|
Part tolerances are allowable
variations in the dimensions of manufactured components. They are expressed as
plus or minus values, or as a range of measurements. If a part is out of
tolerance, problems may occur. For example, if a rubber door seal on a machine is
out of tolerance, the door may be difficult to shut or fail to provide proper
sealing and insulation.
Engineers and product designers understand
part tolerances for metal components, but may be less familiar with tolerances
for rubber parts such as door seals. Rubber has different properties than
metal, of course, and is more sensitive to environmental conditions such as temperature.
The type of rubber and the kind of tooling that is used during manufacturing
also affects part tolerance.
Solving Part Tolerance
Challenges
Sometimes, an engineer or product
designer specifies a tolerance that would be fine for a metal part, but not for
a rubber profile. An experienced custom fabricator can review your drawings or
CAD files, and suggest modifications. By examining tolerance tables from the Rubber Manufacturers Association (RMA), the
custom fabricator can then recommend solutions that support your product
design.
The RMA is well-known for its work
regarding tire safety, but this trade organization supports many manufacturers
of rubber products. The RMA's tolerance tables help during part
design and production, and provide a "common language" that the entire supply
chain can understand. Buyers of industrial rubber products don't need the in-depth
knowledge of a custom fabricator, but it helps to understand the basics.
Molded and Extruded
Parts
The RMA Handbook contains tolerance
tables for both molded and extruded rubber. Molding and extrusion are different
processes, but both subject rubber parts to physical changes. For example,
during molding, the rubber expands as it vulcanizes. The recipe for the rubber
determines the temperature at which curing or vulcanization occurs. Later, when
the molded part
cools, it shrinks.
Extrusion also causes changes in
part size, and extruded parts can swell or shrink depending on the compound
that's used. As extruders know, the shape of the die isn't the shape of the
final part. There are other considerations, too. Solid
extrusions are denser and easier to control. Extruded sponge and foam exhibit
larger variances because of the chemical reaction that creates the cavities.
Tolerance Types and RMA
Classes
As the RMA Handbook indicates,
extruded parts have tolerances for shape or cross-section, cut-length, and
angle cut. In the case of cut-lengths, it's important to remember that rubber
stretches. Tight tolerances are possible with metal channels, but not with
rubber extrusions that expand or contract with temperature changes. Also, the
tolerances on angle cuts used with spliced gaskets aren't the same as the tolerances
for cut lengths. Consequently, each process has its own tolerance chart.
Buyers of industrial
rubber products also need to understand that the RMA Handbook divides, for
example, solid extruded parts into three classes: high precision (1), precision
(2), and commercial (3). That's why when you receive a quote for a rubber part,
you may see a line such as "per RMA-E2". This refers to the specific RMA tolerance
table (Table 13, which uses a lettered identifier of E), as well as the
precision class (2).
Join the Conversation
Do you have questions about part
tolerances for rubber components such as seals and insulation? Would you like
to learn more about molding and extrusion, and why choosing
the right rubber is so important? Do you have questions about RMA tolerance
tables, such as which ones apply to your design? I hope you'll comment on this
blog entry.
About the Author: Doug
Sharpe is the President of Elasto Proxy, Inc. (Boisbriand,
Quebec, Canada), supplier of sealing solutions and custom-fabricated rubber and
plastic parts to a variety of industries, including automotive and mobile
specialty vehicles.
| 
