Tractor Systems and Components Questions and Answers

Answer: A

The tractive efficiency is about 45%.

Tractive efficiency is output (drawbar) power divided by input (axle) power, which reduces to the ratio of net pull to gross pull times the travel ratio \((1 - s)\).

\[ \begin{aligned} NT &= 0.35 \times 28 = 9.8\ \text{kN} \\ GT &= \frac{T}{r} = \frac{10.7}{0.60} = 17.83\ \text{kN} \\ TE &= \frac{9.8}{17.83}(1 - 0.18) \\ &= 0.550 \times 0.82 = 0.451 \end{aligned} \]

So TE is nearest to 45%. 55% is the pull ratio alone, forgetting that 18% slip wastes part of the wheel speed. 35% is the coefficient of net traction mistaken for efficiency, and 58% has no basis in these data.

Answer: B

The pin diameter is about 10.6 mm.

Rated torque at the PTO, then the 150% torque at which the pin must shear:

\[ \begin{aligned} \omega &= \frac{2(3.14)(540)}{60} = 56.52\ \text{rad/s} \\ T &= \frac{40\,000}{56.52} = 707.7\ \text{N.m} \\ T_{f} &= 1.5 \times 707.7 = 1061.6\ \text{N.m} \end{aligned} \]

The pin sits 60 mm from the shaft axis and is loaded in single shear by the force \(F = T_{f}/r\):

\[ \begin{aligned} F &= \frac{1061.6}{0.060} = 17\,693\ \text{N} \\ A &= \frac{F}{\tau} = \frac{17\,693}{200} = 88.5\ \text{mm}^{2} \\ d &= \sqrt{\frac{4A}{\pi}} = \sqrt{112.7} \approx 10.6\ \text{mm} \end{aligned} \]

Forgetting the 150% overload factor and sizing for the rated torque gives about 8.7 mm, close to the 8.6 option. 7.5 and 6.1 mm would shear well below even the rated torque.

Answer: A

Lateral stability on a turn is increased by increasing the radius of the turn.

On a turn, centrifugal force \(mv^{2}/r\) acts outwards at the centre of gravity and tends to overturn the tractor sideways. A larger turning radius r reduces this force at a given speed, so the tractor is less likely to tip.

Raising the centre of gravity increases the overturning moment, so it reduces stability. A front end loader, especially with its bucket raised, raises the centre of gravity too. Adding weight increases both the overturning force and the restoring moment in proportion, so it does not by itself improve lateral stability unless it lowers the centre of gravity.

Answer: B

The pulling power is Zero.

In traction mechanics, a wheel is called self-propelled when the torque applied to it is just enough to overcome its own motion (rolling) resistance and keep it moving. All the torque is used up in moving the wheel itself, so nothing is left to pull a load and the net pull (drawbar pull) is zero.

Only when more torque is applied than the motion resistance requires does the wheel develop a positive pull. Five, two and three are numbers with no meaning in this definition.

Answer: A

The draft control system was invented by Harry Ferguson.

The Irish engineer Harry Ferguson developed the three-point linkage with automatic draft control in the 1920s. The draft of the implement is sensed through the top link, and the hydraulic system raises or lowers the implement to keep the load steady, transferring weight to the tractor's rear wheels for better traction. It became the basis of the Ferguson System used on almost all modern tractors.

Brixius is known for equations predicting tyre traction, not for draft control, and the other names are not linked with this invention.

Answer: C

They are Electric generators.

An electric dynamometer drives a generator from the engine or power take-off; the electrical output is dissipated in a bank of resistance coils, and the power absorbed is measured precisely from the electrical quantities and the reaction torque. Because the load can be adjusted finely and read accurately, this type is preferred for precise test work.

A Prony brake is a simple friction brake suited to rough measurements. Hydraulic pumps and magnetic drag (eddy current) units are other absorption devices, but they do not use resistance coils.