Rotary expansible chamber devices – Methods
Reexamination Certificate
2002-07-15
2003-05-20
Vrablik, John J. (Department: 3748)
Rotary expansible chamber devices
Methods
C418S002000
Reexamination Certificate
active
06565337
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to rotary vane pumps having self-lubricating sliding vanes. More particularly, the present invention is directed to a method and apparatus for inspecting the sliding vanes in a rotary vane pump to determine the amount of wear to the vanes without having to disassemble the pump housing.
BACKGROUND OF THE INVENTION
Sliding rotary vane pumps have been used for several years for a multitude of mechanical and industrial applications and can be exposed to a wide range of environmental conditions. These pumps can be used in both gas and liquid pumping applications. One type of sliding rotary vane pump is a dry air pump. In the general aviation field prior to the early 1960's, the vacuum systems which powered gyros were driven by pumps which were lubricated by oil and referred to in the art as wet pumps. In the 1960's, the oil lubricated, or wet vane vacuum pumps, were replaced by dry vacuum pumps constructed of carbon vanes and rotors which were self-lubricating. To this present day, the standard dry vacuum pumps in the market comprise mechanical carbon rotors and vanes operating in a hardened metal ellipsoidal cavity. These pumps provide a power source for, among other things, gyroscopically controlled, pneumatically operated flight instruments.
A dry air type rotary vane pump usually has a rotor with slots with a radial component of alignment with respect to the rotor's axis of rotation, vanes which reciprocate within these slots, and a chamber contour within which the vane tips trace their path as they rotate and reciprocate within their rotor slots. The reciprocating vanes thus extend and retract synchronously with the relative rotation of the rotor and the shape of the chamber surface in such a way as to create cascading cells of compression and/or expansion, thereby providing the essential components of a pumping machine.
Because dry air pumps do not use a liquid lubricant, other forms of dry lubrication have been developed. For example, vanes for rotary pumps have been manufactured from carbon material as disclosed in U.S. Pat. No. 3,191,852 issued to Kaatz, et al. on Jun. 29, 1965. These vanes are fabricated by compressing carbon, graphite and various organic binders under high pressure and temperature. U.S. Pat. No. 4,804,317 issued to Smart, et al. on Feb. 14, 1989, a carbon composite material has been used for the side plates and vanes of the rotary pump. A composite carbon part is fabricated by combining carbon based tensile strength fibers (in a cloth weave) with graphite and an organic binder. Although providing improved performance over the prior carbon parts, similar wear, chipping and fracture problems exist with composite carbon parts. U.S. Pat. No. 4,820,140 issued to Bishop on Apr. 11, 1989, discloses a self-lubricating coating applied to the pump parts to inhibit wear between the slideable vanes and pump rotor. The coating is comprised of a mixture of lead and polytetrafluoroethylene deposited on the surface of the part to be coated.
While these lubricating methods work well for dry pump applications, the nature of the vane lubrication technique is destructive to the pump. Certain parts of these pumps are made of carbon or carbon graphite. These parts rub against other stationary or moving parts of the pump during operation. Graphite from these parts is deposited on the opposing parts by the rubbing action and forms a low friction film between the parts, thereby providing lubrication. The deposited graphite film is itself worn away by continued operation of the pump, and is eventually exhausted out of the pump. The film is replaced by further wear of the carbon graphite parts. Thus, lubrication is provided on a continuous basis that continuously wears away the carbon graphite parts. The vanes of the pump require and provide the majority of lubrication. Therefore, the vanes wear and lose length as the pump operates. At some point in time, the length of the vanes will become so short that they will not slide properly in the slot, which may lead to pump failure.
Failure of a dry air pump while in service can render one or more aircraft systems inoperative. In addition, most pump failures occur in flight. Dry air pump performance is generally unaffected by wear on the vanes until total failure. Moreover, pump efficiency does not typically degrade enough to be noticed by the pilot until total failure occurs. Usually, pump operation is monitored based on the aircraft's vacuum gauge. If the pump is not operating correctly, the vacuum gauge will indicate such. However, this generally does not occur until near complete failure of the pump.
A correlation exists between the remaining length of the vanes and the expected future operational life of the pump. The inventor has determined that the incidence of structural failure of the vane/rotor combination begins to increase appreciably after the vanes wear to a certain length. The incidence of failures continues to increase and the rate of failure per unit time increases dramatically as the vanes continue to wear shorter.
The inventor has studied various dry air pump failures and determined that until the vane reaches about 74% of its original length, failure due to mechanical malfunction arising from reduced vane length is unlikely. The total failure rate from all causes for pumps with vanes having remaining lengths about equal to or greater than 74% is less than about 5% of the operating population. By the time remaining vane length reaches about 64% of the original length, about 50% of installed pumps have failed, and more than 90% of those failures can be traced to malfunctions relating to vane length. When the remaining vane length falls below 64% of the original length, more than 98% of the installed pumps studied have failed, and 95% of those failures are related to vane length.
While vane wear occurring as a result of graphite deposition for lubrication is normal, fairly predictable, and reasonably slow, vane wear can be accelerated if the carbon graphite parts rub against roughened interior surfaces of the pump. Roughness of the interior surfaces can occur through many different causes, such as elevated temperatures and pressures, dirty filters, etc. Regardless of whether the vane wear is normal, or abnormally accelerated, when the vane length reaches a certain percentage of the original length, the likelihood of pump failure increases significantly.
The current state of the art relating to dry air pump performance and efficiency does not adequately address how to determine when the vanes of the pump have reached a point requiring pump replacement or repair. Presently, there is no effective and simple way to inspect the state or rate of wear of the vanes in this type of pump. There is also no simple and cost effective way to determine the remaining useful life of a dry air pump. Currently, to ensure proper pump performance, the operation time for dry air pumps is monitored. When the number of hours of pump usage reaches a predetermined and arbitrary figure, the pump is removed and a new pump is installed. This is neither cost effective nor efficient since the pump may have a significant amount of usage time still available, or, if wear was abnormally fast, would not be done in time.
What is lacking in the art is a simple and inexpensive way of determine vane length in a pump to determine the state of wear, the rate of wear, and potential remaining life of dry air rotary pump vanes. Such a feature would allow, in some cases, a knowledgeable technician to determine whether other pump or related system failures or malfunctions are attributable to vane length. Thus, opportunity arises to remove from service pumps likely to fail. In addition, opportunities arise to make adjustments or repairs to related aircraft systems to correct other malfunctions determined by inspection of the dry air pump. By correcting system malfunctions that might cause the pump to operate in an overload condition, pump life may be extended, and unscheduled downtime for the
Adams, Schwartz & Evans P.A.
Vrablik John J.
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