An aluminium pneumatic cylinder tube is the extruded barrel that the piston slides in: usually 6063-T5 or 6061-T6, with a precision-finished bore held to H9–H11 and a hard-anodised wearing surface. This guide explains the profile types used for ISO 15552, ISO 6432 and compact cylinders, how to specify the bore, and what to check before you order barrel stock from Metline.
The barrel of a pneumatic cylinder is a pressure vessel, a bearing surface and a mounting frame at the same time. It holds compressed air at typically 6–10 bar, guides a piston seal over millions of strokes, and carries the end caps, tie rods or screws, sensor slots and mounting accessories that hold the whole actuator together.
Aluminium took over this job from steel and brass because an extruded 6xxx-series profile can deliver all of those features in one section. The die forms the bore, the outer shape, the tie-rod or screw channels and the T-slots for reed or magnetic sensors in a single pass, so the machinist only has to cut to length, finish the bore, drill and tap the ends, and anodise. The result is a barrel that weighs roughly a third of an equivalent steel tube, does not rust in humid factories, and lets a magnetic piston ring be sensed through the wall.
Because the piston seal rides directly on the bore, the inside diameter, roundness, straightness and surface finish matter far more than on structural tube. A barrel that is slightly oval, wavy or rough will leak past the seal, wear it quickly or cause stick-slip at low speed. Most of this guide is therefore about how the bore is made and specified.
Almost every aluminium cylinder barrel is a 6xxx alloy. They extrude well into complex hollow sections, can be press-quenched at the extrusion press, and respond well to hard anodising. The choice between them is a balance of extrudability, strength and anodised appearance.
| Alloy and temper | Min. tensile (EN 755-2) | Min. 0.2 % yield | Typical tensile | Where it is used |
|---|---|---|---|---|
| 6063-T5 | 175 MPa (wall ≤ 3 mm) | 130 MPa | 186 MPa | Standard ISO profile barrels and round tubes; best surface and anodised colour |
| 6063-T6 | 215 MPa (wall ≤ 10 mm) | 170 MPa | 241 MPa | Barrels that need a little more strength or a harder base under the anodic film |
| 6061-T6 / T6511 | 260 MPa | 240 MPa | 310 MPa | Larger bores, higher pressures, heavy-duty and hydraulic-pneumatic cylinders |
| 6005A-T6 | 255 MPa (hollow, wall ≤ 5 mm) | 215 MPa | — | Thin-wall complex profiles where 6061 is hard to extrude |
| 6082-T6 | 290 MPa (profile, wall ≤ 5 mm) | 250 MPa | — | European heavy-duty barrels and end caps |
Minimums depend on wall thickness band and are confirmed on the mill certificate. Typical values are from Aluminum Association / ASM data and are not guaranteed.
6063 is the default because it gives the smoothest extruded surface, the most uniform anodised colour and the easiest die design for thin webs and sensor slots. Its lower strength is rarely a limit at normal air pressures: a Ø63 mm barrel with a 2.5 mm wall at 10 bar sees a hoop stress of only about 13 MPa. Move to 6061 or 6082 when the bore is large, the wall is thin relative to the bore, the cylinder is a hydro-pneumatic or high-pressure design, or the barrel also carries significant bending loads from side-mounted guides.
Temper matters for the bore finish too. A harder T6 base supports a hard-anodised film better and is less likely to tear during honing or roller burnishing, while T5 is easier to burnish to a mirror finish. Many cylinder makers standardise on 6063-T5 for ISO profile barrels and 6061-T6 for round heavy-duty tubes.
Square outer shape with integral screw channels and sensor slots, for bores Ø32 to Ø320 mm. ISO 15552 replaced the older ISO 6431 and VDMA 24562 and fixes the mounting interface, not the barrel section, so each cylinder maker has its own profile.
Round bore with four external lobes or grooves that locate the tie rods. Common in SI/SU-style cylinders where tie rods clamp the end caps.
Plain round barrel for ISO 6432 mini cylinders (Ø8–Ø25 mm), round-body cylinders and stainless-end-cap designs. Ends are rolled, crimped or threaded.
Short, square section with through-holes for screws, used for short strokes where installed length is limited (ISO 21287 compact cylinders and similar).
Wide sections with two bores or guide-rod passages for anti-rotation and side-load capacity in pick-and-place and stopper cylinders.
Slotted barrels for band-type rodless cylinders, oval bores and customer-specific actuator housings, made from a new die to drawing.
For square ISO profiles, the bore and the external interface are what the standard controls. Typical external widths run from about 44 mm for a Ø32 bore to about 134 mm for a Ø125 bore, but you should always send the drawing of your own section: two barrels for the same bore from different cylinder makers are rarely interchangeable.
Square and tie-rod profiles are supplied to your drawing. Weight per metre is calculated from the section area on the drawing.
For plain round barrels, the weight per metre follows directly from the outside and inside diameters and the density of 2.70 g/cm³. The table uses common bore/OD pairs found in round-body cylinders.
| Bore (mm) | OD (mm) | Wall (mm) | Weight (kg/m) |
|---|---|---|---|
| Ø16 | Ø20 | 2 | 0.31 |
| Ø20 | Ø25 | 2.5 | 0.48 |
| Ø25 | Ø29 | 2 | 0.46 |
| Ø32 | Ø37 | 2.5 | 0.73 |
| Ø40 | Ø44 | 2 | 0.71 |
| Ø50 | Ø54 | 2 | 0.88 |
| Ø63 | Ø68 | 2.5 | 1.39 |
| Ø80 | Ø86 | 3 | 2.11 |
| Ø100 | Ø106 | 3 | 2.62 |
| Ø125 | Ø133 | 4 | 4.38 |
| Ø160 | Ø170 | 5 | 7.00 |
| Ø200 | Ø210 | 5 | 8.69 |
| Ø250 | Ø262 | 6 | 13.03 |
The bore tolerance is normally given as an ISO 286 hole class. "H" means the bore can be larger than nominal but never smaller, so the seal always has its design squeeze. The number sets the band width. Values below are the standard ISO 286-2 upper deviations (the lower deviation is zero).
| Nominal bore | H8 | H9 | H10 | H11 |
|---|---|---|---|---|
| > 10 – 18 mm | +0.027 | +0.043 | +0.070 | +0.110 |
| > 18 – 30 mm | +0.033 | +0.052 | +0.084 | +0.130 |
| > 30 – 50 mm | +0.039 | +0.062 | +0.100 | +0.160 |
| > 50 – 80 mm | +0.046 | +0.074 | +0.120 | +0.190 |
| > 80 – 120 mm | +0.054 | +0.087 | +0.140 | +0.220 |
| > 120 – 180 mm | +0.063 | +0.100 | +0.160 | +0.250 |
| > 180 – 250 mm | +0.072 | +0.115 | +0.185 | +0.290 |
| > 250 – 315 mm | +0.081 | +0.130 | +0.210 | +0.320 |
An H9 Ø63 bore, for example, may measure 63.000 to 63.074 mm. H9 suits most double-acting cylinders with lip seals. H8 is used where low friction, low leakage or slow, smooth motion is required; H10–H11 can be acceptable for short-stroke or single-acting designs with forgiving seals. Specify roundness and straightness separately: tolerance class alone does not stop a barrel from being oval or bowed within the band.
Agree measuring method and sampling plan on the drawing; the values above are typical industry practice, not a standard requirement.
| Finish | Typical film | Purpose |
|---|---|---|
| Clear (natural) anodising | 10 – 20 µm | Corrosion protection and clean appearance for standard barrels |
| Hard anodising (Type III) | 25 – 60 µm | High hardness and wear resistance in the bore; extends seal and barrel life |
| Black or coloured anodising | 10 – 25 µm | Machine-builder colour schemes; dyed after anodising, then sealed |
| Bead blast + anodise | 10 – 20 µm | Uniform matt outer surface that hides handling marks |
| PTFE-impregnated hard anodise | 25 – 50 µm | Lower friction and better dry-running for low-lubrication cylinders |
Hard anodising slightly reduces the bore diameter because the film grows both into and out of the metal, roughly half each way. Machine the bore allowing for this, or specify the bore tolerance after anodising. Hard anodic films on 6063 and 6061 are typically several times harder than the base metal, which is why they protect the bore so well. Sealing improves corrosion resistance but can lower film hardness slightly, so many cylinder makers leave the bore unsealed or use a light seal.
Aluminium also suits hydro-pneumatic boosters and some low-pressure hydraulic cylinders when 6061-T6 or 6082-T6 is used with an adequate wall. For high-pressure hydraulic service, steel barrels remain the norm; aluminium is chosen there only after a proper pressure and fatigue calculation.
6063-T5 is the most common, because it extrudes complex ISO profiles well and gives a smooth, evenly anodised surface. 6061-T6 or 6082-T6 is used for large bores, thin walls relative to bore, or higher pressures.
H9 to ISO 286 is the usual choice for double-acting cylinders with lip seals. H8 is used for low-friction or precision cylinders, and H10–H11 only for forgiving short-stroke designs. Always state whether the tolerance applies before or after anodising.
Hard anodising creates a dense oxide layer, typically 25–50 µm, that is far harder than the base aluminium. It resists wear from the piston seal and protects against corrosion, extending both seal and barrel life.
ISO 15552 replaced both. It keeps the same mounting interface for bores Ø32–Ø320 mm, so cylinders and accessories remain interchangeable, but it does not define the barrel cross-section, which varies by manufacturer.
Yes. Send the section drawing; if no existing die matches, a new die is made to the drawing. Sample lengths can be supplied for trials before series production.
For a round tube, weight per metre is π × (OD² − bore²) ÷ 4 × 0.0027 kg, with dimensions in millimetres. A Ø63 bore × Ø68 OD tube weighs about 1.39 kg/m. Square profiles are calculated from the drawing section area.
Send your profile drawing, bore tolerance, finish and quantities. Our team will review the section and reply with options and a delivered price.
