Rajasthan PSC — Assistant Agriculture Engineer (Agriculture Engineering syllabus) · India · Rajasthan Public Service Commission
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
How our questions for Rajasthan PSC spread across the units of Hydraulic Drives & Controls. Difficulty: 17 easy, 20 medium, 4 hard.
| Tractor hydraulic systems | 11 | |
| Principles of fluid power | 6 | |
| Hydraulic cylinders | 5 | |
| Open-centre, closed-centre and load-sensing circuits | 5 | |
| Hydraulic motors | 4 | |
| Hydraulic pumps: gear, vane and piston | 3 | |
| Pressure relief and flow control valves | 3 | |
| Directional control valves | 2 | |
| 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.
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
After fitting a replacement piston motor to a harvester's header drive, a mechanic plugs its case-drain port because no drain hose came with it. The likely result is
A. case pressure builds with leakage oil until the shaft seal blows out
Internal leakage collects in the motor case; if it cannot drain freely to tank, case pressure builds until the shaft seal, which is rated for low pressure, fails. Plugging the drain does not make leakage disappear or raise speed, and it does not change the direction of rotation.
The lift cylinder that raises a tractor's rear three-point linkage is normally single-acting. This means that
A. oil pressure raises the linkage and the implement lowers under its own weight
A single-acting cylinder is powered in one direction only; on the lift it raises the linkage and the implement lowers by gravity, with a drop-rate valve controlling the speed. A double-acting cylinder has power both ways. Hitch categories concern pin sizes, and no tractor lift uses a spring to raise the load.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
In an open-centre tractor hydraulic system with all control valves in neutral,
A. pump flow returns to tank at low pressure through the valve centres
Open-centre valves have a through passage in neutral, so a fixed pump circulates its full flow to tank at only the pressure needed to overcome line and valve losses. High standby pressure with no flow describes a pressure-compensated closed-centre system; flow over the relief would overheat the oil; accumulator unloading is a different closed-centre arrangement.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
Which of the following is the main reason open-centre systems remain common on small and medium tractors?
C. They are simple and cheap, and gear pumps tolerate contaminated oil well
A fixed gear pump with open-centre valves is the lowest-cost system and the most forgiving of dirt. Load sensing is usually more efficient at part load, open-centre functions do interact when operated together, and instant full pressure is a feature of closed-centre constant-pressure systems.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
On a tractor three-point linkage, the speed at which the implement descends when the lowering command is given is limited by
B. a drop-rate valve that throttles oil leaving the lift cylinder
The lift cylinder is single-acting, so the implement falls by gravity as oil leaves the cylinder; a drop-rate valve throttles that oil to set lowering speed. The relief valve limits maximum pressure, the pump does not push the linkage down, and top-link length sets implement pitch.
Further reading: Goering et al., Off-Road Vehicle Engineering Principles
Ten here, 41 in the app. The Laisha app has every practice question we have written to Rajasthan PSC in this subject, marked as you go, with timed papers, a jump-to-unanswered list and a certificate when you pass at 70 per cent.
Free for ten days, then one subscription of US$10 a month. We would rather say so here than have you find it later.
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