a mechanical drive in which motion is transmitted or converted by the frictional forces between rolling bodies—cylinders, cones, and the like—that are pressed against each other.

Friction drives may be used to transmit motion between shafts having parallel or transverse axes, to convert rotary motion to helical motion, and to convert rotary motion to translatory motion. They may have fixed or variable gear ratios. The advantages of friction drives include simplicity of fabricating the rolling bodies, uniformity of rotation, and the feasibility of continuously variable regulation of rotational speed. Among the disadvantages are the heavy loads imposed on shafts and bearings, the need for devices to press the rolling bodies against each other, and the absence of a rigid kinematic coupling.

Friction drives with a fixed gear ratio are used in instruments because the small forces required to press the rolling bodies together pose no difficulties. Wheel-to-rail and wheel-to-road drives are widely used in self-propelled transportation. In machine building, friction drives with variable gear ratios are most often used for continuously variable speed regulation. Depending on the shape of the principal rolling body (for which the rolling radius is varied), continuously variable friction drives are divided into disk, cone, sphere, and toroidal types (seeCONTINUOUSLY VARIABLE TRANSMISSION).

Friction drives are produced for powers ranging from infinitely small values (in instruments) to hundred of kilowatts, ordinarily up to 20 kilowatts. The gear ratio may reach 1:7 in power transmissions, 1:15 in unloaded shafts, and 1:25 in hand-operated instrument drives. In simple continuously variable friction drives (with two rolling bodies), the widest range of gear ratios has a highest ratio five times greater than the lowest; in duplex drives (with intermediate rolling bodies), the factor reaches 15, but is usually between four and eight. In simple friction drives the rolling bodies are pressed together by a constant force; in the more complicated types a self-tightening wedge mechanism increases the force with an increase in the torque being transmitted. The shape of the rolling bodies is chosen so as to reduce or eliminate slippage, which is a function of the difference between the linear velocities of the contacting bodies.

The rolling pairs in drives that operate primarily in oil are made of extremely hard steel and require very accurate fabrication; those designed to be operated dry are made of steel or plastic such as resin-impregnated fabric laminates and special friction plastics.