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Construction Mechanic Advanced - Construction methods and practices | Next
Timing Diagrams |
DRAWBAR AND BELT
HORSEPOWER
There are two kinds of horsepower commonly
used by manufacturers in rating the power of
construction vehicles: drawbar and belt
horsepower.
DRAWBAR HORSEPOWER is the power
that can be exerted in pulling a load.
Specifications of the Caterpillar D-8 H series with
a D-342 engine, for example, rate the drawbar
horsepower at 180.
BELT HORSEPOWER is equivalent to the
rated engine power except in cases where the belt
pulley is driven through a gear train. In that case,
there is a slight loss of power caused by gear
friction. Also, while there may be some belt-pulley
slippage, it is disregarded in arriving at the belt
horsepower rating.
The national Automotive Chamber of Com-
merce has developed a simplified method of
determining taxable horsepower based on the bore
of the engine and the number of cylinders. This
specification is listed in most manufacturers’
manuals, but it does not truly represent
the actual horsepower of modern high-speed,
high-compression engines. It is used for licensing
purposes only in some states.
GRAPHS AND DIAGRAMS
Graphs and diagrams are abbreviated methods
of recording operational and maintenance
data.
Manufacturers’ operational and maintenance
manuals often contain graphs and diagrams. The
technical bulletins, prepared chiefly for tune-up
mechanics, may use a particular graph or diagram
to eliminate pages of written description that
otherwise would be necessary.
PERFORMANCE CURVES
Figures 3-2 and 3-3 are examples of graphs
that describe engine performance in terms of
brake horsepower and fuel consumption.
Dynamometer tests provide the data used in
plotting the performance curves for each engine.
Figure 3-2.—Performance curves of a typical six-cylinder gasoline engine.
3-3
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