Jaws of life school project, Grade 7: how to make it, step by step

The jaws of life school project is the Grade 7 Technology project about hydraulics, part of Mechanical Systems and Control. Learners build a model of the rescue tool that firefighters use to free people trapped in a crashed car, and drive it with water in two syringes. It is a good project because every part does a job you can point at: the syringes move the water, the water moves a piston, and a linkage turns that push into jaws that close.

This guide covers what the real tool does, how the syringe model works, the build in eleven steps, and the part the marks are actually for — explaining why it works.

What is the jaws of life?

The jaws of life are hydraulic rescue tools. Firefighters and rescue teams use them to cut and force open crashed cars, so that people trapped inside can be lifted out safely. The two best-known tools are the cutter, which bites through metal such as a car's roof pillars, and the spreader, which forces metal apart, for example to open a jammed door. Both are driven by a pump that pushes hydraulic fluid into the tool at very high pressure. "Jaws of life" started as a brand name and is now the everyday name for all of these tools.

Watch the whole build first

The 3D animation that comes with the Slotbok kit: every part going on in order, then the cutter and the spreader working. Two and a half minutes, worth watching once before you open the box.

The same build plays full-screen on the jaws of life assembly video page.

What is in the kit

The Slotbok jaws of life kit is cut by laser from 3 mm MDF and builds a cutter 345 × 140 × 62 mm, with jaws that open to 147 mm:

• 82 laser-cut pieces on four 200 × 300 mm sheets, every part labelled
• Two 20 ml syringes and a silicone tube
• Four bamboo dowels: the jaw pivot, the crosshead pin and two jaw pins
• Wood glue and a sandpaper strip
• A set of cutting jaws and a set of spreading jaws

You add the water yourself; tap water is fine.

How the jaws of life model works

Push on a liquid in a closed space and the pressure spreads equally through all of it. That is the whole principle of hydraulics, and the model uses it in five moves:

1. You push the master syringe. This is the one in your hand.
2. Water drives the slave piston. The water cannot squash, so it flows down the tube and pushes out the plunger of the slave syringe inside the tool.
3. The crosshead slides forward. It is clipped onto the slave plunger and slides inside the body.
4. The link arms swing. Four link arms join the crosshead to the back of the jaws.
5. The jaws bite shut. Each jaw turns on the pivot dowel, so the forward push of the crosshead becomes a closing movement at the tips.

Pull the master plunger back and the same chain runs in reverse: the water flows back, the crosshead slides back, and the jaws open.

Why water and not air

Air squashes; water does not. Fill the syringes with air and most of your push goes into compressing it: the jaws move late, move less and feel spongy. Water passes nearly all of the movement through to the other syringe. A system that uses a liquid is hydraulic; one that uses a gas, such as air, is pneumatic.

Try it: leave a bubble in the tube and work the jaws. The sponginess you feel is the bubble being squashed, and that one observation is worth a paragraph in the write-up.

Levers and linkages in the jaws

Each jaw is a lever, and the pivot dowel is its fulcrum. The link arm pushes on the jaw close to the pivot, so a small movement there becomes a big movement at the tip, which is why the jaws open so wide. The trade is force: the grip is strongest close to the pivot and weakest at the tips, just like a pair of scissors. That is why a real cutter bites thick metal deep in its jaws.

The crosshead and the link arms together are a linkage: they turn the straight push of the piston into the turning movement of the jaws.

Force and distance

Both syringes in the kit are 20 ml, so they move one-for-one: push the master plunger 2 cm and the slave plunger moves 2 cm. With a smaller master syringe driving a larger slave, the force on the jaws would go up but they would move less; swap them round and you get the opposite. That trade is the heart of hydraulics, and saying what would happen with different syringe sizes is the easiest way to show you understand it.

How to make the jaws of life, step by step

The build follows the kit's 3D animation: ten steps to the cutter, a test, then one more step for the spreader. Before you start, press every part out of its sheet with the press-out tools, and sand the tabs lightly so they slide in rather than being forced.

1. Glue the crosshead

Stack the 12 crosshead blocks and glue each layer, keeping the T-slots lined up. Wipe off any glue that squeezes into the slots: the crosshead has to slide freely later.

2. Base and first side

Lay out base plate 1 and slot side wall 2 into it.

3. Syringe holders F and G

Drop holder F into its slot, then holder G behind it. They hold the slave syringe still, so that only its plunger moves.

4. Syringe and crosshead

Set the slave syringe on its side and slide nose piece E onto the barrel. Clip the crosshead onto the plunger disc, then lower the whole unit into the body.

5. Second side

Slot side wall 3 onto the base tabs.

6. Top cover

Press cover 4 down onto the tabs. The body is now closed, with the slave syringe inside.

7. Jaw plates on the pivot

Turn the body onto its side. Stack four spacer washers, then the four cutting jaws, then four more spacers on top, and push the pivot dowel through with a lock washer on each end.

8. Link arms

Fit the four link arms, two per side, and push the crosshead dowel through the link ends.

9. Jaw pins and spacers

Fit the upper jaw pin with its column of spacers, then the lower jaw pin the same way. Trim the dowels flush.

10. Master syringe and tube

Push the tube onto both syringe nozzles. Fill the circuit with water and squeeze out every bubble of air.

Test the cutter

Push the master plunger and the jaws close. Pull it back and they open wide. That is hydraulics: the water does the work.

11. Swap to the spreading jaws

Pull out the jaw pins and the link arms, fit the four spreading jaws, and rebuild the pins, spacers and links. Now push the plunger and the jaws force apart: you have built a spreader.

If it does not work properly

• Spongy, slow or late — there is air in the circuit. Take the tube off, refill with water and squeeze the bubbles out.
• Water leaking at a nozzle — the tube has to be a tight push fit on both nozzles. A loose tube loses pressure and the jaws go weak.
• The crosshead sticks — check for dried glue in the T-slots, and that the top cover is pressed fully down and square.
• The jaws bind — something on the pivot is too tight. The jaws should turn freely on the dowel; ease tight parts with the sandpaper.

What to write down while you build

Most of the marks for a Technology project are for showing what you did and what you worked out. Note these as you go:

• How far the crosshead moves for one full push of the master syringe. The ruler engraved along the side of the body makes this easy to measure.
• How many millilitres of water you pushed, and how far the jaw tips moved.
• What happens to the feel of the jaws when there is a bubble in the tube.
• Where each jaw acts as a lever, where its fulcrum is, and where it grips hardest.
• How the spreader differs from the cutter, and which rescue job each one would do.
• Your design brief, specifications and constraints in your own words: who the tool is for, what it must do, and what limited your design.
• Anything that went wrong and what you did about it. This is the easiest part of a report to write and the one most learners leave out.

A photograph at the end of each step 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.

Frequently asked questions

What is the jaws of life used for?

To rescue people trapped in vehicles after a crash. Rescue teams use the cutter to cut through a car's pillars and roof, and the spreader to force doors open or push metal away from the people inside.

How does the jaws of life work?

A pump pushes hydraulic fluid into a cylinder in the tool. The fluid pushes a piston, and a linkage turns the piston's straight push into the closing or opening of the jaws. The syringe model works the same way, with your hand as the pump and water as the fluid.

Is the jaws of life hydraulic or pneumatic?

Hydraulic. It uses a liquid, because a liquid does not squash, so the push reaches the jaws without being lost. Some newer tools carry a battery to run the pump, but the jaws are still moved by hydraulic fluid. A pneumatic tool would use compressed air, which is springier.

How do you make a jaws of life with syringes?

Join two syringes with a tube, fill them with water and push out the air. Fix one syringe inside the body of the tool so that its plunger drives a sliding piece, link that piece to two jaws that turn on a pivot, and keep the other syringe in your hand. The eleven steps above show how the Slotbok kit does it.

What materials do you need for the jaws of life project?

Two syringes of the same size, a tube that fits their nozzles tightly, water, and a stiff material for the body and the jaws. The Slotbok kit uses laser-cut 3 mm MDF with bamboo dowels as pins. If you build one from scratch, thick cardboard or ice-cream sticks, kebab sticks or split pins for the pivots, and wood glue are the usual choices.

Where can I buy a jaws of life school project kit?

The Slotbok jaws of life kit is cut in our workshop in Gauteng and delivered across South Africa, with every part, both syringes and the tube in the box.

What information do I need for the jaws of life school project?

Four things: what the real jaws of life is and who uses it, how hydraulics works (a liquid does not squash, so a push at one end comes out at the other), how the levers and the linkage in the jaws turn that push into a bite, and how your own model was built and tested. The sections above cover each of them, in that order.

What should my jaws of life Grade 7 project answers explain?

The questions on a jaws of life task usually ask why the system uses water and not air, what an air bubble in the tube does, where the fulcrum of each jaw is and where it grips hardest, how the cutter differs from the spreader, and what would change with syringes of different sizes. Answer them from your own model: measure, test and write down what you saw.

Where can I find pictures for my jaws of life project?

The best pictures for the report are your own: a photograph at the end of each build step and one of the finished model working. The pictures on this page show the model’s parts and how it works, and your textbook will have photographs of rescue teams using real cutters and spreaders.

Afrikaans terme

If you are doing this project in Afrikaans: Graad 7 Tegnologie (Grade 7 Technology) · skoolprojek (school project) · meganiese stelsels en beheer (mechanical systems and control) · hidroulies (hydraulic) · pneumaties (pneumatic) · spuit (syringe) · buis (tube) · suier (piston / plunger) · druk (pressure) · krag (force) · hefboom (lever) · spilpunt (pivot) · meganiese voordeel (mechanical advantage).

Watch the jaws of life build on TikTok and Instagram

The same build as a short vertical video, made for your phone. Follow Slotbok on TikTok, Instagram and YouTube to see each new kit as it comes out.

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.

• Jaws of Life School Project Kit — Grade 7
• All Grade 7 Technology projects

Other build guides: electromagnetic crane kit, Grade 7 · cell phone tower kit, Grade 7 · hydraulic arm kit, Grade 9, another syringe hydraulics project · bridge kit, Grade 9.

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