Rotary Shaking Forces

Foundation designers, particularly ones who are familiar with reciprocating compressors, will often ask about the shaking forces exhibited by

Position 1

Position 2

Figure 9-6. Counterweight positions with respect to the crankthrow. This applies only to the maximum counterweights.

Position 1

Position 2

Figure 9-6. Counterweight positions with respect to the crankthrow. This applies only to the maximum counterweights.

the rotary or dynamic compressors. Normally these forces are rather insignificant relative to the weight of the compressor. To a designer, a numerical value has more meaning and allows him to decide the relative significance to the foundation design.

Rotary and dynamic compressors have in common the force due to unbalance. As discussed, the objective is to balance a rotor to a level of .lg or better, which was previously stated to be a force equal to or less than 10% of the rotor weight.

This represents a shaking force at a frequency equal to rotor speed. For foundation design, a value of 5 to 10 times the residual unbalance or lA to 1 times rotor weight at operating speed would be a reasonable design value. The direction of the force is perpendicular to the shaft, and operates as a rotating vector which can be centered between the bearings.

The dynamic compressors, axial and centrifugal, exhibit one other non-steady force, which is often questioned by foundation designers. This is the torque reaction force at the casing feet and, thus, the foundation due to surge. As a rule of thumb, a value of lA the maximum steady-state torque reaction at the casing feet can be used. The frequency is the surge frequency, which is less than operating speed but not readily predictable, because it is heavily dependent on the connected system.

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