I needed to create some actuators for an animatronics project I'm working on. Air muscles are very powerful actuators that work very similar to a human muscle and have a phenomenal strength to weight ratio- they can exert a pulling force up to 400 times their own weight. They will work when twisted or bent and can work under water. They're also easy and cheap to make!Air muscles (also known as a McKibben artificial muscle or braided pneumatic actuators) were originally developed by J.L. McKibben in the 1950's as an orthotic appliance for polio patients. Here's how they work:The muscle consists of a rubber tube (bladder or core) that is surrounded by a tubular braided fiber mesh sleeve. When the bladder is inflated the mesh expands radially and contracts axially (since the mesh fibers are inextensible), shortening the overall length of the muscle and subsequently producing a pulling force. Air muscles have performance characteristics very similar to human muscles- the force exerted decreases as the muscle contracts. This is due to the change in the interweave angle of the braided mesh as the muscle contracts- as the mesh expands radially in a scissors like motion it exerts less force due to the weave angle becoming increasingly shallow as the muscle contracts (see the diagram below- figure A shows that the muscle will contract to a greater degree than figure C given an equal increase in bladder pressure).The videos show this effect as well. Air muscles can contract up to 40% of their length, depending on the method and materials of their construction.Gas law states that if you increase pressure you also increase the volume of an expandable cylinder (provided temperature is constant.) The expanding volume of the bladder is ultimately constrained by the physical properties of the braided mesh sleeve so in order to create a greater pulling force you need to be able to increase the effective volume of the bladder- the pulling force of the muscle is a function of the length and diameter of the muscle as well as its ability to contract due to the properties of the mesh sleeve (construction material, number of fibers, interweave angle) and bladder material. I constructed two different sized muscles using similar materials to demonstrate this principle- they both were operated at the same air pressure (60psi) but had different diameters and lengths. The small muscle really starts to struggle when some weight is put on it while the larger muscle has no problems at all. Here are a couple of videos showing both of the constructed air muscles in action. Now let's go make some muscles!
Step 1: Materials
All of the materials are readily available on Amazon.com, with the exception of the 3/8" braided nylon mesh- it is available from electronics suppliers. Amazon does sell a braided sleeving kit with several sizes of braided mesh but the exact material is not stated-AmazonYou'll need an air source:I used a small air tank with a pressure regulator but you can also use a bicycle air pump (you will have to make an adapter to make it work with the 1/4" poly hose. Air tank- AmazonPressure regulator (will require a 1/8" NPT female to 1/4" NPT male adapter)- Amazon1/4" high pressure poly tubing- Amazonmultitool (screwdriver, scissors, pliers, wire cutters)- Amazonlighterfor the small muscle:1/4" silicone or latex tubing- Amazon3/8" braided nylon mesh sleeve (see above) 1/8" small hose barb (brass or nylon)- Amazon small bolt (10-24 thread by 3/8 in length works well)- Amazon steel safety wire- Amazonfor the large muscle:3/8" silicone or latex tubing- Amazon 1/2" braided nylon mesh sleeve- Amazon 1/8" or similar sized drill bit- Amazon21/64" drill bit- Amazon1/8" x 27 NPT tap- Amazon 1/8" hose barb x 1/8" pipe thread adapter- Amazonsmall hose clamps- Amazon3/4" aluminum or plastic rod to construct the muscle ends- AmazonSafety note- make sure you wear safety glasses when testing your air muscles! A high pressure hose that pops off a loose fitting could cause a serious injury!
Step 2: Making the small muscle
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First cut a small length of the 1/4" silicone tubing. Now insert the small bolt into one end of the tubing and the hose barb into the other end.
Now cut the 3/8" braided sleeve about two inches longer than the silicone tube and use a lighter to melt the ends of the braided sleeve so it doesn't fray apart.
Slide the braided sleeve over the silicone tubing and wrap each end of the tube with the safety wire and tighten it.
Now make some wire loops and wrap them around each end of the braided sleeve. As an alternative to using wire loops on the ends of the muscle, you can make the sleeve longer and then fold it back over the end of the muscle, forming a loop ( you have to push the air fitting through)- then tighten the wire around it.
Now connect your 1/4" high pressure tubing and pump a little air into the muscle to make sure it inflates without leaking.
To test the air muscle you have to stretch it to its full length by putting a load on it- this will allow it maximum contraction when it's pressurized. Start adding air (up to about 60psi) and watch the muscle contract!
Step 3: Making the large air muscle
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To make the large muscle I turned some barbed ends from some 3/4" aluminum rod- plastic will also work. One end is solid. The other end has a 1/8" air hole drilled in it and then is tapped for a 1/8" hose barb pipe thread adapter. This is done by drilling a 21/64" hole perpendicular to the 1/8" air hole. Then use a 1/8" pipe thread tap to tap the 21/64" hole for the hose barb fitting.
Now cut a 8" length of 3/8" rubber tubing for the air bladder and slide one end over one of the machined fittings. Then cut some 1/2" braided sleeve 10" long (remember to melt the ends with a lighter) and slide it over the rubber tube. Then slide the opposite end of the rubber tube over the remaining machined air fitting. Now securely clamp each end of the tubing using hose clamps.
The larger muscle works just like smaller version- just add air and watch it contract. Once you put it under load you'll immediately realize this larger muscle is much stronger!