This invention relates to an improved sliding contact for conducting electrical power from a fixed power system to a rotating ice protection system. The invention is especially adapted for use with an aircraft propeller ice protection system.
The hazards of aircraft flight in atmospheric icing conditions are well known. Through the years, various workers in the art have developed various techniques for removing or preventing ice accumulation encountered during flight. Certain of the techniques are particularly adapted to protect specific parts of an aircraft. An electrothermal propeller de-icing system is an example of a specialized system.
In an electrothermal propeller de-icing system, electrothermal de-icers are bonded to the inboard leading edge portions of the individual blades of an aircraft propeller. An example of an electrothermal propeller de-icer is presented in U.S. Pat. No. 4,386,749 issued Jun. 7, 1983 to Sweet et al. (the '749 patent). Electrical power is supplied to the individual de-icers through flexible wire harnesses that act as jumpers between each blade and the propeller bulkhead. The harnesses must be flexible since each blade must be able to rotate about its axis in order to effectuate pitch changes. An example of a wire harness is presented in
U.S. Pat. No. 5,020,741, issued Jun. 4, 1991, to Ziegler et al. (the '741 patent).
Electrical power is conducted to the propeller ice protection system through a sliding contact comprising a slip ring assembly and a brush assembly. A typical arrangement is presented in U.S. Pat. No. 4,136,295, issued Jan. 23, 1979, to Sweet (the '295 patent). The slip ring assembly is mounted to the back of the propeller bulkhead facing the aircraft engine. According to a very common embodiment, the slip ring assembly includes a metal dish having an annular channel that receives a plurality of conductive slip rings formed from a copper alloy. The rings are potted into the channel with an epoxy compound which also provides the necessary dielectric insulation between the individual rings and the channel. The wire harnesses are electrically connected to the individual rings. In such manner, electrical power is transferred from the slip rings to the individual de-icers.
Electrical power is transferred to the slip rings through a brush assembly. A particular system may have one or more brush assemblies. Each assembly typically includes a housing that slidably receives two or more electrical brushes. The brushes are normally composed of a carbon based matrix. Carbon brushes and various carbon based brush compounds are well known in the brush and commutation art. A spring biases each brush against a slip ring thereby forming a sliding contact. Some form of electrical power connection is attached to the housing. The power connection connects the brush assembly to the aircraft power system and may take the form of a shell-type connector or a number of a individual studs. The aircraft power system typically includes a timing and switching device that switches power supply from brush to brush which thereby switches power to the various de-icers in a predetermined timed sequence. Flexible shunts conduct electrical power from the power connection to each brush. The electrical power is then conducted from the brush through the sliding contact into the slip ring.
There are many brush manufacturers that have developed a variety of brush compounds for a variety of applications. There are five basic categories of brushes: carbon, carbon-graphite, electrographite, graphite, and metal-graphite. The term "carbon" has a broad meaning that refers to any brush having any quantity of carbon in it regardless of the quantity of other materials. The term also has a narrower meaning to refer to a brush predominantly composed of amorphous carbon such as petroleum coke. Carbon-graphite refers to a brush composed of a mixture of carbon (as defined above) and graphite. Electrographite refers to a brush composed of carbon subjected to intense heat in an electric furnace that graphitizes the carbonaceous binder. Graphite refers to a brush that is predominantly graphite mined from the ground or manufactured in an electric furnace. Metal-graphite refers to graphite brushes having a quantity of metal such as silver or copper.
Likewise, slip rings formed from various materials are known in the art. Such materials include gunmetal according to ASTM B143-1A or 1B (88% copper, 8% tin, 4% zinc), leaded gunmetal according to ASTM B144-3D (86% copper, 7% tin, 5% zinc, 2% lead), phosphor bronze (commonly 90% copper, 10% tin, 0.4% phosphorus), cupro-nickel (94% copper, 4% nickel, 2% tin), monel (29% copper, 68% nickel, 3% combination of carbon, iron and manganese), steel (usually 0.21% carbon or 1% carbon and 1% chromium), and stainless steel (18% chromium, 8% nickel or 25% chromium, 12% nickel or 17% chromium, 10% nickel).
There are many combinations of brush and slip ring material possible. Regardless of the combination, formation of a slip ring surface film on the slip ring is regarded as crucial for sliding contact performance. The slip ring surface film includes three layers. The first layer is an oxide layer overlying the slip ring surface. The second layer is a graphitic film overlying the oxide layer. The third layer is a layer of free particles and absorbed water vapor. Unacceptable brush and slip ring wear results if a slip ring surface film does not form. This problem is particularly severe at high altitudes where the sliding contact encounters rarified atmosphere, extremely low humidity, and sub-zero temperatures. These conditions severely impede formation of a slip ring surface film. Special brush compounds for high altitude applications were developed in response to this problem. These compounds have ingredients that maintain performance of the sliding contact at high altitudes.
A metal-graphite brush in combination with a copper alloy slip ring is generally preferred in the industry for use in a propeller ice protection system. The brush metal is usually silver or copper. According to a known combination, a silver-graphite brush impregnated with about 31% silver forms a sliding contact with an electrolytic tough pitch copper slip ring according to Copper Development Association C11000. The slip ring has a surface hardness on the order of a Rockwell F40-F55. Hardened slip rings have also been used in the art.
Brush and slip ring wear in propeller ice protection system applications tends to be more severe than in other sliding contact applications. Some aircraft have exhibited more of a problem with slip ring wear than others. The source of the problem has been elusive. Brush and slip ring wear can seriously shorten the life of a sliding contact in a propeller ice protection system, and can greatly increase maintenance costs. Therefore, a sliding contact having minimized brush and ring wear is desired for propeller ice protection system applications.