Machine Design

Variable-Pitch Drives

Variable-pitch drives are the simplest of all mechanical adjustable-speed mechanisms. Most of these drives use rubber belts. However, chain types are unmatched in regard to its particular strong points.

Belt and chain adjustable-speed drives are "adjustable" in two senses. One type, with stationary control can be adjusted only when not in motion. The pulleys have to be stopped and repositioned manually, usually with at least partial disassembly. The motion-control drives can be adjusted while running, and it is this type that is usually meant when the term "adjustable speed" is used. All subsequent comments here refer to motion-control drives.

Belt and chain adjustable-speed drives differ from gear drives in that gears provide only a specific number of drive ratios, while belts and chains provide any reduction ratio continuously over a specified range. An adjustable ratio is provided by adjustable-pitch, conical sheaves that can move laterally on shafts. The sheave separation determines the depth at which a belt or chain rides inside the sheave, thereby providing various "effective" sheave diameters. Output may be step-up or step-down. Chain and belt drives also are usually less costly than gear drives. However, they are not as durable as gears and may "wander" a bit from a set ratio while gears provide an exact ratio.

Rubber belt drives typically are rated from 25 to 50 hp at 4,000 to 12,000 rpm. But some units are rated as low as 2.5 hp for small motor scooters, others are rated at over 80 hp for large combines and racing snowmobiles. The largest industrial drives, however, are normally rated at about 100 hp. The typical price for a medium range belt is from $40 to $80, making the rubber belt one of the least-expensive types of adjustable-speed drives.

Belt drives are usually adjusted manually with vernier mechanisms for precise control. Drives may be operated remotely with a small electric motor attached to the drive-control screw. In addition, belt drives may be designed to vary ratios automatically in response to torque or speed changes.

Efficiency of belt drives is high -- usually about 95%. And belts provide good overload and jam protection because the belt slips when overloaded. However, prolonged slip can cause excessive heat and belt wear.

Even when belt-drive operation is within rated speed and horsepower conditions, heat buildup is still a major factor contributing to belt deterioration. Thus, most belt drives must be well ventilated to dissipate belt heat. This heat and belt wear tend to produce 5 to 10% variation in drive ratio, so speed-control accuracy is relatively low for most models.

There are three standard types of belts for adjustable-speed drives. Classical V-belts are used for limited speed variations to about 1:2. Industrial variable-speed V-belts with wide, thin cross sections are used for applications requiring significant speed variation to 8:1 and transmit up to 32 hp. Agricultural belts are generally thicker than industrial types and can transmit higher horsepower with maximum speed variation to about 4:1. In addition to these, there are also belts custom designed for high-stress applications such as automobiles, recreational vehicles, and heavy machinery.

Industrial variable-speed: These drives are widely used because of their simplicity, efficiency, and wide power range. Until recently, the classical variable-speed belt drive used splined hubs to transmit power and change position. But the hubs require greasing about every 100 hr otherwise they can freeze into position and become single speed devices. Extended use also can freeze pulleys through fretting corrosion.

One solution to this problem is a coating of solid lubricant, which allows the sheaves to move even after long periods at one speed. Also, interchangeable collets enable one variable sheave to work on a variety of shaft sizes, rather than marrying it to one. Recent designs have passed the 50-hp limit and now transmit up to 100 hp. Ratios are also creeping up; previously, 8:1 was tops, now it is 11:1.

One variation on the movable pulley wall is the movable friction surface. Here, the friction surface is divided into segments that move radially to change pulley diameter. A cone-shaped device is driven axially to wedge out friction surfaces while keeping them centered in the same plane. Very fine adjustment is made easily. For instance, a 1-rpm change, less than 0.1%, is possible at full speed. Also, V-ribbed belts contact a greater surface area. Hence, smaller units generally transmit more power. Maximum ratios are somewhat limited, usually less than 2:1.

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