrust under operational conditions.

Operating Mechanism:

1. The standard stationary pad carrier shoe is replaced by a forged steel ring containing a series of interconnected hydraulic cylinders and pistons on its reverse face.
2. A high-pressure hydraulic hand pump, precision pressure gauge, and calibrated relief valve are connected to the cylinder manifold.
3. During sea trials with the ship running at the desired engine speed, oil is pumped into the cylinders.
4. The hydraulic pistons advance slightly, taking the thrust load off the mechanical liners. The entire thrust pad assembly floats on the hydraulic oil film.
5. The exact propulsion thrust force is determined directly by multiplying the hydraulic gauge pressure (typically up to 175 bar) by the total cylinder piston area.
6. A built-in safety relief valve prevents over-pumping once half the axial clearance has been traversed.

9. Troubleshooting a Hot Thrust Bearing

Because the thrust bearing operates under the highest specific unit pressures and smallest clearances in the entire propulsion shafting, any operating irregularity manifests rapidly as elevated temperature.

Emergency Response Procedure:

1. Reduce Engine RPM Immediately: Slow the main engine to reduce propeller thrust load and decrease frictional heat generation while maintaining sufficient rotation for hydrodynamic oil wedge action.
2. Verify Oil Supply and Reservoir Level: Check the sump sight glass. On force-lubricated blocks, verify header tank flow, supply pressure, and filter differential. On self-contained blocks, ensure the peripheral oil scraper is intact and cascading oil freely.
3. Inspect Cooling Medium: Check cooling water flow through the internal cooling coils. Feel the inlet and outlet pipes. Marine growth or scale accumulation causes heat exchange loss; conversely, a burst cooling tube will contaminate the oil sump with water (indicated by cloudy, emulsified oil).
4. Inspect Oil Sample: Draw a sample from the sump drain cock. Check for water contamination, burnt odour, discolouration, or sparkling white-metal debris indicating pad wiping.
5. Check Pad Thermocouple Trends: Modern installations feature embedded PT100 temperature sensors in both ahead and astern pads. Compare individual pad temperatures to distinguish overall cooling failure from localized pad loading due to shaft cocking.
6. Verify Axial Clearance and Jacking Records: Compare recent clearance logs against original installation benchmarks. Check for signs of loose holding-down bolts, shifted taper wedges, or thrust block rocking on the tank top.

Service constraint: Never attempt to re-shim or dismantle thrust pad carriers at sea without full shaft immobilisation and classification society approval. Pad renewal or liner adjustment is strictly a harbour or drydock evolution with the shaft line properly supported by jacks.