ASRB NET — Farm Machinery & Power / Land & Water Management Engineering / Agricultural Structures & Process Engineering · India · Agricultural Scientists Recruitment Board (ICAR)
Original practice questions written to the published syllabus · not past papersOil hydraulics on tractors and machines: pumps, motors, cylinders, valves and circuits.
In the exams: NCEES PE Machinery Systems (hydraulic components and circuits, explicit); elective in the ICAR 6th Deans' Committee B.Tech
ASRB NET and PRC ABELE (power and machinery paper)
Lecturer- and licence-level understanding of components and circuits, standards for linkage and couplers, efficiency and heat calculations, and the reasons behind design choices.
How our questions for ASRB NET spread across the units of Hydraulic Drives & Controls. Difficulty: 1 easy, 20 medium, 22 hard.
| Hydraulic pumps: gear, vane and piston | 6 | |
| Pressure relief and flow control valves | 6 | |
| Open-centre, closed-centre and load-sensing circuits | 6 | |
| Tractor hydraulic systems | 6 | |
| Principles of fluid power | 5 | |
| Hydraulic motors | 5 | |
| Hydraulic cylinders | 4 | |
| Directional control valves | 3 | |
| Pressure drop and heat generation | 2 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A hydrostatic transmission with a variable pump and a fixed motor, operating at a fixed maximum pressure, is best described as a
A. constant-torque drive whose speed rises with pump displacement
With a fixed motor displacement, output torque = Vm·Δp/(2π) is fixed by the pressure limit, while speed follows pump flow; power therefore rises with speed. A fixed pump with a variable motor gives constant power, with torque falling as motor displacement is reduced to raise speed.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
In a closed-loop hydrostatic transmission on a self-propelled sprayer, the charge pump's job is to
A. keep the low-pressure side above a set minimum and make up leakage losses
In a closed loop the motor returns oil directly to the pump inlet, so leakage to the cases must be made up and the low side must be kept pressurised to prevent cavitation; the charge pump does both and brings in filtered, cooled oil. The main pump drives the motors, crossover relief valves deal with pressure peaks, and case drains go through the cooler to the reservoir.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
Compared with a direct-acting relief valve of the same size, a pilot-operated relief valve
D. has smaller pressure override and can be vented to unload the pump
A small pilot sets the pressure and controls a main poppet held by a light spring, so pressure changes little with flow, and connecting the vent port to tank unloads the pump. Direct-acting valves respond faster but override more. Pilot-operated valves are adjustable and suit large flows.
Further reading: Esposito, Fluid Power with Applications
A cylinder (80 mm bore, 56 mm rod) is speed-controlled by a meter-out valve on the rod-end line. The relief is set at 160 bar. With no external load, pressure in the rod end during extension can reach about
A. 314 bar
With no load, the cap-end force p × Ap must be balanced by rod-end pressure on the annulus: p_rod = 160 × Ap/Aa = 160 × 5027/2564 = 314 bar. This intensification can exceed the ratings of seals and hoses. Assuming 160 bar ignores the area ratio; 82 inverts it; using the rod area gives 327.
Further reading: Esposito, Fluid Power with Applications
In a load-sensing valve bank serving several functions, the purpose of the shuttle valves in the signal network is to
C. pass the highest working load pressure to the pump's load-sensing control
Shuttle valves compare pairs of pressures and pass the higher; cascaded, they deliver the highest load pressure to the pump so it can supply every function. Averaging or passing the lowest would starve the most heavily loaded function. Steering priority is handled by a priority valve.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
Mineral oil (ρ = 870 kg/m³, cp = 1.88 kJ/kg·K) passes through a relief valve from 140 bar to tank pressure without doing work. Assuming all the energy stays in the oil, its temperature rise is about
B. 8.6 °C
ΔT = Δp/(ρ·cp) = 140 × 10⁵/(870 × 1880) = 8.6 °C. Using cp in kJ instead of J gives 8560; a bar-to-pascal slip gives 0.86; using water's cp (4.18 kJ/kg·K) gives 3.8.
Further reading: Esposito, Fluid Power with Applications
On small and medium tractors with a mechanical draft-control system, the draft signal is traditionally taken from
C. the compressive force in the top link acting on a calibrated spring
With a mounted implement the top link is usually in compression and the force rises with draft; a spring at its tractor end deflects and moves the control valve. Larger tractors sense lower-link forces, now with electronic load pins. Lift-cylinder pressure reflects implement weight more than draft, and engine-speed sensing is not the basis of draft control.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
In an electronic hitch control system on a large tractor, the draft force is usually measured by
C. load pins, force sensors fitted in the lower-link mounting points
Electronic draft control measures lower-link force with load pins that sense shear or bending. A rockshaft position sensor provides the position signal; radar ground speed is used for slip control; the lever potentiometer gives the set-point, not the measured draft.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
An electronic hitch has slip control enabled at 15 % wheel slip. While ploughing, slip rises to 20 % on a wet patch. The system responds by
A. raising the implement slightly to cut draft until slip falls back below the limit
Slip control compares driven-wheel speed with true ground speed (from radar or GNSS); when slip exceeds the set limit the hitch is raised a little to reduce draft. Lowering would raise draft and slip further. Differential lock and engine-speed management are separate functions.
Which pump type is most tolerant of moderately contaminated oil and is therefore common on low-cost implements and small tractors?
C. External gear pump
Gear pumps have robust geometry and tolerate dirt better than piston or vane pumps, whose close-fitting pistons, shoes and vane tips are damaged quickly by particles. That, with low cost, is why they dominate open-centre systems. Contamination tolerance does not excuse poor filtration.
Further reading: Esposito, Fluid Power with Applications
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