GEAR TRAINS 433 9.1 ROLLING CYLINDERS The simplest means of transferring rotary motion from one shaft to another is a pair of rolling cylinders.They may be an external set of rolling cylinders as shown in Figure 9-1a or an internal set as in Figure 9-1b.Provided that sufficient friction is available at the rolling interface,this mechanism will work quite well.There will be no slip between the cylinders until the maximum available frictional force at the joint is exceeded by the demands of torque transfer. A variation on this mechanism is what causes your car or bicycle to move along the road.Your tire is one rolling cylinder and the road the other(very large radius) one.Friction is all that prevents slip between the two,and it works well unless the (a)Extemal set friction coefficient is reduced by the presence of ice or other slippery substances.In fact,some early automobiles had rolling cylinder drives inside the transmission.as do some present-day snowblowers and garden tractors which use a rubber-coated wheel rolling against a steel disk to transmit power from the engine to the wheels. A variant on the rolling cylinder drive is the flat or vee belt as shown in Figure 9-2. This mechanism also transfers power through friction and is capable of quite large pow- er levels,provided enough belt cross section is provided.Friction belts are used in a wide variety of applications from small sewing machines to the alternator drive on your car,to multihorsepower generators and pumps.Whenever absolute phasing is not required and power levels are moderate,a friction belt drive may be the best choice.They are relative- ly quiet running,require no lubrication,and are inexpensive compared to gears and chain (b)Intemnal set drives. FIGURE 9-1 Both rolling cylinders and belt(or chain)drives have effective linkage equivalents Rolling cylinders as shown in Figure 9-3.These effective linkages are valid only for one instantaneous position but nevertheless show that these devices are just another variation of the four- bar linkage in disguise. FIGURE 9-2 A two-groove vee belt drive Courtesy of T.B.Wood's Sons Co..Chambersburg.PA