Bob Paugh and Ken Sterry
Pawling Engineered Products
Pawling, N.Y.
But what if the piece is too
fragile or irregular for point-source gripping,
or the factory environment is too corrosive
or hostile? Or perhaps you want to
cut costs and parts from the clamp or just
make it better for a small added cost?
Maybe you should be thinking of a bladder
clamp, which is essentially a rubber
pillow that inflates and deflates on command.
Bladder clamps reduce breakage in
ice-cream cone bakeries, lug 500lb engine
blocks around foundries, and manipulate
brittle graphite electrodes for blast
furnaces. They’re also beating corrosion
in paper-making and handling machines
and accurately positioning aerospace
parts during adhesive bonding and riveting.
And in many observatories, bladder
clamps handle the delicate job of shuttling
giant — not to mention brittle and
priceless — telescope mirrors in and out
of place for periodic cleaning. Breaking
a mirror there goes way beyond seven
years of bad luck.
Unlike most other clamps and grippers,
bladder clamps distribute force evenly
over the entire contact area and conform
to mildly irregular surfaces. This latter
feature is often the main reason they are
used. By nature, bladder clamps also
damp out vibrations. And being nonmetallic,
they introduce no contaminating
lubricants or sources of sparking and
electrical shorts. This is why they’re often
found in munitions and fuel handling and
other “clean” operations.
Bladder clamps are inexpensive compared
with automated mechanical, pneumatic,
and magnetic clamps. They also fit
more easily into cramped spaces.
Four Basic Profiles
Once you settle on a bladder clamp in
principle, it will pay to begin with a standard
profile if at all possible. Here are the
four most popular profiles for bladder
clamps:
Footed clamps simplify mounting and the bulb portion inflates and moves outward.
This is the profile used in most conveyor
stops and part positioners. The top
surface can have a gripping surface that
ensures contact despite grit.
Low-profile clamps are usually used
in hoop-type clamps, where they snap-fit
into a retention groove.
Convoluted cross clamps offer greater
stroke and higher forces than the other
types. They can be found in load locks in
processing equipment and tray positioners
in hospital sterilizers.
Channel-style clamps fit into standard
aluminum channel stock. They provide
large contact areas, which translate into
higher forces.
Heavy-duty clamps deliver higher
forces but shorter strokes. The sides
elongate during inflation while the flat
top surface remains unchanged. The actuator
is usually installed in a rigid channel,
which holds it in place and ensures
all internal pressure is converted to upward
force.
The most popular materials used for
bladder clamps are elastomers. But for
flammable, explosive, or chip-handling
applications, specify a conductive elastomer
to avoid sparking or static. And
for heavy loads, high clamping forces, or
large-scale clamping, like structural plastic
layups or airframe assembly, specify
reinforced elastomers.
The maximum motion for a bladder
clamp using standard profiles and materials
is about 2.5 in. And because the
clamps are made of rubber, which will cycle,
the upper service temperature limit
is 400°F. Sharp edges, corners, and rough
surfaces should not touch the bladder
clamp. And bladder clamps are not recommended
for precise positioning unless
the workpiece is going to be pushed
against a rigid stop,
Most bladder clamps are specified as
assemblies with the bladder installed
in a rigid retainer. If you decide to make
the retainer yourself, be sure all areas
that touch the bladder are machined to
63 μin. or better with an axial lay. This
will protect against piercing the bladder
during assembly or use.
Is It A Fit?
Is a bladder clamp right for your application? Here are some indicators:
Fragile part or surface that can’t be
marred. Bladder clamps give a gentle,
even grip. Examples: handling green
ceramic ware, kiln loading of grinding
wheels.
Parts too irregular for rigid clamps.
Conformal clamping surfaces distribute
forces evenly for heavy lifts. Example:
moving engine-block castings
Uniform pressure required over large
area. Examples: vacuum forming large
plastic structures, airframe assembly.
Explosive or flammable service. Rubber
bladder clamps eliminate conductive
metals and sparks, and can be made
intrinsically safe. Examples: munitions
and rocket fuel fabrication.
Contamination not tolerable. Rubber
parts don’t corrode or contaminate,
and require no lubricants or coatings.
Examples: food and pharmaceutical processing and
packaging,
clean-room
part handling,
semiconductor-foundry
equipment.
Tight fit,
tight budget.
Bladder clamps
fit into tighter
physical and
budget envelopes
than mechanical,
magnetic,
or pneumatic
clamps.
All you need
to operate is a
hose leading
to a standard
compressed-air
source.
Too many
manual clamps.
A single, quickacting
bladder
clamp saves
time compared
with an array
of mechanical
clamps individually
dogged
down. Example:
doors and
hatches in isolation chambers.
If you think a bladder clamp may meet
your need, collect a few facts about the
requirements and involve an experienced
vendor early on. They probably
have solved dozens of problems much
like the ones that seem so new to you.
Here’s the information to have on hand
for that first inquiry:
- Part dimensions and weight
- Available envelope for the gripper
- Surface areas available for clamping
- Desired force and stroke
- Ambient service conditions (temperature, corrosion)
- Duty cycle, expected life
- Specs on compressed air supply.