Fluid / Contact Patch
Traction fluid is an elastohydrodynamic fluid specifically designed for use in traction drives. Elastrohydrodynamic lubrication (EHL) is a regime of lubrication that occurs in rolling contact areas. In this regime, shear resistance of the fluid is several orders of magnitude greater than classical hydrodynamic theory would predict. In EHL regimes, the fluid in the contact patch is under high pressure (>1 GPa) and behaves like an elastic solid over a short duration of time (≤1msec). This regime occurs in ball bearings and between gear teeth, as well as in traction drives.
Early traction drives relied on hydrodamic lubrication, which limited the amount of force that could be transferred across the contact patch. In hydrodynamic lubrication, normal forces pressing the rolling elements together are relatively small, the fluid molecules are disordered and tangled, and the force transferred across the fluid film is also small. In EHL, normal forces are high, the fluid gap is orders of magnitude smaller, and the fluid molecules assume a stacked arrangement that makes the fluid behave like an elastic solid capable of transferring large amounts of force through the contact patch. The development of elastohydrodynamic traction fluids allows current traction drives to transfer large amounts of torque.
Traction oil consists of synthetic oil with additives that significantly increase the coefficient of traction under high pressure. In addition to transferring power through traction, the fluid also lubricates surfaces, bearings and gears and transfers heat created by power losses. The traction coefficient of this fluid is non-linear and is dependent on a number of factors, including Hertz pressure, fluid temperature, fluid properties, entrainment velocity, and relative motion between the rolling bodies, or creep. A sample traction curve is shown in Figure 3.
The traction coefficient dependence on temperature is of special significance for automotive applications. Traction oils have historically had high viscosity at -40C, making them difficult to pump to the transmission. In addition, the traction performance falls off at elevated temperatures common to the engine compartment. Recent developments of traction fluids have improved the viscosity at -40C while also decreasing the sensitivity of traction performance to elevated temperatures.
Figure 3 – Typical traction curve for traction oil. Mu is a function of % creep, or the difference in speed between the driving element and the driven element (click image to enlarge).
The contact patch is the small area of contact between the elements of the traction drive. Typically, the rolling elements are ball-on-cone (forming an elliptical contact patch) or cone-on-cone (forming a cylindrical contact patch). The size of the patch is determined by the geometry of the rolling elements combined with the contact pressure, which causes elastic deformation of the two elements. The contact pressure is distributed through the contact patch according to Hertz theory (see Figure 4).
Figure 4– Contact force is distributed through the contact patch according to Hertz theory. Ball on cone geometry shown (click image to enlarge).
The shear force created by the driving member on the contact patch is resolved into traction force as well as losses. The traction force consists of the sum of the forces in the rolling direction. These are the forces that are transferred from the driving member to the driven member. Losses can be divided into three primary sources (see Figure 5).
- Slip: speed variation between rolling elements
- Side Slip: relative motion orthogonal to the rolling direction
- Spin: relative speed variation within the contact
These losses occur as the fluid film is sheared. This shearing can occur in the direction of rolling (slip) as well as in non-rolling directions. Some slip is normal in the operation of a traction drive and is related to how much torque is being transferred. The side slip and spin terms are related to the geometry of the contact patch as well as the different axis of rotation for the two rolling bodies.
The energy lost through the shearing of the fluid is converted into heat in the fluid. Some of the heat transfers into the rolling elements, and the remainder of the heat is retained by the fluid and exits the contact patch.Because the traction performance of the fluid is degraded with elevated temperature, the fluid is typically sent through a heat exchanger to remove some of this heat prior to resupply to the drive.
Figure 5 – Contact patch traction losses. Direction of rolling is along the X-axis (click image to enlarge).