Patent Application
20150292540
This application relates to clamps, particularly to clamps for beams subjected to high cycle fatigue. The ability to clamp a beam of irregular cross-section in a confined space such that it transfers high loads and moments over millions of stress cycles is desirable for a number of reasons. Machinery found in many industries incorporate beam-like structures subjected to high cycle fatigue. Oftentimes these components would be easier to manufacture and/or maintain if the beam could be easily disassembled from its end-constraint. Examples include aero- and hydro-turbine blades or foils; material-carrying buckets in bucket elevators; bars in trough-chain conveyors, and etc.
Traditionally, beams have been attached to their endplates by an adhesive joint, welded joint, or fastener joint. Adhesive joints require special surface preparation for the adhesive to work properly and adhesives tend to be messy. Also, due to the low strength of adhesive joints, they require a substantial surface area to support significant loads. Welded joints only work for metal to metal attachments and have poor fatigue properties. Joints made with fasteners, such as a bolted joint, require features to be put in the mating parts such as holes and flanges. These extra features create stress concentrations, which are a problem for high cycle fatigue. Wedge clamps are an alternative to a fixed joint.
The present invention, therefore, provides a wedge clamp system for use in a variety of demanding applications using various types of beams with various cross-sectional shapes.
Wedge clamp system
As shown in
Endplate 130 has a pocket 131 that accepts wedge 120 and beam 140. Placing wedge 120 and beam 140 into pocket 131 creates three interfaces: interface 301 between endplate inclined face 132 and wedge inclined face 128, interface 302 between wedge second face 122 and beam curved upper surface 146, and interface 303 between beam lower surface 148 and endplate lower face 134. Interfaces 301, 302, and 303 provide large contact areas to hold wedge 120, endplate 130, and beam 140 in intimate contact by the friction between the parts at the respective mating interfaces 301, 302, and 303. The frictional force is generated by forcing wedge 120 into pocket 131 such that wedge inclined face 128 slides relative to mating endplate inclined face 132 on endplate 130 resulting in compressive stresses at interfaces 301, 302, and 303. In another aspect of the invention, the frictional force on the three mating interfaces 301, 302, and 303 can be further augmented with an adhesive. In another aspect, the frictional force can be augmented by providing a rough surface on either or both of the face 134 or beam lower surface 148 comprising interface 303. The frictional force can be further modified by other methods known to those skilled in the art, as well as combinations of any of the foregoing.
As shown in
Endplate 130 can be made from a material that has the strength necessary to support the endplate stresses in wedge clamp system 10. The highest endplate stresses occur at the two ends where the endplate inclined face 132 and the endplate lower lower face 134 meet. For clamp systems that support significant moments, this stress can exceed 100 MPa. Low strength steels such as 1018 steel can have yield strengths as low as 200 MPa and are usually sufficient. However, for high-cycle fatigue situations or higher clamping forces, it may be necessary to use higher strength steels such as 4140 or 17-4Ph, which can have yield strengths higher than 1000 MPa. The stress can also be reduced by providing additional material around the endplate pocket 131. Similarly, beam 140 can be made from any material as long as its strength can support the clamp induced stresses, including difficult to clamp materials such as fiber reinforced plastics, other composites, and ceramics. This is due, in part, to the large, distributed surface contact area at the interfaces 302 and 303, which results in low surface contact stresses, usually less than 5 MPa. Such low stresses are easily sustainable by virtually all engineering materials including polymers. In fact, it has been found that stress concentrations are surprisingly negligible with wedge clamp system 10, and therefore issues such as localized buckling of composite fibers or crack initiation in metals are not an observed problem even over very high cycle fatigue lifetimes of over 100 million cycles.
Beam 140 has an irregular cross-sectional shape that is asymmetrical about the mid-plane. Many other cross-sectional shapes can be envisioned and are acceptable including shapes that are symmetric about the mid-chord, such as might be found in the blades of an impulse turbine. The curved upper surface 146 that mates with wedge second face 122 can have an irregular geometry along the length of the wedge and can follow any path comprised of straight sections, arcs, or splines, or any combination of the foregoing. The beam lower surface 148 can likewise follow any path and does not have to follow the beam curved upper surface 146.
In wedge clamp system 10, wedge 120 is preferably made from a compliant material such as plastic, which allows it to absorb large tolerances in the shape of wedge 120 or endplate 130. Cross-sectional shape tolerances on the order of a millimeter are easily absorbed with a plastic wedge. In one aspect of the invention, wedge 120 can be made of Delrin, acetal, or nylon. For applications with tighter tolerances such as on the order of a tenth of a millimeter, wedge 120 can be made from stiffer materials such as aluminum, bronze, steel, or others. In another aspect, a wedge with a variable modulus of elasticity along its length could be used, for example, in situations where the wedge has to bend around tight radii. In this situation, it might be useful to have the wedge be relatively stiff over the flatter sections and more compliant over the tight bends.
Referring now to
The manner of using wedge clamp system 10 to secure beam 140 involves placing beam 140 into pocket 131 of endplate 130 and using force to draw wedge 120 into position. If plug 402 is required, it can be fit into hollow beam 2140 before assembly with endplate 130. Proper function of wedge clamp system 10 depends on the wedge angle 307, clamp distance 306, frictional properties of the mating materials, and force used to place the wedge 120 in position. The wedge angle 307 is an important parameter for proper function of wedge clamp system 10. Wedge angle 307 is defined as the angle of interface 301 relative to interface 302 in
There are also upper limits to the wedge angle 307 depending on which embodiment is used. For the first embodiment shown in
It is apparent from the preceding paragraph that frictional properties of interfaces 301 and 302 are also important for proper function of wedge clamp 10. Everything else being equal, a lower coefficient of friction results in a greater clamp force. A lower coefficient of friction can also result in a lower upper limit on wedge angle. This is usually not a concern, especially when fasteners 201 or similar are used and left in place. It is, therefore, desirable to use mating material combinations at interfaces 301 and 302 that minimize friction such as typical between plastics and metals. For material combinations such as steel on steel that have a high coefficient of friction, it is also possible to lube the surfaces prior to assembly to reduce friction; in the case of steel on steel, coefficient of friction will drop from 0.74 dry to 0.16 when lubed. To this end, it is also desirable that the mating surfaces are as smooth as practical without undue expense. For example, it has been found that standard machining surface finishes are adequate. Optimized surfaces that promote sliding in one direction (wedge 120 insertion) and hinder sliding in the opposite direction (wedge 120 extraction) are also possible and could enhance clamp performance. These optimized surfaces could comprise macroscopic features such as a sawtooth profile or microscopic features such as directional whiskers similar to those used on ski skins. Interface 303 typically does not experience sliding during assembly or operation. Therefore, to prevent sliding at interface 303, the coefficient of friction should be maximized. In one aspect of the invention, wedge clamp system 10 generally provides sufficient clamp force such that interface 303 does not need to be altered to provide the requisite friction force. In another aspect, the friction at interface 303 can be increased by applying an adhesive, roughing endplate lower face 134 or beam lower surface 148, or other methods.
Another important parameter for proper function of wedge clamp system 10 is the clamp distance 306 of wedge clamp system 10, or the longitudinal distance that wedge 120 and beam 140 extend into pocket 131. A greater clamp distance 306 allows wedge clamp system 10 to sustain greater beam moment loads. In one aspect of the invention, a clamp distance 306 of 35 mm will support a moment of 1000 N-m whereas a clamp distance of 50 mm will support a moment of 1400 N-m. In another aspect of the invention, it is desirable to limit the clamp distance 306 to only support the expected beam moments and loads to reduce the part size and cost of wedge clamp system 10.
The press force used to draw wedge 120 into endplate pocket 131 during assembly is another important parameter in proper function of wedge clamp system 10. A greater press force during assembly results in a greater clamp force. The press force should be selected to result in a clamp force that is sufficient to support the beam moments and loads expected. The press force can be achieved with fasteners 201 or clamps as described above. In one aspect of the invention with coefficient of friction of 0.25, wedge angle 307 of 10 degrees, and clamp distance 306 of 35 mm, a press force of 21.5 kN results in a clamp force of 31 kN and moment carrying capacity of 1000 N-m. Press force is linear with clamp force and so doubling the press force would double the clamp force and therefore double the moment carrying capacity of the clamp system.
Unlike glued or welded joints, it is very easy to disassemble the wedge clamp system 10, if necessary. If using fasteners 201, as illustrated in
Referring now to
Assembly of this beam and wedge clamp system 3010 is no different from that of a cantilever beam except that there are two wedge clamp systems 10. This system can, optionally, be assembled in a jig that sets the endplate to endplate distance, which allows for a tight tolerance on endplate to endplate distance despite having a loose tolerance on beam length. The beam 3140 can be a simple extruded or pultruded profile that is roughly cut to length with no additional features. If necessary, it is simple to disassemble all parts as described above for the cantilever beam example.
This invention was made with Government support under grant Award Number DE-SC0003343 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61977196 | Apr 2014 | US |
