A single acting disk harrow having 5 disks in each gang with a gang angle of 20º is operated at a forward speed of 3 km·h⁻¹ and a depth of 10 cm. The horizontal soil reaction force acting parallel to the face of each disk is found to be 250 N. The drawbar power required to pull the harrow, in kW, is __________. (Rounded off to 2 decimal places)
Official Answer Key:
1.94 to 1.98
Step-by-Step Solution & Explanation
Step 1 — total number of disks. A single-acting disk harrow has two gangs (throwing soil in opposite directions), each carrying 5 disks:
Total disks = 2 × 5 = 10
Step 2 — resolve the soil reaction into the direction of travel. The force of 250 N acts parallel to the face of each disk, which is set at the gang angle of 20° to the line of travel. The component opposing forward motion (i.e. the draft contribution) is:
F_draft per disk = 250 × cos 20° = 250 × 0.93969 = 234.9 N
Step 3 — total draft:
D = 10 × 234.9 = 2349.2 N
Step 4 — forward speed:
v = 3 km·h⁻¹ = 3/3.6 = 0.8333 m·s⁻¹
Step 5 — drawbar power:
P = D × v = 2349.2 × 0.8333 = 1957.7 W = 1.96 kW
(The 10 cm depth is contextual data and does not enter the calculation, since the soil reaction force is already given.)
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