Air Cylinder tube and process for producing same
The present invention relates to air cylinder tube for air cylinders, hydraulic cylinders or the like, for example, for use in industrial robots, vehicles and other industrial machines, and to a process for producing the same.
Generally, cylinder tubes of the type mentioned are formed with an abrasion-resistant hard film on the inner surface to withstand the reciprocation of pistons. Conventional cylinder tubes are produced by subjecting the inner and outer surfaces of an aluminum tube to anodic oxidation by the low-temperature sulfuric acid electrolytic process or electrolytic spontaneous color forming process to form a hard or superhard oxide film on the surfaces.
Thus, the conventional cylinder tube has not only on the inner surface but also on the outer surface the hard to superhard oxide film which is usually of black to blackish brown or like dark color and which therefore impairs the appearance of the tube. Since it is impossible to change the color to a light color, the tube has the problem of being unable to meet requirements as to the design. The conventional tube has another problem in that it is expensive to make because the outside oxide film is as thick as the inside film and has a thickness of at least about 20 μm.
With the cylinder tube of the present invention, the outside oxide film has a hardness not lower than 150 HV to less than 350 HV, preferably 150 to 250 HV. The film is semihard or has a usual hardness. The outside oxide film, if less than 150 HV in hardness, is too soft to permit defacement, whereas when not lower than 350 HV, the film assumes a black to blackish brown or like dark color, giving an impaired appearance to the tube, hence undesirable. On the other hand, the inside oxide film has a hardness not lower than 350 HV to not higher than 650 HV, preferably 350 to 600 HV. If the hardness is less than 350 HV, the film will not be fully resistant to abrasion, whereas if it is in excess of 650 HV, the anodic oxidation treatment requires a longer period of time, making the cylinder tube costly, hence undesirable.
The outside oxide film on the cylinder tube is not smaller than 5 μm to not larger than 20 μm, preferably to 15 μm, in thickness. If the thickness is less than 5 μm, insufficient corrosion resistance will result, while thicknesses exceeding 20 μm result in an increased cost and are therefore undesirable. The inside Oxide film has a thickness of not smaller than 10 μm to not larger than 50 μm, preferably 25 to 45 μm. When the thickness is less than 10 μm, the inside oxide film will be lower.,in hardness and abrasion resistance, whereas thicknesses exceeding 50 μm are costly and not desirable.
The outside oxide film thus formed on the cylinder tube is transparent or semi-transparent, giving the tube outer surface one of silver white color and gray color, especially light gray. The inside oxide film has one of black color and brackish brown color.
When desired, the outside oxide film may be further colored blue, green or the like.
The cylinder tube of the present invention has an appearance of light color, silver white or gray, so that the outer surface can be further colored electrolytically or by dyeing in various colors including primary colors to meet design requirements. The outside oxide film, which has a reduced thickness, renders the cylinder tube less costly to make. Nevertheless, the inside oxide film has outstanding abrasion resistance and high durability to fully withstand the reciprocation of the piston.
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