Aux 2 Unit 2 Hydraulic crane Q2 Answer

  1. With reference to a crane operated by a constant pressure hydraulic system incorporating unidirectional, fixed displacement pumps that run continuously, explain EACH of the following:
    (a) the purpose of the accumulator;(2)
    (b) how the hydraulic pressure is regulated;(2)
    (c) how the speed and direction of the hoist motor is varied;(3)
    (d) how the torque available from the hoist motor can be varied.(3)

Great question — this is about a constant pressure hydraulic system with fixed-displacement, unidirectional pumps (common on marine cranes). Here’s a clear breakdown:

(a) The purpose of the accumulator (2 marks)

  • Stores pressurised hydraulic fluid and maintains system pressure when demand is low.
  • Provides instantaneous flow to meet sudden load changes without requiring the pumps to change output.
  • Reduces pressure fluctuations and protects against pump wear and shock loads.

(b) How the hydraulic pressure is regulated (2 marks)

  • The fixed displacement pumps run continuously, so excess flow is diverted back to tank through a pressure control valve (e.g., pressure relief or unloading valve).
  • The valve opens when the accumulator is charged to system pressure, maintaining a constant predetermined system pressure (e.g., 150–200 bar).

(c) How the speed and direction of the hoist motor is varied (3 marks)

  • Direction: Achieved by a directional control valve (DCV), which routes flow to either side of the motor (forward or reverse).
  • Speed: Controlled by flow control valves or servo/proportional valves that regulate the flow rate to the motor.
  • Since motor speed is proportional to flow, varying flow controls the hoisting speed (slower/faster).

(d) How the torque available from the hoist motor can be varied (3 marks)

  • Torque depends on pressure × motor displacement.
  • In constant pressure systems, torque is varied by using a variable displacement motor:
    • At high displacement → high torque, low speed.
    • At low displacement → low torque, high speed.
  • Alternatively, pressure regulators can adjust the system pressure to influence maximum torque output.