Thrust bearing
Thrust blocks: The main thrust block transfers forward or astern propeller thrust to the hull and limits axial movement of the shaft. Some axial clearance is essential to allow formation of an oil film in the wedge shape between the collar and the thrust pads (Figure 8.6). This clearance is also needed to allow for expansion as parts warm up to operating temperature. The actual clearance required, depends on dimensions of pads, speed, thrust load and the type of oil employed. High bearing temperature, power loss and failure can result if axial clearance is too small.
A larger than necessary clearance will not cause harm to the thrust bearing pads, but axial movement of the shaft must be limited for the protection of main machinery.
The accepted method of checking thrust clearance, involves jacking the shaft axially to the end of its travel in one direction and then back to the limit of travel in the other. Total movement of the thrust shaft (about 1 mm being typical) is registered on a dial gauge. Feelers can be used as an alternative, between thrust ring and casing. Use of feelers in the thrust pad/collar gap is likely to cause damage and may give a false reading.
Thrust block position
The siting of the main thrust block close to the propulsion machinery, reduces any problems due to differential expansion of the shaft and the hull. The low hull temperature of midship engined refrigerated cargo ships, caused a contraction relative to the shaft of perhaps 20mm (|"). Variations can be caused by changes in water temperature or heating of fuel tanks.
Other problems associated with the stern tube end of the shafting system include whirl of the tailshaft, relative movement of the hull and misalignment due to droop from propeller weight. Some thrusts are housed in the after end of large slow speed diesels or against gear boxes. Deformation produced by the thrust load, can cause misalignment problems, unless suitable stiffening is employed (particularly with an end of gearbox installation).
Thrust block support
The substantial double bottom structure under the main propulsion machinery, provides an ideal foundation for the thrust block and a further reason for siting it close to the engine. The upright thrust block and any supporting stool, must have adequate strength to withstand the effect of loading which tends to cause a forward tilt. This results in lift of the aft journal of the block (unless not fitted) and misalignment of the shaft.
Axial vibration of the shaft system, caused by slackening of the propeller blade load as it turns in the sternframe or by the splay of diesel engine crankwebs, is normally damped by the thrustblock. Serious vibration problems have sometimes caused thrust block rock, panting of the tank top and structural damage.
Thrust pads
The pivot position of thrust pads may be central or offset. Offset pads are interchangeable in thrust blocks for direct reversing engines, where the direction of load and rotation changes. Offset pads for non-reversing engine and controllable pitch propeller installations are not interchangeable. Two sets are required. Pads with a central pivot position are interchangeable.
Some modern thrust blocks are fitted with circular pads (Figure 8.7) instead of those with the familiar kidney shape. A comparison of the pressure contours on the conventional kidney shaped pads and the circular type shows why the latter are effective.
A larger than necessary clearance will not cause harm to the thrust bearing pads, but axial movement of the shaft must be limited for the protection of main machinery.
The accepted method of checking thrust clearance, involves jacking the shaft axially to the end of its travel in one direction and then back to the limit of travel in the other. Total movement of the thrust shaft (about 1 mm being typical) is registered on a dial gauge. Feelers can be used as an alternative, between thrust ring and casing. Use of feelers in the thrust pad/collar gap is likely to cause damage and may give a false reading.
Thrust block position
The siting of the main thrust block close to the propulsion machinery, reduces any problems due to differential expansion of the shaft and the hull. The low hull temperature of midship engined refrigerated cargo ships, caused a contraction relative to the shaft of perhaps 20mm (|"). Variations can be caused by changes in water temperature or heating of fuel tanks.
Other problems associated with the stern tube end of the shafting system include whirl of the tailshaft, relative movement of the hull and misalignment due to droop from propeller weight. Some thrusts are housed in the after end of large slow speed diesels or against gear boxes. Deformation produced by the thrust load, can cause misalignment problems, unless suitable stiffening is employed (particularly with an end of gearbox installation).
Thrust block support
The substantial double bottom structure under the main propulsion machinery, provides an ideal foundation for the thrust block and a further reason for siting it close to the engine. The upright thrust block and any supporting stool, must have adequate strength to withstand the effect of loading which tends to cause a forward tilt. This results in lift of the aft journal of the block (unless not fitted) and misalignment of the shaft.
Axial vibration of the shaft system, caused by slackening of the propeller blade load as it turns in the sternframe or by the splay of diesel engine crankwebs, is normally damped by the thrustblock. Serious vibration problems have sometimes caused thrust block rock, panting of the tank top and structural damage.
Thrust pads
The pivot position of thrust pads may be central or offset. Offset pads are interchangeable in thrust blocks for direct reversing engines, where the direction of load and rotation changes. Offset pads for non-reversing engine and controllable pitch propeller installations are not interchangeable. Two sets are required. Pads with a central pivot position are interchangeable.
Some modern thrust blocks are fitted with circular pads (Figure 8.7) instead of those with the familiar kidney shape. A comparison of the pressure contours on the conventional kidney shaped pads and the circular type shows why the latter are effective.
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