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4.2 How to calculate required pulling force
The required pulling force (RPF) can be calculated according the formula:
Where:
M = the load’s weight in kg.
α = gradient stated in degrees.
Rr = roll resistance.
Ff = friction factor for rope sheaves and snatch blocks etc.
g ≈ 10 (correct 9.8).
M The gross weight in kg of the object that shall be pulled.
α The surface gradient.
RPF (kN) =
M (sinα + Rr) × (1 + Ff) g
1,000
Firm road = 0.050
Grass = 0.175
Gravel = 0.250
Sand = 0.325
Shallow mud = 0.425
Deep mud = 0.625
α = 10° (≈ 18%) sinα = 0.17
α = 15° (≈ 27%) sinα = 0.26
α = 20° (≈ 36%) sinα = 0.34
α = 25° ( ≈ 47%) sinα = 0.42
α = 30° ( ≈ 58%) sinα = 0.50
Rr The roll resistance for wheeled and tracked vehicles on different surfaces.
(If a tracked vehicle has immobilized tracks, you should increase the values
below by 40%.)
Ff The friction factor depends on the number of rope sheaves and rope guides.
Calculate with a loss of 2% of the pulling force for each 45° turn of the rope.
For a snatch block with 180° turn of the rope the loss of pulling force is approx.
8-10%. Add 5% for adverse conditions.
Winch basics
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