Pneumatics - Connections

The connections are just as vital as the larger items such as the cylinders and valves, but are overlooked often. There are a bewildering array of them to choose from; everything from plumbers connections through to very high pressure stainless steel fittings. For the main circuit, where you will be using around 10 bar normally, regular off the shelf low pressure fittings and tubes are fine. The picture here shows a selection. The two at the top are push-fit plastic fittings. These are great because they fit together so easily - just cut the pipe and push it into the neck of the connection. You can easily re-configure the system just by unplugging everything and starting again. This is useful when re-routing is needed in a robot.

The two on the left of the picture are a female-female and male-male connector. The male-male is of the brass screw-connection type. You can get all types of adaptors and connectors. For normal low pressure pneumatics these are sized using an imperial system. G1/8, G1/4, G1/2, etc. The 'G' is a type of parallel thread - ISO228 (BS2779/DIN259). The fraction is the diameter of the connection. G1/4 is a 1/4" nominal inside diameter. There are also terms such as NPT (North American Parallel Thread) and BSP (British Standard Parallel Thread). Most pneumatics are BSP. NPT and BSP are not compatible. NPT is often used in hydraulic systems. You may also see the terms extended: eg. BSPT or BSPP. 'T' means 'tapered' and 'P' means 'parallel'. This is just referring to the sides of the threaded section - do they converge or remain parallel. These terms can be very confusing at first, so choose carefully.

The final three fittings are brass and steel barbed fittings. These you push the tube over the top of the barbs to fit the tube. Generally you cannot pull the tube off and push it back on again as the pushing causes the tube to change shape. This means you would use them for semi-permenant connections only. You can increase the pressure they can contain to some extent by using hose-locks to hold the connections in place.

The tubing comes in a variety of colours and materials. Most popular is the nylon tube (seen here in black and blue) which can easily be cut using a knife or cutting tool, can be bent without causing kinks or squashes and is relatively cheap. I bought one 25m roll of this and it's still going strong. This is suitable for all low pressure pneumatics and all of the fittings in the top picture. It cannot be used for high pressure pneumatics at all. The device on the end of the black tube is a resistor (see the cylinder section). The top part fits in the port and the bottom end is the screw that you use to adjust the 'resistance'. When choosing tubing, you need to consider how much airflow you require and what the fittings on your valves and cylinders are. Although you can get convertor fittings, there is no point having a G1/2 port input and using G1/8 tube as the tube will never deliver enough airflow. Likewise the reverse is true - if the tube is bigger than the port, all the airflow will be held up by the bottle neck in the port. Below is a table of the theoretical airflow (dm³/s in free air) through nylon tubing for an air velocity of 30 m/s.

Tube size Line pressure (bar)

o/d bore 1 2 3 4 5 6 7 8 9 10

4mm 2.5mm 0.293 0.438 0.583 0.729 0.874 1.02 1.16 1.31 1.46 1.60

5mm 3.3mm 0.510 0.763 1.02 0.27 1.52 1.78 2.03 2.28 2.54 2.79

6mm 4mm 0.749 1.112 1.49 1.87 2.24 2.61 2.98 3.35 3.73 4.10

8mm 5.5mm 1.42 2.12 2.82 3.53 4.23 4.93 5.64 6.34 7.05 7.75

10mm 7mm 2.29 3.43 4.57 5.71 6.85 7.99 9.13 10.3 11.4 12.6

12mm 8.5mm 3.38 5.06 6.74 8.42 10.1 11.8 13.6 15.1 16.8 18.5

3/16" 1/8" 0.472 0.706 0.941 1.18 1.41 1.64 1.88 2.11 2.35 2.58

5/16" 7/32" 1.45 2.16 2.88 3.60 4.32 5.04 5.75 6.47 7.19 7.91

3/8" 1/4" 1.89 2.83 3.76 4.70 5.64 6.58 7.52 8.45 9.39 10.3

1/2" 3/8" 4.25 6.36 8.47 10.6 12.7 14.8 16.9 19.0 21.1 23.2