Quick questions on Hydraulic lifting and Pascal's principle: HSC Engineering Studies Lifting Devices

Pressure applied to an enclosed incompressible fluid is transmitted undiminished and equally in all directions to every part of the fluid and to the walls of the container.

Hydraulic fluid (mineral oil or water glycol) has very low compressibility (about 0.5 percent volume change per 100 bar). For practical purposes the volume is constant, so:

A complete hydraulic lifting circuit has:

A 20-tonne excavator (Caterpillar 320, Komatsu PC200, Hitachi ZX200) uses three main hydraulic cylinders: boom, stick (arm), and bucket. Working pressure is about 30 MPa (300 bar). Bucket curl forces of 100 kN and breakout forces of 130 kN are produced by 130 mm bore cylinders. Hydraulic systems on construction equipment dominate this duty class because they pack high power density (about 1 kW per kg of cylinder, versus 0.3 kW per kg for an equivalent electric motor and gearbox).

Hydraulic lifting is governed by AS1418 (lifts and hoists). Critical safety items:

Hydraulic bottle jacks (5 to 50 tonne) are standard automotive workshop equipment, with manual lever pumps that drive a small input piston into a much larger output piston. Forklifts by Toyota Material Handling Australia, Linde and CrownLift use double-acting hydraulics with sequenced cylinders for the lift and tilt mast. Hydraulic platform lifts for accessibility are common in Australian commercial buildings; they use AS1735-compliant valves and brake systems.

The input piston must move further than the output piston, by the same area ratio. Energy is conserved.

For HSC purposes it is incompressible. In real hydraulics, compressibility matters at very high pressures and during transient response.

Without a check valve, the load would fall back through the pump as soon as effort was removed. :::