206 LOGGING 
track which rotates around the idler, the rollers, and the 
sprocket driven by engine power. This endless, flexible belt 
corresponds to the rim of a wheel but its inner face serves as a track 
on which the machine itself travels like a locomotive on a rack- 
and-pinion railroad. Its weight is carried on flanged rollers, and 
a positive drive is provided by the meshing of the driving sprocket 
teeth with the pins of the track. On some tractors the frames of 
the crawler members are built in one piece and are rigid, while in 
others the frames are made in two sections so pivoted together 
that the traction members are somewhat flexible in order that 
the traction device may better adhere to an uneven road surface. 
The chief features of a crawler tractor are : 
(1) A larger area of ground contact than is available under 
any other principle of construction. Modern 5- and 10-ton 
machines of full crawler type, i. e., without front steering wheels, 
have traction members from 10 to 12 inches wide and the length 
of track in contact with the ground may be as great as 7 feet. 
(2) The ground pressure of a crawler tractor is extemely low, 
ordinarily from 4 to 9 pounds per square inch. This is many 
times less than the pressure of wheeled tractors, from two to five 
times less than the ground pressure of a horse, and about the same 
as the pressure of a man's foot. With "swamp special" shoes, 
much wider than the standard size, the ground pressure of crawler 
tractors may be decreased to 2 or 3 pounds per square inch, 
and the tractor then can go over soft and swampy places where 
animals could not be used. 
(3) The long crawler members enable a tractor to bridge 
uneven areas, which results in a great ecomony of power and makes 
possible the successful use of such a tractor on broken ground, 
even where there are ditches and deep holes. Wheeled tractors 
are useless under such conditions. 
(4) The ease of steering. Many tractors which are mounted 
only on two crawler devices may be turned as sharply as desired 
and turned around almost within their own length. This is 
done by the application of the driving power forward or backward 
to one of the traction members while the other one is slowed 
down by braking, released, or completely stopped. 
(5) Crawler tractors, due to their better contact with the 
ground, deliver a larger per cent of motor power to the draw-bar 
than wheeled tractors. The difference in favor of the crawler 
