Experimental Investigation of Vibration Characteristics, Dynamic Balancing, and Critical Speed Analysis

Vibration in rotating machinery is the most significant contributor to mechanical failure, downtime, and maintenance cost in industrial plants — with unbalance, misalignment, and bearing defects collectively responsible for over 70 percent of rotating equipment failures. This paper presents a systematic experimental investigation of the vibration characteristics, critical speed behaviour, and dynamic balancing of a flexible multi-disk rotor system representative of industrial pump, compressor, and turbine rotors. The experimental rotor comprises a 40 mm diameter steel shaft of 860 mm length supporting three discs at defined span locations, mounted on two rolling element bearings with an overhang extension. Vibration measurements are conducted using proximity probes (shaft displacement) and accelerometers (bearing housing vibration) with 16-channel simultaneous data acquisition across the full speed range from 0 to 4000 rpm. Frequency Response Function (FRF) analysis is used to identify natural frequencies and damping ratios; a Campbell diagram is constructed to identify critical speeds and potential resonance conditions from integer-order excitations. Modal analysis identifies five natural frequencies: 48.0 Hz (1st bending, 2880 rpm critical speed), 124.2 Hz, 228.6 Hz, 186.4 Hz (torsional), and 312.8 Hz (axial). Two-plane ISO 1940 dynamic balancing reduces peak bearing vibration by 85.0% (from 186 µm to 28 µm at the drive-end bearing) and brings residual unbalance to ISO Grade G1.0 — well within the G6.3 design specification. FFT spectrum analysis before and after balancing identifies residual 2× excitation from angular misalignment as the next priority for vibration reduction. The complete experimental methodology, instrumentation setup, balancing procedure, and diagnostic interpretation provide a practical template for rotating machinery vibration management in industrial practice.