Frequently asked questions – Airsprings General

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General

Q

What is an air spring?

 

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?

 

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?

 

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?

 

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?

 

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?

 

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?

 

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?

 

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?

 

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?

 

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?

 

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?

 

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?

 

-         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?

 

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??

 

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?

 

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)?

 

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?

 

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?

 

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?

 

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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