homemade lego techic pneumatic air muscle lifting 550g!

Homemade Lego Pneumatic Air Muscle

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see also:
Lego Pnuematics
  original air muscle

Homebrew science
  Air Muscles

How I made a Lego compressed air muscle.


In the process of building J5, and researching how to make his air systems controllable off a Lego Mindstorms RCX, I found air muscles. An air muscle works by compressed air inflating a rubber tube inside a braided sleeve. As the tube expands, the sleeve expands in diameter and gets shorter... the same sort of way that our muscles work.

An air muscle can be very powerful for its weight, commercially available air muscles are capable of lifting 400 (or more) times their own weight over 30% of their length.

J5, built out of Lego, is going to be about 1m tall... for a Lego mobile construction this will mean a weight to strength ratio close to shaking itself to bits. The best solution would be to have all the weight (motors, batteries, etc) in the lower body. Currently there are 2 Lego technic 9v motors in the head controlling the eyelid movement... the 2 motors weigh in at over 100g. This weight makes the head and neck rock widly whenever the head moves. An air muscle could be the solution to reducing J5's upper body weight especially in the head and arms.

Click on a picture to enlarge it in a new window.

Caution: messing about with compressed air can be very dangerous. Although the air pressures are relatively low with Lego Pneumatics, I have had hoses pop off of joints and whip. The use of eye protection is highly recommended.

Materials for the air muscle.

Materials for the Lego air muscle. Click to enlarge in a new window. Materials for the Lego air muscle. Click to enlarge in a new window.
  • 3/32" (3mm ish) aluminum tube at least 30mm long.
  • a short length of Lego Pneumatic hose.
  • 4mm dia aluminium rod, about 110mm length
  • a large paper clip
  • small file (metal nail file is ok)
  • a bottle of latex liquid rubber (air drying).
  • 5mm dia braided (expandable) sleeving, about 85mm length.
  • 1 Lego Technic grey piston rod.
  • 1 Lego Technic 3 long axle.
  • 1 Lego Technic bush.
  • 2 small cable ties.
  • B&Q Poly Power Glue (not shown.)


  • needle nose pliers
  • junior hack saw
  • a craft knife
  • hobby vice (not essential)
  • thin tipped centre punch
  • hobby drill
  • hobby drill stand (not essential)
  • cocktail stick
  • cling film
  • a small sheet of plastic (ice cream tub)
  • Blutac
  • talcum powder
  • rags or tissues

Safety equipment.

  • Eye protectors
  • Latex gloves

Components and build.

Making the components.

Cut the aluminium tube into 2 lengths, 10mm, 20mm.

Tidy up the ends of each section of tube.

The 4mm aluminium rod will become the mould for the rubber tube. File 2 notches in either end of the rod, and re-shape the paper clip so it grips the notches in the rod. The paper clip will be used to dip and hang the rod, so make sure it's tight... it may be a good idea to have a jug and funnel standing by just in case the rod drops into the latex!

Dip the aluminium rod in the latex so that it gets completely covered... lift out and drain for a few seconds... and then hang to dry. Follow the instructions on the bottle for dipping time (usually you can re-dip after 20 - 25 mins)... the tube needs to be dipped 4 times.
To help prevent the tube getting thicker at one end put the paper clip on the other end before each dip. After the final dip, leave to cure for at least 12 hours.

Slide the bush into the middle of the 3 long axle, and drill a hole at right angles to the axle and slightly bigger than the aluminium tube through the center.

Using a small file, prepare both small aluminium tubes.
Rough up half of the 10mm tube and the middle third of the 20mm tube.

Wrap the Lego Technic piston rod in cling film. The holes on the sides need to be well wrapped.

Caution: B&Q Poly Power Glue is harmful.
The use of protective gloves is strongly recommended

Mix up a small amount of the Poly Glue (on the small plastic sheet with the cocktail stick), this will be used to set the short lengths of aluminium tube.
I use B&Q's Poly Power Glue for alot of things... it's stonger than epoxy resin... and it can bond flat aluminium sheets together, the aluminium bends before the glue gives up!

Using the cocktail stick, fill the piston rod axle (x) hole with Poly Glue, try not to get air bubbles trapped in the glue. Next insert the roughed up end of the 10mm aluminium tube into the hole. Set the piston rod on a piece of blutac so that the aluminium tube remains vertical.

Use a small piece of blutac to cover one end of the 20mm aluminium tube. Push some Poly Glue into the hole in the axle/bush and push the 20mm aluminium tube through. Dab glue all around the bush and aluminium tube, leaving at least 5mm of tube at both ends free of glue. Wipe the end that was pushed through the axle, and leave to cure for at least 12 hours.

Once the latex has cured, run the craft knife around the tube to trim off the ends. Remove the ends. Talc stops the latex from sticking to itself and will be needed as a release agent. Use plently of talc and patience to remove the tube from the aluminium mould.

Cut the latex tube to the required length. I'm still trying to work out the optimum length for the sleeving I'm using, and this model uses 85mm length for the latex tube and sleeving.

I used a fine tipped centre punch to flare the ends of the aluminium tube.

Cut 2 5mm lengths of the Lego Pneumatic hose, these will be used as spacers over the aluiminium tube for the rubber tube and expandable sleeving.

Assembling the air muscle.

Put the 5mm Lego Pneumaitc hose and latex tube onto the flared aluminium pipe in the piston rod. Slide the sleeving over the latex tube, and clamp the ends with a cable tie. Slide the other 5mm piece of Lego hose over one of the flared aluminium tube ends from the axle/bush piece and insert into the latex tube, line up the sleeving and cable tie the ends.

Testing the air muscle.

Caution: the use of eye protection is strongly recommended.

The first test is to find out the perfomance (or failure) under extreme conditions. A Lego Pneumatic pump is connected to a switch and the air muscle. The air muscle is at maximum contraction (20mm) after 8 pumps... 9 pumps and air escapes from the pump or a hose pops off.

The test rig for the dead lift test consists of a vice and 2 500g reels of wire... 1kg

The air muscle lifted the 1kg weight 17mm!

I did try adding weight, however the air muscle was pulled apart.

Weighing in at 2.73 grammes and capable of lifting 1kg means this little air muscle is capable of lifting over 360 times its own weight.


I feel that my objective of building an air muscle compatible with the Lego Technic Pneumatic elements has been acheived.

I am currently making and testing various sizes of air muscle to be used in the construction of J5

If you try this at home, take care a whipping hose can easily cause eye damage...
take sensible precautions...
you have been warned!


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