FIELD OF THE INVENTION This invention relates to methods for manufacture of hydraulic and pneumatic cylinders and plungers with polymer coating, and specifically to cost-effective methods for mass production of such devices.

BACKGROUND OF THE INVENTION Hydraulic and pneumatic cylinders which convert liquid or gaseous pressure and movement of mechanical into one another by means of plungers moving back and forth in cylinder housings are quite common and have been known for a long time. Such machines work as motors when the pressure of the hydraulic liquid or gas (air for example) is to be converted into movement. They work as pumps when a movement is to be converted into an increase in pressure of the working fluid.

The sliding contact surfaces in the cylinders in which the plungers move suffer extreme stress caused by high sliding speeds and bearing pressures. In order to keep wear and tear in such machines as low as possible, it is known to insert sleeves or bushings in the cylinder housing in which the plungers are able to move. It is also known to cover the friction surface of the cylinder housing bore and the plunger with a suitable plating or polymer coating.

The plunger in such machines may be in the form of a piston with a head and retractable rod, or may be just an integral rod. The piston or plunger may be equipped with sealing rings made of rubber or elastic polymer or may have a metal external surface sliding along a metal internal surface of the cylinder.

US 4,577, 549 discloses a hydraulic cylinder and a piston provided with an autocatalytically or electrolessly deposited relatively thin plated coating, of nickel

phosphorous matrix containing sub-micron particles of a polyfluorocarbon resin, preferably polyfluoroethylene. The plated coating provides a good wearing surface having self-lubricating properties obtained due to the exposed polyfluorocarbon resin particles smearing the surface of the coating, and reducing considerably squeaking noise during operation of the cylinder.

US 5,947, 003 discloses a hydraulic machine where a piston sliding in a cylinder bore has a layer of a friction-reducing plastics material from the group of high-strength thermoplastics. Because the piston is provided only with a friction-reducing layer of a plastics material, but does not consist entirely of the plastics material, it is possible to build it with the necessary strength. The plastics is injection-molded. The hydraulic machine can be operated with fluids that have no lubricating properties, such as water.

RU 2091182 describes a method for application of polymer coatings on internal and external surfaces of cylinders. A liquid polymer composition is poured under pressure or vacuum in an annular slot formed between the coated surface and a counterpart mandrel, then the composition sets (polymerizes) and the mandrel is withdrawn. The mandrel is manufactured with precise and smooth finished surface, thereby providing a geometrically accurate and smooth coating which does not need further finishing.

SUMMARY OF THE INVENTION In accordance with one aspect of the present invention, there is provided a hydraulic or pneumatic cylinder comprising a tubular housing and a plunger adapted to move in the housing under the pressure of a working fluid. The housing and the plunger have matching friction surfaces complying to a standard diameter tolerance and a standard surface roughness. The housing and the plunger are manufactured from blanks with deviations of diameter at least an order of magnitude larger than the standard tolerance and with surface roughness at least an order of magnitude larger than the standard roughness. The friction surfaces are

formed by a polymer coating providing the standard tolerance and the standard roughness. One of the friction surfaces may be equipped with sealing rings.

Preferably, the standard diameter tolerance is H8, h8 ISO, the standard surface roughness is Ra 0.1 to 0. 2 Ilm. Preferably the blanks are made of stock hot-rolled or cold-drawn steel tubes and bars, or of stock extruded aluminum tubes, or of extruded or injection-molded polymer tubes, without any mechanical working of the surface.

Preferably, the polymer coating is 0.3 to 3 mm thick and comprises: a synthetic thermosetting resin selected from the group of epoxy, phenol (such as novolak), polyurethane, silicone resins; a solid lubricant selected from the group of molybdenum disulfide, boron nitride, polytetrafluoroethylene; and a filler selected from the group of graphite, glass, fiber or powder carbon.

According to another aspect of the present invention, the hydraulic or pneumatic cylinder has also a predetermined degree of operational performance defined by the low friction between the housing and the plunger (or respective seal rings), wear resistance, and corrosion resistance to the working fluid. The blanks are manufactured from a low-grade material providing by itself a lower degree of performance, while the polymer coating provides the predetermined degree of performance. Preferably, the blanks are made of one of the following: cheap low-grade carbon steels, light aluminum alloys, magnesium alloys, or plastic materials.

The polymer coating is applied on the blanks in a precision mold with finished surface. The coated housings and plungers retain the surface quality of the mold, have reduced friction, and are protected from wear and corrosion. Thereby, the expensive mechanical working of each housing or plunger is avoided.

According to the invention, stock tube and bar profiles from low-grade materials may be used for mass production of quality hydraulic and pneumatic cylinders, even for non-lubricating working fluids, such as water.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, preferred embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a sectional elevation and a cross-section of a hydraulic or pneumatic cylinder in accordance with the present invention.

Fig. 2 is a schematic illustration of a method for applying a polymer coating.

DETAILED DESCRIPTION OF THE INVENTION With regard to Fig. 1, it is shown in sectional elevation the structure of a hydraulic or pneumatic cylinder 10 according to the present invention. The cylinder 10 comprises a housing 12 with flanges 14 and 16, and nipples 18 and 20. The internal surface 22 of the housing 12 is covered by a polymer coating 24 with friction surface 26. Preferably, the internal surface 22 is formed with a plurality of annular or spiral notches 27 for better retaining of the coating 24. The housing 12 is closed by a closure 28 with a connection element (eye) 30, and by a flange 32 with seal 34. The cylinder 10 also comprises a plunger 36 with friction surface 38 and a retractable rod 40 extending through the seal 34 and having a connection element 42. The plunger or piston may be equipped with sealing rings, or sealing rings may be fitted to the internal surface of the housing. The seal 34 may have a coating similar to the coating 24.

In operation, pressurized working fluid is supplied through the nipples 18 and 20 above or beneath the plunger 36 and the latter moves downwards or upwards, respectively. The interacting surfaces 26 and 38 of the housing and of the plunger, respectively constitute a friction pair working under high load and mechanical stress. The surfaces of the rod 40 and the seal 34 may be in similar situation. Consequently, in order to provide for efficient performance of the cylinder, these surfaces must comply to rigid dimensional tolerances, such as H8,

h8 ISO for hydraulic cylinders; H10, hlO ISO for pneumatic cylinders; and to roughness standards, such as Ra = 0. 1-0. 2 um.

This is achieved by the polymer coating 24 applied on the relatively rough surface 22 (the roughness is exaggerated in the figure), in a manner described below. With such coating, the cylinder housing 12 and the plunger 36 may be manufactured, without any mechanical works to improve the surface, from blanks such as: - cut off from stock hot-rolled steel tubes or bars with roughness Ra 6. 3. elm ; - cut off from stock cold-drawn steel tubes or bars with Ra 0. 2 6. 3 - cut off from aluminum extruded tubes with Ra 0. 5 12. 5 pm ; and - extruded or injection-molded plastics.

The above stock tubes, bars and blanks may have deviations of the external diameter, such as 1%, and deviations of wall thickness, such as 10%. Such large deviations of geometric shape are compensated, in accordance with the present invention, by the polymer coating 24 with thickness 0.3 to 3 mm, whereby the above-mentioned rigid dimensional tolerances and roughness standards are met.

The operational performance of a friction pair such as 26-38 in a hydraulic or pneumatic cylinder depends on the friction coefficient of the surfaces in the pair, on their wear resistance and their corrosion resistance to the working fluid. The composition of the polymer coating 24 is designed for friction-reduction and comprises synthetic thermosetting resin with high adhesion selected from the group of epoxy resin, phenol resins such as novolak, polyurethane, silicone resins; solid lubricants from the group of molybdenum disulfide, boron nitride, polytetra- fluoroethylene; and fillers such as graphite, glass or carbon in powder or fiber form.

The polymer coating also has wear-reduction properties and protects the underlying surface from corrosion. In view of this, low-grade construction materials may be used for blanks instead of expensive alloys. Specifically, we have found that regular hydraulic cylinders, which are currently made mainly of high-grade carbon and alloy steels, can be made of cheap low-grade carbon steel (AISI-SAE 10XX-

15XX). Also, such cylinders can be made of light alloys such as aluminum, magnesium and titanium alloys, and plastic materials. In this case, the weight of the cylinders is significantly lower at the same cost.

Corrosion-resistant hydraulic cylinders, which are currently made of stainless steel, can be made of cheap low-grade carbon steel, light aluminum alloys, titanium alloys, and plastic materials.

Regular pneumatic cylinders, which are currently made of thick-wall aluminum alloy extrusion profiles, can be made of cheap low-grade carbon steel thin-wall tubes.

Corrosion-resistant pneumatic cylinders, which are currently made of stainless steel, can be made of cheap low-grade carbon steel, light aluminum and magnesium alloys, and plastic materials.

With reference to Fig. 2, it is shown a method for application of polymer coating to the surface of a plunger or cylinder housing of a hydraulic or pneumatic cylinder, similar to RU 2091182. A plunger blank 52 is fixed in a cylindrical mold 56 closed by a base 58 with a feed channel 60, and by a cover 62 with exhaust openings 64. The mold has heaters 65. The internal surface 66 of the mold 56 is machined and finished according to dimension tolerances and roughness standards applicable to a plunger friction surface, such as H8 ISO and Ra = 0. 1 0. 2 Sun. The external surface 68 of the plunger blank 52 is not precisely machined or finished but the plunger is precisely positioned in the mold with respect to the surface 64 to form an annular gap 70 enveloping the plunger blank 52.

A viscous polymer compound 72 is prepared in the container 74 having heaters 75. The compound, as described above, comprises synthetic thermosetting resin with high adhesion selected from the group of epoxy resin, phenol resins such as novolak, polyurethane, silicone resins; solid lubricants from the group of molybdenum disulfide, boron nitride, polytetra-fluoroethylene; fillers such as graphite, glass or carbon in powder or fiber form, and viscosity regulators such as the known phthalate plasticizers. The compound is poured into the gap 70 via a conduit 76 and the channel 60, either free or under high pressure 0. 01, 5 Mpa, in

the temperature range 20-100°C. After filling the gap 70, the compound sets at temperatures 20=150°C and is post-cured at temperatures 60-150°C. Thereby, a friction-reducing, wear-resistant, and corrosion-resistant polymer coating 80 is obtained. The coating adheres to the plunger blank surface 68 due to the adhesive properties of the polymer matrix, but for better adhesion, a plurality of annular notches 82 may be cut on the blank surface. The coating 80 also compensates for the roughness and dimensional tolerances of the blank. The external friction surface of the ready plunger retains the quality of the internal finished mold surface 64.

Thus, the expensive mechanical working of each housing and plunger in a mass production process is avoided.

Although the above method was illustrated by an application of polymer coating on the external surface of a plunger, it is obvious for a person skilled in the art how to use the method for coating the internal surface of a cylinder.

It should be understood that the above method is applicable to other friction pairs or members of such pairs, for example in piston pumps, shock absorbers, cylinder dampers, cylindrical guides, telescopic assemblies, etc.