Thrust block

2019-06-12T15:51:48Z

199.101.176.172: /* Improved Michell thrust blocks */Corrected ref.

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.

== Early thrust boxes ==
[[File:Thrust box.jpg|thumb|right|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 bearing]]s stacked on the same shaft.<ref>{{cite book
|title=Iron and Steamship Archaeology: Success and Failure of the [[SS Xantho]]
|last=McCarthy |first=Mike
|publisher=Springer
|year=2000
|isbn=0-306-46365-2
}}</ref><ref name="Powerhouse, thrust bearing model" >{{cite web
|title=Model marine screw engine, twin compound, with thrust block and propeller
|url=http://www.powerhousemuseum.com/collection/database/?irn=206711
|publisher=[[Powerhouse Museum]]
}}</ref> 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.<ref name="Evers, Steam and the Steam Engine: Land and Marine" >{{cite book
|title=Steam and the Steam Engine: Land and Marine
|year=1875
|author=Evers, Henry
|publisher=Williams Collins |location=Glasgow
}}</ref> 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 bearing]]s 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.

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.

== Improved Michell thrust blocks ==
[[File:Thrust blocks at the Science Museum.jpg|thumb|right|Michell thrust blocks at the [[Science Museum (London)|London Science Museum]]]]
Improved understanding of the theory of lubrication films (initially by [[Osborne Reynolds|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.

In 1905, Australian engineer [[Anthony Michell|George Michell]]<ref name="ADB" >{{cite book
|title=Michell, Anthony George Maldon
|author=Sydney Walker
|work=Australian dictionary of biography (1891-1939)
|pages=492–494.
}}</ref> obtained a patent for the thrust block.<ref name="Powerhouse, Michell" >{{cite web
|url=http://www.powerhousemuseum.com/australia_innovates/?behaviour=view_article&Section_id=1080&article_id=10086
|title=The Michell thrust bearing (1907)
|publisher=Powerhouse Museum
}}</ref> (Subsequently, [[United States|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 bearing#Michell/Kingsbury tilting-pad fluid bearings|fluid-film thrust bearing]].<ref name=Kingsbury>{{cite web |url=http://www.kingsbury.com/ |title=Kingsbury, Inc. |accessdate=2015-02-27}}</ref><ref>{{Cite patent|US|947242}}</ref>)

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]]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]] [[U-boat#World War I|U-boat]]s were using them.<ref name="Lienhard" >{{cite episode
|title=Engines of our Ingenuity: Anthony G.M. Michell
|station=KUHF-FM
|author=John Lienhard
|url=http://www.uh.edu/engines/epi581.htm
}}</ref> After this they were soon adopted widely. The large single pad illustrated is a model of one used in the battlecruiser {{HMS|Hood|51|6}}, once the pride of the [[Royal Navy]].

Michell Bearings continue in production today under the same name,<ref name="Michell Bearings" >{{cite web |title=Company history |publisher=Michell Bearings
|url=https://www.michellbearings.com/about-us/our-heritage/
}}</ref> first as part of [[Rolls-Royce Group plc|Rolls-Royce marine systems]].,<ref name="Rolls-Royce" >{{cite web
|title=Main propulsion thrust blocks
|url=http://marine.rolls-royce.com/marine-propeller-thrust-bearings/
|publisher=Rolls-Royce marine systems
}}</ref> since 2016 as part of [[British Engines Group]].<ref name="British Engines" >{{cite web
|title=Michell Bearings joins the British Engines Group
|url=http://www.britishengines.co.uk/blog/michell-bearings-joins-the-british-engines-group/
|publisher=British Engines Group
}}</ref>

== References ==
{{commons category|Thrust blocks}}
{{reflist}}

[[Category:Marine propulsion]]
[[Category:Bearings (mechanical)]]

Catalog - User contributions [en]

Thrust block