C. Matter and Energy · Fluids

Hydraulic and Pneumatic Systems

C2.9 C2.7 C2.4 A1.3 A1.4 A1.5 A3.1About 60 minutes

Learning goals

I can describe the differences between hydraulic and pneumatic systems.
I can choose the better type of system for a job and justify my choice.
I can use an engineering design process to build and test a syringe-powered device.
I can describe skilled trades that work with hydraulic and pneumatic systems.

Words to know

hydraulic system
A system that uses a liquid (usually oil) under pressure to move things and multiply force.

pneumatic system
A system that uses a gas (usually compressed air) under pressure to move things.

compressor
A machine that squeezes air into a tank, storing it under high pressure for a pneumatic system.

cylinder
A tube in which a piston slides. Fluid pushed into the cylinder moves the piston.

valve
A device that opens, closes or controls the path of a fluid.

criteria and constraints
Criteria are what a design must do to succeed. Constraints are the limits you must work within, like materials, time or cost.

Two machines, two fluids

Watch a construction site. The excavator's huge arm digs smoothly into frozen ground and lifts a bucket of rock. Nearby, a carpenter's nail gun goes pfft, pfft, pfft as she fastens boards. Both machines use a fluid under pressure. But the excavator uses oil, and the nail gun uses air. Why?

A yellow excavator digging into a pile of earth with its arm and bucket. — Look for the shiny rods and black hoses on the arm. They are hydraulic cylinders: oil under pressure pushes them out to move the arm and bucket. AI-generated image

Hydraulic and pneumatic systems

A hydraulic system uses a liquid, usually a special oil. Hydro comes from the Greek word for water.
A pneumatic system uses a gas, usually compressed air. Pneuma comes from the Greek word for air or breath.

Both types have similar parts:

a pump (hydraulic) or compressor (pneumatic) that puts the fluid under pressure;
a reservoir (for oil) or tank (for compressed air);
hoses or pipes to carry the fluid;
valves to control where the fluid goes;
cylinders and pistons (or motors) that do the moving.

Both use Pascal's law: pressure in the fluid is passed on through the hoses to the piston. The big difference comes from compressibility, which you studied in the lesson Squeeze It: Compressibility of Liquids and Gases.

Both systems have the same kinds of parts. The difference is what happens to the fluid: oil is reused, air is let out.

Feature	Hydraulic (liquid)	Pneumatic (gas)
Fluid	liquid, usually oil	gas, usually air
Compressible?	nearly incompressible	very compressible
Force	very large forces; good for heavy loads	smaller forces
Motion	firm, steady and precise; holds a load in place well	fast, springy, cushioned; harder to stop at an exact spot
Where the fluid goes	recycled back to a reservoir in a closed loop	used air is let out into the room (the hiss)
If it leaks	oil can make a mess, be a slip hazard and pollute soil or water	air leaks are cleaner, though they waste energy and can be noisy
Examples	excavators, backhoes, car brakes, car lifts, garbage truck compactors, snowplow blades, barber chairs	air brakes on trucks and buses, nail guns, jackhammers, dentist's drills, tire inflators, factory grippers

Comparing hydraulic and pneumatic systems.

Worked example

Let's choose the right system

For each job, choose hydraulic or pneumatic, and give a reason. (a) Lifting heavy rock at a mine. (b) A machine in a bakery that quickly pushes boxes off a conveyor belt. (c) The brakes on a transport truck that pulls a trailer.

Step 1. Ask: how big is the force? Huge forces suit hydraulics. Smaller, quick pushes suit pneumatics.
Step 2. Ask: what if it leaks? Oil leaks are a problem near food. Air leaks are clean.
Step 3. (a) Mine: very heavy loads, and the arm must hold the rock steady → hydraulic.
Step 4. (b) Bakery: small, fast, repeated pushes, and no oil near the food → pneumatic.
Step 5. (c) Transport truck: big trucks use pneumatic air brakes. Air is easy to connect from the truck to the trailer with hoses, a small leak doesn't let the brake fluid run out, and the system is designed so that if air pressure is lost, strong springs put the brakes on, a safety feature.

(a) Hydraulic: huge, steady force. (b) Pneumatic: fast, light, clean. (c) Pneumatic air brakes: easy to connect to trailers, and designed to brake if air pressure is lost.

People who work with fluid power

Many skilled trades in Ontario use hydraulics and pneumatics every day:

Heavy equipment technicians repair the hydraulic systems on excavators, loaders and snowplows.
Automotive service technicians and truck and coach technicians maintain brakes, including bleeding air from hydraulic brake lines and fixing air brake systems.
Millwrights install and maintain the machines in factories, many of them run by compressed air or hydraulics.
Agricultural equipment technicians fix the hydraulics that lift and steer farm machinery.

These jobs need a solid understanding of pressure, force and how fluids flow.

Safety around fluid power

Hydraulic and pneumatic systems can hold very high pressures. Never point an air hose at anyone, and never put your hand near a leaking hydraulic hose: a fine jet of high-pressure fluid can injure skin. Professionals release the pressure before working on a system.

Just for fun

What kind of music do pneumatic tools play?

Air guitar, of course. The hydraulic tools tried to join the band, but their music was way too heavy.

Hands-on activity

Design challenge: a syringe-powered lift or arm

Ask a grown-up to help with cutting and with any hot glue.

Problem: design and build a device that uses fluid power to lift a small object (like an eraser or a juice box) at least 5 cm.

Materials: 2 to 4 needle-free medicine syringes, clear plastic tubing that fits the syringe tips, water, cardboard, craft sticks, tape or glue, a brass paper fastener or a pencil for a pivot, scissors.

Criteria: lifts the object at least 5 cm; can be controlled by pushing one plunger; holds the object up for 10 seconds.

Constraints: only the materials listed; build time about 45 minutes; no electricity.

Define and research: look at pictures of excavators or car lifts. Where are the cylinders?
Plan: sketch two ideas with labels. Choose one and explain why.
Build: fill one syringe and the tubing with water (no air bubbles), and connect a second syringe. Fix one syringe to your structure so its plunger pushes the lifting part.
Test: does it meet each criterion? Record the results.
Compare fluids: empty the system and repeat the test with air instead of water. Describe the difference.
Improve and communicate: make one improvement, test again, and write a short report or make a labelled diagram for a younger student.

What to look for: with water (hydraulic), the lift moves firmly and holds its position. With air (pneumatic), the lift feels springy, moves after a delay and may sink back a little, because the air compresses. Using a small syringe to push a large one gives more force but less movement (Pascal's law).

Safety: needle-free syringes only. A grown-up does the cutting and any hot gluing. Work over a towel and wipe up spills.

Your syringe lift, two ways. Water can't be squeezed, so the far plunger moves right away. Air gets squeezed first, so the far plunger moves less and feels springy.

Your turn

What fluid does a hydraulic system use? A pneumatic system?

Which type of system can usually produce larger forces? Why?

Name two examples of hydraulic systems and two of pneumatic systems.

Why do pneumatic tools make a hissing sound?

Give one advantage of pneumatic systems in a food factory.

Name a skilled trade that works with hydraulic systems.

More practice: warm-up

"Pneuma" comes from the Greek word for...

What machine fills the tank of a pneumatic system?

Which is springier: a hydraulic or a pneumatic system?

What part controls where a fluid goes?

Is a car's brake system usually hydraulic or pneumatic?

More practice: on your own

In the design challenge, why did the lift feel springy when filled with air?

A snowplow driver needs to hold the blade at an exact height while clearing a road. Hydraulic or pneumatic? Why?

Give one disadvantage of hydraulic systems.

Give one disadvantage of pneumatic systems.

A dentist's drill spins very fast and must be light to hold. Why is compressed air a good choice?

Challenge

In your syringe lift, you connected a 10 mL syringe (plunger area about 1.5 cm²) to a 60 mL syringe (plunger area about 6 cm²). If you push the small one with 6 N, about what force does the large one give? What's the catch?

Transport trucks' air brakes are designed so springs apply the brakes if air pressure is lost. Why is this a smart "fail-safe" design?

A company wants a robot that sorts small, light parcels very quickly in a warehouse, plus a press that squashes cars at a scrapyard. Recommend a system for each and give two reasons each.

Check your understanding

Quiz

A pneumatic system uses...

Hydraulic systems can produce very large, steady forces mainly because...

Which is a hydraulic system?

Why does a pneumatic lift feel springy?

Which job is best suited to a pneumatic system?

Which skilled trade repairs the hydraulics on excavators and loaders?

For parents and teachers

Big idea (C2.9): hydraulic systems use liquids (nearly incompressible: large, precise forces) and pneumatic systems use gases (compressible: fast, springy, clean). Both apply Pascal's law (C2.7). The design challenge follows the engineering design process (A1.3) with safe use of tools (A1.4) and a report (A1.5). The trades section addresses A3.1.

How to help: watch for these systems on a walk: garbage trucks, snowplows, buses kneeling or opening doors, construction equipment. Ask "Liquid or air? How can you tell?" (Hissing usually means air.)

Common difficulties:

Thinking "hydraulic" means water. It means a liquid; real systems usually use oil.
Thinking pneumatic systems are weaker in every way. They are faster, lighter and cleaner, which is often what matters.

Safety: you help with cutting and hot glue. Use needle-free syringes only. Teach your child never to touch a leaking hydraulic hose or point an air hose at anyone.