Traction fluids in infinitely variable transmissions · 2019-10-17 · torque converter. The...
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80 NOVEMBER 2006 TRIBOLOGY & LUBRICATION TECHNOLOGY n the search for better energy efficiency, most manufacturers of vehicle transmissions are developing either continuously vari- able transmissions (CVTs) or infinite- ly variable transmissions (IVTs). Both types require fluids with high metal- to-metal friction coefficients, which are also called traction coefficients. U.K.-based Torotrak has developed an IVT that provides all ratios, from full reverse through zero-speed to high overdrive, without the need for a starting device such as a clutch or torque converter. The running engine is connected to the road wheels even while the vehicle is stationary. Toro- trak claims the absence of a clutch or torque convert- er gives significant savings in cost, weight and, thus, fuel economy. The Torotrak IVT has a variator, consisting of two outer input toroidal discs, that are engine-driven via a torsional damper and two output toroidal discs between which are six rollers. The angle at which the rollers are inclined governs the ratio between the speed of rotation of the input and output discs. Within the geometric limits of each cavity, the rollers can have any angle of inclination, providing a continuous range of drive ratios. Forces are trans- mitted between the discs and rollers by the traction fluid. Transmission ratio changes are achieved, under computer control, by raising or lower- ing the hydraulic pressure, and this torque control distinguishes the Toro- trak IVT from all conventional trans- missions, including belt CVTs. Traction fluids have historically been naphthenic oils. During the 1960s, Monsanto started research on pressure-stiffening lubricants, result- ing in the development of synthetic hydrocarbons based mostly on deri- vatized cyclohexanes and decalins that possess relatively high coeffi- cients of traction, Ct. Other compa- nies have now developed similar syn- thetic cycloaliphatic hydrocarbon traction fluids. Because of the pressures involved, traction fluids function in the elasto- hydrodynamic region, and it is the combination of phy- sical and chemical properties of the lubricant in this re- gime that controls the performance of traction drives. Sev- eral theories at- tempt to explain the properties of trac- tion fluids. The lead- ing one suggests that under high con- tact stress (pressure) and high shear rates of rolling contact, the viscosity of the lubricant film in the contact area increases to a glassy solid state. This “pad” of solid film transmits the tangential force with a shear re- sistance far beyond the capability of a liquid film in addition to preventing metal-to-metal contact, minimizing wear and, when compressed in sur- face cracks, inhibiting the propagation of fatigue cracks. The more “solid” the fluid becomes under pressure, the higher 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 after the pressure is relaxed. The output of a 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 other common physical properties. Typical- ly, variations in normal load, rolling speed and temperature all have some effect on a lubricant’s Ct, depending on the design of the drive and the rolling motion in the contact area. It has been shown that molecular geometry has an important effect on the coefficient of traction. The ability of molecules to arrange themselves into an apparent glassy state under high shear increases the coefficient of traction. Commercial traction lubri- cants, based on cycloaliphatic hydro- carbons, have been used successfully in industrial, automotive and aircraft CVTs. At present, Shell is working closely with Torotrak on the develop- ment and application of traction flu- ids for the company’s IVT, although they have not disclosed the type of fluid being used in field trial units. Although more work remains to be done to determine the operating life- times of IVTs and the traction fluids used in them, the future for these transmissions could be very promis- ing. The fluids must function as good lubricants, but with exceptionally high frictional properties in the pres- sure contacts of the IVT. << David Whitby is chief executive of Pathmas- ter Marketing Ltd. in Surrey, England. You can contact him at pathmaster@dial. pipex.com. Because of the pressures involved, traction fluids function in the elastohydro- dynamic region and it is the combination of physical and chemical properties of the lubricant in this regime that controls the performance of traction drives. Worldwide I By R. David Whitby Traction fluids in infinitely variable transmissions
Transcript of 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 [email protected].
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.
Worldwide
IBy R. David Whitby
Traction fluids in infinitelyvariable transmissions
