Return to index: [Subject] [Thread] [Date] [Author]

RE: Vibrating Equipments

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Harold Sprague wrote:

> The only thing similar to a rule of thumb is contained in ACI 350 Section
> 2.9 - Impact, vibration, torque, and seismic loads (only 3 pages long).
> There is Table 2.9.2(a) "Natural frequencies of beams" and Table 2.9.2(b)
> "Recommended maximum structural deflection for given equipment operating
> speeds".  

ACI 350 does provide some useful formulas for vibration design - however 
these are basically just modifications of the natural frequency formula 
using f=((g/delta)^1/2)/2*pi. These formulas would still require very stiff 
support structures for even minor vibrating equipment. 

The problem frequently arises when the frequency of a piece of equipment 
is not less than 1.5x the structure natural frequency. But if it is not a 
large or heavy piece of equipment, it becomes questionable as to whether 
the extra cost of stiffening the structure is cost effective. The only 
rule of thumb I have come across is that electrical motors are generally 
a well balanced piece of equipment and generally should not require 
design for an unbalanced condition. 

Another design method commonly used is inclusion of an "impact factor" such 
as specified in AISC ASD A4.2. (i.e., 20 percent impact "for supports of 
light machinery, shaft or motor driven" and 50 percent impact "for supports 
of reciprocating machinery or power driven units"). Again, no limits are 
given as to what constitutes "light machinery" or when a full dynamic 
analysis is needed. Ultimately it comes down to "engineering judgement" but 
different engineers can come up with different answers.