Air Isolators Design & Fitting Considerations
Vibration isolation and damping
Air isolators are mounting elements of very low natural frequency that obtain their spring force from the compression of the gases they contain.
Machine mounts based on air isolators effectively suppress the transmission of vibration and structure-borne sound to the surroundings (active isolation).
Air isolators can also reduce the effects of vibration by isolating sensitive equipment from the source (passive isolation).
This vibration isolation is not to be confused with damping, which is the conversion of kinetic energy into heat.
For instance in the area of resonance, damping limits vibration amplitudes to a permissible ratio. Lehr’s damping ratio D of standard air springs is 0.03. The damping of air isolators can be adapted to meet special needs by modifying its design. With the non-wearing air damping design (two-chamber principle) damping up to D = 0.15 is available.
Stability and height of the centre of gravity
Air isolators should always be mounted so that the shortest distance between points of support is at least twice the height of the centre of gravity above the plane of support. This minimizes wobble and prevents operational problems caused by machine instability. The below figure show the position of the mounting elements for a small bottom surface and high centre of gravity.
The lateral stiffness of air isolators differs greatly from type to type. For the individual air isolator types, the following lateral stiffness values – relative to the axial stiffness – can be expected. The specified percentages are for the recommended operating height for vibration isolation.
- Single convolution air isolators 30 to 60%
- Double convolution air isolators 5 to 30%
- Belted air isolator 30 to 50%
Triple convolution air isolators, rolling lobe air isolators and sleeve type air isolators have no positive lateral stiffness and can only be used for vibration isolation with lateral guidance. Lateral guidance can also be achieved on rolling lobe air isolators and sleeve type air isolators with the use of a restraining cylinder which turns the air isolator into a kind of guided diaphragm. Because of their low natural frequency, both types are excellent vibration isolators.
Air isolators can be supplied with air in various ways.
- Tank valve
For applications involving a constant load and where small differences in height are permissible, a tank valve can be employed. After the initial inflation, the air pressure or the operating height should be checked regularly and topped up if necessary.
- Pressure regulating system
When several air isolators are linked to a common pressure regulating valve, any lost air is replenished automatically and requires no maintenance. This system makes it possible to level machines with unknown weight distribution. The air isolators are combined into three groups and the pressure control valves are individually set in accordance with the distribution of weight (see below). This type of air supply can be employed only for convolution air springs and sleeve type air isolators – so only for air isolators for which the load capacity decreases as the operating height increases.
- Height regulating system
If the height regulation has to be exceptionally accurate or if rolling lobe air springs are used for vibration isolation, automatic height adjustment valves are required. Height regulation must always be carried out with three control valves so that the level of the machine can be adjusted via three points (see below).
Unlike other suspension elements with static compression, air isolators can also function as lifting elements, which makes them much easier to install. There is no need for an extra lifting device to raise the machine since the isolators need not be pre-tensioned, nor is it necessary to spend a great deal of time levelling and aligning. The system has to be levelled only once via air pressure or height adjustment valves. On machines that generate vibrations, the air inlet should be located on the stationary side of the isolator.
Air isolators permit motion in all directions. Depending on the expected dynamic vibrations that occur during operation as well as when starting and stopping, machines with isolators should be equipped with vertical and horizontal stops. Clearance can range from 10 to 30 mm, depending on the size of the isolators. Accordingly, all machine or device connections must be constructed flexibly so as not to impair the high isolating effect of the air isolators.
ContiTech SCHWINGMETALL® mounts are suitable for this purpose. They are available as cone mounts, impact mounts, and heavy-duty impact mounts in various sizes, elastomer hardness and types.
Auxiliary volume, change in natural frequency
The spring stiffness of an air isolator results from the compression of the air volume it contains. The axial stiffness can be reduced further by using an auxiliary volume. Depending on the dimensioning of the connection line between air isolator and auxiliary volume, a non-wearing, non-ageing air suspension system is created. If the connection line has a shut off system, the air spring suspension system can be switched between two vertical natural frequencies.
Air isolator systems
Air isolators are available as complete systems including height regulating system and all required accessories.