Frequently asked questions – Airsprings General |
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General |
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Q |
What is an air spring? |
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An air spring is a rubber bellow with 2 solid end plates,
one of which has a threaded hole for an air fitting. Most air springs are 2 ply (inner rubber, first ply
fabric reinforced rubber, second ply fabric reinforced rubber, outer rubber)
although some 3 and 4 ply are available A 2 Ply airspring generally has a maximum pressure of
around 7 bars, giving a burst pressure around 21 bar. A high strength airspring (additional ply’s) have a
maximum pressure of around 12 bars. |
Q |
What kinds of rubber compounds are used? |
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Standard
rubber : - 40°C to + 70°C Butyl
: - 25°C to + 90°C, for higher than normal temperature applications Epichlorohydrin
: - 20°C to + 115°C , is the best with Oils (hydrocarbons) |
Q |
What kinds of end plates are used? |
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The most common material is mild steel but alumimuim and
plastics / nylon are also common. Aluminuim, Plastics and Stainless Steel are used where
rust is a problem. |
Q |
What kind of air holes are used? |
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Threaded air holes range from 1/8” to 1”, some
are holes in the endplate while others are inside a combination stud –
a stud with a thread inside and outside. The inside thread is for the air
fitting, the outside to secure the top plate to its device. |
Q |
What are airsprings generally used for? |
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Air springs are used on vehicles [for chassis, cabin and
seat suspension], in industry as rubber actuators (like a pneumatic cylinder)
or vibration isolators for various applications like [Lifting, Locking, Shock
absorber, Isolation of vibrating machines] and also for railway train
suspension |
Q |
What are the minimum & maximum diameters of
available airsprings? |
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59mm (holds 95 kg at 7 bars, needs
9kg to compress at 0 bars) to 890mm
(holds 37 tons at 7 bars, needs 169kg to compress at 0 bar ) Force
is proportional to area (3.1417 x radius x radius) Instead
of one large airspring, 2 cheaper, smaller airsprings with the same total
area can often be used in a design |
Q |
What types of airspring are available? |
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Rollover sleeves start the smallest. The rubber section
looks like a circular tin can, with a solid top and a solid piston at the
base (usually steel, plastic or aluminuim). The rubber rolls over itself at
the base to shorten or lengthen the airspring These sleeves start small at cabmounts and seat mounts
(59 to 145mm diameters), medium size for 4x4 suspension (151 to 241mm) and
end at large diameter truck and trailor suspension air springs (255 to
361mm). Some of the larger truck sleeves have cheaper “service
assemblies”, usually consisting of a different part number with an
identical ribber section, indentical top plate and no piston. Single convolutes are airsprings which look like a
doughnut. Double convolutes are like two doughnuts joined to each
other and are used to extend the air spring stroke. Triple convolutes are like three doughnuts joined to each
other and are used to further extend the air spring stroke. They are mostly used in industry but also for some truck
lift axles. Convolutes maintain their doughnut type shape. They are often available with bolt on top/bottom plates
with beadrings and sometime girdle hoops, which allows the user to
manufacture their own plates. On many models, the rubber bellow only (without plates
and beadrings) is available as a cheaper “service assembly” They can operate at angles up to 10-30 degrees and have
the plates out of alignment between 10-40mm, or 25mm per convolution. A bus bellow is like a vertically extended single
convolute without a top or bottom plate (the top plate and bottom piston are
considered part of the bus). The inside rubber diameters are usually steel
reinforced to avoid strech. They operate like rollover sleeves. |
Q |
What does an old, used airspring look like? |
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It usually becomes shorter and fatter, when uninflated,
to the extent that it no longer looks like the original part, although the
maximum diameter seldom changes. |
Q |
What are the advantages of airsprings over more
traditional actuators? |
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Air
spring rubber actuators, compared with products like (rigid actuators,
cylinders and springs etc.) offer numerous advantages such as : -
Flexibility (no angle or alignment problems), Compactness, Improved Power /
price ratio, No maintenance or
lubrication required, Durability, Reliability, Resistance to aggressive or
hot conditions, Widest size range, Cheaper than pneumatic cylinders,
especially in large sizes, no internal rods or pistons, Friction free for
immediate response, Flexible media (works with air, nitrogen, water, glycol
and certain oils), Angular capability, Side loading misalignment, Compact
starting height, Factory sealed & tested |
Q |
What are the advantages of airsprings over more
traditional isolators? |
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Air
spring rubber isolators, compared with products like coil springs offer numerous advantages such
as : -
Unsurpassed Isolation Capability
which can even be improved with auxiliary tanks -
Constant Isolation efficiency, has
the same natural frequency for different loads, so the same isolator can be
used at different mounting points on an unevenly loaded machine. -
Accurate height control by
regulating the pressure. Eliminates fatigue and permanent set. -
Wide size range -
Compact installed height (at least
25% of coil springs) -
Extended machine life (from better
isolation) -
Effective Noise reduction -
Versatile |
Q |
What are some example industrial applications? |
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As Actuators: Lift
Tables, Clamp devices, Bumper and actuator, Levelling platforms, Quick lock
devices, Webb Tensioning devices, Pressure compensators, Gate valve operator,
Glueing Press, Die Stripper, Knife spring Actuator, Quench tank actuator,
Paper Sizing press, Pickling Tank actuator, Calendar Pressure rolls, Core
straightener, Belt take up, Roller Friction brake, Pipe Indexing threading
Oscillating doctor for calendar rolls, Forming press, Cable tensioning, Bag
flattener, Torsional friction brake, Hinged gate, Hot foil stamping press,
Heavy duty sealer, Pivoted clamping device, Actauted roller stop, Hinged
actuated gravity gate, Vertical actuated drive table, Scissor lift, Case
packer, Conveyor transfer actuator, Vacuum pump, Inflatable chuck, Cam
follower, Robotic coupling, Conveyor end stop, Screening Machine Cover clamp,
Pipe Crawler, Sheet welding clamp. Swash plate motor, Magazine cutter, Wave
poer module, Check valve lapping, Srlf aligning device, Air blast generator,
Solar tracker As
Vibration Isolators: Oscillating
Conveyors, Vibrating bin hoppers, Vibrating screen, Motor and blower, Control
panel, Shock impact isolation, Protective boot and flexible connector, Mountain bike suspension, Shock
absorber test system, |
Q |
What are some example industries? |
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Undersea diamond mining (pressure compensators) Soft drink can making machines (redraw carriage) Fruit packing (conveyers and sorting
machines) Brick Making (aggregate settling) Movie Making (Camera antivibration
and special effects) Laminated Board Making (Presses) Paper Making (Roll Tensioners) Cement making (Vibrators) |
Q |
What precautions should I take in using air springs? |
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Airsprings should not be pressurised
unless they are restricted by an outside frame or by a suitable load. -
Strokes should be limited by the direct
use of bump stops or external stops. -
When stacking airsprings, special
care must be taken to ensure the airsprings are guided and fixed. -
An Airspring is a single acting air
actuator and must not be used below atmospheric pressure. -
Please check the over-pressure in
case of rapid compression. |
Q |
What are the height related measurements for an
airspring? |
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Minimum Height : The squashed height of the airspring {a
height less than which can damage the airspring]. Some airsprings have
internal rubber bumpers to avoid damage. A bumpstop is often external e.g.
most vehicles have bumpstops to limit the minimum height of the suspension.
The piston, if applicable, is included in minimum height. If the airspring is “overspecced” for the application,
then the pressure at minimum height can act as a limiter to avoid damage by
compression beyond minimum height. Design Height : The “normal” height of the
airspring [a “middle” height for which the airspring is designed,
often the middle of the operating range]. This is usually the distance
between the outsides of the top and bottom plate (or piston) when the machine
is normally loaded. For a vehicle suspension, this would be the inside
distance between the top and bottom brackets when normally loaded. Design height is normally a range of around 20 to 25% of
the total stroke, with the specified Design height in the middle of this
range. If the airspring is used as a vibration isolator, we
recommend that the airspring is used at this height. Maximum Height : The height at maximum extension of the
airspring [a height if exceeded, can cause damage to the airspring when the
rubber tears out of the top/bottom crimps or clamps]. Beyond maximum height,
the load carrying capability and vertical stability of the air spring often
drops off sharply. Stroke: This is the operating range [i.e. Maximum minus
Minimum] Piston Height: This is relevant for rollover sleeves. It
is included in all 3 heights above. |
Q |
What are the diameter related measurements for an
airspring?? |
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For accurate diameter measurement, measure the
circumference [around the outside] then divide by 3.1417 to get the diameter. Normal
Diameter: This is the outside
diameter without any pressure inside the airspring. This measurement is often
used for aftermarket replacement. It usually refers to the point at which the
airspring is widest. Inside
Diameter: This is usually relevant for bus bellows, some of which
are tapered and have smaller bottom inside diameters. This measurement is
important for aftermarket replacement. Top
plate Diameter: The widest distance across the top
solid plate. This measurement is important for aftermarket replacement. Bottom
plate Diameter: The widest distance across the
bottom solid plate (or piston, where applicable). This measurement is
important for aftermarket replacement. |
Q |
What additional measurements are important for
aftermarket replacement? |
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On the top
plate, the number of studs or holes. The diagonal (where applicable) distance between the
studs or holes (Centre to Centre) The type of Air fitting (Hole or Combination stud) and
its internal diameter and placement on the top plate. On the bottom
plate (or piston), the number of studs or holes The diagonal (where applicable) distance between the
studs or holes (Centre to Centre) |
Q |
What is important
when reading an airspring spec (usually PDF’s)? |
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The measurements given above. The force you need at minimum, design and maximum heights
will give you the pressure required for that height. Overspec is often not a problem, except to achieving the
minimum might reqire external springs Underspec (The force required needs more than 7 bar
maximum pressure) means a bigger airspring or more of the same airsprings. You need to have the required pressure available in your
airline (at your tank / compressor). You can usually interpolate the force tables to get the
measurement you require e.g. if force at 7 bar = 1400kg then at 4 bar it
would be 4 / 7 X 1400 = 800kg If the graph gives 14Kn then that is 1400Kg or 1.4 Metric
tons |
Q |
How do rollover sleeves differ from convolutes in the
force supported at different heights? |
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A rollover sleeve generally maintains its diameter over
different heights if the piston is parallel sided and as a result, across the
operating height range, will support the same force for a given pressure. A convolute , for a given pressure, will generally
support a higher force as its gets shorter (and fatter). |
Q |
How meaningful are the numbers on the rubber for part
replacement? |
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Goodyear, Conti and Firestone produce a rubber component
and fit different top plates and pistons to get different airsprings, so the
rubber number helps to narrow down the correct part number, but additional
details are required about to top plate and piston. Airtech and Blacktech print the full number on the
rubber. Goodyear have had a fabric label sticking out of the top
plate for about 3 years, which can be read if the airspring is installed. Firestone and Contitech airsprings must either be
uninstalled to read the label on the top plate or measured. |
Q |
How do I calculate what vibration isolation I will get? |
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Download this Vibration
Isolation extract from Firestones engineering Manual. (807 Kb
pdf) Download the complete Firestone
metric engineering manual.
(3745 Kb pdf) Download the Firestone
Mini Airspring metric Brochure (565Kb pdf) |
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