Hydraulics moves force through a liquid that cannot be squeezed, so oil moves a piston exactly as far as the oil put in, holds a load still when the valves are shut and responds at once. Force is pressure times piston area, so 20 MPa on a 50 mm bore gives 39.3 kN, about 4 tonnes-force. The price is that a pinhole in a pressurised line throws an invisible jet that injects oil through skin, so never search for a leak with a hand.
Part of Hydraulic Systems
Hydraulics transmits force and position through a confined liquid. A pump pressurises the oil, valves direct it, and a cylinder turns the pressure back into a push.
An Incompressible Fluid Carries Position as Well as Force
Where air is a spring (Pneumatic Principles), oil is a rod. Push 1 mL of oil into a cylinder and the piston advances by exactly the volume of 1 mL divided by its area. Close the valve and the piston stays where it is. This is why a hydraulic jack holds a car and a hydraulic press stops at a set position.
The stiffness can be put in numbers. Oil’s bulk modulus is about 1.5 GPa (recalled from engineering tables, not from this corpus), which means a pressure rise Δp shrinks its volume by Δp / 1.5 x 10⁹. A 200 mm column of oil in a 50 mm bore (area 0.0019635 m²) carrying 500 N more load:
- Extra pressure = 500 / 0.0019635 = 254,600 Pa
- Volume change = 254,600 / 1.5 x 10⁹ = 1.7 x 10⁻⁴ of the column
- Piston sinks 200 mm x 1.7 x 10⁻⁴ = 0.034 mm
A column of air under the same extra load sinks 53 mm, about 1,600 times further. That stiffness is what hydraulics buys.
Force Is Pressure Times Area, With Units Kept Straight
Force (N) = pressure (Pa) x area (m²). 1 MPa = 1,000,000 Pa = 10 bar.
Worked example: 50 mm bore at 20 MPa.
- Area = π / 4 x 0.05² = 0.0019635 m²
- Force = 20,000,000 x 0.0019635 = 39,270 N, 39.3 kN
- In tonnes-force: 39,270 / 9,810 = 4.0 (a tonne-force is 9,810 N)
Doubling the bore quadruples the force. The same bore at the 6 bar of an air line gives only 1,178 N, a factor of 33 less, which is why hydraulics is the choice for presses and jacks (Hydraulic vs Pneumatic).
Speed Comes From Flow, Not Pressure
Because the oil does not compress, a cylinder’s speed is the flow into it divided by its area. A hand pump delivering 6.3 mL per stroke at 60 strokes a minute supplies 378 mL/min; into a 100 mm cylinder (area 78.5 cm²) that moves the piston 378 / 78.5 = 4.8 cm per minute. Force is cheap and speed is expensive, which is the trade the next articles measure (Force Multiplication).
The Parts of a System
A reservoir feeds a pump that raises the pressure, a relief valve limits it, a directional valve sends oil to one side of the cylinder or the other, and the oil returns to the reservoir through a filter. Every part has a pressure rating, and the system can run at no more than the weakest. The corpus’s own pipe and hose methods top out far below tens of megapascals: wrought-iron pipe at 3-8 bar (Metal Pipes) and fabric-braid hose at 3-10 bar (Hoses and Tubing). A system working at 20 MPa needs parts with a marked rating at least that high, and salvaged hydraulic equipment (Heavy Equipment) is the usual source. Keep the pressurised volume short and rigid, with the pump bolted close to the cylinder.
Common Mistakes
- Air in the oil. A bubble compresses, and the stiffness is lost and the movement jerks. Bleed the system at its high points.
- Forgetting that pressure acts on every wall. The end cap of a 100 mm cylinder at 20 MPa is pushed off by 157 kN, and its threads or tie rods must hold that.
- Running a system at the pump’s maximum pressure with parts of unknown rating.
Safety
A pinhole leak injects oil through skin
Oil leaking from a pressurised line can leave a pinhole as a jet too fine to see, and it can pass through skin and inject oil into the tissue. The entry wound looks trivial and may not hurt, yet this is a surgical emergency that can cost the limb. Never search for a leak with a hand or finger. Pass a piece of card or wood across the suspect fitting and look for oil on it, with the system at the lowest pressure that shows the leak and everyone out of the line of the jet. Anyone who may have been hit needs urgent surgical care at once; do not wait for pain or swelling, and do not try to squeeze the oil out. Relieve system pressure before tightening or removing any fitting.
Never stand where a rod, a load or a hose would travel
A cylinder under load can drop it if a seal or valve leaks. Support any raised load mechanically and stay out of the line of force. See Hydraulic Jack.
What’s Next
- Learn next: Pascal’s Law — why pressure is the same everywhere in the fluid
- Combine with: Hydraulic Cylinders — turning pressure into a push