MACHINE BUILDING, FRAMEWORKS & SAFETY FEATURE
Building up to reliable electric (EV) and hybrid electric vehicle (HEV) design Growth in the electric and hybrid vehicle market is adding a whole
new dimension to the age-old efficiency, cost and reliability challenges faced by powertrain engineers. Solutions to these, however, often lie in the innovative details of bearing design, says Phil Burge, SKF
F
riction in bearings is a key issue in the
transmission architectures of all vehicles, whether they are powered by IC engines, electric motors or both. It generates energy losses which lower the mileage per battery charge or increase fuel consumption
and emissions. At the
same time, friction accelerates wear, shortens component life and compromises reliability. Although electric vehicle (EV) sales are rising,
pressure to make hybrid EV (HEV) and IC-only powertrains more efficient will continue for many years to come. Looking further ahead, the anticipated trend of shared mobility based on autonomous vehicles will extend annual driving distances considerably and demand greater component longevity.
UP TO THE CHALLENGE Examples of SKF friction-reducing advances in the IC engine market include a bearing package, developed in co-operation with a European OEM, for a balancing shaft application. Replacing the previous journal bearings with SKF rolling element bearings has not only reduced drivetrain friction but has cut CO2
emissions by 1%. The challenge from another OEM was to
improve its transmission architecture’s efficiency without adding to its size. Collaboration with SKF produced a solution in which customised compact ball bearings, made from sheet metal, were integrated at two points in the wet double clutch system. Eliminating rubber-to-metal contact in the dynamic seals reduced friction and cut power losses by 8.5%, while lowering CO2
emissions and extending design life to 300,000km.
HIGH-SPEED STRESSES Electric powertrains are so different from IC engine set-ups in their structure and functioning that key components such as bearings need to be redesigned or modified. In particular, the bearing units must withstand much higher speeds, acceleration and temperatures, as EV motors run about three times faster than their typical industrial counterparts. Using its own proprietary simulation software, SKF has optimised bearing component designs to
cope with these strains. One result is a re-engineered polymer cage for steel ball bearings. Special greases which can maintain their viscosity and lubricant properties under such extreme conditions can also be specified.
CURRENT LEAKAGE The fast running of EV motors boosts power and efficiency, without the need to increase current or torque, but it also increases voltage switching frequency. This, in turn, leads to electrical discharge. If leaking current is conducted through the bearings, it may cause surface pitting and ultimately result in catastrophic failure. SKF can address this by replacing steel balls with
ceramic versions, which are non-conductive, to create hybrid bearings. Ceramic ball bearings have further advantages, including a 40% lower density which enables cooler running temperatures. They are lighter and harder, require less lubrication and offer up to ten times longer life expectancy. Replacement of conventional bearings with SKF hybrids is easy, as their dimensions are identical, and they come complete with the cage and grease adaptations mentioned earlier. The downside is that hybrid bearings are
more expensive, but this should be set against the efficiency, longevity and reliability benefits they bring. In most cases, EV designers find the price off-putting and tend to use them only after their steel bearings have failed. Some of the larger EV manufacturers have access to advanced simulation software which allows
the prediction of current leakage and bearing failure. With that knowledge, they can incorporate hybrid bearings in their designs right from the start, if it makes financial sense. To provide a lower-cost option, SKF is working
on an alternative approach, without ceramics, which will create a non-damaging path for discharge currents. This has an added advantage over hybrids in that it also prevents current from reaching beyond the bearing and causing damage in the gearbox area. In the more distant future, large EV manufacturers who design full systems may be able to ‘design out’ current leakage problems at their source. However for those who have to source components such as control units and gearbox units externally, solutions similar to SKF’s will still be needed.
A CONTINUING QUEST SKF has been designing bearings and other components for vehicles ever since the first automotive revolution, and today the company supplies products to most EV manufacturers. SKF’s deep insight into this sector’s challenges, supported by heavy investment in research, development and application support, is invaluable in solving problems for OEMs and their suppliers. Working with companies across the sector,
SKF continues its quest to make powertrains more efficient, affordable and reliable.
SKF
www.skf.com/uk SICK’S ADVANCED IO-LINK ENCODERS STREAMLINE MACHINE CONTROLS
SICK has released advanced variants of its AHS36 and AHM36 IO-Link absolute encoders that can harness valuable diagnostic insights for better machine control and availability. With versatile programming options, they make it easy to integrate data from encoders into localised and ‘edge computing’ configurations to improve the efficiency of automated plant. The AHS36 14-bit (singleturn) and AHM36 26-bit (multiturn) IO-Link absolute
encoders achieve higher-level fieldbus integration via an IO-Link Master. Now, the AHS36 and AHM36 IO-Link Advanced variants enable users to use diagnostic information, such as temperature values, to streamline machine control and optimise maintenance regimes. Two banks of eight programmable CAM switches enable the free programming, via SOPAS, of up to 16 position ranges on the encoders. Limit values can be defined to set alarms or switch processes. The encoders’ digital inputs and outputs also enable triggering using simple push-pull I/O, which can be used to create Smart Tasks. A choice of two motion timers help govern service and replacement routines. These have
an operating range from -40˚C and +85˚C and IP66 and IP67 ratings, while the Inox variant provides a stainless-steel IP69K-rated housing. An IO-Link Master performs connection to higher-level Ethernet interface.
SICK
www.sick.co.uk DESIGN SOLUTIONS | JUNE 2020 13
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