Steering Toward Lower NVH

March 15, 2018

Dr. Alfred Lethbridge

EVs have set new benchmarks in acceptable cabin noise thresholds, putting a greater premium on steering yoke bearing design. (Image: Tesla)

As regulations on automotive noise have become more stringent over the years, engine noise has been reduced significantly. This trend has increased with the rise of EVs and hybrid vehicles, modern quiet ICEs and stop-start systems. The overall impact has been positive for the environment on and around roads, as high levels of noise can cause human stress and other disruptions. This is also good news for drivers who want the solace of a quiet cabin. However, one problem with increasingly quiet vehicle propulsion systems is that the small noises—the little rattles and squeaks—become more obvious.

Steering rack schematic with the steering yoke sliding layer shown in red. An example of a steering yoke is shown top-left (image: Saint-Gobain).

Improvements in road conditions and tire and suspension technology has meant that the driving experience has become smoother. This means that automotive systems that have a small effect on the comfort of a drive now become more obvious. This puts further demand on automotive parts to ensure a high-quality brand perception. Vehicle manufacturers need confidence that the components they purchase are going to deliver.

As Automotive Engineering readers know, only when engineers fully understand the challenges their customers face can they then provide effective solutions. Such is the case with evaluating the customer’s steering system with thorough testing and detailed investigation. Saint-Gobain engineers have found that that two important features of a steering system that can affect quality are noise and steering feel. And often the solution is a systems-based, rather than a component-based, approach.

Finite-element analysis of the pressure exerted due to a 1 kN load from the rack shaft on the two yoke shapes shown here: left, line contact; right, greater contact (image: Saint-Gobain).

The steering yoke bearing (also known as the steering rack guide bearing) as seen in the Figure 1 illustration, performs a critical function in the steering system. The yoke bearing is used to ensure constant meshing between the rack and pinion gear teeth. If the gear teeth were to disengage, then the entire steering system would fail. The steering yoke bearing has to be pushed against the steering rack by a spring with typically 300 N (67.4 lb) of spring force (see https://www.bearings.saint-gobain.com/your-industry/automotive/chassis/steering-yoke-guide-bearing ).

The force applied to the steering yoke generates friction on the steering rack, which affects steering “feel.” Therefore, it is necessary to ensure that the yoke bearing’s sliding layer is well designed. Generally, yoke bearings will comprise a PTFE layer with a metal backing. The yoke bearing shape is typically designed so that there is line contact between the yoke bearing sliding layer and the steering rack. This creates a high pressure on the sliding surface which increases wear.

As the material is worn away, clearance can be produced between the rack and pinion’s gear teeth. Clearance here can cause “teeth chatter” or rattle.

Steering yoke bearings that are tailored to individual customer needs and specifications are more likely to ensure a high-quality product throughout the lifetime of the vehicle. As an example, Figure 2 shows the finite element analysis (FEA) of the bearing pressure generated by two yoke bearing shapes—one shape with poor conformity to the rack that gives line contact, and one shape with higher rack conformity giving greater contact.

Steering rack yokes with greater contact shapes reduce the pressure and, therefore, the wear on the part. This results in less rattle in the steering system.

Durability tests are performed so the effect of wear on the performance can be discerned. The friction force of (a) the dynamic friction force and (b) the stick-slip friction force were measured where Fstick-slip= Fpeak- Fdynamic Fstick-slip= Fpeak- Fdynamic Fstick-slip = Fpeak — Fdynamic (image: Saint-Gobain).

Another problem with a high wear rate is that the friction will increase over time. Analyses have determined that bearings that are made with sintered bronze and PTFE often have a thin PTFE layer, which when worn, exposes the bronze layer to the steering rack. This, in turn, increases the friction in the system.

The greater contact bearings that have a lower wear rate ensure that the metal part of the structure is not exposed. The effect of a durability test on the friction and stick-slip of a sintered bronze and PTFE product designed with line contact and two Norglide yoke bearings designed with greater contact, is shown in Figure 3.

The sintered bronze and PTFE-based bearing has a significantly increasing friction value throughout the lifetime of the part, but the Norglide solutions maintain a consistent friction and stick-slip value.

By working in close collaboration with customers, engineers taking a systems approach can help to solve the problems that customers have with custom made solutions. Thorough testing gives confidence to the customers in the parts they buy.

About the author: Dr. Lethbridge is an NVH Engineer responsible for Norglide bearings and Rencol tolerance rings at Saint-Gobain. He is based in Bristol, U.K. He earned his Ph.D in Physics from the University of Exeter where he specialized in optical waves and oscillations in resonant systems.