Some pretty clever tribology has gone into the development of a new
continuously variable transmission system whose novel traction drive
mechanism is being hailed as the world's first power roller bearing
NSK-RHP is claiming a world first for its new Powertoros continuously
variable transmission (CVT) system for cars with engine capacities of
three litres or more. The half toroidal CVT comprises power rollers
sandwiched between input and output discs; the action of changing the
angles of the power rollers changes the rotational radius at the contact
area, and with it the relative speed. When the input disc and the first
power roller make contact at an area of small radius, and the output disc
and second power roller make contact at an area of large radius,
deceleration results. Reversing these relationships produces acceleration.
In all cases, smooth changes in the power roller angle yields a smooth,
continuous change in speed. The sensitivity of the system is such that
moving a power roller within just a single millimetre range in relation
to the input and output discs, generates enough force to make the power
roller turn on its own and change the angle of its orientation with the
discs. The result is a transmission system that allows rapid, indeed
unprecedented speed changes.
The traction system is predicated on the concept: 'if only gear teeth
could be made infinitesimally small'. Under heavy loads, a lubricating
film just a thousandth of a millimetre thick is formed at the contact
points between the discs and power rollers. This special oil becomes
glass-like under high pressure at the contact points, enabling the
transmission of power from disc to oil and from oil to power roller. The
oil film does not break down under any driving conditions, thereby
preventing abrasion and seizure.
Critical to the performance of Powertoros is the steel that makes up its
components. NSK needed steel that was already highly 'pure', to be purer
still, and whose structure would render indentations non-destructive -
all to prevent potentially damaging flaking. The company's first step was
to develop a method of measuring the non-metallic impurities within the
steel - a process involving surface image analysis, electron beam melting
and sampling, and extreme value calculation. The next step was to ensure
a reduction in the stress concentrations surrounding indentations. By
increasing retained austenite, a soft metal structure, NSK produced a
more resilient material without compromising its hardness.
The major components of the CVT are made from highly pure steel that is
forged, machined and heat-treated prior to 'super-finishing' and
post-processing. The super high precision processing of the traction
surfaces at its heart involves processes developed by NSK for the
mass-production of precision bearing components. In post-processing,
moreover, the topology of the traction surfaces and osculation details of
their contact is controlled within a few microns. This, plus the surface
finish and its characteristics, are absolutely critical, as they have a
direct influence on the CVT's traction performance and durability.