Thrust block

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A thrust block, also known as a thrust box, is a specialised form of thrust bearing used in ships, to resist the thrust of the propeller shaft and transmit it to the hull.

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 predominantly axial load.

Contents

Early thrust boxes

Multi-collar thrust box, with shaft Thrust box.jpg
Multi-collar thrust box, with shaft

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. [1] [2] 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. [3] This shaft was often a short section of removable shaft called the thrust shaft, linking the engine ahead to the propeller 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. Lubrication was by an oil bath in the box and a plentiful volume was important for cooling purposes too.

Although lignum vitae wood was used for the radial stave bearings in the stuffing box, 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.

Lignum vitae type of wood

Lignum vitae is a wood, also called guayacan or guaiacum, and in parts of Europe known as Pockholz, from trees of the genus Guaiacum. The trees are indigenous to the Caribbean and the northern coast of South America and have been an important export crop to Europe since the beginning of the 16th century. The wood was once very important for applications requiring a material with its extraordinary combination of strength, toughness, and density. It is also the national tree of the Bahamas and the Jamaican national flower.

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. A large housing is made with grooves running along the shaft, these grooves being filled with strips of suitable material, originally wood.

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.

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, gauged with the engineer's hand.

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 maximum operating temperature. Outside this range of safe operating temperatures the device may fail.

Improved Michell thrust blocks

Michell thrust blocks at the London Science Museum Thrust blocks at the Science Museum.jpg
Michell thrust blocks at the London Science Museum

Improved understanding of the theory of lubrication films (initially by Reynolds) 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.

Osborne Reynolds Anglo-Irish innovator (1842-1912)

Osborne Reynolds FRS was a prominent Irish-born British innovator in the understanding of fluid dynamics. Separately, his studies of heat transfer between solids and fluids brought improvements in boiler and condenser design. He spent his entire career at what is now the University of Manchester.

In 1905, Australian engineer George Michell [4] obtained a patent for the thrust block. [5] (Subsequently, American engineer Albert Kingsbury 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. [6] [7] )

Anthony George Maldon Michell FRS was an Australian mechanical engineer of the early 20th century.

United States Federal republic in North America

The United States of America (USA), commonly known as the United States or America, is a country comprising 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico. The State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The extremely diverse geography, climate, and wildlife of the United States make it one of the world's 17 megadiverse countries.

Engineer Professional practitioner of engineering and its sub classes

Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build, and test machines, systems, structures and materials to fulfill objectives and requirements while considering the limitations imposed by practicality, regulation, safety, and cost. The word engineer is derived from the Latin words ingeniare and ingenium ("cleverness"). The foundational qualifications of an engineer typically include a four-year bachelor's degree in an engineering discipline, or in some jurisdictions, a master's degree in an engineering discipline plus four to six years of peer-reviewed professional practice and passage of engineering board examinations.

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 turbines, with their higher propeller speeds. Despite this, there was some reluctance to adopt them in their homeland, until the discovery that World War I U-boats were using them. [8] After this they were soon adopted widely. The large single pad illustrated is a model of one used in the battlecruiser HMS Hood, once the pride of the Royal Navy.

Michell Bearings continue in production today under the same name, [9] first as part of Rolls-Royce marine systems., [10] since 2016 as part of British Engines Group. [11]

Related Research Articles

Propeller fan that transmits rotational motion into thrust

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics, like those of aircraft wings, can be modelled by Bernoulli's principle and Newton's third law. Most marine propellers are screw propellers with fixed helical blades rotating around a horizontal axis or propeller shaft.

Steamship Type of steam powered vessel

A steamship, often referred to as a steamer, is a type of steam-powered vessel, typically ocean-faring and seaworthy, that is propelled by one or more steam engines that typically move (turn) propellers or paddlewheels. The first steamships came into practical usage during the early 1800s; however, there were exceptions that came before. Steamships usually use the prefix designations of "PS" for paddle steamer or "SS" for screw steamer. As paddle steamers became less common, "SS" is assumed by many to stand for "steam ship". Ships powered by internal combustion engines use a prefix such as "MV" for motor vessel, so it is not correct to use "SS" for most modern vessels.

Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings.

Bearing (mechanical) machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts

A bearing is a machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or to the directions of the loads (forces) applied to the parts.

Plain bearing simplest type of bearing, comprising just a bearing surface and no rolling elements

A plain bearing, or more commonly sliding bearing and slide bearing, is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.

Rolling-element bearing bearing which carries a load by placing rolling elements (such as balls, rollers) between two bearing rings called races. The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding

A rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements between two bearing rings called races. The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding.

Babbitt (alloy) alloys used for the bearing surface in a plain bearing

Babbitt, also called Babbitt metal or bearing metal, is any of several alloys used for the bearing surface in a plain bearing.

Tapered roller bearing

Tapered roller bearings are rolling element bearings that can support axial forces as well as radial forces.

A linear-motion bearing or linear slide is a bearing designed to provide free motion in one direction. There are many different types of linear motion bearings.

Reduction drive Device, containing a different gears.

A reduction drive is a mechanical device to shift rotational speed. A planetary reduction drive is a small scale version using ball bearings in an epicyclic arrangement instead of toothed gears.

Albert Kingsbury engineer and inventor

Albert Kingsbury 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 bearing which uses a thin film of oil to support weights of up to 220 tons. This bearing extended the service life of many types of machinery during the early 20th century. It was primarily outfitted on Navy ships during World War I and World War II.

Yaw bearing

The yaw bearing is the most crucial and cost intensive component of a yaw system found on modern horizontal axis wind turbines. The yaw bearing must cope with enormous static and dynamic loads and moments during the wind turbine operation, and provide smooth rotation characteristics for the orientation of the nacelle under all weather conditions. It has also to be corrosion and wear resistant and extremely long lasting. It should last for the service life of the wind turbine) while being cost effective.

Accessory drive

The accessory drive is a gearbox that forms part of a gas turbine engine. Although not part of the engine's core, it drives the accessories, fuel pumps etc., that are otherwise essential for the operation of the engine or the aircraft on which it is mounted. Accessory drives on large engines handle between 400–500 hp.

High-speed steam engine

High-speed steam engines were one of the final developments of the stationary steam engine. They ran at a high speed, of several hundred rpm, which was needed by tasks such as electricity generation.

Total-loss oiling system Engine lubrication system

A total-loss oiling system is an engine lubrication system whereby oil is introduced into the engine, and then either burned or ejected overboard. Now rare in four-stroke engines, total loss oiling is still used in many two-stroke engines.

The Michel engine was an unusual form of opposed-piston engine. It was unique in that its cylinders, instead of being open-ended cylinders containing two pistons, were instead joined in a Y-shape and had three pistons working within them.

A cannon bearing or cannon box bearing is an arrangement of bearings on a shaft, usually an axle, where two bearings are mounted in an enclosed tube.

References

  1. McCarthy, Mike (2000). Iron and Steamship Archaeology: Success and Failure of the SS Xantho . Springer. ISBN   0-306-46365-2.
  2. "Model marine screw engine, twin compound, with thrust block and propeller". Powerhouse Museum.
  3. Evers, Henry (1875). Steam and the Steam Engine: Land and Marine. Glasgow: Williams Collins.
  4. Sydney Walker. Michell, Anthony George Maldon. Australian dictionary of biography (1891-1939). pp. 492–494.
  5. "The Michell thrust bearing (1907)". Powerhouse Museum.
  6. "Kingsbury, Inc" . Retrieved 2015-02-27.
  7. US 947242
  8. John Lienhard. "Engines of our Ingenuity: Anthony G.M. Michell". KUHF-FM.Missing or empty |series= (help)
  9. "Company history". Michell Bearings.
  10. "Main propulsion thrust blocks". Rolls-Royce marine systems.
  11. "Michell Bearings joins the British Engines Group". British Engines Group.