How an air cylinder behaves

This page gathers the points to settle first when choosing an air cylinder. The thrust can be calculated, but the speed it can move at and the temperature it can work in differ from maker to maker, so it separates what the calculation settles from what only a catalogue can settle.

Working out the thrust

The force the compressed air puts on the piston is the pressure area times the pressure. On the extend stroke the whole face of the piston takes the pressure.

A = \pi \left(\frac{D}{2}\right)^2 \qquad F = A \times P

$D$ is the tube bore [mm] and $P$ the working pressure [MPa]. Because 1 MPa = 1 N/mm², multiplying the area [mm²] by it gives the force [N] directly.

On the retract stroke the cross-section of the rod is subtracted. That is why, at the same pressure, the retract thrust is the smaller of the two.

A' = \pi \left(\frac{D}{2}\right)^2 - \pi \left(\frac{d}{2}\right)^2

Load factor

A design does not use up the whole theoretical thrust. Friction and the delay in the pressure building up mean that a margin is needed. How that margin is taken is the Load factor $\eta$ .

Static clamping or slow motion: about 0.7
General transfer: about 0.5
Fast motion: about 0.35
With impact: 0.3 or less

Mass a bore can lift

The table applies the formula above to give a guide to the mass each bore can lift vertically. You can change the working pressure and the load factor to see the effect.

使用圧力 P [MPa]

負荷率 η

チューブ内径 D [mm]	受圧面積 A [mm²]	理論推力 F [N]	負荷率を掛けた推力 [N]	持ち上げ質量の目安 [kg]
2.5	4.909	2.454	1.718	1.752e-1
4	12.57	6.283	4.398	4.485e-1
6	28.27	14.14	9.896	1.009
8	50.27	25.13	17.59	1.794
10	78.54	39.27	27.49	2.803
12	113.1	56.55	39.58	4.036
16	201.1	100.5	70.37	7.176
20	314.2	157.1	110	11.21
25	490.9	245.4	171.8	17.52
32	804.2	402.1	281.5	28.7
40	1257	628.3	439.8	44.85
50	1963	981.7	687.2	70.08
63	3117	1559	1091	111.3
80	5027	2513	1759	179.4
100	7854	3927	2749	280.3
125	1.227e+4	6136	4295	438
140	1.539e+4	7697	5388	549.4
160	2.011e+4	1.005e+4	7037	717.6
180	2.545e+4	1.272e+4	8906	908.2
200	3.142e+4	1.571e+4	1.100e+4	1121
250	4.909e+4	2.454e+4	1.718e+4	1752
300	7.069e+4	3.534e+4	2.474e+4	2523

The figures are a guide for lifting vertically. The thrust needed to move a load horizontally follows from friction and from the acceleration, not from the mass, so this table cannot give it.

Check these in the catalogue

The following differ from maker to maker and from series to series. They cannot be settled by calculation, so check the catalogue for the series you are using.

Minimum operating pressure: the lowest pressure at which the cylinder starts to move. It is set by the friction of the seals, so it differs from series to series. It matters when you work at low pressure
Speed range: the upper and lower limits on speed. If you work at low speed, check the lower limit at which stick-slip (juddering) does not appear
Ambient and fluid temperature: set by the material of the seals. A low-temperature or high-temperature version is sometimes offered separately
Life: given as a travel distance. It is usually published together with the test conditions, and a different condition gives a different result. Note that it is not a guaranteed value
Allowable load at the rod end: the limit on the side load that can be put on the rod. It falls as the stroke gets longer

Air cylinder selection

Sizes a cylinder from its specification by working out thrust, absorbed energy and buckling.

How an air cylinder behaves

Working out the thrust

Load factor

Mass a bore can lift

Check these in the catalogue

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