TRACTORS 
205 
railroad spur lines on up-grade hauls which are too steep for a 
railroad. 
Prior to the World War crawler tractors were rarely used for 
logging purposes, hut at the present time, especially in the hard- 
wood and pine regions, there are 20(^0 or more of these machines 
in use. Crawler tractors were first used only for hauling timber, 
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Fig. 59. — A Holt 10-ton Caterpillar Tractor showing the General Features 
of one of the Traction Members. 
but they are now employed for skidding and yarding, for con- 
struction work on logging roads, and in other auxiliary work. 
The principle of the crawler device was invented in England 
about 150 years ago, but its successful application to tractors 
was made in America about the beginning of the present century, 
when a steam log hauler was built for work on snow and iced 
roads. The crawler traction device was later improved and suc- 
cessfully used on tractors with internal combustion motors. 
At present there are about twenty American models of crawler 
tractors which are used by loggers. 
The modern traction device of a crawler-type tractor has steel 
links or shoes pivoted together by steel pins in an endless belt or 
