Everything about Driving Wheel totally explained
On a
steam locomotive, a
driving wheel is a powered
wheel which is driven by the
locomotive's
pistons (or
turbine, in the case of a
steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled together with
side rods (also known as
coupling rods); normally one pair is directly driven by the
main rod (or
connecting rod) which is connected to the end of the
piston rod; power is transmitted to the others through the side rods.
On
Diesel and
Electric locomotives the driving wheels may be directly driven by the
traction motors. Coupling rods are not usually used, and it's quite common for each axle to have its own motor. However,
shunting locomotives sometimes use
jackshaft drive and coupling rods.
On an
articulated locomotive or a
duplex locomotive driving wheels are grouped into sets which are linked together within the set.
Diameter
Driving wheels are generally larger than
leading or
trailing wheels. Since a conventional steam locomotive is directly driven, one of the few ways to '
gear' a locomotive for a particular performance goal is to size the driving wheels appropriately. Freight locomotives generally had driving wheels between 40 and 60
inches (1016 to 1524
mm) in diameter; dual-purpose locomotives generally between 60 and 70 inches (1524 to 1778 mm), and passenger locomotives between 70 and 100 inches (1778 to 2540 mm) or so. Some long wheelbase locomotives (four or more coupled axles) were equipped with
blind drivers. These were driving wheels without the usual
flanges, which allowed them to negotiate tighter curves without binding.
The driving wheels on express passenger locomotives have come down in diameter over the years, for example from 8' 1" (2464 mm) on the
GNR Stirling 4-2-2 of 1870 to 6' 2" (1880 mm) on the
SR Merchant Navy Class of 1941. This is because improvements in valve design allowed for higher piston speeds.
Balancing
» Main article: Engine balance
On steam locomotives the driving wheels have balance weights to balance the weight of the coupling and connecting rods. The crescent-shaped balance weight is clearly visible in the picture on the right.
Whyte notation
In the
Whyte notation, driving wheels are designated by the middle number or numbers in the set. The
UIC classification system counts the number of axles rather than the number of wheels and driving wheels are designated by letters rather than numbers. The suffix 'o' is used to indicate independently powered axles.
The number of driving wheels on locomotives varied quite a bit. Some early locomotives had as few as two driving wheels (one axle). The largest number of total driving wheels was 24 (twelve axles) on the
2-8-8-8-2 and
2-8-8-8-4 locomotives. The largest number of coupled driving wheels was 14 (seven axles) on the ill-fated AA20
4-14-4 locomotive.
Other uses of the term driving wheel
The term
driving wheel is sometimes used to denote the
drive sprocket which moves the
track on tracked vehicles such as
tanks and
bulldozers.
Further Information
Get more info on 'Driving Wheel'.
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