{{Short description|Tool used in railway engineering}} {{More citations needed|date=December 2009}} [[File:14 inch 50 caliber railway gun Mk I right elevation diagram.jpeg|thumb|right|Span bolsters can be seen at both ends of this diagram of the [[14"/50 caliber railway gun]].]] [[File:CPOX820.jpg|thumb|Span bolsters (white/orange) in a [[Schnabel car]]]] A '''span bolster''', in [[rail terminology]], is a beam or frame used to link two trucks ([[US English|US]]), or [[bogie]]s ([[British English|UK]]), so that they can be articulated together and be joined to the [[locomotive]] or [[railroad car]] at one rotating mounting point. In effect, they make one "super-truck" out of the two, while permitting each truck to move relative to the other.<ref name="vd">{{cite book | last = Dukkipati | first = Rao | title = Vehicle dynamics | publisher = CRC Press Narosa Pub. House | location = Boca Raton, Fla. New Delhi | year = 2000 | isbn = 084930976X |page=317}}</ref>
==Use== ===Locomotives=== The most common use on [[locomotive]]s is to give a more flexible alternative to a four-axle truck; two two-axle trucks linked by a span bolster allows the wheels to follow a curve better, without excessive side forces or the need for [[lateral motion device|lateral motion]] of the axles in the truck. The use of a span bolster is normally signified in the [[AAR wheel arrangement]] notation by a + sign; thus a locomotive with two span bolsters, each with two two-axle trucks (the most common arrangement) is a [[AAR wheel arrangement#B.2BB-B.2BB|B+B-B+B]].
===Freight cars=== Span bolsters have been used for some high-capacity [[freight car]]s. The most common use is for large [[flatcar]]s to haul very heavy loads. These cars need many axles to distribute the weight. Specialized [[Schnabel car]]s with up to 72 wheels have been built for carrying heavy items. Often the span bolsters connecting the trucks are connected by additional span bolsters to the car. The use of span-bolster-connected trucks is essential to allow the load to be transported through curves.
== See also == * [[Whippletree (mechanism)]]
==References== <references />
{{DEFAULTSORT:Span Bolster}} [[Category:Rail technologies]] [[Category:Bogie]]