When we hear these words, we cringe. Not because we can't successfully press a
bearing; we can. We cringe because
processes involving press-fit components are often among the most
underestimated, yet critical functions in the manufacturing process. In this article, we'll discuss some key
considerations that apply to all pressing applications.
But Force Is Important, Too…
Press It Until…?
The first part of the process that must be defined is
whether we are pressing the object to a hard stop or to a distance. A hard stop may be a step or shoulder on the
part, the bottom of a bore, or a mechanical hard stop in the press itself. The second method entails pressing the object
to a distance, or depth. Assuming that
the press has adequate tonnage, the press stroke will continue until a signal
indicates that the distance or depth falls within a given parameter and that
the press may now return. This signal
may come from an external proximity switch, mechanical switch, optical switch,
or LVDT.
But Force Is Important, Too…
The next major consideration is force. Outside dimension minus inside dimension
yields the level of interference fit and, subsequently, the amount of force
required to press the two parts together.
The reasons why interference and force are so important are pressed part
retention and structural integrity. Too
little force and the pressed part (bearing, gear, etc.) may simply fall free of
its mate. Too much force and one or both
parts may suffer irreparable, and maybe even undetected, structural integrity
damage (cracked housings, collapsed dimensions, etc.).
Closing the Loop : Force
over Distance…
Since distance and force are both important to the process,
we may need to set parameters for each factor.
Whereas a switch or LVDT confirms the distance, a load cell will provide
feedback on the amount of force encountered during the cycle. Correlating these two parameters together to
create a Pass or Fail condition closes the loop. Force-over-distance logic holds that the part
was pressed to the right depth, encountered forces that were within spec, and
that these two events occurred at the right time relative to one another.
This graph illustrates possible outcomes of a force-over-distance application:
What Source for Force…?
The three most common sources of force in a press are
pneumatic, hydraulic, and ball screw.
Pneumatics are most commonly used in applications with lower forces
where the part is pressed to a hard stop and the position confirmed by a
switch. In higher force applications, it
is not uncommon to get erratic or jerky motion due to the compressibility of
air. An air-over-oil reservoir can smooth
out this motion but force will still be limited.
Hydraulic cylinders are more often used in medium to high
force applications. Hydraulics can
provide high pressing forces (100,000 lbs. plus), extremely smooth operation,
high speed, and when combined with a servo positioning valve and integral LVDT,
surprising positional accuracy.
For the ultimate in positional accuracy, try a ball screw
press driven by a servomotor with encoder.
The downsides of the ball screw are that it usually gives up tonnage to
the hydraulic cylinder, often gives up speed as well, and will be more
expensive to purchase. But you can't
argue with the accuracy!
Last, there is a hybrid system commonly referred to as an
air-over-oil intensified cylinder. This
hybrid technology combines the cleanliness of pneumatics with the force of
hydraulics. There is a catch, however:
while the overall stroke may be considerable (10" plus), the actual power
stroke wherein the intensified forces are generated may only be 1" or
so. These systems require only
compressed air and conventional pneumatic valves and plumbing to operate. When combined with an integral LVDT and
precision pressure switch, these cylinders can deliver incredibly accurate
results.
Pulling it All Together…
Pulling it All Together…
In summary, if you're going to press-fit two parts together,
then you must first decide if you are pressing to a hard stop or to a depth,
and to what level of accuracy. The
second consideration is the amount of force that will be required under maximum
tolerance buildup conditions and whether this force should be a condition for
passing or failing the finished assembly.
The third consideration is the method by which you will press these
parts—pneumatic, hydraulic, or ball screw.
Our salesmen are always just an email away to help execute a successful project. If you have automation jobs you need a quote on reach out to Jay at machines@turnermc.com If you have build to print jobs be sure to give Mike a shout out at dedication@turnermc.comWe'd love to show you what makes us stand out from the rest. Check us out by following us on Facebook and see our videos on Youtube! Curious about us? Visit our website at TurnerMC.com and see what our culture is about!
