421 OF ENGINEERING F0RMULÆ.
Friction of Plane Surfaces. (‘ Trans. Inst. C. E.,’ UH.)
Kimball reports that whilst the coefficient oi friction as the
velocity increases beyond the limit due to the maximum
coefficient, it decreases towards a constant value. The coeffi-
cient also slightly decreases with the pressure.
Coefficients with |-inch shafting on cast-iron bearings
Velocity, feet 1 per minilte J 1 3 5 7 10 15 20 30 40 60 8p 100
Coefficient per) cent J 15- 12-2 10-4 9-3 6-e 5-8 5'4 5-3 5-2 5-1
With pressures from 23J- lbs. to 1961 lbs. per square inch of
longitudinal section of journal, the coefficient at very low
speed decreased by from 6 to 11 per cent.
For the greatest pressures the coefficient appears to become
stationary. .
From experiments byPoirée andBochet on waggon friction
on rails, the coefficients from 900 to 4000 feet per minutø
decrease from -21 to -14 respectively.
Fkictional Resistance of Pneumatic Foundations.
(Schmoll.)
The tube or caisson must be vertical and free, neither
resting on the guide chains or on its lower edge, but only kept
In equilibrium by the friction of Its surface.
Table of Coefficients of Friction. Dry Material. Wet Material.
A > a .So® £3 a kc fl fl Så -w i-u « > a •h o ® fcs a I During ! I Motion.
Material. Soil.
Sheet iron without rivets Crravel and sand •4015 •4583 •3348 •4409
„ with rivets .. •3965 •4911 •467? •6481
Cast iron unplaned .. •3677 •4668 •3646 •4963
Granite roughly worked It '4266 •5368 •4104 •4800
Pine, sawn .. ,, ,, Sand .. •. •4088 •5109 •4106 •4985
Sheet iron without rivets •5361 •6313 •3655 ■3247
„ with rivets .. .. • • • •7269 •8391 •5156 •4977
Cast iron unplaned .. J> • • • • ■5636 •6063 •4744 •3796
Granite roughly worked .* • • • • •6473 •7000 •4728 •5291
Pine, sawn • • •• •6633 •7340 •5787 -4793
Contrary to Morin’s experience, the friction from rest is
smaller than the resistance during motion.