11.2 Basic Hydraulic Systems
Key Takeaways
- Pascal's law (pressure is transmitted undiminished throughout a confined fluid) is the basis for force multiplication: F = P x A
- A 1 in² input piston at 100 psi and a connected 10 in² output piston produce a 10:1 force multiplication - 100 lb in, 1,000 lb out - but the input piston travels 10 times farther
- Pressure determines force; flow rate (gpm) determines actuator speed - never confuse the two on the exam
- The reservoir stores fluid, cools it, releases entrained air, settles contaminants, and is typically sized at 2-3 times pump flow rate
- Hydraulic pressures (1,000-5,000+ psi) are far higher than pneumatic pressures (90-125 psi), and any suspected fluid-injection injury is a medical emergency regardless of how the wound looks
Why Hydraulics Matters on the Exam
Where pneumatics trades force for speed and simplicity, hydraulics trades higher cost and complexity for enormous force in a compact package — hydraulic jacks lift multi-ton loads with hand pressure, and industrial hydraulic power units drive presses, cranes, and machine-positioning cylinders at pressures many times higher than any pneumatic system. NCCER Module 15409, Basic Hydraulic Systems, is scored alongside pneumatics in the smallest official domain, but it introduces the physics (Pascal's law), the major components, and the safety hazards a millwright must recognize before ever working on a pressurized hydraulic line.
Pascal's Law and Force Multiplication
Hydraulics uses a confined, essentially incompressible liquid — hydraulic fluid or oil — to transmit force. The physics is governed by Pascal's law: pressure applied anywhere to a confined fluid is transmitted undiminished in all directions throughout that fluid. Because pressure (P) equals force (F) divided by area (A), rearranging gives a formula every millwright should be able to apply on the exam:
F = P x A
Worked example: a hydraulic cylinder has a piston with a 5 in² working area. At a system pressure of 1,000 psi, the cylinder produces 1,000 x 5 = 5,000 lb of force. Because pressure is transmitted equally, a small-diameter input piston connected through a confined fluid to a large-diameter output piston multiplies force: if a 1 in² input piston is pushed with 100 lb, the fluid pressure is 100 psi throughout the system, and a connected 10 in² output piston produces 100 x 10 = 1,000 lb — a 10:1 mechanical advantage. The trade-off (conservation of energy) is that the small piston must travel 10 times farther than the large piston moves, since the same volume of fluid is displaced on both sides.
Major System Components
| Component | Function |
|---|---|
| Reservoir | Stores fluid, dissipates heat generated by system operation, lets entrained air rise out and contaminants settle, and supplies steady flow to the pump inlet. Typically sized to hold 2-3 times the pump's rated flow rate (gpm) in reserve. |
| Pump | Converts the mechanical energy of a prime mover (usually an electric motor) into fluid flow. Common types: gear (simple, moderate pressure, tolerant of contamination), vane (quiet, moderate-to-high pressure), piston (highest pressure and efficiency, most expensive, most contamination-sensitive). |
| Directional control valve | A spool valve that routes pressurized fluid to the correct port of a cylinder or motor to control direction of motion. |
| Pressure relief valve | Protects the system from overpressure by diverting flow back to the reservoir once a preset maximum pressure is reached; set slightly above normal operating pressure. |
| Flow control valve | Restricts flow rate to a cylinder or motor to regulate actuator speed. |
| Check valve | Permits flow in one direction only, commonly used to hold a load in position if pump pressure is lost. |
| Filter | Removes particulate contamination; located at the pump suction (strainer), on the pressure line, and/or on the return line. |
| Accumulator | Stores fluid energy under pressure (bladder, piston, or diaphragm type) to absorb shock loads or supply short bursts of extra flow. |
| Actuator | Converts fluid pressure back into mechanical work — linear (cylinders, single- or double-acting) or rotary (hydraulic motors). |
Flow Rate, Speed, and the Pressure/Flow Distinction
This is one of the most heavily tested conceptual traps in fluid power: pressure creates force; flow rate creates speed. Raising system pressure does not make a cylinder move faster — it only lets the cylinder produce more force at the same speed. Cylinder speed is set entirely by how much fluid volume per minute reaches it.
Worked example: a pump delivers 10 gallons per minute (gpm) to a cylinder with a 4 in² piston area. Since 1 gallon = 231 in³, 10 gpm = 2,310 in³/min. Cylinder speed = flow / area = 2,310 / 4 = 577.5 in/min (about 48 ft/min). If that same pump were replaced with a higher-pressure-rated pump delivering the same 10 gpm, the cylinder would move at the same speed but could push a heavier load before stalling.
Fluid Properties and Cleanliness
Hydraulic fluid is selected by viscosity, most commonly graded by ISO VG number (for example, ISO VG 32, 46, or 68 — the number approximates centistokes viscosity at 40°C); using fluid that is too thin increases internal leakage and wear, while fluid that is too thick increases pump cavitation risk and power loss to friction. Fluid cleanliness is rated using the ISO 4406 code (three numbers, such as 18/16/13, representing particle counts at three size ranges) — dirtier fluid is the single largest cause of premature component wear in hydraulic systems, since internal clearances in a piston pump can be only a few thousandths of an inch.
Safety: Fluid Injection Injury
Industrial hydraulic systems commonly operate at 1,000-5,000 psi (some systems reach 10,000+ psi) — far above the roughly 90-125 psi typical of shop pneumatic air. At those pressures, a pinhole leak in a hose or fitting can inject fluid through skin at pressures as low as 100 psi, producing what looks like a minor puncture wound but causing severe internal tissue damage. Any suspected high-pressure fluid injection injury requires immediate emergency medical attention, even if the wound looks trivial — never wave a hand near a suspected leak to "find" it; use cardboard instead.
Key Takeaways
- Pascal's law (pressure is transmitted undiminished throughout a confined fluid) is the basis for force multiplication: F = P x A.
- A 1 in² input piston at 100 psi and a connected 10 in² output piston produce a 10:1 force multiplication — 100 lb in, 1,000 lb out — but the input piston travels 10 times farther.
- Pressure determines force; flow rate (gpm) determines actuator speed — never confuse the two on the exam.
- The reservoir stores fluid, cools it, releases entrained air, settles contaminants, and is typically sized at 2-3 times pump flow rate.
- Hydraulic pressures (1,000-5,000+ psi) are far higher than pneumatic pressures (90-125 psi), and any suspected fluid-injection injury is a medical emergency regardless of how the wound looks.
A hydraulic cylinder has a 5 in² piston and the system runs at 1,200 psi. How much force does the cylinder produce?
A pump delivers 20 gallons per minute (gpm) to a cylinder with a 5 in² piston area. Using 1 gallon = 231 in³, approximately how fast does the cylinder extend?
Which system property primarily determines how fast a hydraulic cylinder moves, independent of the load it is pushing?