|Title of Invention||
A SILVER-GRAPHITE BRUSH BLOCK AND A METHOD OF PRODUCING THE SAME
|Abstract||This invention relates to a silver - graphite brush Mock. The graphite is subjected to surface coating with solver prior to admixing the same with silver. This improves the physical and mechanical properties of the block brush. This invention also includes a method of making silver-graphite brush blocks. graphite is sur&ce coated by suspending it in an aqueous bath containing sugar, tartaric acid, ethyl alcohol, silver nitrate and ammonium nitrate. The surface coated graphite is admixed with silver powder and subjected to known steps of drying, sintering and annealing. The new brush blocks exhibit better wear rate, minimum resistance, low coefficient of friction and mmimum contact voltage drop.|
This invention relates to a silver-graphite brush block and a method of producing the same.
Silver-graphite brush blocks are used for precision electric and electronic rotating systems. Slip ring units of radars and launchers are provided with silver graphite brush blocks.
Silver-graphite brush blocks are conventionally made by blending silver and graphite powders and cold pressing the blend into the desired shape and size in a die. Green compacts thus obtained are sintered in a protective atmosphere, shaped and annealed subsequently to achieve the desired density and size. One of the major drawbacks noticed in the product made by this method is the lack of a metallurgical bond between the silver matrix and graphite particulates. The silver graphite brush block produced by this method exhibits high resistivity. Moreover, due to loosely bonded graphite particles in the brush block, dust formation occurs during rotation resulting in high wear rate and short circuiting of channels. Presence of graphite particles on the surface of these brush blocks makes soldering with Copper Berrylium strips, difficult.
Physical properties like electrical conductivity, mechanical strength and wear resistance of a composite material follow the rule that the property of the composite is equal to the additive of the products of individual properties of the components and their volume fractions provided coherent/semi-coherent interfaces are formed between the matrix and the
second phase. Deviations from the calculated values may be attributed to improper interfaces.
The objectives of the present mvention are in developing brush blocks exhibiting minimum wear rate resulting in longer life under normal conditions of use. The brush blocks must produce minimum possible dust formation during operation and exhibit minimum resistivity. Contact voltage drop must be minimum and the coefficient of friction must be very low. Further, when mounted on a rotor part, the brush blocks must not leave any markings on the rotor parts.
Silver and graphite form an immiscible system, in the composite resulting in the formation of incoherent interface between the two components.
This imperfection namely incoherent interfaces in the microstructure of the composite body is reflected in lowering of the physical properties like electrical conductivity, mechanical strength and wear resistance. In order to provide better interfaces between silver and graphite which would overcome the drawbacks noticed in the conventional brush blocks, the graphite used in making the composite is subjected a special treatment before admixing it with silver.
Precoating of graphite is effected by suspending graphite powder in a bath to evenly coat the powder with silver. When this process is completed, the graphite powder is washed, dried and heated in a protective atmosphere before blending it with silver powder. This mixture is then processed to form silver graphite brush blocks by compacting, sintering, shaping and anneaUng.
Surface treatment of graphite powder prior to brush block manufacture is effected by suspending the powder in a mixture of solutions 1,2 and 3. Solution (1) consists of sugar, tartaric acid, ethyl alcohol dissolved in water. Solution (2) consists of silver nitrate and ammonium nitrate dissolved in water and solution (3) is an aqueous alkali such as sodium hydroxide. These three solutions are admixed in any desired range and graphite particles are suspended therein and agitated to surface coat the particles with silver. Silver coated graphite thus obtained are separated from the bath and dried in a protective atmosphere, preferably in argon by heating to 110° to 130°C . The dried silver coated graphite is mixed with silver powder upto 10% by wt of graphite powder is preferably used and this mixture is compacted to provide a green compact which is shaped as desired. This green compact is sintered and densified in a protective atmosphere by heating. The atmosphere for sintering may be argon and may be heated to 850°C to 900°C for a period of 2 to 2 1/2 hours. The sintered matter is then compacted and shaped to the desired density and dimension. Annealing is carried out under a protective atmosphere, preferably in an argon atmosphere at 600 to 650** for 2 to 2 1/2 hours.
The silver-graphite brush block according to this invention comprises a compacted sintered and annealed silver-graphite brush block wherein said graphite powder in the compact is provided with a surface coating of silver prior to admixture with silver powder.
This invention also relates to a method of producing a silver-graphite brush block comprising the steps of surface treating graphite powder prior to admixture with silver powder to provide a coating of silver thereon, admixing said silver coated graphite with silver powder, and subsequently compacting, sintering, shaping and annealing said mixture to form the silver graphite brush block.
The solutions for effecting silver coating preferably consists of the following composition: Solution: 1
Sugar : 2 gms Tartaric acid : 0.3 gm Ethyl alcohol: 5cc Distilled water 45 cc Solution 2:
AgNO3: 2 gms NH4N03:3gms Distilled water: 50 cc Solution 3:
NaOH: 5 grams Distilled water : 50 cc The above solutions are mixed together for silver coating operation.
The graph given in the accompanying drawings illustrates the improved properties of the brush produced with silver coated graphite particles when compared with conventional brush blocks.
This description and the Expended claims do not exclude obvious alterations, modifications and equivalents known to persons skilled in the art.
1. A method of producing a silver-graphite brush block comprising the steps of admixing silver surface coated graphite with silver particles, compacting, sintering, shaping and annealing said silver graphite mixture to produce said brush blocks.
2. The method as claimed in claim 1, wherein graphite is surface coated with silver by suspending the same in an aqueous solution of sugar, tartaric acid, ethyl alcohol, silver nitrate, ammonium nitrate and sodium hydroxide.
3. The method as claimed in claims 1 and 2, wherein said silver surface coated graphite particles are separated, washed and dried in an inert atmosphere at 110 to 130°C prior to admixture with silver particles.
4. The method as claimed in claim 3, wherein said inert atmosphere is made of argon gas.
5. The method as claimed in claims 1 to 4, wherein said silver-graphite mixture is compacted and densified by sintering in an argon atmosphere.
6. The method as claimed in claim 5, wherein said densification is effected by heating the compact to 850 to 900°C in an argon atmosphere for 2 to 2 1/2 hours.
7. The method as claimed in claims 1 to 5, wherein said compacted and
sintered brush block is annealed in an inert atmosphere for relieving stress
8. The method as claimed in claims 1 to 7, wherein annealing is carried out in
an argon atmosphere at a temperature range of 600 to 650°C for a period of
2 to 2 1/2 hours.
9. The method as claimed in claims 1 to 8, wherein 10% by weight of surface
coated graphite is admixed with silver powder for producing said compact.
10. A silver-graphite brush block produced by the method as claimed in any
one of the preceding claims.
11. A method of producing a silver-graphite brush block, substantially as
hereinabove described and illustrated with reference to the accompanying
|Indian Patent Application Number||985/MAS/2000|
|PG Journal Number||50/2007|
|Date of Filing||21-Nov-2000|
|Name of Patentee||M/S. INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||DEPARTMENT OF SPACE, ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE - 560 094|
|PCT International Classification Number||B22F3/00|
|PCT International Application Number||N/A|
|PCT International Filing date|