The Foundations of Frames

Dr. Dave:? This is a good commentary on a subject often ignored by converters, especially smaller firms that wish to minimize their capital investment.? Personally, I think it is pretty hard to make a machine foundation too big.? The great advantages a good foundation provides is vibration management and minimizing deflection.? The forces generated by rotating masses can be fantastic even with well balanced equipment.? Vibration management is almost always better accomplished by adding mass, which reduces the natural frequency of the system.? This results in the operating machine passing through the system natural frequency of the system at a low rotating speed.? The other technique would be to add stiffness to the structure, which increases the system natural frequency of the system, with the idea of designing a system that vibrates at a higher frequency than operations occur at.? This is difficult to engineer.? Concrete is relatively cheap and weighs a lot, which makes adding mass the preferred route.? Minimizing deflection is important for maintaining alignment.? A good foundation with good soil conditions underneath goes far in assuring this.? Deflection happens easily - sometimes equipment changes position (for example, the very heavy extruder carriage on an extrusion coater moves in/out in the cross machine direction), or maybe a loaded lift truck drives by.? Isolating the equipment foundation from these operational disturbances eliminates many operating problems.? Now, this all works great when the equipment is installed on grade.? Much equipment is not, and for good reason.? Creating a stiff structural system with a large mass and equipment installed on a raised level that works from a web processing perspective is a structural engineering challenge that goes far beyond meeting building code requirements.? This is sometimes difficult for technically unsophisticated plant personnel who are in the position of making engineering decisions to fully understand.