Grade 9 technology project: hydraulics, and how to build a working hydraulic arm

Hydraulics is the Grade 9 Technology topic that sits under Mechanical Systems and Control, and the hydraulic arm is the model most learners build for it. No batteries, no motors, no circuit — push a syringe at one end and an arm on the other end of a tube moves. Everything that happens is something you can see and explain.

Watch the whole build first

How a hydraulic system actually works

Press on a liquid in a closed container and the pressure does not stay where you pushed it. It spreads out equally through the whole liquid, and pushes on every surface it touches. That is the principle the entire topic rests on.

So if you join two syringes with a tube and fill the whole thing with water, pushing the plunger on the first syringe raises the plunger on the second. The water has nowhere to go and will not squash, so it has to move the other plunger instead.

That pair — one syringe you push, one syringe that does the work, and a tube between them — is a complete hydraulic system. The model just has three of them.

Why water and not air

This is the question most likely to come up, and the answer is short: air squashes, water does not.

Push on a syringe full of air and most of your effort goes into compressing the air; the far end moves late, moves less, and feels spongy. Water has almost nowhere to compress to, so nearly all of the movement you put in comes straight out the other end. That is also the difference between hydraulic (liquid) and pneumatic (gas) systems in the real world — and it is why diggers use oil and not air.

Try it. Leave one bubble in a tube and work the arm. The sponginess you feel is the bubble being squashed instead of the arm moving, and that single observation is worth a paragraph in the write-up.

Force and distance

The six syringes in this kit are all the same size, so each pair moves one-for-one: push the control plunger 2 cm and the cylinder moves 2 cm.

Change that and you get the interesting part. A small syringe pushing a large one gives more force at the far end, but the large plunger moves a shorter distance. Swap them and you get the opposite. You never get both — force and distance trade against each other. That trade is the heart of the Grade 9 hydraulics section, and the easiest way to show you understand it is to say what would happen if the syringes were different sizes.

What is in the kit

The Slotbok hydraulic arm kit is cut by laser from 3 mm MDF, and it builds a model 430 × 150 × 265 mm:

• 120 pieces across four 3 mm MDF sheets, every part labelled
• Six 5 ml syringes and three lengths of tube
• Thirteen bamboo sticks for the pivots and axles
• Four M4 screws and a screwdriver
• Glue and sandpaper
• A two-page assembly guide

You add the water yourself. Food colouring is not in the box and is not necessary, but a drop of it makes the water visible in the tubes, which looks better in a photograph and makes the system much easier to explain to someone holding the model.

Three pairs, three joints

Six syringes and three tubes make three pairs. Each pair has a control syringe on the chassis and a cylinder on the arm, joined by one tube, and each pair drives one joint:

• The boom — the big arm that lifts from the base
• The stick — the middle section that reaches out and back
• The bucket — the scoop at the end that tips

Those are the real names for the three parts of an excavator arm, and using them is free marks. Three independent circuits, three controls, one arm that can reach, lift and tip without a single motor.

The build order

The kit builds in thirteen steps, and the assembly guide in the box takes them one at a time. Three things are worth knowing before you start:

1. Press the parts out carefully. Work along the tabs rather than forcing a piece out of the middle, and sand any rough edges. Everything after this is easier if the parts are clean.
2. Fill the syringes before you fit them. Draw water into a syringe, push the tube on, and draw water through until the tube and both syringes are full with no bubble. Doing this on the bench is far easier than doing it once the pair is mounted on the arm.
3. Leave one plunger part-way out. A pair that is bottomed out at one end can only move in one direction. Set each pair with the control plunger roughly half-way so the joint can go both ways.

If it does not move properly

• Spongy, slow, or lazy coming back — there is air in that pair. Pull it all out, refill with no bubble, refit.
• Water leaking at the tube — the tube has to be a tight push fit on the syringe nozzle. A loose one loses pressure and the joint goes weak.
• A joint that will only move one way — the plunger has run out of travel. Reset it half-way.
• A joint that binds — a pivot is too tight. The bamboo sticks should turn freely in their holes, not grip.

What to write down while you build

Most of the marks are for the write-up, not the model. Worth noting as you go:

• How far the control plunger moves, and how far the cylinder moves. Are they the same? Why?
• What happens to the feel of the arm when there is a bubble in a tube.
• What each of the three pairs controls, named: boom, stick, bucket.
• The heaviest thing the arm can lift, and where you had to hold it from.
• What you would expect to change if one syringe in a pair were twice the size of the other.
• Anything that went wrong and what you did about it — the easiest marks in the report, and the ones most often left out.

A photograph at the end of each stage gives you pictures for the report without stopping to take notes.

Worth saying plainly: copying someone else's answers is the one way to lose marks you had already earned. Everything here is so you can explain your own model in your own words.

Afrikaans terme

If you are doing this project in Afrikaans: Graad 9 Tegnologie (Grade 9 Technology) · skoolprojek (school project) · hidrouliese arm (hydraulic arm) · meganiese stelsels en beheer (mechanical systems and control) · suier (plunger / piston).

Get the kit

The kit is cut in our own workshop in Gauteng, and every design is built once by hand before it is packed, so the slots and tabs are known to fit.

• Hydraulic Arm School Project Kit — Grade 9
• All Grade 9 Technology projects
• The mine headgear build guide, if you need a frame-structure project instead

Questions about a kit, a class set or delivery to your town: message us on WhatsApp.