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Whenever their is a movement, friction comes into action. The same is the case with earth moving equipments. We term this as rolling resistance and this has to be overcomed by vehicle engine so that it can move on that surface. This is the formula to calculate rolling resistance
R=RfW + RpPW
R = rolling resistance, lb (N)
p = tire penetration, in (mm)
Rf = rolling-resistance factor, lb/ton (N/tonne)
W = weight on wheels, ton (tonne)
Rp = tire-penetration factor, lb/ton in (N/tonne mm) penetration
Rf usually is taken as 40 lb/ton (or 2 percent lb/lb) (173 N/t) and Rp as 30 lb/ton in (1.5% lb/lb in) (3288 N/t mm).
So the above equation becomes
R=(2%+1.5 % p) W’=R’W’
W’ = weight on wheels, lb(N)
R’ = 2% + 1.5%p.
Case – When we have a slope
G = RgsW
G = grade resistance, lb(N)
Rg = grade-resistance factor = 20 lb/ton (86.3 N/t) = 1%
s = percent grade which is positive for uphill motion and negative for downhill motion.
The total road resistance is calculated by adding the rolling and grade resistances and is given by:
T = (R’+Rg s )W’ =(2%b + 1.5%p + 1%s)W’
Loss due to Altitude
Generally we take 3 percent pull loss for each 1000 ft (305 m) above 2500 ft (762 m).
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