Afriction drive[1][2] orfriction engine is a type oftransmission that utilises thestatic friction of two smooth surfaces (instead of contactpressure of meshing teeth) to transfertorque between two rotating parts.
This type of mechanism is also called atraction drive, although this term often refers specifically to drives where a layer oftraction fluid (that becomes momentarily solid under pressure) is used to increase thefriction coefficient between the two parts, to 0.1 or more.[3][4][5]
In general, least one of the two parts is rigid, and it may be anysolid of revolution, such as a disk, cylinder, or cone. While the bulk of the rigid part(s) may be constructed of any hard material, such as metal or plastic, at least one of the surfaces where they come into contact usually is coated with some high-friction material, such ashard rubber (or, in early systems, paper orleather).
The most common example of friction drive is apulley and (smooth)belt pair. However, the name "friction drive" is more often used when both parts are rigid.
A friction drive was patented by C. W. Hunt in 1887.
The simplest form of friction drive is a pair of cylinders mounted on parallel axes, pressing against each others.
Themechanical advantage of this system is determined by the ratior between the radiiR1 andR2 of the two cylinders. Namely, the torquesT andT on the two axles will satisfyT1/T2 =r =R1/R2, while therotation speedsω1 andω2 will satisfyω1/ω2 = 1/r =R2/R1.
This system is used, for example, to drive the spools in sometape recorders. In a typical configuration, one of the cylinders is the axle of an electric motor, say 1mm in diameter, while the other is a disk with a hard rubber rim, say 50 mm in diameter, yielding a 1:50 mechanical advantage. A gear train with the same ratio would require either much larger gears or very fine teeth, which would be much more expensive and delicate to produce.
Another important application ismotorized bicycles, where a rubber-coated roller connected to the motor drives one of the bike wheels by friction. Other examples are add-on motors to operate hand wheels.[6]
Another important type of friction drive consists of a wheel rolling over a mostly flat surface or mostly straight track. This could be seen as a limiting case of the parallel-cylinder arrangement where one of the cylinders has practically infinite radius. In this case the goal is not to transfer torque but to convert torque into linear force, and the rotary motion of the wheel to relative linear motion of the two parts.
The most common example is of course wheeled vehicles traveling on roads or railway tracks. But the same arrangement is also used in many other applications, such asconveyor belts[7] androller coasters.
Another important type of friction drive consists of two discs with perpendicular axes, arranged so that the edge of one wheel presses against the flat face of the other. Friction between the two wheels causes torque applied to one wheel to be transferred to the other.
Themechanical advantage of the system is determined by the ratior between the radiusR1 of the first wheel and the distanceR2 on the second wheel between its axis and the point of contact. If the first disk is mounted so that it can slide along its shaft, the radiusR2, and therefore the ratior, can be varied continuously even while the disks are rotating. The system is then acontinuously variable transmission that requires noclutch.
One problem with this design is that the inner and outer edges of the first disk are moving at the same speed, but the speed on the surface of the second disk increases proportionally with distance from the axis. This speed mismatch causes considerable friction at the edges of the first disk, wearing it out and wasting mechanical power as heat. Since these effects are proportional to the contact pressure and to the relative speed of the rubbing surfaces, they limit the torque and rotational speed that can be transferred.
This kind of friction drive was once employed in earlyautomobiles, but today the system is most commonly used onscooters, particularly go-peds, as a substitute for a chain and gear system. It is mechanically identical to aball-and-disk integrator, but is designed to handle highertorque levels.
Friction drive has been most successfully used in low-power applications, such as drivingphonograph turntables.
Automobiles using this drive system included theAnglo-Dane, theArista, theArmadale, theAstra, theAllvelo, theBukh & Gry, theCartercar, theCrown 12HP Model Two (1905-1906), theDavis Totem, theG.W.K., theKelsey, theLambert, theLuLu, theMetz, theNer-a Car, theRichardson, and theTuricum. The Turicum's friction drive consisted of a flat steel disk coupled directly to the engine. This primary disk subsequently drove a smallerleather covered wheel oriented normal to its surface. Assuming a constant rotational velocity on the primary wheel, the angular velocity on the disk's surface will increase proportionally to the distance from the center of rotation. Therefore, positioning the smaller wheel at different points along the larger wheel's surface varies thegear ratio. Furthermore, since there are no limitations beyond the minimum and maximum positions, the gear ratios are infinitely adjustable. The Lambert's friction drive (illustrated) was similar but used an aluminium-faced driving disk and a fiber-faced driven wheel.
Plymouth Locomotive Works's first three models, the AL, BL and CL were equipped with a friction drive.[8][9]
Early models of thepermanent way maintenance ganger'sWickham trolley used a vee-twinJAP engine. This drove through a large flat flywheel and a friction drive.[10]
While mechanically simple, friction drives have a number of limitations. The first is that the amount of torque that can be transferred is a function of the area of thecontact patch between the two members and the pressure applied to it by the two parts. Increasing either parameter increases the maximum transmitted torque, but also increases losses due to friction and local deformation of the material at the contact.
Friction drives also suffer from slippage when overloaded, as the static friction limit is reached. They also accumulate small amounts of slippage in normal operation because local deformation of the surface changes the effective radius of the part. Thus they cannot be used for precise rotational positioning, unless the motor uses somefeedback scheme to compensate for those errors.
On the other hand, there are applications where slippage is desired, as it prevents damage to the mechanism in case of overload.
