Boundary Lubrication
In this type of lubrication, a thin layer of lubricant film physically adheres to the surfaces by molecular forces, such as van der Waals forces, or by chemical forces (chemisorption). Typical boundary lubricants are oils, fatty oils, fatty acids, and soaps. Boundary films can form rapidly on clean surfaces, although reactivity on some metals such as titanium and stainless steel is very low; lubrication may then be enhanced by the formation of boundary films on tool and die surfaces instead of on the surface of the workpiece.
An important distinction is that, unlike in full-fluid film lubrication, where the bulk properties of the lubricant (such as viscosity) are important, in boundary lubrication, the chemical aspects of the lubricant and its reactivity with metal surfaces are important; viscosity has a secondary role. In the boundary lubrication area, the coefficient of friction usually ranges from 0.1 and 0.4, depending on the strength and thickness of the boundary film. Boundary lubrication is often observed and practiced in metalworking operations.
Wear rate in this regime depends on the rate at which films are destroyed by rubbing off, or by desorption owing to excessive temperature generated during the metalworking process. If the protective boundary layer is destroyed, then friction and wear will be high. The adherence and strength of this film is therefore a very important factor in the effectiveness of boundary lubrication. The role of pressure, speed, and viscosity on film thickness should also be recognized.
(Halmos G. T., Roll forming handbook, sector 7 p. 4)
Mixed Lubrication
The film thickness in thick-film lubrication can be reduced by decreasing the viscosity (such as owing to temperature rise), decreasing the sliding speed, or increasing the load. The surfaces become close to each other and the normal load between the tool or die, and the workpiece is supported partly by the fluid film in hydrodynamic pockets in the surface roughness of the interfaces and partly by metal-to-metal contact of the surfaces. This is generally referred to as mixed lubrication and also as the thin film or quasihydrodynamic regime.
The film thickness is less than three times the surface roughness; the coefficient of friction may be as high as about 0.4 (thus, forces and power consumption may increase substantially), and wear can be significant. There is an optimum roughness for effective lubricant entrapment, with a recommended roughness of usually 15 m. The hydrodynamic pockets also serve as reservoirs for supplying lubricant to those regions at the interface that are starved for lubricants.
(Halmos G. T., Roll forming handbook, sector 7 p. 4)
Folded Free Loop
To avoid making pits in the plant floor, very seldom the excess loop length is allowed to create a free folding loop (Figure 4.23). The loops, in a box-type structure, are supported at the strip edges. The equipment supplier occasionally uses folded free loop during setup tests where the floor does not have a pit. This system is not recommended in production lines, because the strip surface can be damaged and it is difficult to control the loop and the speed of the operation.
(Halmos G. T., Roll forming handbook, sector 4 p. 13)
Forced Loop
To reduce the space requirement and to avoid the use of pits, the loop can be forced upwards (Figure 4.24). The weight of the strip and the top roll is balanced by counter weights or by cylinders. It can be economically used in front of roll formers but the speed of the ascending and descending top roll can restrict its application in lines with fast accelerating press feeders.
It is the tool and equipment designer’s function to evaluate all factors, to consider the advantages and disadvantages, and to select the best process. Usually, the production speed, length, and variations of the pattern (e.g., hole pattern created by each stroke), tolerance requirements, method of synchronizing the cutoff, or other operations with the previous operation, the type and thickness of material, the shape of notching or punching, and the price will determine the technology. It is feasible to process one product in tight line and another one in loose line in the same roll forming line.
(Halmos G. T., Roll forming handbook, sector 4 p. 13)
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