Gear for use in vacuum space

Abstract: The peripheral teeth (11) of a gear body (10) are made of alloy tool steel and coated with a manganese phosphate layer (12) and then with a solid lubricating film (13). The alloy tool steel of which the teeth (11) of the gear body (10) cooperates with the manganese phosphate layer (12) to act on the solid lubricating film (13). The taking-in force of the solid lubricating film (13) can be thus increased at the surface thereof. The bonding strength of the solid lubricating film (13) can be increased as well. This can prevent the solid lubricating films (13) on teeth (11) of the two gear bodies (10) engaged from being separated from each other.

Description: The present invention relates to a gear for use in gear transmission systems built in vacuum environment such as in space. In the case of this gear intended to use in vacuum environment, its gear body must be usually made of stainless steel, nitrided steel or others having a relatively long life even if it is used together with its paired one without any lubricating matter. This is because the space in which it is used is vacuum and also because the use of any lubricating matter such as grease is difficult there. Particularly when it is used in the field of space development, a solid lubricating film consisting of mainly gold, silver, molybdenum disulfide or others must be formed on those teeth of it which will be engaged with teeth of its paired gear to keep them lubricated while they are turning. This is because it is quite difficult to check and maintain them and also because it is required that they will be highly durable under vacuum circumstances and against high load added and that they will not contaminate any other components at their peripheral region. Machines and devices which will be launched into space by the spacecraft have been made larger and larger in size these days in the field of space development. As they become larger in size, load unexpectedly higher is sometimes added to the gear transmission system at the drive section for their space manipulators, for example. It is, therefore, required that those gears which will be used as components of the gear transmission system in space can be kept reliably operative for a long time under this unexpectedly high load added. When high load is added to the above-described gears, however, their solid lubricating films become liable to separate from each other, thereby making it difficult to keep them reliably operative for a long time. From Patent Abstract of Japan, Vol. 12, No. 312(C523) there is known the use of a phosphate coating layer on a gear coated with a resin having excellent damping characteristics, in order to ensure bonding of the metal, which forms the gear, and the resin. There is not mentioned any material to be used for the tooth body, and as the resin coating there is mentioned a heat and wear resistant fluororesin layer. Finally, there is no mention that respective gears can be used in vacuum space. Furthermore, from EP-A-250 283 there is known forming a solid lubricating film on the teeth of a gear in a harmonic drive which can be used in a vacuum and which includes a bearing. According to said document the teeth of ring gears are covered with PTFE. Only for covering the balls of the bearing there is used molybdenum disulfide. Therefore, according to said document molybdenum disulfide is intentionally not used for engaging teeth. The object of the present invention is to provide a gear for use in vacuum space having on it a solid lubricating film not separated from that on another gear engaged to lubricate the gears, so that they are durable for a longer time even under unexpectedly high load added in vacuum space. This object is achieved in one embodiment according to claim 1, and in another embodiment according to claim 2. According to the invention the gear body is formed using the alloy tool steel, the manganese phosphate layer and the molybdenum-disulfide-bonded film being sequentially formed on the surface of the gear body. The co-operation of the alloy steel and the manganese phosphate layer results in high durability of the molybdenum-disulfide-bonded film. At the same time the taking-in force of the solid lubricating film is increased at the surface thereof and the bonding strength of it increased as well. The solid lubricating film on the gear body can be therefore prevented from separating from that on another gear body engaged to thereby make them more durable for a longer time. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figure 1 is a schematic view showing gears engaged with each other, each gear being an embodiment of the present invention and intended to use in vacuum environment; Figure 2 is a characteristic graph showing lubricities of the gears shown in Fig. 1; Fig. 3 is a characteristic graph showing lubricities of two gears engaged with each other, one of said gears being made of nitrided steel while the other of stainless steel and both of them having a first manganese phosphate layer and a second solid lubricating film formed on the first layer but their second solid lubricating films being different in composition; and Fig. 4 is a characteristic graph showing lubricities of gears engaged with each other, each being made of stainless steel and having the first manganese phosphate layer and the second solid lubricating film formed on the first layer, but each second solid lubricating film being different in composition. Some embodiments of the present invention will be described in detail with reference with the accompanying drawings. Fig. 1 shows gears engaged with each other, each gear being an embodiment of the present invention and intended to use in vacuum environment. At least teeth 11 of each gear body 10 are made of alloy tool steel. The alloy tool steel has a composition containing each of C, Si, Mn, Cr, Mo, P, S and V at a predetermined amount. A first manganese phosphate layer 12 is formed on each of teeth 11 of the gear body 10. A second solid lubricating film 13 consisting of molybdenum disulfide is formed on the first manganese phosphate layer 12. The first manganese phosphate layer 12 cooperates with the alloy tool steel, of which the teeth 11 of the gear body 10 are made, to increase the taking-in force of the second solid lubricating film 13. The bonding of the solid lubricating film 13 can be thus increased at the surface thereof. The second solid lubricating films 13 on the teeth 11 of both gear bodies 10 engaged can be thus prevented from separating from each other. This merit was confirmed by the following test wherein it was examined how the gears wore out. Three kinds of molybdenum-disulfide-bonded films A, C and D were prepared using molybdenum disulfide as their main lubricant but different in composition depending upon organic binders used. These molybdenum-disulfide-bonded films A, C and D were used as the second solid lubricating film 13 formed on the first manganese phosphate layer 12. Each paired gears having one of these molybdenum-disulfide-bonded films A, C and D were rolled and slid in vacuum and at normal temperature, having a load plane pressure of 60kgf under a load of 10kgf, a slip ratio of 10% and a relative slip speed of 0.26m/s. Results thus obtained are shown in Fig. 2. It was confirmed that the bonded films A, C and D had life times of 245 hours, 390 hours and 270 hours until they were broken up. It was also detected how the frictional factor of each paired gears changed. Then, the manganese phosphate layer 12 was formed on each conventional gear made of nitrided steel and a chromium-nitrate-treated layer was formed on each gear made of stainless steel. Each of the molybdenum-disulfide-bonded films A, C and D was further formed, as the second solid lubricating film 13, on these gears of two kinds. They were rolled and slid same as described above. Results thus obtained are shown in Fig. 3. As apparent from Fig. 3, it can be understood that the bonded films A, C and D have life times of 68 hours, 89 hours and 123 hours until they are broken up and that their lives are, therefore, shorter as compared with those shown in Fig. 2. It can also be understood that the frictional factors change to a little greater extent in the cases of those paired gears coated with the molybdenum-disulfide-bonded films C and D in Fig. 3. Further then, the chromium nitrate layer was formed on gears each made of stainless steel. Each of the molybdenum-disulfide-bonded films A, C and D was further formed, as the second solid lubricating film 13, on each paired gears which are to be engaged with each other. The pairs of these gears were tested same as described above. Results thus obtained are shown in Fig. 4. It can be understood from Fig. 4 that the bonded films A, C and D have life times of 46 hours, 62 hours and 237.5 hours until they are broken up and that their lives are, therefore, shorter as compared with those shown in Fig. 2. It can also be understood that each paired gears have a more unstable frictional factor. According to the gear of the present invention which is intended to use in vacuum environment, at least teeth 11 of its gear body 10 are made of alloy tool steel. In addition, it has the first manganese phosphate layer 12 and the second solid lubricating film 13 formed on the layer 12. The alloy tool steel of which the teeth 11 of the gear body 10 are made can therefore cooperate with the manganese phosphate layer 12 to act on the second solid lubricating film 13. The taking-in force of the solid lubricating film 13 can be thus increased at the surface thereof and its bonding strength can be increased as well. The solid lubricating films 13 on the teeth 11 of the two gears 10 engaged can be therefore prevented from separating from each other and their lives can be made longer accordingly. Although the teeth 11 of the gear body 10 have been made of alloy tool steel and coated with the manganese phosphate layer 12 and then with the solid lubricating film 13 in the above-described cases, the gear itself may be made of alloy tool steel and coated, on its surface, with the manganese phosphate layer 12 and then with the solid lubricating film 13. In other words, the present invention can achieve same merits even if only those portions of the teeth of the gear which is to be engaged with the other one are made of alloy tool steel and coated with the manganese phosphate layer 12 and then with the solid lubricating film 13. Or the gear itself may be made of alloy tool steel and only those portions of its teeth which are to be engaged with teeth of the other gear may be coated with the manganese phosphate layer 12 and then with the solid lubricating film 13.

Claims: A gear for use in vacuum space, to be engaged with another gear to transmit rotation force, said gear being characterized in that its gear body (10) is made of alloy tool steel and coated with a manganese phosphate layer (12) and then with a solid lubricating film (13) consisting of mainly molybdenum disulfide. A gear for use in vacuum space, to be engaged with another gear to transmit rotation force, said gear being characterized in that at least those portions of teeth (11) of its gear body (10) which are to be engaged with the teeth of the other gear are made of alloy tool steel and coated with a manganese phosphate layer (12) and then with a solid lubricating film (13) made of mainly molybdenum disulfide.

Inventors: HONDA TOSHIO C O INTELLECTUAL, HONDA, TOSHIO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: F16H55
IPC codes: F16H55, F16H57