Development of Friction Drive Transmission

Friction drive is a mechanical device that utilizes friction force to transmit torque and power. Since the power is transferred through shearing a thin layer of highly pressurized lubricant film formed between the mating surfaces. Friction drive possesses desired performance attributes that pertain to its unique operating principles. These attributes include high mechanical efficiency, minimal backlash, low noise and vibration and high-speed capability. The power density of a friction drive can be very high when operated at elevated speeds. These performance features, in conjunction with its inherent manufacturing simplicity, make friction drives suitable candidates for a host of applications. The current global technology trend towards electrification and increasing use of electric machines in auxiliary drives for both automotive and industrial applications presents a good opportunity for friction drives as a cost-effective alternative to conventional gear drives. The smooth high-speed performance feature of friction drives allows the use of more efficient high-speed motors to reduce motor size and thus improve system power density. A novel cylindrical friction drive was developed [1,2] for electric oil pump applications. The friction drive is to be integrated with an electric motor to provide necessary speed reduction. The friction drive, as shown in Figure 1, is comprised of an outer ring, a sun roller, a loading planet, two supporting planets and a stationary carrier. The sun roller is set eccentric to the outer ring to generate a wedge gap that facilitates a torque actuated loading mechanism for the friction drive. The loading planet is properly assembled in the wedge gap with frictional contact with the sun roller and the outer ring and is elastically supported on the carrier. By altering the ratio of the support stiffness to contact stiffness, the actual operating friction coefficient of the friction drive can be changed to suit for desired performance regardless the wedge angle. This provides a grater freedom for design optimization. Design analysis was presented and a FE model was developed to quantify design parameters. Prototypes of the friction drive were fabricated for testing. Major geometry parameters are listed in Table 1. Extensive testing was conducted to evaluate its performance. Figure 2 shows the schematic of test apparatus. It is comprised of a drive motor, a high-speed spindle, and a hydraulic brake pump. The motor drives the spindle through a rubber belt and a pair of pulleys. The spindle shaft connects to the input shaft of the friction drive thought an input torque meter. The output shaft of the friction drive couples to the hydraulic pump through an output torque meter. The torque meters pick up both speed and torque signals at input and output shafts of the friction drive, respectively. Thermo-couples are mounted to monitor temperatures at planet support shafts and at bearings of input and output shafts. An accelerometer was placed on the back plate of a mounting bracket to which the friction drive was bolted. It monitors the vibration signals of the friction drive for reference and safety purposes. A data acquisition system was used to collect and process all signals at predetermined sampling rate. The friction drive offered a consistent smooth and quite performance over a wide range of operating conditions. It was capable of operating at an elevated speed of up to 12000 rpm with adequate thermal characteristics. Figure 3 shows the steady sate temperature contour map as function of input shaft speed and output shaft torque. Results demonstrated that the friction drive has high power transmission efficiency under various test conditions. The peak efficiency exceeded 97%. Figure 4 plots the overall system efficiency as a function of output torque for various input speeds. Results also confirmed that the stiffness of the elastic support has an important impact on performance. The elastic support stiffness, in conjunction with, the contact stiffness determines the actual operating friction coefficient at the frictional contacts.