Joints and connections – Utilizing thermal characteristic – e.g. – expansion or... – Members having different coefficients of expansion
Reexamination Certificate
1998-12-14
2001-05-01
Saether, Flemming (Department: 3627)
Joints and connections
Utilizing thermal characteristic, e.g., expansion or...
Members having different coefficients of expansion
C411S531000, C411S534000, C411S545000
Reexamination Certificate
active
06224288
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of mechanical washers and bearings and mechanical assemblies subjected to thermal expansion stresses and differential motion during temperature variations.
BACKGROUND OF THE INVENTION
Changes in temperature can cause excessive thermal stresses at the attachment points between arbitrary parallel plates of a mechanical assembly, such as a top component plate and a bottom base, both rigidly fastened together. An example of such a mechanical assembly is a high precision optical instrument assembly or inertial guidance unit on a spacecraft. The unit has an aluminum housing bolted to a graphite-epoxy honeycomb support base structure. The instrument and/or the support base are subjected to large changes in orbital temperature with resulting differential expansions between the base structure and the instrument. The top component plate and the bottom base plate could be fastened together using conventional fastening bolts, nuts and washers, through aligned holes extending through the component and base plate. The top component plate and the bottom base plate may have substantially different coefficients of thermal expansion. The resulting unequal amounts of expansion or contraction, if constrained, can cause significant locally induced strains and stresses. During temperature variations, the top component plate may expand and contract at a different rate than the bottom base plate, thereby tending to misalign the base holes and placing stresses upon the bolts extending through the fastening holes, leading to stress and even eventual failure of the bolt. The differential in the thermal expansion can also lead to damage to the component plate and base plate, or may result in misalignment of precision mounted instruments. These and other disadvantages are eliminated or reduced using the invention.
SUMMARY OF THE INVENTION
An object of the invention is to provide a linear slider washer bearing that reduces stresses in an assembly having fastening bolts extending between a component plate and a base plate, both expanding and contracting at differing rates during temperature variations.
Another object of the invention is to provide a method of rigidly fastening the component and base plates together using a linear flexure washer bearing, thus enabling the component and base plates to expand and contract at differing rates during temperature variations without inducing excessive stresses in the component or base plate.
The present invention is directed to a corrugated slider washer bearing. The corrugated slider washer bearing comprises a pair of sliding washer plates each having one or more mating parallel extending corrugated grooves. A bolt hole extends through the top and bottom washer plates for fastening a component plate and a base plate together. The top washer plate mates with the bottom washer plate, and the top washer plate can unidirectionally slide upon the bottom washer plate. The top washer plate can slide relative to the bottom plate along one direction in the presence of horizontal loads upon the top washer plate relative to the bottom washer plate.
The sliding plates are formed to be disposed in alignment around a bolt hole having a larger diameter than a fastening bolt extending through the bolt hole in the top component plate and into the bottom base plate into which is rigidly fastened the bolt. The bolt fastens together the top component plate, the bottom base plates and the two flexure washer bearings. In the preferred form, a bottom slider bearing is disposed between the bottom base plate and the top component plate and another top slider bearing is disposed between the bolt head and the top component plate, with the bolt rigidly fastened into the bottom base plate. As the top component plate expands or contracts relative to the bottom base plate during temperature variations, the bottom slider washer plate of the bottom slider washer bearing remains rigidly affixed to the bottom component plate and the top slider washer plate of the top slider washer bearing remains rigidly affixed to the bolt head, as the bottom washer plate of the top washer bearing and the top washer plate of the bottom washer bearing remain affixed to the top component plate, to enable the expansion and contraction of the top component plate in plane relative to the base plate without placing excessive stresses upon the bolt. As the top component plate expands or contracts, the hole in the top component plate moves in misalignment relative to the hole in the bottom base plate. At all times, the bolt remains in a rigid vertical and orthogonal position relative to the horizontally extending assembly. The hole in the component plate being larger than the bolt stem creates sufficient space for the relative movement without the top component plate inducing excessive stresses in the fastening bolt. The corrugated grooves in the slider washer bearing enable sliding movement along one direction of the thermal expansion and contraction of the base and component plates, while constraining sliding motions in all other directions.
The invention minimizes interface stresses by allowing relative differential motion between the component plate and base plate fastened by a bolt. The full strength of the bolt is available to support the component plate. The strength of the bolted assembly is not compromised in the presence of relative thermal expansion and contraction. The sliding of the washer bearing avoids excessive friction shear forces. The washer bearings function as thin spacers between the component and base plates, thus enabling high preload tightening of the bolted assembly. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.
REFERENCES:
patent: 627938 (1899-06-01), Long
patent: 629699 (1899-07-01), McQueen
patent: 2952344 (1960-09-01), Pope
patent: 4214444 (1980-07-01), Fujioka
patent: 5364214 (1994-11-01), Fazekas
patent: 5385433 (1995-01-01), Calandra
patent: 5439309 (1995-08-01), Lhernould
Pan Robert B.
Postma Robert W.
Reid Derrick Michael
Saether Flemming
The Aerospace Corporation
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