Fluid reaction surfaces (i.e. – impellers) – Support mounting – carrier or fairing structure
Reexamination Certificate
1999-12-03
2001-09-18
Ryznic, John E. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Support mounting, carrier or fairing structure
Reexamination Certificate
active
06290467
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to centrifugal gas compressors and, more particularly, to an impeller and shaft assembly used in a high-speed gas compressor in a refrigeration plant or other chiller.
Centrifugal gas compressors have one or more impellers rotated in a cavity for compressing a gas, such as refrigerant vapor. The one or more impellers are mounted on a pinion shaft that is turned by a motor. In centrifugal gas compressors, it is important that the impellers and pinion shaft mounting arrangements are simple and efficient to manufacture, install and operate. In particular, overly complex attachment arrangements involving the machining of complementary grooves and threads in male and female parts pose a greater burden on highly skilled machinists, a resource that is both finite and costly. More particularly, such arrangements are more likely to be damaged during transport, installation and normal running of the compressor.
U.S. Pat. No. 4,257,744 describes an impeller and shaft assembly that includes a cap screw, a Belleville washer or spring, a deformable socket machined into the rear of an impeller, a drive shaft with a frusto-conical shaped extremity, and a steel washer. The impeller has an axial bore extending through its center and a counterbored recess at its front. The frusto-conical shaped extremity includes axially extended grooves that are circumferentially spaced and alternate with intervening lands. A high torque applied to the cap screw results in plastic deformation of the lining of the socket in the rear of the impeller.
The manufacture of the frusto-conical shaped extremity is complex and adds to the cost of the impeller and shaft assembly. In addition, the counterbored recess is sized to accommodate the cap screw. As a result, the protective steel washer and single spring are both sized to correspond to the cross-section area of the counterbored recess and screw cap. Thus, the torque results in a clamping force being directly transmitted from the cap screw, without dissipation, through the single Belleville washer and steel washer. This arrangement may damage the single Belleville washer and cause stress fractures in the front face of the impeller immediately around the counterbored recess, necessitating the costly replacement of the entire impeller. Thus, there is a need for a simple impeller and shaft assembly that minimizes the risk of damage to the front face of the impeller necessitating the costly replacement of the entire impeller.
Maintenance personnel may use an ordinary wrench when a torque wrench is more appropriate. Dramatic over or under-torquing of pinion shafts in centrifugal impeller configurations leads to increased maintenance and downtime costs. An impeller assembly that is less vulnerable to such problems is needed.
Additionally, cap screws increase the diameter of the impeller eye. The impeller eye is the terminal area on the cap screw end which is located radially inward of the impeller contour.
Other factors are the effect of thermal expansion of the aluminum impeller versus the steel drive shaft, and the fretting between the parts.
BRIEF SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to provide a simpler and improved impeller and shaft assembly.
Another object is to provide an impeller and shaft assembly that employs an arrangement that more effectively dissipates the clamping load.
Yet another object is to avoid stress fractures in the front face of the impeller leading to replacement of the entire impeller.
A further object is to provide an impeller assembly that is less prone to damage resulting from failure to use a torque wrench.
It is an object, feature and advantage of the present invention to expand the impeller contour into the area of the impeller eye. It is a further object of the invention to provide a contour to the fastener or washer located in that eye area. It is still a further object, advantage and feature of the invention that the contour added in the area of the impeller eye should be continuous with the contour of the impeller itself.
It is an object, feature and advantage of the present invention to provide a collapsible washer that counteracts the effects of thermal expansion between the aluminum impeller versus a steel drive shaft.
It is a further object, feature and advantage of the present invention to reduce fretting between the components of a high speed impeller, shaft and fasteners.
At least one of these objects is addressed, in whole or in part, by the present invention. The invention is a rotatable impeller assembly for a refrigerant compressor. The assembly includes an impeller, a protective washer, a contoured spacer body, and at least one spring. (In this specification, an element introduced with an article “a,” “an,” or “the,” such as “a spring” or “the bore,” should be read to include one or more of the element.)
The impeller has an axial bore through it, a front face intersecting with the axial bore, and a rear face that is adapted to fit the driving end of a rotatable shaft. The protective washer is seated against the front face of the impeller. The rear face of the protective washer is seated against the front face of the impeller. The protective washer has an aperture registered with the axial bore. The contoured spacer body has a front face, a rear face, a recessed spring bearing surface in its rear face, a spring spacing abutment positioned to seat against the protective washer, and a central bore. At least one spring is seated between the protective washer and the spring bearing surface to provide a spacer assembly. The protective washer is used to keep the at least one spring from damaging the impeller.
A fastener (such as a bolt), including a headed front end and a rear end, is positioned through the axial and central bores. The rear end of the fastener is connected to the rotatable shaft. The headed front end of the fastener is seated against the front face of the contoured spacer body to provide a clamping load. The front face of the contoured spacer body may further comprise a recess sized to accommodate the headed front end of the fastener.
An advantage of this invention is that the cross-section area of the headed front end of the fastener does not govern the cross-section area of the protective washer and the at least one spring. Instead, the protective washer and the spring are sized to correspond to the much larger cross-section area of the rear face of the contoured spacer body, which itself closely matches the cross-section area of the front face of the impeller. Hence, the clamping load, after bolt tightening, is dissipated over a relatively large area of the front face of the impeller.
This arrangement has two immediate and very advantageous consequences. First, the front face of the impeller is less likely to suffer stress fractures. Second, even in the event that the clamping load causes stress fractures in the region immediately around the headed front end, such damage will only require the replacement of the contoured spacer body rather than the replacement of the impeller.
Another advantage of this invention is that the use of a collapsible washer counteracts the effects of the thermal expansion caused by the difference in materials between an aluminum impeller and a steel drive shaft. The use of the collapsible washer also reduces fretting between the parts since the washer absorbs some of the tension generated in axial directions.
Yet another advantage of the present invention is that the impeller contour is extended closer to the axis of impeller rotation. This is accomplished by modifying the contoured spacer body to extend the impeller contour over the fastener and washer area.
A further advantage of the present arrangement is that a maintenance engineer not using a torque wrench is far less likely to damage the impeller shaft assembly by applying too great a clamping load at the headed front end of the fastener. This is because in one aspect of the invention the contoured spacer body includes a spring spacing abutment
Dewhirst Randy E.
Harrison Mark W.
American Standard International Inc.
Beres William J.
O'Driscoll William
Ryznic John E.
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