Fluid Power Fundamentals and Pascal's Law

What Is Fluid Power?

Hydraulic elevator systems move the car by pushing fluid — almost always a petroleum-based or biodegradable hydraulic oil — through a sealed circuit under pressure. Understanding how that pressure is created, transmitted, and controlled is the foundation of every hydraulic elevator task you will perform as an apprentice and throughout your career.

Pascal's Law

Blaise Pascal, a seventeenth-century French mathematician, established the principle that governs every hydraulic system: pressure applied to a confined fluid is transmitted equally and undiminished in all directions throughout the fluid and acts with equal force on equal areas.

In plain trade language: squeeze oil in one place and the same squeeze shows up everywhere in the circuit at the same time. This is why a relatively small pump can lift a fully loaded elevator car weighing several thousand pounds — the pump builds pressure throughout the oil, and that pressure pushes on the full cross-sectional area of the jack cylinder.

Key Relationships: Pressure, Force, and Area

Three quantities are linked by a simple formula you will use constantly:

P = F ÷ A

• P = Pressure in pounds per square inch (PSI)
• F = Force (or load) in pounds (lb)
• A = Area in square inches (in²)

Rearrange the formula to solve for any unknown:

• Force: F = P × A
• Area: A = F ÷ P

To find the area of a circular cylinder bore, use:

A = π × r² (where r = radius = bore diameter ÷ 2, and π ≈ 3.1416)

Flow Rate and Car Speed

Pressure determines whether the car can lift the load. Flow rate — measured in gallons per minute (GPM) — determines how fast the car moves. More GPM from the pump means faster upward travel. The control valve regulates GPM during leveling and stopping to give passengers a smooth ride. Pressure and flow rate work together: you need adequate pressure to overcome the load, and the right flow rate to control speed.

How a Hydraulic Elevator Works

Up Travel

The electric motor drives the pump, which draws oil from the reservoir (tank) and forces it through the control valve into the bottom of the jack cylinder. As oil enters, it pushes the plunger (piston rod) upward, raising the car. Pressure must be high enough to support the total load: car weight plus rated passenger or freight capacity plus the weight of the plunger itself.

Down Travel

The motor stops. The control valve opens a metered path back to the tank. Gravity and the weight of the car push the plunger down, forcing oil back through the valve to the reservoir. The valve controls the rate of descent so the car lowers smoothly and levels accurately at each floor.

In-Ground vs. Holeless Hydraulic Elevators

In-ground (conventional) hydraulic elevators have the jack cylinder buried in a casing below the pit floor. The plunger rises directly from the ground, giving a clean, single-stage lift. Concern about underground oil leaks led to the development of double-bottom cylinders and PVC liners required by modern codes.

Holeless hydraulic elevators eliminate the in-ground cylinder entirely. Two common configurations exist:

• Above-ground telescoping jack — a multi-stage plunger that extends upward inside the hoistway, mounted beside or behind the car.
• Roped hydraulic — the jack acts on a rope-and-sheave arrangement, allowing a shorter jack stroke to move the car a greater distance.

Both designs keep all hydraulic components above the pit floor, removing the underground leak risk.

Safety Considerations

Lockout/Tagout (LOTO): Before working on any hydraulic component, de-energize the pump motor at the disconnect, verify zero energy, and apply your personal lock and tag per your employer's written LOTO program and OSHA requirements. Hydraulic systems store energy under pressure even after the motor stops — always verify pressure is relieved before opening any fitting or line.

PPE: Wear safety glasses or a face shield when working near hydraulic lines. A pinhole leak in a pressurized line can inject fluid through skin — this is a medical emergency. Wear chemical-resistant gloves when handling hydraulic oil.

Pit Safety: The elevator pit must be entered only when the car is secured and cannot move. Use pit stop switches and follow your employer's confined-space and pit-entry procedures.

Spill Control: Hydraulic oil is a slip and environmental hazard. Have absorbent material available and dispose of used oil according to local regulations.

Connecting Theory to the Trade

When you troubleshoot a hydraulic elevator that will not lift its rated load, Pascal's Law points you directly to the variables: Is the pump building adequate pressure? Is the relief valve set correctly and not bypassing oil? Is the cylinder bore the right size for the design pressure? Every diagnostic step traces back to P = F ÷ A and the fundamentals you learn in this lesson.