Joints and connections – Utilizing thermal characteristic – e.g. – expansion or... – Members having different coefficients of expansion
Reexamination Certificate
1998-10-20
2001-01-09
Kim, Harry C. (Department: 3629)
Joints and connections
Utilizing thermal characteristic, e.g., expansion or...
Members having different coefficients of expansion
C403S404000, C403S408100
Reexamination Certificate
active
06171009
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to temperature compensation of a joint made from materials having dissimilar temperature coefficients of expansion.
BACKGROUND OF THE INVENTION
Increased sophistication of spacecraft and aircraft has resulted in improved weight-to-payload ratios. A part of this improvement is attributable to the use of lightweight composite materials for many structural portions of the vehicle. The use of composites for pressure vessels is well known. For example, U.S. Pat. No. 5,427,334, issued Jun. 27, 1995 in the name of Rauscher, Jr. describes a method for manufacturing a composite propellant vessel with a built-in vane.
One of the problems with the use of such composites on spacecraft or aircraft is that of the relatively great coefficient of thermal or temperature expansion (CTE) of composites, relative to other materials, such as steel. This problem is particularly acute, even in terrestrial applications, when the composite materials are used in or about cryogenic materials such as liquid hydrogen. The structure including the composite materials must initially be fabricated at temperatures at which humans can work, and may then be subject to extreme temperatures, such as −423° F. (−253° C.), when filled with cryogen. Extremely high temperatures may also be encountered under other conditions. The CTE of composite materials such as carbon-fiber reinforced polymer is large relative to other materials through the thickness, or in the direction a fastener would travel, such as steel, and its resistance to crushing forces is much lower.
When a steel screw or bolt is used with a nut to fasten together two structures, such as a hatch on a port of a pressure vessel, where both are made from composite material, the assembly is performed at room temperature, and if the nut is tightened to provide a force which is just below the force which would crush the composite material, the joint will loosen at cryogenic temperatures because the composite contracts more than the bolt. If the joint is exposed to a temperature significantly higher than room temperature, the joint will be crushed, because the composite material expands more than the bolt. Belleville washers are sometimes used to maintain tension in such structures, but tensioning the washer increases the tension in the bolt beyond that required for simply tightening the joint, thereby requiring a larger-diameter, and hence heavier, bolt. Also, Belleville washers must undesirably be carefully designed to the application.
Improved joints between composites are desired.
SUMMARY OF THE INVENTION
A joint subject to temperature variation includes a sheet of material defining a through aperture for an elongated fastener. The material of the sheet has a particular temperature coefficient of expansion, and is subject to damage under compressive forces greater than a given level. An elongated fastener includes a body portion, part of which extends through the aperture. The fastener also includes a head at one end and an other threaded end, and has a temperature coefficient of expansion less than the particular temperature coefficient of expansion of the material. The fastener further includes a nut threaded onto the threaded end. When the nut is tightened at an assembly temperature within the range of variation, it compresses that portion of the sheet of material lying in the region between the head and the nut to a value of compressive force less than the given level. Under these conditions, temperatures different from the assembly temperature may cause the material of the sheet to expand or contract at a rate greater than the corresponding expansion or contraction rate of the fastener so as to result in compression exceeding the given level. A collar surrounds a portion of the body of the fastener, and is located either (one of) between (a) the nut and the sheet of material, or (and) (b) the head and the material. The collar has a temperature coefficient of expansion which is less than that of the fastener, and has a length, which is in the direction of the axis of elongation of the body of the fastener, selected so that the difference between a first product and a second product is greater than or equal to the product of the particular coefficient of expansion of the material multiplied by the thickness of the material. In this selection, the first product is the product of the coefficient of expansion of the fastener multiplied by the length of that portion of the body of the fastener lying between facing sides of the head and the nut. In this selection, the second product is the product of the coefficient of expansion of the collar, multiplied by the length of the collar.
In a particular avatar of the invention, the material of the sheet is carbon fiber reinforced laminate having a temperature coefficient of expansion of about 16, the body portion of the fastener is made from steel having a temperature coefficient of expansion of about 6.2, and the collar is made from INVAR alloy having a coefficient of expansion of about 1.2.
In a manifestation of the invention, the collar includes a body portion and an enlarged flange portion at one end. The flange portion is oriented to lie adjacent the sheet material, for spreading the compressive forces over a larger area than that of the body of the collar.
A method according to a mode of the invention fastens together multiple layers of sheet material having a particular CTE, to form a joined structure for operation over a particular temperature range. The sheets are of various standardized thicknesses. The material of the sheets has a particular temperature coefficient of expansion, and is subject to damage under compressive forces greater than a given level. The method includes the step of juxtaposing the desired number of the layers of sheet material, to thereby form juxtaposed layers having the desired total thickness. The next step is the procuring of an elongated fastener having a body, a head at one end of the body, and a threaded portion at an other end of the body, and a nut. At least the body portion of the elongated fastener has a CTE which is less than the particular CTE. If necessary, an aperture is defined through the juxtaposed layers. The body of the fastener is placed through the aperture, and the nut is threaded onto the threaded end. At a given temperature within the particular temperature range, the nut is tightened, to create a tensile force in the body portion of the fastener which is insufficient to cause the compressive forces of the given level. Between the step of placing the body of the fastener and the step of tightening the nut, one compensation collar is placed between one of (a) the head of the fastener and the juxtaposed layers and (b) the nut and the juxtaposed layers, for each of the layers of material in the juxtaposed layers. Each of the compensation collars has a CTE less than that of the fastener. Each of the compensation collars has an axial dimension which is selected in conjunction with the known thickness of the corresponding one of the sheets of material, the known CTEs of the material and the length of the body of the fastener, to compensate for differences between the CTEs of the thickness of the layer of material and the CTE of the corresponding length of fastener body, in such a manner as to effectively eliminate changes in the tensile force with changes in temperature from the given temperature, at least over the desired range of temperatures.
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patent: 5467592 (1
Fierke T. G.
Kim Harry C.
Lockheed Martin Corporation
Meise W. H.
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