Troubleshooting power steering pump noise and vibration issues Part 1: The power assist steering system by Jerry Nessler

Jerry Nessler is our NVH Test guru,� and as such he is extremely well respected, both by colleagues and customers, and maybe even a bit feared (but it goes with the territory, I say). So I asked him to develop for the blog a series of articles illustrating typical NVH troubleshooting tips and I know this will be a real treat for those of you who are getting into sound and vibration and need a primer on how to approach a problem. His first series of troubleshooting tips relates to power steering system noise in vehicles, but the approach and the techniques that he uses are equally applicable to other systems and components causing noise or vibration concerns. Enjoy!

. . . Gabriella Cerrato-Jay

There are two types of power assist steering systems used in automotive applications, the power rack and pinion steering and the recirculating ball steering (see the Automotive Handbook by Bosch). The power rack and pinion system utilizes a hydraulic cylinder integral with the rack assembly. A rotary spool valve directs pressurized fluid to the cylinder as a function of torque applied at the steering wheel to generate steering assist. The recirculating ball steering system utilizes a series of steel balls between the steering shaft and rack piston. The rack piston moves up or down assisted by hydraulic pressure regulated by a control valve. Both systems utilize a power steering pump driven off the engine by a belt. A schematic of the hydraulic system for a power rack and pinion steering is shown below (reference ZF Driveline and Chassis Technology)

The speed of the power steering pump is typically slightly faster than engine speed. The fact that the engine speed and the power steering pump speed are different can be used to separate out harmonics of the pump from engine harmonics. The engine acoustic and vibration spectrum typically contains harmonics of running speed with the 2nd order dominant for 4 cylinder engines and 3rd order dominant for six cylinder engines due to engine firing pulses. There is frequency content from the engine excitation well into the kHz (kiloHertz) frequency range. The power steering pump also has its own characteristic frequency spectrum. A power steering pump is a positive displacement pump, typically containing 10 vanes. The acoustic and vibration frequency spectrum generated by the power steering pump will contain harmonics of the vane pass frequency which will be multiples of the 10th order of the pump. Typically the 10th through 60th orders of the pump speed are dominant with the 10th order most dominant. However, acoustic and structural resonances can result in any order being dominant due to the amplification of the resonance. The noise and vibration concerns associated to the power steering system that can be experienced inside the vehicle are :

* Shudder - Vibration felt (tactile) in the steering wheel during either stationary steering maneuver or low speed steering maneuver. The frequency range typically associated with shudder is below 50 Hz. Some of the sources of shudder have been attributed to engine speed variation at idle causing flow variations from the pump at engine firing frequency, and a system instability controlled by the gain associated with the boost curve of the rotary spool valve.

* Moan - Low-frequency ( f < 400 Hz) tonal noise perceived in the passenger compartment at idle and low speed. The frequency of the moan is associated with the 10th, 20th, and 30th orders of a 10 vane pump. The path to the vehicle structure is fluid borne as a result of pressure ripple.

* Whine - High-frequency ( 1000 < f < 4000 Hz) tonal noise occurring at high engine speeds. The frequency of whine aligns either with one of the higher harmonics of the steering pump (i.e. 40th, 50th, 60th for a 10 vane pump) or with a system resonance. The path to the receiver is generally airborne.

* Hiss - Noise due to fluid flow across valve. Frequency range typically above 2000 Hz.

Here are recordings of noise at the driver position inside a vehicle when turning the steering wheel, illustrating moan, whine and acceptable noise (i.e. lack of perceived moan or whine).