Course Work: MSE 480 Materials Dissection
Selection Process
To begin the process of choosing a material, we must plot these properties for as many materials as we have data, we do this using CES Selector 4.0 software by Granta Design Limited. Because they are directly proportional, we may substitute tensile strength for shear strength, and so we plot hardness against the tensile strength.
In our initial plot, we see that we are faced with a tremendous amount of materials, to limit this; we must look for a general constraint on pad design, the hardness. By setting our upper limit on hardness to an order of magnitude lower than the hardness of 6000 series aluminum, we can safely assum that the pad will wear instead of the rim. So, we replot the data with this new constraint.
In order to further refine our list of potential materials we must turn to other
properties, which will affect the final performance. Tantamount to brake performance
is of course the ability to cause friction with the rim, so we must look for
a material with a high coefficient of friction with aluminum. Since the CES
software does not offer any information on the coefficient of friction, we must
move to another source of information, "Machinery's Handbook 22nd Edition",
for some guidelines on friction. Looking at table 2 (Oberg,
pg. 436), we can quickly begin to eliminate families of materials, such
as woods and ceramics. The two families of materials with the highest coefficient
of friction with metals, aside from other metals, which can be discounted because
of hardness, were leather, and rubbers, with rubbers having a significantly
higher coefficient of friction. Of the two families of materials, we can easily
eliminate leather, as it is not possible to process leather into the complex
shape needed for a brake pad.
Now that we have limited our materials to one type, we can return to our materials
index, and choose the most appropriate rubber from the data we have available.
There is one difficulty with this, because of rubbers' unique properties, it
is very difficult to obtain hardness data because of the inability to leave
a permanent indentation. As a result we must compare rubbers based on their
tensile strength, and then compare their relative hardness as given by the CES
database. We find that of all the traditional rubbers, filled butyl rubber has
the highest tensile strength, nearly twice that of the closest competitor, unfilled
butyl rubber. Also, its relative wear charactaristics are considered good, as
opposed to poor or average for all other rubbers. Thus, we would choose filled
butyl rubber to make our brake pad. However, there is one nontraditional alternative
that was found by examining our materials index plot, High Impact copolymer
modified rubbers. These materials boast hardness values twice that of filled
butyl rubbers, as well as high hardness values.
Next: Final Materials
this site last updated 10-21-2002 by pmiska@deepthought.org