*Article* **Nonlinear Response of Tilting Pad Journal Bearings to Harmonic Excitation** †

#### **Enrico Ciulli \* and Paola Forte \***

Department of Civil and Industrial Engineering, University of Pisa, Largo Lazzarino, 56122 Pisa, Italy **\*** Correspondence:enrico.ciulli@unipi.it(E.C.);paola.forte@unipi.it(P.F.)


Received: 19 April 2019; Accepted: 14 June 2019; Published: 17 June 2019

**Abstract:** In the experimental identification of dynamic bearing coe fficients, usually small perturbations around the static equilibrium position are assumed and linear coe fficients are considered. In the literature, studies on non-linear e ffects in plain journal bearings, especially from a numerical point of view, are reported. Few similar studies can be found on tilting pad journal bearings (TPJB). The present work reports some peculiar aspects observed during the experimental identification procedure of TPJB linear dynamic coe fficients. The tests are performed on a test bench designed for large size journal bearings operating at high peripheral speeds and static loads. A quasi-static procedure is developed to quickly check the results obtained from the usually adopted dynamic excitation. It consists of applying a slowly rotating force to the floating stator and measuring the relative displacement of the stator from the rotating shaft. Di fferent levels of static and dynamic load are applied to two di fferent TPJBs with four and five pads. Deformed orbits have been observed increasing the ratio between dynamic load and static load, suggesting the presence of non-linearity. Similar results are obtained with simple analytical models assuming suitably tuned non-linear sti ffness terms.

**Keywords:** tilting pad journal bearing; nonlinear behavior; experimental characterization
