EP0019605B1

EP0019605B1 - A rotary vane machine

Info

Publication number: EP0019605B1
Application number: EP80850073A
Authority: EP
Inventors: George Anthony Cooper; Jacques Berlie
Original assignee: Institut Cerac SA
Current assignee: Institut Cerac SA
Priority date: 1979-05-16
Filing date: 1980-05-12
Publication date: 1983-03-02
Also published as: AU536155B2; ES8101727A1; ES491491A0; JPS55153885A; BR8003080A; SE427491B; EP0019605A1; NO801324L; SU1056920A3; DE3062178D1; SE7904291L

Description

The present invention relates to a rotary vane machine according to the prior art part of claim 1; it is intended for operation with a dry and oil-free compressible working medium, e.g. compressed air, at high speeds.
A considerable amount of experiments has been made during several decades in order to find out how a vane machine shoud be designed in order to operate without oil lubrication over a substantial period of time. So far no solution has been found having a service life directly comparable with the service life of oil lubricated machines. In US patent 3 335 944 it has been suggested to use a vane material consisting of a mixture of polytetrafluoroethylene PTFE and ground glass. FR-A-1 574 999 describes a rotary vane machine where the vanes comprise a matrix of PTFE reinforced with glass fibre and having molybdenumdisulfide as lubricant. No figures are given regarding the amounts of the materials. GB-A-1 336 128 describes a material, example 2, comprising PTFE and carbon and glass fibres and furthermore it is compression moulded under the conventional conditions pertaining for PTFE followed by sintering. 1596 by weight of bronze powder. Since bronze is much denser than the other_'- materials in the example the figure is much lower when calculated according to volume. GB-A-1 324 248 describes for dry operation a vane material having a matrix of lead oxide and not more than 20% of PTFE and also another one having a matrix of PTFE and a filler of lead oxide; it describes furthermore that bronze may be substituted for lead.
The main object of the present invention has been to find a material combination which is sufficiently hard to make it wear resistant without being brittle so that pieces are broken away from the vanes. Since a working medium which is believed to be dry and oil-free often contains small amounts of moisture and oil, the vane machine according to the invention should be capable of operation also under these circumstances wihout any serious effects on the service life.
As mentioned above nobody has found a suitable material combination for the vanes of a rotary vane machine before the present inventⁱon was made, in spite of all the efforts that have been made during several decades. One important problem has been the high sliding speeds which often exist between the vanes and the cylinder housing. The sliding speed can for instance be up to and even above 20 m/sec in a rotary vane motor for a hand-held grinder. Since the contact pressure between the substantially radially movable vanes and the cylindrical countersurface is approximatively proportional to the square of the sliding speed it is obviously desirable to make the vanes as light as possible in order to counteract the effect of high sliding speeds.
According to the present invention a rotary vane machine is created in which the substantially radially movable vanes are provided with portions, at least at the surface in sealing contact with the cylinder housing of a wear resistant material according to the prior art part of claim 1 and characterized thereby that the material comprises, between 5 and 3096 by volume of glass or carbon as hard phases and from 10 to 2096 by volume of lead or lead oxide as inorganic solid lubricant whereas the cooperating surface of the cylinder is made of steel or cast iron.
Particularly good performance has been obtained with lead or lead oxides as solid lubricant in the wear resistant material when the cylinder is made of steel or cast iron. No significant difference in performance has been found between non-oxidised lead metal, semi-oxidised lead metal and lead oxide.
In bearing technology lead and tin are recommended because they are soft and ductile metals. However, these recommendations cannot be applied in the present case because the addition of tin to a mixture of polytetrafluoroethylene and glass seriously impairs the performance. The wear rate is about three times as high with 1096 by volume of tin in the material than without. As mentioned above lead does not have to be in its metal form. Furthermore, as mentioned above, it is desirable to make the vanes as light as possible. The addition of lead constitutes a considerable step away from this idea. It is, therefore, quite surprising that the addition of lead decreases the wear rate of the vanes. Improvements by about a factor ten have been observed in high speed tests with lead or lead oxide as solid lubricant. Tests have shown that the wear rates obtained with the invention, under dry and oil-free conditions, are as low as the wear rates obtained with conventional phenolic vanes with full oil lubrication.
Fig. 1 is a longitudinal section through an air motor according to 1-1 in Fig. 2. Fig. 2 is a cross-section through the motor according to 2-2 in Fig. 1. Fig. 3 shows the wear rates obtained in a series of tests with different amounts of lead. Fig. 4 shows the wear rates as a function of the amount of lead.
The air motor shown in Figs. 1 and 2 comprises a cylinder housing 11 provided with two end plates 13. A rotor 16 having an outgoing shaft 12 is eccentrically journalled relative to cylinder 11 in the end plates 13 by means of bearings 14. The rotor is further provided with a number of vanes 20 which are movable in substantially radial slots 19. Substantially radial in this context means that the slots may deviate from the radial direction with up to 30°. The housing is provided with an inlet port 30 for compressed air and outlet ports 31.
Fig. 3 shows the wear rates obtained with different amounts of semi-oxidised lead metal. These tests have been made with an air motor operating unloaded at 10000 rpm, corresponding to a sliding speed of 20 m/sec. The ratio of glass fibre, cut into short pieces, to polytetrafluoroethylene (PTFE) has been kept constant at one to three. All amounts are given in percent by volume (v/o). In Fig. 4 the wear rates in ,um/hr are shown as a function of the amount of lead. The wear rate obtained without lead is the wear rate obtainable with the combination of materials disclosed in the above mentioned U.S. patent 3 335 944. The wear rate obtained with one percent of lead (18 ,um/hr) has been deleted when Fig. 4 was plotted because it is very unlikely that there should be a peak value in that position. This value is probably wrong and no efforts have been made to obtain the correct value since it is not, from a practical point of view, interesting to know this value precisely. As can be seen in Figs. 3 and 4 the wear resistant material should contain from 10 to 20% by volume of lead.
Tests have also been made, at the above mentioned operating speeds, with carbon fibre instead of glass as the hard phase material. In these tests two varieties commonly known as type I (high modulus, moderate strength). and type II (high strength, moderate modulus) were used. As in the above mentioned tests with glass the ratio of carbon fibres to PTFE was kept constant at one to three. Tests were made with up to 20 v/o of lead with the type II fibres. A wear rate of 4 µm/hr was obtained with 60 v/o PTFE, 20 v/o carbon fibre type II and 20 v/o lead. With a composition comprising 60 v/o PTFE, 20 v/o carbon fibre type I and 20 v/o lead a wear rate of 1.5,um/hr was obtained. It can be seen from these results that very good performance is obtainable with carbon fibres as hard phases. The best value obtained with type I fibres is as good as the value obtained with glass.

Claims

A rotary vane machine comprising a cylinder (11) having two opposite end plates (13), inlet (30) and outlet means (31) for a dry and oil-free compressible working medium, a rotor (16) eccentrically journalled in the cylinder (11), a number of vanes (20) slidable in substantially radial slots (19) in the rotor (16) to maintain sealing contact with the cylinder (11) during rotation of the rotor (16), each vane being provided with wear resistant material at least at the surface in sealing contact with the cylinder, said wear resistant material comprising a matrix of a fluorocarbon polymer, glass or carbon as hard phases and an inorganic solid lubricant, characterized thereby that the wear resistant material comprises from 10 to 20% by volume of lead or lead oxide as inorganic solid lubricant and between 5 and 30% by volume of glass or carbon, whereas the cooperating surface of the cylinder is made of steel or cast iron.