Machine Foundation Design for Vibrations
Machine Foundation Design for Vibrations
(OP)
How are rotating unbalanced forces calculated by equipment manufacturers? A 10 megawatt generator for example.
We frequesntly design foundations for large rotating equipment. Generators, pumps, compressors etc. A recent example was a smaller 5MW combustion turbine generator set.
We use the standard techniques found in text books such as "Vibrations of Soils and Foundation". We frequently estimate the dynamic unbalanced force for the equipment (say the generator) using this formula
Unbalanced Force=Rotor Weight*Rotor Speed(rpm)/6000.
Or by using the rotor shaft eccentricity (provided by the manufacturer) and mass to come up with unbalanced force using this formula F=m(e)(4*Pi^2)*(f^2). m=mass, e=eccetricity of the shaft, f=frequency of rotation.
Sometimes we get manufacturers data giving us the unbalanced force directly which is usually just one number such as, "1248 pounds at 1800RPM" for a generator lets say. This information is usually always a lower value than the equation above estimates.
Recently a generator manufacturer gave us forces much higher than the formula would estimate and gave different forces for vertical and horizontal directions. They indicated that the forces were calculated using bearing stiffness(k) which was different in each direction but that they assumed a constant displacement(x) in each direction. The result of calculating F=kx gave high horizontal forces and even higher vertical forces.
Can someone familiar with machine design discuss how unbalanced forces (for use by the structural engineer) are usually determined by manufacturers?
It seems that the basic unbalanced rotor force based on shaft eccentricity is always the same in each direction and that would represent the basic vibration load we are interested in structurally.
Thanks
We frequesntly design foundations for large rotating equipment. Generators, pumps, compressors etc. A recent example was a smaller 5MW combustion turbine generator set.
We use the standard techniques found in text books such as "Vibrations of Soils and Foundation". We frequently estimate the dynamic unbalanced force for the equipment (say the generator) using this formula
Unbalanced Force=Rotor Weight*Rotor Speed(rpm)/6000.
Or by using the rotor shaft eccentricity (provided by the manufacturer) and mass to come up with unbalanced force using this formula F=m(e)(4*Pi^2)*(f^2). m=mass, e=eccetricity of the shaft, f=frequency of rotation.
Sometimes we get manufacturers data giving us the unbalanced force directly which is usually just one number such as, "1248 pounds at 1800RPM" for a generator lets say. This information is usually always a lower value than the equation above estimates.
Recently a generator manufacturer gave us forces much higher than the formula would estimate and gave different forces for vertical and horizontal directions. They indicated that the forces were calculated using bearing stiffness(k) which was different in each direction but that they assumed a constant displacement(x) in each direction. The result of calculating F=kx gave high horizontal forces and even higher vertical forces.
Can someone familiar with machine design discuss how unbalanced forces (for use by the structural engineer) are usually determined by manufacturers?
It seems that the basic unbalanced rotor force based on shaft eccentricity is always the same in each direction and that would represent the basic vibration load we are interested in structurally.
Thanks





RE: Machine Foundation Design for Vibrations
I can see no reasonable explanation for that equation as a general purpose description. It isn't dimensionally consistent.
Cheers
Greg Locock
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RE: Machine Foundation Design for Vibrations
It's really a simplified version of the second equation with an assumed value for e and unit conversions from frequency in hertz to RPM and mass to weight in lbs. I quoted it because it's a common foundation design text book formula that many in my field will recognize.
The error in this equation is usually the overly conservative assumption of e.
That's where I'm trying to learn more from some machine design specialists.
How do they determine the value of unbalanced force given their technical knowledge of the equipment.
RE: Machine Foundation Design for Vibrations
For big reciprocating equipment it includes the inertial effects that the configuration does not counteract.
htt
RE: Machine Foundation Design for Vibrations
RE: Machine Foundation Design for Vibrations
ACI 351.3 R-04 "Foundations for Dynamic Equipment."
RE: Machine Foundation Design for Vibrations
I'm looking for a technical understanding of how the data we use as input, the unbalanced force, is actually determined by manufacturers when they supply it.
I can better detect bad data from suppliers if I understand more about what they are doing.