Patent Application
20060191095
The present invention relates to a composite buckling spring for use in a compression spring wiper arm assembly.
Over center compression spring wiper arms, where the spring contacts the head casting above the pivot, require a compression spring approximately thirteen millimeters (13 mm) in diameter with clearances of one millimeter (1 mm) to two millimeter (2 mm), and the spring being approximately forty millimeters to fifty millimeters (40 mm-50 mm) long plus extra length for a stiffening rod to keep the spring in place. It would be desirable to provide a compression spring with a very constant generated force.
U.S. Pat. No. 4,991,251 illustrates a wiper arm having a service up feature for the windshield wiper of an automotive vehicle. The service up feature is a European automotive requirement. Typically, an over center tension spring, such as that illustrated in U.S. Pat. No. 4,947,508 allows the wiper arm to be pivoted about the head which is fixed to the vehicle, away from the windshield to a locked, over center spring position to fix the wiper arm and attached blade in a position spaced from the windshield. Various automotive manufactures use a compression spring for the service up feature on automotive windshield wipers. There are numerous problems with the use of a compression spring, such as a wide range of spring constant variability from spring to spring as well as the loss of spring force over time. The compression spring also functions to maintain the outer end of the wiper blade against the windshield with appropriate wiping force as the wiper blade moves across the generally curved windshield.
A composite pultruded plastic fiber linkage has been proposed in U.S. Pat. No. 6,148,470. The advantage of a composite pultruded plastic fiber linkage is that it provides a repeatable spring force which does not deteriorate over time. The pultruded linkage of U.S. Pat. No. 6,148,470 is also less expensive than corresponding metal linkages.
The present invention eliminates the over center service up spring end clip by using a beam placed in compression. The beam can be a separate composite pultruded member, or alternatively the beam can be integrally stamped from a steel wiper arm. In either case, a beam compression spring is formed in association with the wiper arm to provide a service up position and can be biased between the wiper arm and the wiper linkage head. The beam and the wiper arm can be formed of steel material or a composite pultruded material. The beam can be a loose piece attached to the arm or integrally formed with the arm In either case, the beam defines a spring force biased against the wiper head and wiper arm
The present invention can use a flat composite pultrusion, thin, wide as permitted in the arm construction, and approximately sixty millimeters (60 mm) to approximately 100 millimeters (100 mm) long. The compression spring beam is placed in compression in a buckled situation. The force generated can be very constant and a pivot point between the wiper arm and wiper linkage head can be placed to operate at a fairly constant offset for the rise and fall projection of the arm through active use. The pultruded compression spring beam has the ability to deflect substantially without fracture. A thin, axially oriented, or nearly axially oriented, spring is pultruded with 21% thermoset polyester, or similar thermoset or thermoplastic resin with glass fibers oriented within 15° of axial, ±20°. These strips can be cut to the required design length to buckle per a Euler formula Pcr=(π2×EI)/L2. A tab or other protrusion can be used to insure single direction buckling of the beam compression spring. Opposing sockets can be positioned so as to give a working range load, by way of example and not limitation ±10° of perpendicular to an axis of a load, a very constant moment. This configuration can provide a tip load of ±1½% in ±10° arm motion and ±6% in ±20° travel. It should be recognized that the design can be altered to vary the loads outside the close intolerance ranges provided, by way of example and not limitation, the present invention can be configured to keep compressive stresses below approximately 60,000 lbs per square inch (psi) for high cycles and near or under 100,000 lbs per square inch (psi) for low cycle operation. It should be noted that the load drops to zero rapidly once the beam compression spring becomes unbuckled which can be advantages for assembly or a no-load “park” position. By way of example and not limitation, compressive load of a pultruded axially oriented glass E-glass 113 yield 75% by weight, 21% thermoset polyester by weight, 4% fill by weight, 25 millimeter wide, 60 millimeter long, 1.25 millimeter thick σ=51 ksi at 2 millimeters, 98 ksi at 8 millimeter service up position.
A method of weaving and fabricating arm springs according to the present invention can include a tri-axial woven sock made or purchased. The tri-axial woven sock can be run through a thermoset resin bath and cured in a heated pultrusion die. The cured product can then be fed into an extruder to add cylindrical ends to the strip in order to provide a continuous strip that can be cut off in varying widths. The varying widths can provide varying compressive load depending on the width. The weaving of the tri-axially sock provides nearly cross axis perpendicularity, later after being cut off, and leaves the fibers nearly axial to the buckling load to optimize stress and strain characteristics of the pultruded beam compression spring according to the present invention.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.
The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:
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The beam compression spring 38 can be formed of any suitable material using any known manufacturing process. By way of example and not limitation, the beam compression spring 38 can be formed of steel, either separately, or as a stamping with either the wiper arm 26 or linkage head 12, or the beam compression spring 38 can be formed of a composite material, either separately, or as part of either the wiper arm 26 or linkage head 12.
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In the windshield wiper arm assembly 10 according to the present invention, the wiper arm 26 can be coupled to the wiper linkage head 12 through a pivot pin 24 allowing pivoting movement of the wiper arm 26 with respect to the wiper linkage head 12 between a stable windshield engaging position 42 and a stable windshield disengaged position 40. The beam compression spring 38 can bias the wiper arm 26 and the wiper linkage head 12 between the stable windshield engaging position 42 and the stable windshield disengage position 40. The beam compression spring 38 can be integrally formed with one of the wiper arm 26 and the wiper linkage head 12. Alternatively, the beam compression spring can be attachable between the wiper arm 26 and the linkage head 12. The beam compression spring 38 can be formed of a composite pultruded material or can be integrally stamped from a metal material. The beam compression spring 38 can include a cross-woven elongate tri-axial glass fiber sock 52 bathed in a thermoset resin bath 54 and cured in a heated pultrusion die 56 to form a pultruded elongate strip 60. The pultruded elongate strip 60 can be overmolded with thermoplastic extruded cylindrical bearing ends 48, 50 on opposite sides 62, 64. The elongate strip 60 can be cut transversely to form a pultruded beam compression spring 38 of a desired width having overmolded cylindrical bearing ends 48, 50 and cross-woven glass fibers 68 oriented axially between the bearing ends 48, 50 at an angle between approximately 0° and approximately 30°, inclusive.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.
