Isolated Foundations, Reaction Masses and "Inertia Blocks"
for Automotive Testing, Dynamometers, Electro-Dynamic Shakers, Industrial Machine Tools and MRI facilities.

Fabreeka's® expertise and capabilities include over 15 years experience providing the design, analysis and documentation for isolated masses for the aerospace/defense, automotive, industrial machine tool and other industries. Applications include automotive testing, dynamometers, industrial machine tools and roll grinders. In certain applications, it is not desirable or feasible to mount a machine directly on vibration isolators. Instead, the machine it attached to a relatively heavy and rigid block (usually made of concrete) which is then supported by suitable isolators; the use of such a massive block (called an inertia block or reaction mass) is desirable for the following reasons:

1. If a machine (such as a diesel engine, forging hammer or vibration testing machine) generates relatively large forces during its operation, the overall movement of the machine on its isolation system tends to become excessive unless its effective mass is substantially increased. This increase in effective mass can be achieved by attaching the machine rigidly to an inertia block and mounting the inertia block on isolators.
2. In applications in which the frequency of excitation is low, the natural frequency of the system must be very low to provide low transmissibility and therefore good vibration isolation. A problem often arises with a machine intended to be mounted only at its base, because a low-stiffness base-mounted system tends to be unstable and to allow excessive motion. Effective isolation may therefore be difficult to achieve. A mounting arrangement with isolators angled upward may be used to move the system's elastic center closer to the center of gravity. This will reduce the effect of "rocking", improve the vibration isolation and reduce motion. It may be more feasible to mount the machine rigidly to an inertia block (to lower the center of gravity of the assembly) and to suspend the inertia block on isolators located in the same horizontal plane as the center of gravity.
3. Some types of equipment do not operate properly unless supported by a rigid structure. This applies to certain types of machine tools that are not inherently rigid and therefore need a rigid support to maintain the prescribed accuracy. In other types of machinery (such as printing presses) consisting of articulated components, a rigid support may be needed to maintain the proper alignment of working parts.

Fabreeka® provides complete structural analysis and design of foundations including consultation and construction management and vibration site survey/analysis services.

A foundation or mass designed to meet the requirements outlined above may be installed either above the floor level or within a pit below floor level. Isolators employed to support the mass may be made of rubber, steel springs, air springs or other suitable, resilient material. Fabreeka's® INFAB™, FABSORB™ and PAL pneumatic isolators are examples of these types of systems. The required size of the foundation depends on the reason for its use and on the type and size of equipment.

The desired natural frequency for the isolation system usually is established by the operating characteristics of the mounted equipment and the isolation required. The design basis for the seismic block natural frequency assumes that the seismic mass is a rigid body with a stiffness at least many times more than the isolator. In practice, this is a reasonable assumption for inertia blocks.

Figure 1 shows how resilient pads can be used in constructing an inertia block below floor level. A concrete pit of the required size is lined with the resilient material. Then this material is covered with plastic sheeting, and the concrete is poured to form an inertia block. The desired natural frequency can be obtained by using resilient pads of the appropriate thickness and area. To obtain a low natural frequency for the isolated system, a large static deflection is required when using rubber or coil spring isolators. However, no static deflection is required when using pneumatic isolators (air springs) with low natural frequencies.

Figure 1

If the isolators are located substantially below the center of gravity of the inertia block, a tendency toward instability is introduced, an effect which becomes more important if the machine generates large horizontal forces during normal operation. This limitation can be minimized by installing the isolators in positions closer to the upper surface of the inertia block, supported on abutments extending inward from the walls of the pit. A more refined version of this concept is the T-shaped inertia block illustrated in Figure 2. With such a design, it is possible to place the isolators in the same horizontal plane as the center of gravity.

Figure 2

Click to see project overviews/case histories for foundation isolation

• AVL Road Simulator on Isolated Support Foundation at Toyota
• Lansmont Shaker Table Isolated at Western Michigan University
• Four Post Shaker at North American Auto Show
• Multi-Axis Shaker Table Isolated at General Motors
• Electro-Dynamic Shakers at Lockheed Martin Isolated
• Milling Machine Isolated on INFAB™ Foundation Isolation at Composite Optics
• Dynamometer Laboratory Isolated on Seismic Mass with Low Frequency Isolators at Ford Motor Company
• Road Simulators Isolated on Concrete Reaction Mass with Low Frequency Isolators at General Motors
• Multiple Reaction Masses for Test Facility Isolated with Low Frequency Isolators at Mercedes Benz
• Fabreeka® Provides Vibration Isolation for General Electric MRI Units
• Fabreeka® Provides Vibration Isolation for MRI Unit Located on Second Floor of Building

Related magazine articles:

• Isolating Unwanted Vibration - An article reprinted from American Machininst, May 2005, in which Robert Haley, Fabreeka's® Engineering Manager, was consulted.

Please contact Fabreeka's® Engineering Department for assistance with your particular application.