3 Fundamentals of press design

184 Sheet metal forming and blanking
by viscosity demonstrates the greatest dependence on the two parameters:
pressure and temperature. Particularly at contact points with high
surface pressure, attention must be paid not only to the temperature,
but also to the influence of pressure. Lubricants can be classified as follows:
– water soluble,
– non water soluble,
– solid lubricants,
– foils and varnishes.
Liquid lubricants are predominantly oil based. These are generally blends
without a consistent composition, as the portions of paraffin, aromatic
and naphtha content varies according to the origin of the oils. Animalbased,
vegetable and synthetic oils also belong in this category. Pastetype
lubricants are stabilized blends of mineral or synthetic oils, greases
and waxes as well as soap. Powdered or needle shaped hard waxes
and hard soaps are used as solid lubricants. The properties of lubricants
can be altered by additives and adjusted to suit the respective application.
Additives serve to improve load bearing capacities, bonding
strength, and viscosity-temperature-pressure characteristics, and also
assist in preventing corrosion. The additives create either physical
adsorption layers or chemical reaction layers.
By means of so called “friction modifiers” (for example animal and
vegetable fats, fat soap etc. also belong in this category), the lubricant
bonds itself to the metal surface without causing any chemical reaction.
This type of physically-acting additives are, however, inherently temperature-dependent.
The reduction in cohesive and adhesive strength
under increasing temperatures leads to an increase in the coefficient of
friction. The effect of these additives ranges from a total absence of
physical adsorption up to a stable chemical bond (formation of metallic
soap). At high levels of interface pressures, anti-wear additives are
used. They form a wear-reducing protective layer. A sub-division of this
group are the extreme pressure (EP) additives, which form reaction layers
under higher temperatures. With a combination of selective additives,
in this range lubricants can be optimized to suit just about any
particular application.
Based on the discussion given above, the following is a summary of
the performance requirements of lubricants:
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998