💧Physics & Applied Math · 5 min read

Hydraulic Systems

Pascal's Law, cylinders, and how hydraulic elevators work

Hydraulic elevators use fluid pressure to lift the car. Understanding Pascal's Law and how hydraulic cylinders work will help you answer questions about pressure, force, and system behavior. These questions often involve calculating force from pressure and area, or identifying which component does what.

Pascal's Law

Pressure applied to an enclosed, incompressible fluid transmits equally in all directions throughout the fluid. This is the foundational principle of all hydraulic systems.

Practical implication: generate pressure on a small piston → that pressure acts equally on every surface in the system → push on a large piston with that pressure → get a very large output force.

Force = Pressure × Area. A small pump generating 500 PSI in a cylinder with a 20 in² piston produces: F = 500 × 20 = 10,000 lbs of lift force.

Hydraulic Cylinder Operation

Rising: An electric motor drives a hydraulic pump, which forces oil from a reservoir into the cylinder. The increasing oil volume pushes the piston (plunger) upward, raising the car. The motor must overcome the full weight of the car and passengers.

Holding: When the motor stops, a check valve closes automatically, trapping oil in the cylinder. The car holds position without motor power.

Lowering: A solenoid-operated valve opens a controlled path back to the reservoir. Oil drains under the weight of the car at a metered rate — gravity lowers the car, no motor energy needed.

This means hydraulic elevators use motor energy only for upward travel. Descending is essentially free from an energy standpoint.

Hydraulic Force Multiplication

MA = area of output cylinder ÷ area of input cylinder. A small pump piston (2 in²) pushing into a large jack piston (50 in²) gives MA = 25.

Trade-off: the pump piston must move 25 inches for every 1 inch the jack extends. The fluid volume is constant — same volume just redistributed.

Hydraulic Elevator Safety Devices

Pressure relief valve: Limits maximum system pressure. If the pump builds pressure beyond the set point (e.g., a stuck car), the relief valve opens and returns oil to the tank, preventing over-pressure damage.

Check valve: One-way valve. Oil flows into the cylinder freely; it cannot flow back without a deliberate valve opening. This prevents the car from drifting down when the pump is off.

Rupture valve (velocity fuse): Located at the jack, it monitors flow rate. If oil suddenly rushes out at a rate indicating a burst pipe (not normal lowering), it snaps closed, trapping oil in the cylinder and preventing the car from dropping.

Low-oil switch: Stops the pump motor if the oil reservoir level drops too low — prevents cavitation damage to the pump and signals that there may be a leak.

Key Takeaways

✓Force = Pressure × Area. Most common hydraulic calculation on the test.
✓Pascal's Law: same pressure throughout the fluid. Larger piston = larger force.
✓Hydraulic elevators have no counterweight — motor lifts the full load every time.
✓Lowering uses gravity (oil drains out) — no motor energy required.
✓Rupture valve: closes when pipe bursts. Check valve: prevents downward drift when pump is off. Know the difference.

Key Terms

Pascal's Law

Pressure in an enclosed fluid transmits equally in all directions.

Hydraulic jack / plunger

The cylinder-and-piston that lifts the car in a hydraulic elevator.

PSI

Pounds per square inch — unit of hydraulic pressure.

Check valve

One-way valve; allows oil in, prevents it from flowing back without a separate valve opening.

Rupture valve

Safety valve at the jack that closes if abnormal flow indicates a pipe burst.

Pressure relief valve

Limits maximum system pressure; opens to return oil to tank if overpressure occurs.

Hydraulic pump

Motor-driven pump that forces oil into the jack cylinder.

Holeless jack

Hydraulic jack that sits alongside the car in the hoistway — no underground drilling.

Cavitation

Damage from air bubbles collapsing inside a pump when oil is insufficient.

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