Build a simple syringe-powered hydraulic arm

You can explore a simple hydraulic arm by connecting two needle-free syringes with water-filled tubing: operate one by hand and use the other to move a hinged cardboard arm. Start with a single lifting joint rather than a grabbing mechanism. The useful outcome is a small model you can adjust, measure and improve.

This is an illustrative learning exercise, not a historical BTE Academy project or a tested construction kit. Treat the suggested layout as a starting design: check that your syringes move freely and that the arm can pivot before fastening everything permanently.

Keep the first design simple

Your model needs three parts: a stationary base, a hinged lifting arm and a syringe mounted between them. Call the syringe you operate the control syringe and the one attached to the arm the actuator. Keep those labels in your results so your observations are easy to follow.

Syringe systems are also used in TeachEngineering’s artificial-limb modelling activity, which explains liquid-filled subsystems and common problems such as air in the tubing. The single-joint layout below is a separate illustrative investigation. It does not reproduce that activity’s limb design or claim the same curriculum alignment.

Gather the materials

• Two new, needle-free plastic syringes of the same size.
• Flexible tubing that fits their outlets snugly.
• Water, a shallow tray and a cloth for spills.
• Stiff cardboard for a base, upright support and lifting arm.
• Tape, elastic bands and paper fasteners for adjustable connections.
• A ruler, pencil and scissors.

An adult or responsible teacher should choose the materials and supervise cutting, hole-making and assembly. Paper fasteners, tubing and syringe parts can present small-part or sharp-edge hazards; use an age-appropriate work area and keep loose parts away from younger children. Never use needles, medical sharps or previously used medical equipment.

Work over the tray, keep water away from electrical equipment and do not force a sticking plunger or seal the system to build pressure. Point outlets away from faces, keep fingers clear of the hinge and stop if tubing loosens, the structure buckles or a plunger binds. Begin without a payload; the arm itself is enough for the first investigation. These checks are a starting point for the supervising adult’s assessment, not a safety certification.

Assemble and check the movement

1. Make the cardboard mechanism

Cut a broad base, a short upright support and a narrow lifting arm. Fold a foot on the support and tape it firmly to the base; add folded cardboard braces if it wobbles.

Join one end of the arm to the upright with a paper fastener. Leave enough clearance for the arm to rotate freely. Move it by hand through a modest lifting arc. If the cardboard rubs, bends or catches, fix that before adding the syringes.

Mark two possible actuator attachment positions along the arm. Measure each distance from the hinge centre in millimetres and label them A and B. These will become your experimental variable.

2. Prepare the syringe pair

Draw water into one syringe. Connect the tubing and gently push water through it into the tray until the tube is filled. Attach the second syringe with its plunger near the outlet, leaving enough travel for the intended movement and avoiding a hard push against either end stop. Keep the outlets over the tray while making the connections.

Push the control plunger slowly and watch the other plunger. Reverse the movement gently using the other syringe. Look for leaks, visible air pockets or sticking. Refill and reconnect if necessary rather than compensating by pushing harder.

This check separates the water-and-syringe system from the cardboard mechanism. If it will not move smoothly on its own, attaching it to the arm will make troubleshooting harder.

3. Fit the actuator temporarily

Arrange the actuator below the arm so that extending its plunger raises the arm. Loosely secure its barrel near the base and its plunger thumb pad near position A. Use tape loops or elastic bands that allow the syringe to change angle as the arm moves.

Check the proposed motion by hand before tightening the attachments. Avoid a layout that bends the plunger sideways or reaches the end of its travel before the arm finishes moving. Keep the control syringe loose on the tray where you can operate it comfortably.

Test one change at a time

Investigate this question: How does the actuator’s attachment position affect the arm-tip movement for the same control-plunger movement?

First predict whether position A or B will give the larger vertical movement. Sketch your reasoning; a prediction is useful even when the result disagrees.

1. Mark a repeatable starting height for the arm tip.
2. Choose a small control-plunger movement that works comfortably at both attachment positions. Measure it in millimetres.
3. At position A, record the arm-tip height before and after that movement. Subtract the starting height from the finishing height.
4. Return to the same starting arrangement and repeat three times.
5. Move only the arm attachment to position B and repeat the measurements. Keep the base attachment, syringes, water filling and control movement unchanged.

Record the attachment distance, trial number, control movement and vertical tip movement, all in millimetres. Add notes for slipping, bending, leaks or jerky motion. Do not quietly discard an awkward result: identify what happened and repeat it after correcting the fault.

Decide what to improve

Compare the three tip movements at each position. If the readings vary widely, improve the attachments and starting-position checks before drawing a conclusion about the geometry.

Judge the design against more than maximum movement. An arrangement that produces a large lift but repeatedly slips may be less useful than one with a smaller, repeatable lift. Likewise, a change that improves movement but bends the cardboard calls for a stronger structure before any payload test.

Finish with a short design statement: “For the same control movement, position ___ produced ___ mm of tip movement across my trials. I would choose it because ___.” Include an observed drawback and the next single change you would test.