212 
LOGGING 
Table VII 
APPROXIMATE MAXIMUM TRACTIVE POWER OF CRAWLER 
TRACTORS! 
Character of 
Average rolling 
resistance of 
level road in 
lbs. per ton 
Approximate gross load in 
tons which a 5-ton crawler 
tractor with actual draw-bar 
H. P. 25, can move at low 
speed — 2 miles per hour 
Approximate gross load in tons 
which a 10-ton crawler tractor 
with actual draw-bar H. P. 40, 
can move at low speed — 2 
miles per hour 
road surface 
Adverse grade, per cent 
Level 
5 
10 
20 
Level 
5 
10 
20 
Earth road.. 
Deep sand 
or wet clay 
150 
350 
31 
13 
17 
9 
11 
7 
5 
3^ 
49 
21 
25 
14 
15 
10 
61 
4§ 
' These values are computed in the following manner, and the same method may be used in 
approximating the tractive power for speeds other than 2 miles. 
Example: Type of bottom, earth road. Grade 10 per cent. Resistance per ton; rolling 150 
pounds, axle 4 pounds, grade 20 X 10 = 200 pounds, total 354 pounds. Grade resistance of a 
10-ton tractor on a 10 per cent grade, 20 X 10 X 10 = 2000 pounds. Draw-bar pull (pounds) 
40 X 375 
of a 40 horse power tractor = = 7500. (The value 375 is obtained as follows: 
2 
33,000 X 60 
5280 
375 
in which 33,000 represents foot pounds per minute per horse power; 60 equals minutes per hour, 
and 5280 equals feet per mile.) The net draw-bar pull of a 40 h. p. tractor at a speed of 2 miles per 
hour equals 7500 — 2000 or 5500 pounds. The gross load in tons equals 5500 -i- 354 or 15^ tons. 
If vehicles weighing 2 tons each are used with tractors in hauling timber weighing 5 pounds per 
board foot, then on a 10 per cent grade a 40 horse power tractor can haul 4600 board feet on two 
,000 - 8000 
vehicles, I • = 4600 
(31,0 
capped where they cannot get proper footing, such as on loose 
ground and in swamps, and also in heavy work when several 
animals must be used together, since much energy is then wasted, 
due to lack of smiultaneous action. The expense of feeding 
animals when idle, their lower average speed, limited working 
period, and the necessity for hiring animal drivers, also are dis- 
advantages connected with the use of animal draft. Studies 
of logging operations where horses and tractors work imder 
identical conditions indicate that the ratio between the efficiency 
of horses and tractors varies within wide limits, although in most 
cases, in hauling, one horse is equivalent to two or three^ tractor 
draw-bar H. P. This is due chiefly to the ability of a horse to 
