4 Bar Linkage (March 31, 2011 - 01:07)
In all applications of 4-bar linkages one bar is fixed and is termed the frame; see AB in the figure above. The two bars which rotate about the fixed points A and B are termed cranks, while the bar DC opposite the frame is called the coupler.
The characteristics of a given 4-bar linkages clearly depend on the relative lengths a, b, c, d of the bars and which bar is chosen as the frame. For the linkage to exist at all it is clear that no single bar can be longer than the sum of the remaining three, so:
a < b + c + d
b < a + c + d
c < a + b + d
d < a + b + c
but other relevant inequalities are not so obvious.
(Bolt B., Mathematics meets technology, 2007, pg. 82)
Although most types of plastics can be repaired with adhesive materials, hot-air welding is usually preferred for thermoplastics because of its speed and ease. A plastic welder only takes a couple of minutes to heat up to operating temperatures and, once hot, can weld at speeds ranging from 5 to 30 inches per minute, depending on the application. Best of all one can go directly to and from welding to sanding and painting without waiting for the repair to cure (although it might be necessary to allow a few minutes for the welded plastic to cool).
Many adhesives do require mixing and can take anywhere from 30 minutes to several hours to cure, but heat can be used to shorten the curing time. This is not necessarily a disadvantage because repair technicians can always find something else to do while the adhesive cures. As for surface preparation, both welding and adhesives require some grinding, sanding, or trimming for good adhesion. More preparation is generally needed with adhesives, however, because the edges of the damaged area often have to be featheredged before the adhesive is applied. Some adhesives also require reinforcing or support patches behind the damaged area, making the repair a several step process.
To accomplish a plastic weld, either of two types of equipment can be used:
Hot-air welder
Airless
(Scharff R., Mullen K., Corinchock J.A., Complete Automotive Estimating, 1990, pg. 106)
Hooke's Coupling (Universal Joint) (March 31, 2011 - 00:52)
It is a rigid coupling that connects two shafts, whose axes intersect if extended. It consists of two forks which are keyed to the shafts. The two forks are pin joined to a central block, which has two arms at right angle to each other in the form of a cross. The angle between the shafts may be varied even while the shafts are rotating.
(Narayana K.L., Kannaiah P., Reddy K.V., Machine drawing, third edition, 2006, pg. 123)
Stitching (March 3, 2011 - 01:24)
Stitching has been used for more than 20 years to provide through the thickness reinforcement in composite structures, primarily to improve damage tolerance. The major manufacturing advancement in recent years has been the introduction of liquid molding processes which allows stitching of dry preforms rather than prepreg material. This enhances speed, allows stitching through thicker material, and greatly reduces damage to the in-plane fibers. Besides enhancing the damage tolerance, stitching also aids fabrication. Many textile processes generate preforms that cannot serve as the complete structure. Stitching provides a mechanical connection between the preform elements before the resin is introduced, allowing the completed preform to be handled without shifting or damage. In addition, stitching compacts (debulks) the fiber preform closer to the final desired thickness. Therefore, less mechanical compaction needs to be applied to the preform in the tool.
(Campbell F.C., Manufacturing processes for advanced composites, 2004, pg. 312)
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