This document is designed to provide familiarity with the necessary auxiliary equipment for the use of air muscles. We suggest a variety of parts from some of our usual suppliers, to make first designs using muscles easier. However, these should not be taken as definitive guides to available parts.


If you're building a system using muscles to provide the motive force, then there are a number of things that will have to be provided:

• A source of compressed air.
• Piping to deliver this air through the system
• Fixtures to terminate air lines on other components of the system, and to each other.
• Air filtering and regulating equipment.
• Air flow control equipment (Valves)
• Muscles
• Mounting points for the muscles

Sources of compressed air for air muscles

Since the air muscle is rated for use between 1 and 6 bar (15 and 90 psi) a range of air sources are suitable. The normal operating pressure of the muscle is about that of a bicycle tyre.

Sources of compressed air:

• Air can be supplied reliably and continuously by a standard workshop compressor. Note that the usual output of such a compressor is around 8 bar, and therefore the supply will need to be regulated near the muscles.
  
• Air can be supplied occasionally or for test purposes by bicycle pumps or car foot pumps (since the volume of air used can be relatively low, it is feasible to use a manual pump, with a reservoir of air, to supply air).
  
• Air can be supplied portably and simply by canisters of liquefied gas, such as photographic gas canisters. However, these are relatively expensive, and if used to supply a large amount of air, tend to freeze.
  
• Whilst air could be supplied from a high-pressure cylinder, we do not recommend this. Compressed air above 10 bar is something to leave to the experts.

Piping to deliver air

All the air in the system is at 8 bar or less, and therefore standard polyurethane or nylon airline piping can be used. For example, the [SMC] Superflex Nylon Tubing, which comes in dimensions from 4 to 12 mm outside diameter, is ideally suited. See [RS components] section: Pneumatic-Accessories-Tube and Airline Coils]

Note that metric air-line piping is usually specified with outside diameters (OD) of 2.5, 4, 6, 8, 12 mm, and if you use piping of different OD, you will have more problems finding compatible connectors.

Note that certain polyurethane tubings, although more flexible, are not rated for use at temperatures above 25°C: since the air from a compressor can be quite warm, it is essential to check this point when ordering airline for use near the compressor, or in applications where a wider environmental range is required

Fixtures to terminate air lines on other components of the system

There are two separate aspects of this problem: connecting to the tube itself, and connecting to a threaded port on a valve or other piece of equipment.

For connecting to the tube itself, the `one-touch' [SMC] or `super-Speedfit' [CompAir Maxam] can be used. The fitting must match the outside diameter of the airline. The airline is firmly pushed into the fitting, where a retaining collet (not shown) holds the air-line in place. The release bushing is pushed in to free the air-line. It is necessary to ensure that the air line used is clean and dry when the connection is made. It is also necessary for the end of the air line to be cut cleanly and squarely; for this purpose, air-line manufacturers also make tubing cutters [SMC:TK-1 or TK-3, Compair: SSTC1, RS:722-198]

Whilst it is often possible to reduce air-line sizes by inserting a larger air-line into a smaller one, this is not reliable; instead, the use of a plug-in reducer [SMC: KQR series, KQR06-12 reduces 12mm air line to 6mm, KQR04-06 reduces 6mm to 4mm.] is preferable.

Fittings will need to be attached to threaded ports on valves, regulators, and other parts. Threads come in many different shapes, and this can pose severe problems. The most obvious difference between methods of threading joints is the sealing. Either the thread is parallel and the seal is made by a washer at the body end of the thread, or the thread is tapered and sealant (typically PTFE tape [available from any plumbing store]) is applied along the thread to seal it. (However, some threads, e.g. Rp and PF threads, are parallel but sealed along the thread!) In the UK, the threads are historically BSP or BSPT (modern thread is specified as G), which is at a 55° angle; in America, thread is to the American NPT and NPTF standards, which is at a 60° angle. (Trying to mix BSP and NPT results in stripped thread and frustration.) Threaded parts are specified by the type of thread (BSP, BSPT, NPT, NPTF, G, ...) which includes whether the thread is tapered or parallel, and the diameter (1/4, 3/8, 1/2, 1/8, ...).

The simple resolution of the thread problem is the SMC Unithread^TM, which all SMC fittings are supplied with as standard. Male Unithread of the correct diameter is compatible with all female thread types: this eases the job of connecting threads immensely, as it is now only necessary to check the size of port (M3,M5, 1/8", 1/4", 3/8", 1/2" being the alternatives).

Air filtering and regulating equipment

A conventional layout for compressed air has the compressor output (8 or 8.5 bar) piped via 12mm piping to as close as possible to the point of use. Regulators are then used as close to the valves as possible.

The air from a compressor or pump is also likely to be relatively dirty, containing oil, water, and other particulate matter. To remove this a condensate remover (probably with manual drain for workshop use or automatic drain for permanent installations) is mounted near the compressor (at a low point in the air line) and a combination filter and regulator is mounted at or near the point of use. Parts we have used in this context are:

• RS 720-978 a pressure regulator & filter. This regulator has G1/4 fittings, necessitating a G1/4 to speedfit connector,
• SMC KHQ12-U02 RS 726-768 is a 12mm to G1/4 connector.
• SMC KHQ06-U02 RS 726-718 is a 6mm to G1/4 connector.
• RS 721-088 is a suitable gauge for the regulator.

Miniature regulators can be useful in some systems; if the air supply is of good quality (i.e. already filtered) then a preset regulator [SMC ARJ1020F series] can be used, resulting in a lighter and more compact solution.

Air flow control equipment

There are a wide variety of valves available, many of which are suitable. However, considerations of weight, size and mounting will usually determine the choice. Normally-closed 2/2 water solenoid valves [RS 342-023] are cheap, and have high throughput. These valves have 1/2inch BSP threaded ports. A convertor from 1/2BSP to air line [SMC KQF12-04, RS similar 218-1757, convert male 1/2BSP to 12mm air line] permits connecting this to other air-line fittings as required. A 1/2-inch BSP equal socket [RS 789-842, or a plumber's merchant], can be drilled and tapped to accept a 1/4-inch BSP thread, to connect two valves together, providing fill and empty control; however, this requires expertise in thread cutting. It is simpler to fit a BSP to speed-fit convertor, [SMC KQF12-04] two short pieces of 12mm tubing, and a 12mm union Tee [SMC KQT12-00, RS 727-121] which will then allow a 12mm air line to be run to the muscle. Note here that the air in the tube between the valves and the muscle is dead air: it contributes nothing to the performance of the system, except to make the muscle a little springier, and delays the actuation of the muscle.

The Mead Isonic valve [Middlesex], which has Speedfit-type connections on the body of the valve, can also be used. These valves are a plastic-bodied valve, made with great precision, and offer advantages for air use over other valves.

Switching of Valves

Solenoid valves are usually actuated from 12V DC, 24VDC or 240V AC. In most electrical usage, it will be most convenient to use 12V or 24V solenoids, since this reduces the complexity of the driver hardware; 24V DC is the standard for process control electronics. However, if the air quality is poor, or the valves are only driven occasionally, 240V AC solenoids may prove more reliable.

Control boards to switch solenoid valves from computers are widely available; we do not make any specific recommendations.

Muscles

The Shadow Air-muscles come in a variety of standard sizes.. The muscle is mounted under tension, and compressed air applied at between 2 and 6 bar. It will contract by around 25% of its length. If it does not, then it might not have been under enough tension initially (pull the muscle as hard as possible) or it might be under too much loading (try a larger size of muscle!).

Shadow Air Muscle Range

Thumbnails / Products	Braid Diameter	Length	Air fitting size	Pull at 3.5bar	Maximum Pull
	6mm	150mm  (Stretched)	4mm	3 kg	7kg
	20mm	210mm  (Stretched)	4mm	12 kg	20kg
	30mm	290mm  (Stretched)	6mm	35 kg	70kg
Please note: Muscle lengths differing from the list below can also be supplied.

Important points to watch in the use of muscles

Air quality  Because of the construction of the muscles, they will suffer if oil is allowed into them. Prevent this by properly filtering the incoming air.

Bedding In Because the muscles are made of rubber, there is an initial period in which the rubber `beds in'. The first few times it is used, it will stretch by a small percentage. Also, as the muscle becomes warm in use, it will stretch slightly.

Chafing Because of the braiding, if muscles are allowed to chafe against hard or pointed objects, over time the braiding will distort, and the muscle will balloon through the braiding and eventually burst. Prevent this by ensuring that the muscles are free to move.

Environmental conditions We do not recommend that standard muscles be use in ambient conditions exceeding the 0-50°C range. Muscles can be washed with water or mild soap, but harsh detergents are not recommended.

Puncture The lightweight construction of the muscle means that it is vulnerable to sharp objects. We are currently investigating armouring techniques for muscles. Note, however, that muscles do not necessarily suffer catastrophic failure. In the event of a puncture, the muscle will often leak air noisily, whilst continuing to function at lower efficiency.

Pressure The muscles are rated for use up to 6 bar (90psi) when under load. If the muscle is not loaded, i.e. is not under tension, then it should not be taken above 2 bar (30 psi).

Mounting points for the muscles

The muscle is best used to move a lever. Its simplest configuration is when attached with a return spring on opposite sides of a pivot point. Simple installations often manage with elastic bands as return springs!

The muscle is used to raise the lever; the elastic band acts to return it. Note that the muscle is used for the motion that requires greater force: the return for the lever is gravity-assisted. This is a common characteristic of simple muscle designs: the muscle is used to provide the power stroke.

The muscle operates with great strength in the first part of its contraction, but exerts less force as it contracts further. A trade-off can be used between stroke length and force: in the case of the lever above, mounting the muscle closer to the pivot point increases the motion of the end of the arm, but reduces the weight that can be raised:

Conversely, mounting the muscle further away from the pivot point increases the load that can be raised, but decreases the length of the stroke.

The actual attachment of the muscle can be done in several ways. The smallest sizes of muscles have loops at the ends, and these can be simply mounted over bolts or hooks on the machine. If the device is carefully made, and the muscle motion required is known exactly, then mounting bolts can be placed in the body, and the muscle end-loops placed over them.

This technique, shown at the right end of the muscle above, requires the mounting points to be exact; however, it allows for simple replacement of a muscle at the end of its life: the muscle need only be slipped over the bolt, and another placed there.

The alternative technique, shown at the left end of the muscle above, is to use a cable-tie, wire or string to attach the muscle. This can then be adjusted to produce the optimal tension in the muscle. This is great for prototyping with muscles: the system can be constructed very quickly, and the necessary muscle adjustment discovered.

The larger sizes of muscles exert considerably greater forces, and so are not suited to a simple attachment technique like this. Instead, our current designs come with screw-thread ends. These are designed for a firmer anchoring, whether directly bolted to the frame, or attached using high-strength ropes.

Caution: When using muscles to supply large forces (at high pressure, or muscles of large diameter), use appropriately specified fittings.

Adjusting the muscle attachment

Because of the motion of the muscle, it is necessary to carefully adjust the system for best results.

When the muscle is at rest, it should not be limp, but rather taut. This ensures that as the muscle is filled, it begins to move as soon as possible.

As the muscle fills, it contracts. Careful placement of the muscle end about the pivot can make significant differences to the range of motion available.

Once the maximum load on the muscle is known, and the pressure at which it is to be driven is known, then the muscle can be adjusted so that the contraction that the muscle provides is fully utilised in the motion of the pivot. If the muscle cannot be made to reach its full expected contraction, then increasing the pressure slightly, or moving the muscle closer to the pivot, will often help; however, the pressure limits of the muscles must be respected!

Measuring the distance from the mounting point of the muscle when the lever is down and the distance when the lever is up gives the amount of contraction the muscle is expected to produce. Then, this can be compared with the load on the system to predict the pressure required.

Appendix: Sources of supply

RS Components distribute most of the necessary pneumatic and electrical components in their Mechanical catalogue.
RS Components Ltd
PO Box 99
Corby
Northants
NN17 9RS

Tel: 01536 444111
Fax 01536 405415 SMC UK OR SMC Europe produce an excellent range of pneumatic components.
SMC Pneumatics (UK) Ltd
Vincent Avenue
Crownhill
Milton Keynes
MK8 0AN


as do Compair Maxam

CompAir Maxam
Carn Brea
REDRUTH
Cornwall
TR15 3PR

Tel: 0209 712712
Fax: 01209 312579

Photographic duster cans:
Process Supplies (London) Ltd
13-25 Mount Pleasant
London
WC1X 0AR

Phone: 020 7837 2179
Fax: 020 7837 8551
www: www.process-supplies.co.uk