Question

The resultant soil thrust and radial force acting on each gang of a single acting disc harrow are 1200 N, and 600 N respectively. The resultant downward force acting on each gang is 2400 N. The perpendicular distance of the soil thrust from the gang axis is 20 cm. What is the distance between line of action of downward force and centre of the gang to get a uniform depth of cut?

• A. 5 cm
• B. 10 cm
• C. 15 cm
• D. 40 cm

Hint:

Note the simple ratio of forces:
The downward force (2400 N) is exactly twice the soil thrust (1200 N).
Therefore, its moment arm \(x\) must be exactly half of the thrust height (20 cm), which gives \(x = 10\text{ cm}\).

Answer 1

The Correct Option is B

Step 1: Identify the two moments that must balance.
For a disc harrow gang to cut at a uniform depth, the disc must be in rotational equilibrium about its own axis or pivot, meaning the turning effect of the horizontal soil thrust must be cancelled exactly by the turning effect of the vertical downward force.
Step 2: Write the moment due to soil thrust.
The soil thrust of 1200 N acts at a perpendicular distance of 20 cm from the gang axis, giving a moment of \( 1200 \times 20 = 24000 \ \text{N.cm} \). The 600 N radial force passes through the axis in this configuration and does not contribute a moment.
Step 3: Equate it to the moment of the downward force and solve for the unknown distance.
If x is the distance from the centre of the gang to the line of action of the 2400 N downward force, balance requires \( 2400 \times x = 24000 \), so \( x = \dfrac{24000}{2400} = 10 \text{cm} \).
\[ \boxed{10 \ cm} \]