NCEES PE Agricultural and Biological Engineering Mechanics of Tillage & Traction (Soil Dynamics): practice questions

Question 1 · hard

A tractor rated at 45 kW is to pull a mouldboard plough in clay. The farmer wants the widest plough possible. Which reasoning is sound?

1. Choose the width at which the engine's rated power equals draft times speed, since tractive and transmission losses are negligible in heavy clay
2. Choose width from sandy-soil tables, because they give the safest upper bound
3. Estimate draft for local soil, divide drawbar power available at efficient slip by speed, then choose width with a margin
4. Use the widest plough the three-point linkage can lift, since draft depends only on implement mass

Question 2 · hard

A bias-ply tyre (b = 0.45 m, d = 1.6 m, deflection/section height δ/h = 0.2) carries 18 kN on soil with CI = 800 kPa. What is the Brixius mobility number Bn?

1. 32.0
2. 34.7
3. 64.0
4. 17.4

Question 3 · hard

For the same tyre (Bn = 34.7) at 12 % slip, use GTR = 0.88(1 − e^(−0.1Bn))(1 − e^(−7.5s)) + 0.04 and MRR = 1/Bn + 0.04 + 0.5s/√Bn to find the net traction ratio.

1. 0.467
2. 0.546
3. 0.625
4. 0.427

Question 4 · medium

A national test report gives maximum drawbar power on a concrete track. Why will a farmer rarely achieve it in the field on tilled soil?

1. Tractive efficiency on tilled soil is lower than on concrete because of greater sinkage, motion resistance and slip
2. Test tractors use special engines not sold to farmers
3. Drawbar power on concrete is measured at the PTO
4. Concrete test tracks raise engine power by cooling the engine, so the same tractor develops less power in a hot, dusty field

Question 5 · medium

What insertion rate does the ASABE procedure specify for the soil cone penetrometer?

1. About 30 mm/s
2. About 3 mm/s
3. About 300 mm/s
4. About 30 mm/min

Question 6 · medium

A two-wheel-drive tractor must develop 18 kN net pull. On this soil at the target slip, the drive tyres can achieve a net traction ratio of 0.45. What dynamic load must the drive wheels carry?

1. 8.1 kN
2. 32.7 kN
3. 40.0 kN
4. 0.4 kN

Question 7 · medium

A contractor wants to reduce subsoil compaction from a heavy grain cart during a wet harvest. Which measure addresses subsoil compaction most directly?

1. Lower tyre pressure only, keeping the same axle load and the same cart
2. Increase travel speed
3. Add ballast to the tractor
4. Reduce the load per axle, for example by a smaller cart or more axles

Question 8 · medium

Dual wheels on a tractor, compared with singles at the same load and pressure, typically

1. eliminate subsoil compaction completely
2. increase contact pressure
3. reduce contact pressure and rutting but spread stress over a wider zone
4. increase the stress under each tyre because the two contact patches interact

Question 9 · medium

A tyre's load-inflation table gives a minimum pressure for the axle load. Why should a field engineer not go below it to gain traction?

1. Excess deflection overstresses the carcass and sidewalls and can cause failure or rim slip
2. Traction usually falls below that pressure
3. The table value is set only for road-legal reasons and has no bearing on tyre life in the field
4. Lower pressure increases compaction

Question 10 · medium

In mixed control of a tractor hydraulic lift, the system

1. responds partly to draft and partly to implement position, limiting depth variation while still controlling load
2. ignores both draft and position, holding the lift valve open so the implement floats freely on the soil surface at all times
3. responds only to engine speed
4. locks the implement at transport height