The science idea
Hydraulic machines are all around us: the brakes of a car, a dentist’s chair, the arm of a digger or a crane, the lift that raises a car in a garage. They all work by pushing on a liquid in a closed space. In this project you build a small hydraulic arm, controlled by syringes, which is a simple robot with no electricity or electronics at all.
The science is Pascal’s law. Blaise Pascal found that when pressure is applied to a liquid that is enclosed, the pressure is passed on without loss to every part of the liquid and to the walls of the container. A liquid is almost incompressible, which means that it cannot be squashed into a smaller space, so when you push the plunger of one syringe, the water has to go somewhere. It moves along the tube and pushes the plunger of the other syringe out by the same volume.
Pressure is force divided by area, P = F ÷ A. The pressure is the same in the whole liquid, so a syringe with a larger plunger area gets a larger force: F₂ = F₁ × (A₂ ÷ A₁). But the water that leaves the small master syringe must be the same volume as the water that enters the big one, so the big plunger moves less far: A₁ × d₁ = A₂ × d₂. This is the trade-off found in all machines: you can get more force, but you pay for it with distance. A hydraulic jack lifts a car with a small push because the push is made over a long distance.
In a real hydraulic machine there is also friction, and the liquid is usually oil (which does not rust parts and resists squeezing). Air in the tubes is a problem, because air is compressible. A bubble acts like a spring, so part of your push goes into squeezing the bubble instead of moving the arm. That is why you fill the circuit under water and remove all the bubbles, a procedure called bleeding.
The arm is also a lever. If the slave syringe pushes the arm at a point near the pivot, the tip of the arm moves further than the syringe plunger but with less force. Calculating the force and the distance at the tip is a good problem for your report. You can also compare syringes of different sizes to test the area ratio.
Materials and tools
Cost: Under Rs 100
Cardboard, sticks, a split pin and a box for the base are free. Four needle-free syringes and 1 to 2 metres of clear tube cost roughly Rs 40 to Rs 80 from a chemist and an aquarium shop.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Plastic syringes without needles, 10 mL (or 5 mL) | 4 (two working syringes and two master syringes) | Ask a chemist for new needle-free syringes (a few rupees each). Use 2 of one size and 2 of another, for the ratio test. |
| Clear flexible tube (silicone or aquarium air tubing) that fits the nozzle of the syringe tightly | 2 pieces, each about 60 to 80 cm | Aquarium tube or medical IV tube, from a pet or medical store. It must be a snug fit. |
| Stiff cardboard, ice-cream sticks or card strips for the arm and the control panel | Strips about 30 cm × 3 cm, a panel about 20 cm × 15 cm | From cartons. Glue two layers together for strength. |
| Cardboard box (or a tin) for the base | 1 | A shoe box with a few stones or coins inside for weight. |
| Split pin (paper fastener), skewer or bolt for the pivots | 2 or 3 | Stationery box. |
| Strong tape, string, rubber bands, small cable ties | As needed | To fix the syringes and tubes. |
| A bowl of water and some food colour | 1 bowl, a few drops of colour | The colour makes the water visible in the tubes. |
| Small light objects to pick up (eraser, cork, paper cup) | A few | From the house. |
Tools
- Scissors and a craft knife (adult)
- Ruler and pencil
- Awl or a thick nail for the pivot holes (adult)
- A bowl of water for filling
- A towel for spills
Safety first
Read this before you start
- Use only syringes without needles. Never use or look for a syringe needle for this project. Keep the syringes for this project only, and never for medicines or chemicals.
- Do not squirt water at people, at electrical sockets or at electrical devices. Work on a tray with a towel, away from sockets and extension boards.
- An adult should cut the card with the knife and make the pivot holes.
- Small parts (syringe plungers, pins, caps) are a choking risk for young children. Keep them away from them.
- Do not push the plungers with great force. A tube can pop off the nozzle and spray, so hold the joints while you push.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
13 steps. Do them in order, and test each part before you go on to the next.
Plan the arm
Draw the arm on paper, full size. Mark the base, the post, the pivot, the arm (about 25 cm), the gripper, and where each working syringe fixes. Decide how far each syringe has to move.
Tip: Put the lifting syringe about one third of the way along the arm, so a small movement of the plunger raises the tip a good distance.
Build the base and post
Fill a shoe box with stones to make it heavy and tape the lid on. Cut a post from thick card, 15 cm tall, and fix it upright in the base with tape or glue. Support it with triangular card braces on both sides.
Common mistake: A light base or a wobbly post. The arm will tip the whole machine over when you push.
Make the arm
Cut two strips of card, 25 cm × 3 cm, and glue them together as a long strip with a strip of card between them as a spacer. This makes a stiff box-section arm. Make a hole near one end for the pivot.
Tip: Fold the card in a U-shape for extra stiffness. It will not bend when the water pushes on it.
Fix the pivot
Push a split pin or a skewer through the hole in the arm and the top of the post so the arm can swing up and down. It must move freely, but without too much wobble.
Common mistake: Making the pivot tight. A stiff pivot adds friction, and you waste force.
Build the gripper
Make two jaws from card or ice-cream sticks, each 8 cm long. Fix jaw A to the end of the arm. Pivot jaw B on a pin next to it so that the two jaws close like pliers. Add a rubber band to pull the jaws open.
Tip: Stick a strip of rubber or sandpaper inside each jaw so that objects do not slip.
Fix the lifting syringe
Tape a working syringe to the base, pointing up at an angle, with its barrel fixed and its plunger end attached to the underside of the arm, one third of the way from the pivot. Tie the plunger to the arm with a loop of string so the joint can turn.
Common mistake: Fixing the plunger rigidly. The arm and the syringe change angle as the arm rises, so each joint must be able to swing.
Fix the gripper syringe
Tape the second working syringe along the arm, with its plunger pushing on a short lever of jaw B. When the plunger moves out, the jaws close.
Tip: The rubber band opens the gripper again when you pull the master plunger back.
Prepare the control panel
Fix the two master syringes side by side on the cardboard panel with tape or cable ties, plungers facing the same way. Write 1 and 2 on the panel, and label them LIFT and GRIP.
Tip: Fix the panel to a box or the base so it stays in place when you push.
Fill under water
Put a few drops of food colour in a bowl of water. Put the syringes and the tubes under the surface. Draw water into every syringe and tube, tap the barrels so that the bubbles rise, and push the air out. Join each tube to its pair of syringes while everything is still under water.
Common mistake: Joining the tube in the air. A bubble stays trapped, and the arm feels spongy.
Set the plungers
Before you take the circuit out of the water, set each master plunger about half way in and each working plunger about half way out, so there is travel in both directions. Take the syringes out and fix them in place.
Tip: Hold the tube joints with your fingers while moving the circuit from the bowl to the model.
Test and bleed
Push and pull each master plunger slowly. The arm should lift and the jaws should close and open. If it feels spongy or jerky, there is air in the tube. Tilt the tube so that the bubble moves to a syringe, and push it out.
Tip: Move the plungers slowly. Fast pushes make the tube pop off.
Pick things up
Use the two master syringes together to lift a small object, such as an eraser or a cork, and put it down in a cup. Try different loads and see how much the arm can lift.
Common mistake: Overloading the arm. If it sags or the joints slip, use a lighter load or move the lifting syringe further from the pivot.
Test the ratio
Replace one syringe pair with syringes of different sizes. Push the small master by 2 cm and measure how far the larger plunger moves. Compare with the sum d₂ = d₁ × (A₁ ÷ A₂) after measuring the barrel diameters.
Tip: Area = 0.785 × diameter². Measure the inside diameter of each barrel with a ruler held across the opening.
How it should look when it is finished
A cardboard crane about 30 cm tall standing on a weighted box. A long arm, hinged on a post, ends in a two-finger gripper. Two coloured water-filled tubes run from the arm to a small panel carrying two syringes labelled LIFT and GRIP. Pushing the LIFT plunger raises the arm smoothly, and pushing GRIP closes the fingers so the arm can pick up a cork and move it.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
When you push the plunger of the master syringe, you push on the water in the tube. Water cannot be compressed, so the pressure you make is passed on through the whole liquid, and the water moves along the tube and pushes out the plunger of the working syringe.
The working syringe is fixed to the arm. As its plunger comes out, it pushes the arm up around its pivot, or it pushes the lever of the gripper and closes the jaws. When you pull the master plunger back, the water is drawn back, and the rubber band or the weight of the arm pushes the working plunger in again.
Because the pressure is the same in both syringes, the force on each plunger is proportional to its area, and because the volume moved is equal, the distance is inversely proportional to its area. Friction takes some of the force, and air bubbles take some of the movement, so the real arm is less efficient than the ideal sums.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The arm moves with a delay or feels spongy | Air bubbles are trapped in the tube or a syringe. | Bleed the air: push the bubble to a syringe, tap the barrel and push out the air, or refill under water. |
| Water leaks at the nozzle | The tube does not fit tightly. | Use a snugger tube, or wrap the joint with thin tape or a small cable tie. |
| The master moves but the arm does not | The working plunger is at the end of its travel, or the pivot is stiff. | Reset the plungers half way and free the pivot. |
| The arm moves jerkily | Friction in the syringe seals or the pivot, or the arm is too heavy. | Put a drop of cooking oil on the plunger, loosen the pivot and lighten the arm. |
| The tube pops off | Too much pressure, or a loose fit. | Push slower and secure the joint with tape or a cable tie. |
| The whole machine tips over | The base is too light. | Add more weight to the base on the side opposite the arm. |
Test and record
Measure the inside diameter of each barrel in mm. Push the master plunger 20 mm and measure how far the working plunger moves. Area in mm² = 0.785 × diameter². Predicted d₂ = d₁ × A₁ ÷ A₂.
| Syringe pair | Master diameter (mm) | Working diameter (mm) | Master push d₁ (mm) | Predicted d₂ (mm) | Measured d₂ (mm) |
|---|---|---|---|---|---|
| Same size (10 mL and 10 mL) | 20 | ||||
| Small master and large working | 20 | ||||
| Large master and small working | 20 |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- State Pascal’s law.
Pressure applied to an enclosed liquid is passed on equally in all directions and to every part of the liquid and the walls of its container.
- Why is water (or oil) used instead of air?
Liquids are almost incompressible, so the movement of the master plunger is passed on at once. Air can be squeezed, so it makes the system spongy.
- If the working piston has twice the area of the master piston, what happens to force and distance?
The force is twice as large and the distance moved is half as large (ideal case).
- What is meant by bleeding the system?
Removing the air bubbles from the liquid so that the hydraulic system works firmly.
- Give two real examples of hydraulic machines.
A car brake, a hydraulic jack, the arm of an excavator, a dentist’s chair, a garage lift.
- What is pressure and what is its unit?
Pressure is force per unit area, P = F ÷ A. Its SI unit is the pascal (Pa), which is one newton per square metre.
- Why does the arm lift less than the ideal sums say?
Friction in the pivots and syringes, air in the tubes, and flexing of the card all waste force and movement.
- Is a machine that gives more force also giving more work?
No. The work done is not increased. The larger force acts over a smaller distance, so (ignoring friction) the work in equals the work out.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a hydraulic robotic arm using syringes and to verify the relation between area, force and distance in a hydraulic system.
- Apparatus
- Syringes (no needles), clear tubes, cardboard, sticks, split pins, a box, tape, water and food colour, a ruler.
- Procedure
- Describe how you built the base, post, arm and gripper, fixed the syringes, filled the circuit under water, removed the air and tested the arm. Describe how you measured the barrel diameters and the movements. Add the diagrams.
- Observation
- The table of diameters and movements, and a note on what the arm could lift.
- Result
- The measured movement of the working plunger was close to the predicted value: ... mm against ... mm. The arm lifted a load of ... g.
- Conclusion
- A push on one syringe is transmitted by the water to the other (Pascal’s law). A larger plunger gives a larger force and a smaller movement. Air in the system reduces the performance.
Take it further
- Add a third circuit and syringe to turn the whole arm on its base, like a real robot arm.
- Use a bigger working syringe to lift a heavier load, and measure how the lifting distance changes.
- Make a hydraulic jack from two syringes of very different sizes and lift a book.
- Compare the performance when the circuit is filled with water and with air.
- Calculate the force at the tip of the arm using the lever (moments) equation.
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