In the design phase of developing a rotating machine, it is important to consider the effect that the supporting structure has on the rotordynamic behavior of the system. The American Petroleum Institute's (API) Standard Paragraphs state that if the support stiffness is greater than 3.5 times the bearing oil film stiffness, the designer may omit the supports from the rotordynamic analysis. As discussed in this paper, there is concern that machines operating near the second critical speed may

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... be adequately modeled using the rigid support assumption allowed by API. Due to the suspected influence of support dynamics on problem machines in industry, this paper investigates the effectiveness of the support to bearing stiffness ratio threshold of 3.5. A Jeffcott rotor model is used to capture the effect of the support on the separation margin and amplification factors of the first and second critical speeds. The trends are then validated using numerical models of two case studies. Results show that the API support stiffness ratio of 3.5 has limitations as a rigid support threshold and should be used with caution when the second critical speed is near a separation margin boundary. The paper proposes an addendum to the current API specification intended to reduce the risk of separation margin encroachment due to a rigid support assumption. Unlike other papers discussing the influence of flexible supports on rotating machinery, this investigation studies the effects at API's support stiffness ratio of 3.5 and compares them to a rigid support system.