Page 113 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

ROCK, SOIL, AND MUD 3.7

FIGURE 3.5 Weights of materials.

Weight Distribution. The ability of a machine to propel itself on slippery footing is affected by
its weight distribution.
For example, if a machine having a weight of 10 tons on the drive wheels can exert a drawbar
pull of only 4 tons because of wheel slippage, the tractive efficiency of the ground is 40 percent,
or .40. It does not matter whether the whole machine weighs 10 tons or 50 tons, or whether its rim
pull is 5 tons or 20, as for this particular calculation we use only weight holding the drive wheels
(or tracks) against the ground. However, the total weight of the machine and its load make up the
resistance that the drive wheels must move.
Increasing the weight on slipping drive wheels increases drawbar pull in direct proportion, up
to the maximum that can be produced by the engine and gears. Increasing weight on nondrive
wheels increases resistance. Shift of existing weight to the drive wheels increases potential trac-
tion, while shifting weight from drive to nondrive wheels reduces traction. Neither shift affects
resistance.
While resistance to movement is in proportion to the weight of the whole machine, ability to
move the machine, if power is adequate, depends on the weight on the drive units. All-wheel-drive
trucks and most crawlers keep all their weight on the drivers. In other machines weight distribu-
tion is important to performance when traction is limited.
Manufacturers usually provide detailed weight specifications for haulers, showing the total
weight and its distribution, both loaded and empty.