CVT- Continuously Variable Transmission

Milner CVT for High Torque Applications

Contact InterSyn Technologies: Michiel de Rijk


Since the commencement of the Milner CVT project five years ago, development has been concentrated on relatively low power applications (<75kW) due principally to the wishes of the first licensee for the technology, Yamaha. During this period the basic operating principles of the transmission were validated and, by comparing design predictions with practical results, design methods have been improved to the extent that there now exists a sufficiently large knowledge base to justify moving the technology forward into the heavier duty power range associated with automotive transmissions.

The paper discusses the constructional changes developed for heavy-duty applications, describes the design solutions adopted, and looks at the key performance parameters that emerge from analysis of this new version of the Milner CVT.

Basic Operating Principles

The Milner CVT (MCVT) is a rolling traction variator of essentially simple construction. In its original, standard form it is effectively a variable geometry four-point contact ball bearing with the outer race held stationary, drive input to the inner race and power take off from the bearing balls by roller followers located between the balls and mounted on a rotating carrier; see Figure 1.

The substantial contact forces necessary to support rolling traction are contained entirely within the races of the variator assembly. The correct amount of contact force for any input torque is automatically generated by a helical drive mechanism, normally a ball screw, built into the inner race assembly. A similar type of mechanism is employed in the outer race assembly to provide low-load ratio changing under load. How these functions are achieved is now briefly described.

Figure 0. MCVT Cycle transmission

Figure 1. Milner CVT General Arrangement – Original Configuration

Figure 2. Principle of Operation
Rotation of the ratio change lever, typically by electric motor, actuates the outer race ball screw. This changes the axial separation between the outer race halves and causes the planet balls to adjust their radial position and contact points with the raceways. In order to permit this to happen, the inner race ball screw operates (solely under the action of the prevailing forces) to allow the inner race halves to adjust their axial separation and contact points with the planet balls accordingly. The overall function may be likened to that of a normal epicyclic (planetary) gearbox fitted with compound planets (one gear to mesh with the annulus and another with the sun) in which all the gears have variable radii; see Figure 2.

According to the choice of helix angle for the inner race ball screw, the application of input torque automatically develops normal forces at the contact points between the planet balls and raceways that are sufficient to support the tractive forces generated for that level of applied torque. Additionally, the outer race ball screw helix angle may be chosen such that the combination of normal and tractive forces applied to the race half used for ratio changing results in that race more or less floating in the rotational sense. This condition permits low load ratio changing even under the action of large drive torques.

Figure 2b. MCVT transmission animation

Original Configuration of Milner CVT

Tests undertaken on a small MCVT [1] validated basic principles. In this original form the MCVT transmits torque in one direction only and freewheels in the other, in the manner of a sprag clutch. Due to the annular space required by the roller followers, it is normally possible to adopt a maximum of three planets, unless the transmission is restricted to a relatively small ratio range. Photographs of the rolling assembly and rotating assembly (ie the rolling assembly with the outer race set removed) of such a small ratio range unit are shown in Figure 3, illustrating the tight packaging necessary for the adoption of four planets.

Calculation suggests, and tests confirm [1] that the static torque capacity of a unit such as that shown in Figure 3 can be very high. For example, a rolling assembly of 110mm diameter can be engineered to support an input torque of over 250Nm in the worst-case lowest ratio (and up to 750Nm in the higher ratios) before failure. However, various factors cause real-world torque capacity to be significantly less than this and it is consideration these factors that has led to the development of a new form of the transmission.

Figure 3. Example of Rolling/Rotating Assembly – Original Configuration

If the torque requirement of an application is high, but power requirement is low, then this original configuration is normally perfectly adequate. For example, a 75mm diameter MCVT cycle hub transmission has been designed to accept an input torque at the wheel sprocket of 60Nm with lubrication by Santotrac traction grease.

As power (ie speed) is increased, there comes a point at which the lubrication system must be adapted to also serve as a cooling system, involving the use of oil instead of grease. Splash cooling is sometimes appropriate (as for a variable speed crankshaft pulley 10kW/50Nm output torque application with rotating, finned housing, for example) but for more powerful applications some kind of circulatory oil cooling system is necessary. However, in this case, only a basic, low-pressure, low power loss circuit is required (as distinct from the high pressure, high loss circuit required for most CVTs) and therefore adoption of a remote, electrically powered pump is normally the preferred solution.

The inclusion of an oil cooling circuit takes the capability of the original MCVT configuration to at least several tens of kilowatts (any upper limit is as yet unknown) but a number of issues become increasingly difficult to deal with effectively as design size is increased to accommodate the larger power and ratio range requirements of automotive transmissions. These issues include:
      - Losses associated with spherical planets
      - Mass implications of spherical planets
      - Space occupied by roller followers

In addition, of course, automotive transmissions require full bi-directionality of torque and rotation. In order to effectively address all these issues a fundamental design review was undertaken that resulted in the new heavy-duty configuration nowpresented.

The Heavy Duty Milner CVT

Prior to illustrating an example of this new version of the MCVT, key design changes are first described and discussed.

Planet Design

The design review mentioned above identified benefits to be gained by departure from the use of solid spherical planets for large MCVTs, fundamentally on mass grounds, and this opened up new design opportunities that permitted the other principal objectives also to be achieved. A large automotive transmission would require ball planets of several kilograms mass each. Apart from the mass penalty on the transmission as a whole, the centrifugal forces involved at high speed also cause problems. Hollow steel and ceramic balls both offer solutions to the mass problem, but both bring problems of their own, such as availability and cost. Therefore, it was decided to replace the spherical planets with items manufactured from three separate components, enabling redundant mass to be removed.
Having relinquished the geometric constraints inherent in ball planets, the opportunity was taken to shape the rolling surface of the new planet for increased efficiency and to separate the two halves to accommodate a more compact power take off system, resulting in the lenticular shaped, divided planet shown in Figure 4.

At extremes of ratio, ball planets operate under conditions of relatively high contact zone spin. This is because these zones lie close to the axis of planet rotation which means that the plane of the zone lies nearly perpendicular to the rotation axis, as is clear from Figure 2 (analogous to a ball rolling along the walls of a narrow ‘V’ shaped channel; it spins many times before it rolls a distance equal to one diameter). This contact zone spin, a feature of all rolling traction transmissions, is doubly detrimental because it consumes power that both reduces efficiency and has to be removed as heat.

In the new planet design the extreme ratio contact points still lie close to the rotation axis (as they must do to achieve a large ratio range) but contact zone spin is reduced by rotating the plane of the contact zone more towards the rotation axis of the planet and away from the perpendicular to it. This process produces the lenticular shape, as seen in Figure 4, that operates with more of a rolling contact than a spinning one, thereby reducing losses.

Figure 4. Planet Design for Heavy Duty Applications

Power Take Off System

As mentioned previously, the change in planet concept also allowed the incorporation of an improved power take off (PTO) system, one that in general permits the use of one extra planet (which allows five planets to be packaged for small ratio range applications and four planets for large ratio range applications) thereby providing extra torque capacity for a given transmission size. Key features of the new PTO system are illustrated in exploded form in Figure 5.

The three-part planet is push-fit assembled after entering its central shaft (A) into a cylindrical, needle bearing equipped sleeve(B) and after entering the sleeve into a slotted plate (C) provided with one slot for each planet. The slot sides are extended(D) to provide a widened surface on which the sleeve is able to roll a small distance in a radial direction. The plate delivers its power to the PTO hub(E) via holes (F) in the plate that mate with fingers (G) on the hub.

Figure 5. Power Take Off System for Heavy Duty Applications

Bi-Directional Drive System

The form of the transmission shown in Figure 1 is uni-directional because only in one direction of input torque does the ball screw drive the inner race halves together. (In the other direction the inner race halves are driven apart, traction is lost and torque transmission drops to virtually zero.) In order to cause both directions of torque application to drive the inner race halves together, the heavy duty MCVT employs a single ball screw fitted with dual ball nut inner race halves (which may be identical) as illustrated in exploded form in Figure 6.

Inner race travel stops in the form of the collars shown either side of the race set are fixed to the ball screw shaft. For a right hand ball screw helix, as shown, positive torque drives the right race half to its stop and the left race half then supplies the necessary planet clamping forces (and vice versa for negative torque). Each collar engages its half-race via mating pegs with an inclined interface thereby sharing the load input to the half-race with the ball screw. The collar at the free end of the ball screw (left in Figure 6) may be fitted with a lighter duty mounting (such as the shear pin arrangement shown) since this collar is relatively lightly loaded, being subject to engine braking torque only.

Figure 6. Inner Race Component Set for Bi-Directional Heavy Duty Application
In addition to acting as travel stops, the collars perform an important hydraulic damping function as the inner race set runs from one stop to the other upon change of torque direction. The damping action effectively eliminates this backlash (of typically up to three quarters of a revolution) and also provides a hydraulic cushion at the moment metal-to-metal drive is re-established, thereby eliminating the possibility of noise. Oil for the damping system is supplied by the cooling circuit, as shown in the assembly drawing, below, and is admitted to the annular damping chambers via galleries and one-way valves indicated in Section X-X of Figure 6.

Heavy Duty Transmission Layout

Figures 7a and 7b show a typical layout according to the new configuration.

Figure 7b. Layout of Heavy Duty Milner CVT

Figure 7a. Layout of Heavy Duty Milner CVT

The unit shown, which measures 250mm in both diameter and length, has a design input torque rating of 250Nm and an estimated cooling capacity adequate for a power rating of 150kW. Ratings for other sizes are predicted on theoretical grounds to vary according to linear size to the power 2.5 to 3, depending on duty cycle. Thus, for example, a unit of 300mm diameter and length would have a design input torque rating of between 394Nm (=250x(300/250)^2.5) and 432Nm (=250x(300/250)^3). The above ratings are very conservative (static factor-of-safety = 3, fatigue life = 10Grev) and calculation suggests that they could be as much as twice the quoted figures, given adequate cooling.

A worm and wheel type ratio change actuator is illustrated but other types are also suitable. Total travel of the wheel between ratio extremes is about half a revolution, making rapid ratio changes possible, if required, even under load. For example, a 1Nm/3000rpm motor will shift the entire ratio range in one second, even under full load. An advantage of the worm and wheel drive is that no holding current is required to maintain ratio even under full load conditions.

The Figure 7 drawings show the unit in its lowest ratio, which is a reduction of 9.0:1, while Figure 8 shows it in its highest rolling ratio, which is a reduction of 1.28:1, for a ratio range of 7.0, and Figure 9 shows it in an optional lock-up-top condition (which is 1:1, for a ratio range of 9.0, of course). Note that the shift to lock-up is a step shift.

In all conditions except the lowest ratio, the inner race halves are closer together than when in lowest ratio. In these conditions the entire rolling assembly takes up an axial position as determined by the right collar stop under positive drive torque, as shown in Figures 8 and 9, and by the left collar stop under negative drive torque (engine braking).

The maximum coefficient of traction demanded by the design described here is 0.06, which has proved to be satisfactory for earlier MCVT designs when using Santotrac 50 traction fluid for lubrication and cooling. It will be appreciated that the coefficient of traction demanded may be altered simply by changing the helix angle of the inner race ball screw.

As shown in the figures, input and output shafts lie on the same side of the transmission, but for applications where it is advantageous for them to lie on opposite sides this is readily accomplished with only the oil inlet arrangements requiring significant alteration.

With steel as the material for the planets and races, efficiency is predicted to be 90% or greater in all ratios (apart from the optional super-efficient lock-up top condition) but it is known from tests with an earlier MCVT design [1] that silicon nitride components produce an efficiency boost of several percent. It should also be noted that in the MCVT most of the losses occur at the rolling interfaces, with very little power lost elsewhere. This means that efficiency under part- and low-load conditions is much the same as under full load, unlike most other CVTs that are burdened, particularly under lighter load conditions, by the substantial ‘parasitic’ losses of a high-pressure hydraulic control system.

Figure 8. Highest Rolling Ratio

Figure 9. Lock-Up Top Gear


The modifications described make the Milner CVT concept suitable for high torque, high power automotive driveline applications. In this form the Milner CVT offers all the features required of manual, semi-automatic and automatic transmissions in a single package.


[1] ‘Performance Investigations of a Novel Rolling Traction CVT’, Akehurst S, et al, SAE Technical Paper 2001-01-0874

For more information contact:
InterSyn Technologies: Michiel de Rijk

[ this site is under construction]  [created: 14 Nov 2002, updated: 19 Set 2008 ]  [ designed by ]