Motion control must be considered as this creates an effect on the vibratory force of the isolated mass.
The below image shows several options which can be seen for introducing anti vibration mounts on industrial machinery.
a)
Machine sits on an inertia block. Depending on the flexibility of the floor the main supporting structure will to some degree be isolated from the machine vibration.
b)
Machine sits on anti vibration mounts, the level of transferred force that is reduced will depend on the dynamic stiffness of the vibration isolators.
c)
Machine supported with mounts on an inertia block. If the floor is flexible the introduction of the inertia block produces a compound spring system. If the natural frequency of the floor/inertia block is greater than the disturbing frequency of the machine, problems should not occur.
d)
Machine sits on an inertia block supported with anti vibration mounts. Satisfactory degree of vibration isolation will be achieved if the dynamic stiffness of the anti vibration mounts is adequate, whilst the effect if the inertia block will be to reduce the motion of the isolated machine due to the effects of rotary or reciprocating action.
Fig. 2: Different mounting options of anti vibration mounts.
Inertia masses are usually made of concrete with reinforced bars. They are normally used for the following reasons:
1. To Increase the stability of the system.
It is usual to find machines that incorporate fixation holes that are too close together. If we want to provide adequate stability correct spacing of the mounts is advisable. Concrete bases or steel rails can be used that set a distance between the mounts can be used. Fig 3 and Fig 4 show examples of how this can be achieved.
Fig. 3: Boiler supported with Vibration isolators on Vibrabsorber+Sylomer® spring mounts.
Fig. 4: Single cylinder diesel engine isolated on AMC-MECANOCAUCHO BRB mounts with distancing frame.
2. To lower the Centre of Gravity.
Mounting industrial machinery on a substantial concrete base has the effect of lowering the centre of gravity of the complete assembly. This adds to the improvement of the stability provided by extending the width of the base, but also has the effect of reducing the likelihood of a rocking motion. A typical section through such a base is illustrated on Fig.5
Fig. 5: Machine with T section Foundation block to Effect on Centre of Gravity mounting.
3. To give a more even weight Distribution.
Often industrial machinery is much heavier at one end than the other. This means that, if they are mounted directly on anti vibration mounts, very different arrangements are needed at opposite ends of the equipment to cope with uneven weight distribution. If the machine is mounted on a concrete block, the weight distribution will be more even and, providing the block is heavy enough, it may enable symmetrical mounting arrangement to be used. Figure 6&7 illustrates an example where not only uniform loading of the vibration isolators is achieved, but also a better method of supporting the pipework.
Fig. 6: Pump plus Elbow extended to the inertia block.
Fig. 7: If the concrete inertia is thick even if the elbow is at one of the corners, the stability is not compromised. Courtesy picture of 2 AMC 500+Sylomer® under concrete inertia block on the HVAC system on a shopping mall in France.
4. To minimize the effect of External forces.
Although the use of an inertia block does not improve the transmissibility for a given static deflection, it does not mean that a much stiffer vibration isolator can be used for the same static deflection. This is to say if the mass of the installation is doubled, the stiffness of the anti vibration mounts must also be doubled. This means that the equipment is far less susceptible to the effects of external forces such as fan reaction forces and transient torques due to changes in speed or load.
Fig. 8: Installation of a Genset on FZH+Sylomer®jack up mounts embedded on a concrete block on a rooftop of a building. Courtesy picture of BBVA bank headquarters in Lima, Perú.
5. To provide or replace rigidity
An inertia base can be used to provide rigidity for the mounted equipment in the same way that a steel base is used. This consequently leads to reduced wear.
6. To reduce problems due to coupled modes.
A tall piece of equipment will have two rocking sideways coupled frequencies, these may occur at two to three times the frequency of the vertical natural frequency which can lead to resonance problems. Adding an inertia base has the effect of lowering the rocking natural frequencies which helps to avoid this problem.
Fig. 9: Installation of washing machines on a concrete block using Sylomer® SR-11 in 37mm of thickness.
In order to avoid problems of coupled modes, the machine can be installed on a stepped foundation block. In this case, the center of gravity of the system is lower, closer to the plane of the vibration isolators. This results in decoupling of the rocking modes and high levels of movement can be avoided.
Fig 10, shows an example of a large Vee twin piston compressor installed using this stepped inertia mass as this avoids the movement of the pipe connections on the inclined cylinders.
In order to model this system AMC-MECANOCAUCHO applications engineers can help by making a mathematical model of the system using multibody calculation software as shown on the article Vibration isolation of industrial machinery.
Contact form to get in touch with applications engineers of AMC MECANOCAUCHO.
Fig. 10: Piston compressor installation on a stepped inertia block.
7. To minimise the effects of errors in estimated positions in the equipment’s centre of Gravity.
When vibration isolators are being selected, it is necessary to calculate the total load on each anti vibration mount so that the appropriate vibration isolator can be chosen. This normally has to be done before the equipment is available and estimated positions of the centres of gravity of each item has to be used. If this information is inaccurate, the estimated loads may be considerably different from the ones which occur in practice. This may lead to anti vibration mounts being grossly under or overloaded, or to the equipment sitting at an unacceptable tilt. The latter problem becomes increasingly likely as the vibration isolators with high static deflections are used. If a concrete inertia base is used the centre of gravity of this is normally known and accurate and, if the mass of the inertia block is comparable with the mass of the rest of the equipment, it means that, even if the equipment information is not accurate, the possible inaccuracies in the final estimated centre of gravity are small. This reduces the possible errors per vibration isolator loading and reduces the likelihood of a tilted installation. The probability of a tilted installation is also further reduced due to the stiffer springs that will be required to carry the additional weight of the inertia base.
Fig. 11: Installation of a full surface Sylomer SR-11-25. Concrete will be poured so several HVAC machines can sit on top of the inertia Block. The application is on the rooftop of a Shopping Centre.
8. To act as a local acoustic barrier.
When very noisy equipment is mounted directly on the floor of an equipment room, the floor directly under the equipment may be subject to very high sound pressure levels in the immediate vicinity. This local area where the floor is exposed to these high levels may cause problems of noise transmission into the room below. A concrete inertia base can act as an effective barrier, protecting the vulnerable areas of the floor.
Interesting measures to adopt when inertia bases cannot be used.
1. Installing mounts equidistant to the centre of gravity and if possible, on the points where lowest vibration is felt. (nodes).
The position of the antivibration mounts determines the vibration modes of the suspended ensemble. An even load distribution over all the vibration isolators is advisable. One easy way of obtaining this is by installing the antivibration mounts equidistant from the Centre of gravity of the suspended equipment. Following Fig.11 shows the position of the Cog of a machine and the mounts with the position A and B in respect to the centre of gravity. Achieving an equal distance between A and B will help to achieve this goal.
Fig. 11: Equidistant mounts to the center of gravity is achieved when a=b.
Mounts installed at the height of the Centre of gravity provide more stable suspensions and avoid excessive movement of the suspended element, particularly in mobile applications.
If the vibration mounts are positioned at the points where less vibration amplitude is found (nodes), they will be subjected to the lowest vibratory force, this leads to optimized vibration isolation.
Fig. 12: Nodal points expressed as red rectangles on a diesel engine installation.
2. Using anti vibration mounts with high damping (energetic dissipation) devices.
When concrete mass/blocks cannot be installed, the alternative can be to use mounts that incorporate a damping system. The use of additional damping can allow stable solutions. The energetic dissipation often achieved by the transfer of a viscous fluid from one cavity to another as shown on the below video.
VIDEO
Fig. 13: Explanatory video of the process of energetic dissipation using the Vibrabsorber+Sylomer Antiseismic Visco mounts under a dynamic test bench.
AMC-MECANOCAUCHO produces a wide variety of anti vibration mounts that use hydraulic systems to provide additional damping. These vibration isolators can use spring+Sylomer or rubber, maximum load capacities range from 20Kg (44lbs) to 50 tons (112.000Lbs) of load. Their damping system can be adjusted by the viscosity of the fluid and by the orifices where the liquid must flow through. For the case of the Vibrabsorber+Sylomer Visco, this mount incorporates a “MAX-MED-MIN” adjustable damping system, that regulates the passage of flow of viscous fluid. More information can be found by clicking here.
Fig. 14: Vibrabsorber+Sylomer antiseismic Visco .
Fig. 14: Vibrabsorber+Sylomer Visco incorporates an adjustable damping device and a restricted displacement device as shown on this link.
Fig. 15: AMC-MECANOCAUCHO Hydraulic mounts .
Fig. 16: AMC-MECANOCAUCHO Hydraulic cones.
In the below video a stamping press is installed without a concrete inertia block. The video on the left shows the press being installed without a damping system where as the video on the right shows the Vibrabsorber+Sylomer antiseismic visco . The energetic dissipation produced inside the fluid chamber, provides a more stable and safer environment for the worker.
VIDEO
Fig. 17: AMC-MECANOCAUCHO ANTISEISMIC VIBRABSORBER+SYLOMER VISCO SPRING MOUNTS are anti vibration mounts that incorporate an energy dissipation device. This device provides damping to eccentric machinery that requires quick stabilization for proper functioning.
Fig 18 and 19 below shows how hydraulic mounts can be used in 3 cylinder engines. Optimum results are achieved when the mounts are equidistant to the centre of gravity and at the height of the Crankshaft (or centre of gravity).
Fig. 18: Hydraulic mount medium from AMC-MECANOCAUCHO supporting a 3 cyl engine.
Fig. 19: Hydraulic mount medium from AMC-MECANOCAUCHO supporting a 3 cyl engine.
3. Using anti vibration mounts with restricted displacement due to integral snubbers.
Anti vibration mounts can be equipped with different elastic steps. A linear elastical range of load provides the stiffness required for the vibration isolation of the machinery. In the event a high traction or compression forces occurs, the mounts architecture provides a secondary stiffness step. Normally this is achieved with metal parts that enter in contact with a section of the rubber that acts as a snubber/buffer.
Fig. 20: Pattern of elasticity of an AMC-MECANOCAUCHO CB mount.
Fig. 21: Cross section of a CB 76 mount, where the internal architecture is shown.
The progressive stiffness section is achieved when rubber is bulked filling the hollow areas of the anti vibration mount. Figure 22 shows the FEM process of loading of an AMC-MECANOCAUCHO CB mount.
Fig. 22: Loading process of a AMC-MECANOCAUCHO CB mount.
4. Using stabilizers on the highest amplitude points of the suspended element.
Installing stabilizers on the highest amplitude points can be used, nevertheless special care must be taken so this point does not transmit excessive vibration to the adjacent supporting members (Frame). The part should only work when the machine is exposed to shocks, whilst under normal operation this point should have very little contribution, i.e have the lowest stiffness possible. Fig 23 shows a mobile genset installed with a CB mount acting as a multiaxial buffer, on the point where highest mobility is registered.
Fig. 23: Extreme mobility genset using CB mounts as multiaxial stabilizers
AMC MECANOCAUCHO manufactures anti vibration mounts and has a team of application engineers to help your installation needs so do not hesitate to
contact our technical department if you require help on this topic.
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