Resistance (to motion) is futile

4 min read

Bearing companies are increasingly keen to offer engineering expertise when it comes to advising customers about the smooth running – and in many cases increasing functionality – of modern bearing systems.

For some time bearing suppliers have been waving the consultancy flag, and it seems to be paying off after a raft of engineering successes, and accolades, have attributed bearing components to step change improvements. In particular, automotive OEMs have reached out to the expertise on offer.

A recent example comes from bearing experts SKF, who launched an upgraded version of its third generation hub bearing unit (HBU3), designed to reduce preload and CO2 emissions on a variety of vehicles.

For those not in the know, the preload is essentially an interference fit on the actual ball bearings inside a casing. Ball bearings needs some level of clearance when fitted inside a housing, meaning there is some 'give' between the different parts to allow them to freely rotate.

But these internal clearances need to be tightly controlled so there is not too much axial movement, as this would cause vibration and inefficiency. Normally manufacturers induce a permanent thrust load on the bearings to secure them in place. However, that means more friction is produced on rotation.

Getting the balance right has been a constant source of optimisation and trade off for bearing developers over the years. However, SKF has said that it has managed to reduce preload, and its associated friction by 10%, by revising the design of internal components and enhancing the manufacturing and assembly process normally applied at its factory. Though tight-lipped about the details, SKF found the new design and manufacturing technique has also improved reliability and quality of existing hub bearing units.

"The third generation hub bearing unit has been developed while working with leading automotive OEMs," said Massimiliano Leone, customer project manager for the automotive markets at SKF. "As a result, we have produced a unit that is easy to install and replace, that offers a class-leading standard of reliability – and with the latest enhancements – gives both OEMs and their customers the opportunity to improve vehicle performance."

The theme of low-friction is an obvious one for bearing companies and the trend of reducing it has undoubtedly been impressive over the last few decades. Modern bearing design and manufacture has become as advanced as the projects and products they look to enable.

Part of this 'enabling' process is not just direct CO2 savings by decreasing rolling resistance, but also the optimisation of sub-assembly systems that have bearing systems at their heart. And it was this that saw bearing giant NSK develop a low-friction reduction gear for vehicle electric power steering (EPS) systems.

The premise of EPS goes back to the common view that power steering is provided using hydraulic systems powered from the internal combustion engine. However, EPS saves energy here, as the motor that provides power can be turned off when the vehicle is travelling in a straight line. And this results in an improvement in fuel efficiency over hydraulic power steering systems.

EPS is an industry wide trend and the use of this sub-system could eventually make hydraulic power steering systems obsolete, and there are a number of reasons why. The first is the ability for it to be 'smart'. The motor providing the steering assistance can be electronically controlled to respond to a number of driver and environmental inputs including the amount the driver turns the steering wheel (so changing lanes on a motorway to swerving), the vehicle speed and road conditions. Here, the aim is not necessarily to cut CO2, but improve safety as well as drivability. For these reasons the application of EPS is rapidly expanding.

In addition, the system aims to reduce steering fatigue experienced by drivers by making steering smoother during straight motorway type driving, as well as improving comfort when the steering wheel returns from large steering efforts at a junction, for example. One of the key elements in creating this kind of smoothness is the amount of friction in the system.

To aid this, NSK developed a low-friction grease that reduces the frictional force of the reduction gear, thereby reducing the steering effort for the driver and improving the 'feel' of the steering. NSK leveraged its tribology technology here and optimised the formation of oil film by using a new grease that prevents any increase in viscous drag. As a result, oil film is formed in a reliable manner between the face of the gear teeth. The result is an astonishing 17% reduction of friction in the reduction gear.

NSK is not the only one to lean on years of tribological knowhow to create an advanced steering system. Schaeffler has also carried out a project with a similar end in mind. It carried out a collaborative project with the Karlsruhe Institute for Technology (KIT) based in Germany to research and develop technologies for improving the energy efficiency of assisted steering systems, specifically for electric vehicles.

Like NSK the premise was to develop an electric power steering system, with the ultimate end market firmly being electric vehicles. In conventional vehicles, the internal combustion engine not only accelerates the car but also supplies power to on-board assist systems, including the power steering system. In electric vehicles, this energy comes from the battery, which reduces the vehicle's travelling range, so a workaround is an industry necessity.

The project known as 'Intelligent Assisted Steering System with Optimum Energy Efficiency for Electric Vehicles (e²-Lenk)' looks to improve the energy efficiency of steering systems by intelligently controlling the drive torque transmitted to each of the four wheels of the vehicle.

Though Schaeffer is perhaps best known as a bearing and linear system supplier, it is fast being recognised within the automotive industry as transferring that knowledge and engineering expertise to produce automotive sub-systems.

Project manager Dr Marcel Mayer of Schaeffler, explained: "The assisted steering system would require fewer system components in an electric vehicle, which would mean savings in terms of weight and energy. This, in turn, would mean that an electric car would be cheaper and have a greater travelling range."

The basic idea of the e²-Lenk project is simple: the wheels of an electric car are driven individually by electric motors. If the wheels on the left side transmit more drive torque to the road than those on the right side, this will result in acceleration of the vehicle to the right without the need to turn the front wheels or consume additional energy for steering. Tracked vehicles or quadcopters steer using the same principle.

"Steering assistance can be provided while driving by means of an intelligent control system and suitable wheel suspension," said Dr Mayer. "Only steering when stationary remains a challenge with conventional designs."

As part of the project, functional demonstrators are being built, with which the concepts and components – no doubt of which bearings and low friction movement play a vital role – will be validated and optimised in tests.