The axial clearance (oil clearance) in a Mitchell-type main thrust bearing is the total axial movement of the thrust shaft between the ahead and astern thrust pads. Maintaining the correct axial clearance is essential for the following reasons:

1. Formation of the Hydrodynamic Oil Wedge
   • The correct clearance allows the thrust pads to tilt freely on their pivots or ridges.
   • This tilting action draws lubricating oil between the rotating thrust collar and the stationary white-metal pads, forming a pressurized wedge-shaped hydrodynamic oil film.
   • The oil film prevents direct metal-to-metal contact and ensures smooth operation.
2. Prevention of Overheating and Seizure
   • If the clearance is too small, the oil flow between the collar and pads is restricted.
   • The resulting thin oil film produces excessive friction and heat, which can cause wiping (melting or smearing) of the white-metal lining and may eventually lead to bearing seizure.
3. Accommodation of Thermal Expansion
   • During operation, the engine and shafting expand axially due to temperature rise.
   • The axial clearance provides sufficient space to accommodate this thermal expansion without imposing excessive compressive loads on the crankshaft, thrust bearing, or engine bedplate.
4. Control of Crankshaft Axial Movement
   • Excessive clearance caused by wear allows the shafting to move too far in the axial direction.
   • This shifts the crankshaft from its designed position, which may lead to damage to the crank webs, main bearings, and misalignment of connected equipment such as the turning gear.
5. Reduction of Axial Vibrations
   • The correct clearance helps absorb and dampen axial vibrations transmitted from the propeller through the shafting, thereby protecting the main propulsion machinery.

Axial clearance can be measured by the following onboard methods:

1. Feeler Gauge Method (Static)

• Ensure the thrust collar is pressed firmly against one set of thrust pads (ahead or astern).
• Insert a long feeler gauge between the thrust collar and the opposite set of thrust pads.
• The thickness of the feeler gauge that fits snugly without forcing represents the total axial clearance.

2. Dial Gauge Method (Static)

• Mount a dial indicator securely on the thrust block casing using a magnetic base.
• Position the dial gauge tip against a machined surface of the thrust shaft and set the indicator to zero.
• Using a hydraulic jack or suitable levering arrangement, move the shaft fully forward until it contacts the ahead thrust pads and note the reading.
• Move the shaft fully aft until it contacts the astern thrust pads.
• The total movement indicated on the dial gauge represents the total axial clearance.

Removal Procedure

1. Safety and Isolation
   • Stop and isolate the main engine.
   • Engage the turning gear and lock out the starting system.
   • Isolate the lubricating oil system and display appropriate warning notices.
2. Gain Access
   • Remove the thrust bearing top cover using suitable lifting equipment such as the engine room overhead crane.
3. Shift the Shaft
   • Move the thrust shaft axially towards the opposite side of the pads to be removed (for example, move the shaft forward to remove the astern pads), creating sufficient clearance for removal.
4. Remove the Thrust Pads
   • The pads are generally mounted in a carrier ring or retaining ring.
   • Rotate the pad ring or individual pads upward using the provided eyebolts or special lifting tools.
   • Withdraw each thrust pad carefully, one at a time, from the side of the shaft.
   • Mark and keep each pad in its original position (Ahead/Astern and Port/Starboard) to ensure correct reassembly.

Inspection Points

During inspection, particular attention should be given to the following:

1. Condition of the White-Metal Lining
   • Check for scoring, scratches, pitting, erosion, overheating, wiping (melting or smearing), and signs of metal-to-metal contact.
2. Cracks and Delamination
   • Inspect for hairline cracks, fatigue cracks, crazing, or separation of the white-metal lining from the steel or bronze backing.
   • If necessary, carry out a dye penetrant test to detect fine cracks or bonding failure.
3. Pivot or Tilting Surface
   • Examine the pivot button or ridge on the back of the pad for wear or damage.
   • Excessive wear at the pivot prevents proper pad tilting and affects the formation of the hydrodynamic oil wedge.
4. Oil Grooves and Chamfers
   • Ensure that the oil grooves, leading-edge chamfers, and oil passages are clean and free from carbon deposits, sludge, or metal particles that could restrict oil flow and impair lubrication.