Home      Discussion      Topics      Dictionary      Almanac
Signup       Login
Thrust block

Thrust block

Overview
A thrust block, also known as a thrust box, is a specialised form of thrust bearing
Thrust bearing
A thrust bearing is a particular type of rotary bearing. Like other bearings they permit rotation between parts, but they are designed to support a high axial load while doing this.Thrust bearings come in several varieties....

 used in ships, to resist the thrust of the propellor shaft and transmit it to the hull.

Early screw-propelled steamships used a thrust block or thrust box composed of perhaps a dozen lower-rated plain thrust journal bearings stacked on the same shaft. These were problematic in service: they were bulky, difficult to dismantle, wasted power through friction and they had a tendency to overheat.
Discussion
Ask a question about 'Thrust block'
Start a new discussion about 'Thrust block'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Encyclopedia
A thrust block, also known as a thrust box, is a specialised form of thrust bearing
Thrust bearing
A thrust bearing is a particular type of rotary bearing. Like other bearings they permit rotation between parts, but they are designed to support a high axial load while doing this.Thrust bearings come in several varieties....

 used in ships, to resist the thrust of the propellor shaft and transmit it to the hull.

Early thrust boxes


Early screw-propelled steamships used a thrust block or thrust box composed of perhaps a dozen lower-rated plain thrust journal bearings stacked on the same shaft. These were problematic in service: they were bulky, difficult to dismantle, wasted power through friction and they had a tendency to overheat. The thrust box was built of a box-like cast iron housing with a radial bearing at each end and a number of collars formed on the shaft between them. This shaft was often a short section of removable shaft called the thrust shaft, linking the engine ahead to the propellor shaft astern. A series of iron horseshoe-shaped collars fitted over the small diameter of the shaft and bore against the forward face of the shaft's collars. Each horseshoe was faced with a low-friction pad of babbitt metal
Babbitt metal
Babbitt, also called Babbitt metal or bearing metal, is any of several alloys used for the bearing surface in a plain bearing.The original Babbitt metal was invented in 1839 by Isaac Babbitt in Taunton, Massachusetts, USA. Other formulations were later developed...

. Lubrication was by an oil bath in the box and a plentiful volume was important for cooling purposes too.

Although lignum vitae
Lignum vitae
Lignum vitae is a trade wood, also called guayacan or guaiacum, and in parts of Europe known as pockenholz, from trees of the genus Guaiacum. This wood was once very important for applications requiring a material with its extraordinary combination of strength, toughness and density...

 wood was used for the radial stave bearing
Stave bearing
A stave bearing is a simple journal bearing where a shaft rotates in a bearing housing. Rather than the usual arrangement where the fixed part of the bearing surrounds most of the circumference of the shaft in one or two pieces, a stave bearing uses a large number of axial staves to support the shaft...

s in the stuffing box
Stuffing box
A stuffing box is an assembly which is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.-Boats:...

, cooled directly by seawater itself, this material wasn't capable of withstanding the force needed for the thrust blocks of any but the earliest screw vessels.

Each horseshoe was independently adjustable forwards and back, by either wedged gibs or a screwed adjustment. A particular problem with these thrust boxes was in adjusting them so that the force was shared equally between all the collars. Adjustment was often done on the basis of their operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...

, gauged with the engineer's hand.

Improved Michell thrust blocks



Improved understanding of the theory of lubrication films (initially by Reynolds
Osborne Reynolds
Osborne Reynolds FRS was a prominent innovator in the understanding of fluid dynamics. Separately, his studies of heat transfer between solids and fluids brought improvements in boiler and condenser design.-Life:...

) allowed the development of much more efficient bearing surfaces. This allowed the replacement of multiple collars in a thrust box by a single thrust block.

Fluid-film thrust bearings were invented by Australian engineer George Michell
Anthony Michell
Anthony George Maldon Michell FRS was an Australian mechanical engineer of the early 20th century.-Early life:...

 who patented his invention in 1905. Michell bearings contain a number of sector-shaped pads, arranged in a circle around the shaft, and which are free to pivot. These create wedge-shaped films of oil between the pads and a rotating disk on the shaft. Each lubricant "wedge" can only be of a limited length (in the direction of travel, i.e. circumferential) so multiple pads are needed rather than a single ring. No lubrication pump is needed, the rotation of the shaft itself is sufficient.

The need for an efficient thrust block became even more important with the advent of steam turbine
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....

s and their higher propellor speeds. Despite this, there was some reluctance to adopt them in their homeland, until the discovery that World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 U-boats were using them. After this they were soon adopted widely. The large single pad illustrated is a model of one used in the battlecruiser HMS Hood
HMS Hood
Three ships of the Royal Navy have been named HMS Hood after several members of the Hood family, who were notable Navy officers: was a 91-gun second-rate ship of the line, originally laid down as HMS Edgar, but renamed in 1848 and launched in 1859. She was used for harbour service from 1872 and was...

, the pride of the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

.

Michell Bearings continue in production today under the same name, now as part of Rolls-Royce marine systems
Rolls-Royce plc
Rolls-Royce Group plc is a global power systems company headquartered in the City of Westminster, London, United Kingdom. It is the world’s second-largest maker of aircraft engines , and also has major businesses in the marine propulsion and energy sectors. Through its defence-related activities...

.