【正文】 motor speed controls the continuously variable drive ratio. The operator merely works the throttle and brake. Variablediameter Vbelt pulleys connect the motor and chain drive sprocket to give a wide range of speed reduction. The front pulley incorporates a threeball centrifugal clutch which forces the flanges together when the engine speeds up. At idle speed the belt rides on a ball bearing between the retracted flanges of the pulley. During starting and warm up, a lockout prevents the forward clutch from operating. Upon initial engagement, the overall drive ratio is approximately 18:1. As engine speed increases, the belt rides higher up on the forwardpulley flanges until the overall drive ratio bees approximately 6:1. The resulting variations in belt tension are absorbed by the springloaded flanges of the rear pulley. When a clutch is in an idle position, the Vbelt is forced to the outer edge of the rear pulley by a spring force. When the clutchengages, the floating half of the front pulley moves inward, increasing its effective diameter and pulling the belt down between the flanges of the rear pulley. The transmission is torqueresponsive. A sudden engine acceleration increases the effective diameter of the rear pulley, lowering the drive ratio. It works this way: An increase in belt tension rotates the floating flange ahead in relation to the driving flange. The belt now slips slightly on its driver. At this time nylon rollers on the floating flange engage cams on the driving flange, pulling the flanges together and increasing the effective diameter of the pulley. Figure 23 Smoother Drive without Gears 36 Table 21 Timing Belts Timing Belts Timing belts solve the slip problems of flat, O ring, and V belts by using a flexible tooth, molded to a belt that has tension members built in. The teeth are flexible allowing the load to be spread out over all the teeth in contact with the pulley. Timing belts are part of a larger category of power transmission devices called synchronous drives. These belt or cablebased drives have the distinct advantage of not slipping, hence the name synchronous. Synchronous or positive drive also means these belts can even be used in wet conditions,。 and still others directly couple the motor to the output shaft, deal with any misalignment, and exchange speed for torque all in one mechanism. Power transfer mechanisms are normally divided into five general categories: 32 1. Belts (flat, round, Vbelts, timing) 2. Chain (roller, ladder, and timing) 3. plasticandcable chain (bead, ladder, pinned) 4. Friction drives 5. Gears (spur helical, bevel, worm, rack and pinion, and many others) Some of these, like Vbelts and friction drives, can be used to provide the further benefit of mechanically varying the output speed. This ability is not usually required on a mobile robot, indeed it can cause control problems in certain cases because the puter does not have direct control over the actual speed of the output shaft. Other power transfer devices like timing belts, plasticandcable chain, and all types of steel chain connect the input to the output mechanically by means of teeth just like gears. These devices could all be called synchronous because they keep the input and output shafts in synch, but roller chain is usually left out of this category because the rollers allow some relative motion between the chain and the sprocket. The term synchronous is usually applied only to toothed belts which fit on their sprockets much tighter than roller chain. For power transfer methods that require attaching one shaft to another, like motormounted gearboxes driving a separate output shaft, a method to deal with misalignment and vibration should be incorporated. This is done with shaft couplers and flexible drives. In some cases where shock loads might be high, a method of protecting against overloading and breaking the power transfer mechanism should be included. This is done with torque limiters and clutches. Let’s take a look at each method. We’ll start with mechanisms that transfer power between shafts that are not inline, and then look at couplers and torque limiters. Each section has a short discussion on how well that method applies to mobile robots. BELTS Belts are available in at least 4 major variations and many smaller can be used at power levels from fractional horsepower to tens of horsepower. They can be used in variable speed drives, remembering that this may cause control problems in an autonomous robot. They are durable, in most cases quiet, and handle some misalignment. The four variations are ? Flat belt ? Oring belt 33 ? Vbelt ? Timing belt There are many panies that make belts, many of which have excellent web sites on the World Wide Web. Their web sites contain an enormous amount of information about belts of all types. ? ? ? ? ? ? ? Flat Belts Flat belts are an old design that has only limited use today. The belt was originally made flat primarily because the only available durable belt material was leather. In the late 18th and early 19th centuries, it was used extensively in just about every facility that required moving rotating power from one place to another. There are examples running in museums and some period villages, but for the most part flat belts are obsolete. Leather flat belts suffered from relatively short life and moderate efficiency. Having said all that, they are still available for low power devices with the belts now being made of more durable urethane rubber, sometimes reinforced with nylon, Kevlar, or polyester tension members. They require good alignment between the driver and driven pulleys and the pulleys themselves are not actually flat, but slightly convex. While they do work, there are better belt sty