traction fluids in infinitely variable transmissions · 2019-10-17 · torque converter. the...

80 N O V E M B E R 2 0 0 6 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

n the search for better energyefficiency, most manufacturersof vehicle transmissions are

developing either continuously vari-able transmissions (CVTs) or infinite-ly variable transmissions (IVTs). Bothtypes require fluids with high metal-to-metal friction coefficients, whichare also called traction coefficients.

U.K.-based Torotrak has developedan IVT that provides all ratios, fromfull reverse through zero-speed tohigh overdrive, without the need for astarting device such as a clutch ortorque converter. The running engineis connected to the roadwheels even while thevehicle is stationary. Toro-trak claims the absence ofa clutch or torque convert-er gives significant savingsin cost, weight and, thus,fuel economy.

The Torotrak IVT has avariator, consisting of twoouter input toroidal discs,that are engine-driven via a torsionaldamper and two output toroidal discsbetween which are six rollers. Theangle at which the rollers are inclinedgoverns the ratio between the speed ofrotation of the input and output discs.

Within the geometric limits of eachcavity, the rollers can have any angleof inclination, providing a continuousrange of drive ratios. Forces are trans-mitted between the discs and rollersby the traction fluid. Transmissionratio changes are achieved, undercomputer control, by raising or lower-ing the hydraulic pressure, and thistorque control distinguishes the Toro-trak IVT from all conventional trans-missions, including belt CVTs.

Traction fluids have historicallybeen naphthenic oils. During the1960s, Monsanto started research onpressure-stiffening lubricants, result-ing in the development of synthetichydrocarbons based mostly on deri-vatized cyclohexanes and decalinsthat possess relatively high coeffi-cients of traction, Ct. Other compa-nies have now developed similar syn-thetic cycloaliphatic hydrocarbontraction fluids.

Because of the pressures involved,traction fluids function in the elasto-hydrodynamic region, and it is the

combination of phy-sical and chemicalproperties of thelubricant in this re-gime that controlsthe performance oftraction drives. Sev-eral theories at-tempt to explain theproperties of trac-tion fluids. The lead-

ing one suggests that under high con-tact stress (pressure) and high shearrates of rolling contact, the viscosityof the lubricant film in the contactarea increases to a glassy solid state.

This “pad” of solid film transmitsthe tangential force with a shear re-sistance far beyond the capability of aliquid film in addition to preventingmetal-to-metal contact, minimizingwear and, when compressed in sur-face cracks, inhibiting the propagationof fatigue cracks. The more “solid” thefluid becomes under pressure, thehigher will be its Ct, and the more pow-er it will transmit between rolling ele-ments for a given normal contact load.

The lubricant returns to its normal

liquid properties immediately afterthe pressure is relaxed. The output ofa given traction drive design is limit-ed primarily by the traction fluid’s Ct,which has been shown to be indepen-dent of viscosity, viscosity index, pres-sure viscosity, specific gravity or othercommon physical properties. Typical-ly, variations in normal load, rollingspeed and temperature all have someeffect on a lubricant’s Ct, dependingon the design of the drive and therolling motion in the contact area.

It has been shown that moleculargeometry has an important effect onthe coefficient of traction. The abilityof molecules to arrange themselvesinto an apparent glassy state underhigh shear increases the coefficient oftraction. Commercial traction lubri-cants, based on cycloaliphatic hydro-carbons, have been used successfullyin industrial, automotive and aircraftCVTs. At present, Shell is workingclosely with Torotrak on the develop-ment and application of traction flu-ids for the company’s IVT, althoughthey have not disclosed the type offluid being used in field trial units.

Although more work remains to bedone to determine the operating life-times of IVTs and the traction fluidsused in them, the future for thesetransmissions could be very promis-ing. The fluids must function as goodlubricants, but with exceptionallyhigh frictional properties in the pres-sure contacts of the IVT. <<

David Whitby is chief executive of Pathmas-ter Marketing Ltd. in Surrey, England. Youcan contact him at pathmaster@dial.pipex.com.

Because of the pressuresinvolved, traction fluids function in the elastohydro-dynamic region and it is thecombination of physical andchemical properties of thelubricant in this regime thatcontrols the performance oftraction drives.

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IBy R. David Whitby

Traction fluids in infinitelyvariable transmissions