Dry Friction : (15:04 21/04/2011)
Dry friction which is also called "Coulomb" friction, describes the tangential component of the contact force that exists when two dry surfaces move or tend to move relative to one another. If the solid bodies are loaded together and a tangential force (F) is applied then the value of the tangential force that is required to initiate motion is the static friction force . It may take a few miliseconds before relative motion is initiated at the interface.
(Bharat Bhushan, Introduction to Tribology, p.207)
Trasfer Film : (15:11 21/04/2011)
Many plastics sliding against hard mating surfaces result in formation of transfer films of plastic onto the mating surface. The formation and behavior of the transfer films are important factors in the friction and wear of these plastics. Once a transfer film has formed, subsequent interaction occurs between the plastic and a layer of similar material, irrespective of the substrate. On further sliding the plastic may continue to wear by adding material to the transfer film, since the interfacial bond to the counterface is often stronger that that within the bulk of the polymer itself. The trasfer film also wears through generation of wear particles and reaches a steady thickness in an interface with low friction and wear. The coefficien of friction for initial sliding on a clean hard substrate is not particlarly low and the trasfer film is on the order of micrometers thick. As sliding progresses the coefficient of friction drops to a much lower value; the trasfer film becomes much thinner and contains molecular chains strongly oriented parallel to the sliding direction.
(Bharat Bhushan, Introduction to Tribology, p.262)
Through-Shaft Duplex Mills (in Roll Forming) (22:55 21/04/2011)
The through-shaft duplex mill is a combination of the duplex and conventional (standard) mills. The main characteristics of the through-shaft duplex mills are:
* The through-shafts deflect less than the cantilevered shafts.
* Rolls are mounted on sleeves.
* Sleeves are attached to the stand at each side.
* All the operator-side stands are mounted on a common plate, which can slide in or out, thus
changing the distance (roll lengths) between stands. Other types of mills allow both sides to move
more in and out.
* Sleeves with the mounted rolls are keyed to and sliding in and out on the shafts.* Through-shaft can carry center rolls between the two edge-forming rolls. These center rolls can
support or form beads into the products.
The advantages of the through-shaft duplex mill are:
* The reduced shaft deflection permits the forming of thicker/higher strength material than that of
the duplex mills with the same shaft diameters.
* The carrying rolls can support both the top and bottom of the center part of the product.
* It is relatively simple to incorporate forming at the center part of the product.
* Through-shaft duplex mills do not have restriction on the formed width at the strip edges while
duplex mills are restricted to form the relatively narrow edges (4 in. or 100 mm) of the strip.
The disadvantages of the through-shaft duplex mill are:
* More expensive than the duplex mill.
* Slightly larger diameter rolls are required because they are mounted on sleeves.
* Relocating the center rolls can be cumbersome.
Many of the variations described in the duplex mills, such as installing two mills in one line, can also be
applied to through-shaft duplex mill.
(George T. Helmos, Roll Forming Handbook, p. 2-8)
Spiral-Tube Mills (in Roll Forming) (23:01 21/04/2011)
Corrugated spiral tubes with lock-seamed, welded, or other joints are used for rigid culvert pipes, flexible water/electrical hoses, and other applications. The corrugated section is formed in a mill and curved with a special curving head . Most frequently, the edges are lock-seam joined continuously in the curving head (mechanical joint) or they are welded. The diameter of the finished tubes/pipes is changed by altering the “helix angle” (the angle between the entering profile and the exiting tubular product), without changing the width of the roll formed section. Usually, the curving head and the run-out table remain in a fixed position, and the helix angle is changed by “swinging” the mill bed around a pivot point, which is located at the curving head. The same principle is used in the manufacturing of smooth wall lock-formed (or welded) ventilation pipes. For light gauge ventilation pipes, the spiral tube curving is often accomplished with “bronze shoes.” In this case, each diameter (and helix angle) must have its own shoes.
(George T. Helmos, Roll Forming Handbook, p. 2-16)
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