Sec. 10
Page 76
WHEELS A N D TIRES
W H AT HAPPENS WHEN
TIRES GET HOT
A s a tir e b eco m es heated, the air in the tire
expands and the a ir p r e s s u r e is ra ised . This is
norm al u n le ss the p r e ssu r e build-up is e x c e ssiv e .
B u ild -u p s of over 20 lb s. a re e x c e s s iv e , indicating
underinflation, overload , too much speed, too sm all
a tir e o r, m ore often, a combination of th ese fa c
to r s . T h erefo re, p r e s s u r e build-ups of over 20 lbs.
should be studied to determ ine cause and proper
c o r r e c tiv e action. N orm al p r e ssu r e build-ups (not
over 20 lb s.) reduce flex in g . Thus the amount of
heat gen erated allow s the tir e to reach a heat bal
ance (the tem p eratu re at which the rate of heating
equals the rate of cooling) at a low er tem perature
than if the inflation p r e s s u r e w ere m aintained uni
form ly by frequently bleeding out a ir. T ir es should
never be bled. When the p r e ssu r e builds up ex
c e s s iv e ly - reduce the sp eed or the load instead.
A tir e operated for a con sid erab le distance at
sustained highway sp eed s, in a sev e r e ly underin
flated or flat condition, b eco m e s extrem ely hot due
to internal frictio n , and th is heat tran sm its to the
t ir e ’s outer su r fa ce. If and when the outer surface
tem p eratu re rea ch es the com bustion point, the tire
b u r sts into flam e. T his usually occu rs after the
v e h ic le has stopped. Such an occu rren ce usually
in volves a dual a sse m b ly w here one of the tir e s is
fla t and the other is , th erefo re, extrem ely o v e r
loaded (p ossib ly a lso underinflated even for normal
load). In such c a s e s , eith er the tir e which is s e
v e r e ly underinflated or com pletely flat, or the tire
which is ex trem ely overloaded, could get so hot as
to reach the point of spontaneous com bustion.
CAUSE AND PREVENTION OF TIRE FIRES
A m ajor cau se of tir e fir e s is heat caused by
a dragging brake, p a rticu la rly if there is e x c e ssiv e
g r e a se an d /or o il around the brake drum The heat
produced by the dragging brake ignites the grease
or o il which in turn can eventually ignite the tire,
even though the tir e is not overloaded or under
inflated.
R e g a rd less of the ca u se of a tire fir e , the
t ir e ’s internal tem p eratu re is so high that even if
the fla m es a re extinguished, repeated spontaneous
ignition o ccu rs until the internal tem perature de
c r e a s e s su fficie n tly . A fir e extinguisher usually
w ill not have su fficien t capacity to control the fire
until the burning tir e can be rem oved. Operators
should carry sp e c ia l a sb e sto s blankets which, in
c a s e of tir e f ir e , a re u sefu l in preventing the fire
from spreading to v eh icle and cargo. If p o ssib le,
a steady str e a m of w ater should be placed on the
tir e until it is cool enough for rem oval.
The b est p rotection against tire fir e s is to
avoid running on a flat tir e , prevent underinflation
through regu lar checking of inflation p r e s s u r e s , and
avoid accum ulation of g r e a se or o il around brake
drum a r e a s . O perators carryin g cargo which is
ex p lo siv e or read ily com bustible should check tir e s
at 5 0 -m ile in terv a ls and should a lso u se a p r e ssu r e
warning d ev ice.
INSTRUCTIONS FOR M A IN T EN A N C E
OF TIRES O N OFF-THE-HIGHW AY
UNITS IN H IG H W A Y TRANSIT
Many O ff-T he-H ighw ay T ir e s a re designed and
built to giv e m axim um s e r v ic e for mining and con
stru ction op eration s. B eca u se of the extra heavy
con stru ction built into th ese tir e s , sp e c ia l p reca u
tio n s m ust be o b serv ed when equipment is being
m oved o v er the highway for d elivery or when tr a n s
ferrin g to a new job s it e . If th ese p recautions a re
not o b served , the tir e s may build up e x c e s s iv e heat
and cau se prem ature fa ilu r e .
The follow ing recom m en dations a re made in
the in te r e st of p rotectin g your in vestm en t against
lo s s and delay. T h ese recom m endations apply to
a ll tir e s on v e h ic le s in tra n sit, whether driven or
towed.
LOAD AND PRESSURE
1. V eh icle s m ust be empty during tra n sit.
2. Inflation p r e s s u r e is determ ined by cold
p r e s s u r e checks b efore startin g. P r e s s u r e is not
to be reduced by "bleeding” tir e s during tr a n sit.
3. Inflation p r e s s u r e s a re to be determ ined by
u se of the follow ing ch a rts. If load cannot be a c
cu rately estim ated , u se the m axim um inflation
p r e s s u r e shown for the ply rating in question.
4. T ir e s with "dry" b a lla st m ust not be u sed
fo r v e h ic le transport p u rp o ses.
SPEED
R egular Skid Depth T ir e s
1. Maximum highway sp eed 30 mph.
2. Stop for 3 0 - minute cooling p eriod after
each 50 m ile s of driving or b efore 2 h ours’ s u s
tained operation, w hichever com es f ir s t.
3. One hour m inim um m id-day lunch stop
should be o b served during full day op eration s.
E xtra Skid Depth and Sp ecial Compound T ir e s
V e h ic le s equipped with extra skid depth or
s p e c ia l compound tir e s a re not to be driven in
tr a n sit ov er the highway u n le ss the proposed trip
i s review ed and approved by qualified p erso n n el.
Super Deep T read T ir e s
Under no cir cu m sta n ces should the Super Deep
T read tir e s be m oved (roaded) ov er the highway.
V e h icles in tra n sit m ust be con trolled and a c
com panied by resp o n sib le p erson n el in a p ilot ca r
to en force recom m endations and maintain a check
on equipm ent. T his is good in su ran ce for a v a l
uable in vestm en t.
