A thrust block, also known as a thrust box, is a specialised form of

thrust bearing A thrust bearing is a particular type of rotary bearing. Like other bearings they permanently rotate between parts, but they are designed to support a predominantly axial load. Thrust bearings come in several varieties. *''Thrust ball bearing ...

used in ships, to resist the thrust of the propeller 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 bearing A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no roll ...

s 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

propeller shaft A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connect ...

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 or bearing metal is any of several alloys used for the bearing surface in a plain bearing. The original Babbitt alloy was invented in 1839 by Isaac Babbitt in Taunton, Massachusetts, United States. He disclosed one of his alloy recip ...

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

lignum vitae Lignum vitae () is a wood, also called guayacan or guaiacum, and in parts of Europe known as Pockholz or pokhout, from trees of the genus ''Guaiacum''. The trees are indigenous to the Caribbean and the northern coast of South America (e.g: Col ...

wood was used for the radial

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 ...

s in the

stuffing box A stuffing box or gland package 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. Components A stuffing box of a sailing boat ...

, 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 An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

, gauged with the engineer's hand.

Improved Michell thrust blocks

Improved understanding of the theory of lubrication films (initially by

Reynolds Reynolds may refer to: Places Australia *Hundred of Reynolds, a cadastral unit in South Australia *Hundred of Reynolds (Northern Territory), a cadastral unit in the Northern Territory of Australia United States * Reynolds, Mendocino County, Calif ...

) 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. In 1905, Australian engineer George Michell obtained a patent for the thrust block. (Subsequently,

American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the "United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, pe ...

engineer Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the l ...

Albert Kingsbury Albert Kingsbury (23 December 1863 – 28 July 1943) was an American engineer, inventor and entrepreneur. He was responsible for over fifty patents obtained between the years 1902 to 1930. Kingsbury is most famous for his hydrodynamic thrust bear ...

established that tests he conducted in 1898 predated Michell's work. In 1910, Kingsbury was awarded US patent No. 947242 for the fluid-film thrust bearing.) 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 A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam turbin ...

s, with their higher propeller speeds. Despite this, there was some reluctance to adopt them in their homeland, until the discovery that

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare role ...

s were using them. After this they were soon adopted widely. The large single pad illustrated is a model of one used in the battlecruiser , once the pride of the

Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against F ...

. Michell Bearings continue in production today under the same name, first as part of Rolls-Royce marine systems., since 2016 as part of British Engines Group.

References

{{reflist Marine propulsion Bearings (mechanical)