Analysis and Annihilation of Grunt Noise in Hydraulic Power Assisted Steering Systems

Hydraulic steering systems employ pressurized fluid to alleviate the effort needed to steer the wheels of an automobile. Hydraulic Power Assisted Systems (HPAS) are cost-efficient, yet intricate. As customer trends lean towards reduced effort and improved handling, HPAS must intervene more in steering, leading to increased pressure requirements from the HPAS pump. Consequently, HPAS components must withstand higher pressure liquid and operate with closer tolerances, occasionally resulting in anomalous functioning. This paper delves into one anomaly with HPAS systems "Grunt Noise", a phenomenon caused by the resonance of the Torsion bar (T-bar) with fluid pressure pulsations. An extensive study was conducted to identify load conditions for grunt noise generation, examine transfer paths and analyze its frequency band. The study also explored the cascading process, reproducing the vehicle-level phenomenon at the rig-level for the Rack and Pinion and later at Pinion Valve assembly level. Following the analysis, the root cause of grunt noise was meticulously traced from the Steering Wheel to the Rack and Pinion, and further to the T-bar within, validating the approach. The operational principle of the T-bar necessitates twisting, making torsional stiffness a critical factor in grunt noise generation. T-bars with low torsional stiffness are influenced by pressurized liquid, and if the stiffness is overpowered by liquid force, the valve remains partially open for microseconds, resulting in cavitation and excitation that vibrates the entire Rack and Pinion structure. Torsion bars of varying stiffness (by altering the diameter) were evaluated for NVH performance in the same Rack and Pinion assembly to eliminate variability. It was observed by increasing the torsional stiffness and dimensions of the T-bar, the grunt noise in HPAS systems could be mitigated. In conclusion, this paper presents a comprehensive analysis of grunt noise in HPAS systems, highlighting the importance of torsional stiffness in mitigating this issue.