Sheave ratio

During the startup of the line, the extruder was quickly identified as not having enough torque from the motor to make the product. At this point the best option was to change the belt sheave ratio between the motor and gearbox such that addi-... 

At a maximum rate of 440 kg/h, the extruder was rate limited by motor torque and not by the quality of the extrudate. This extruder was fabricated with a gearbox and sheave ratio such that the maximum screw speed was 105 rpm. Since the process was operating at a screw speed of 45 rpm, the sheave ratio could be changed such that the maximum screw speed would be about 75 rpm. This might allow the process to operate at rates up to 700 kg/h. Changing the sheave ratio will increase the torque to the inlet side of the gearbox. Care must be taken to make sure that the additional torque does not harm the gearbox. 

In applications where a measurable fan-speed change can occur (i.e., V-belt or variable-speed drives), care must be taken to ensure that the selected speed does not coincide with any of the fan s critical speeds. For general-purpose fans, the actual running speed is designed to be between 10 and 15% below the first critical speed of the rotating element. If the sheave ratio of a V-belt drive or the actual running speed is increased above the design value, it may coincide with a critical speed. 

The silicon-controlled rectifier with a dc motor has become predominant in adjustable-speed drives for almost all commonly used conveyors when speed adjustment to process conditions is necessary. The low cost of this control device has influenced its use when speed synchronization among conveyors is required. This can also be done, of course, by changing sheave or sprocket ratios. 

For V-bell applications D and d are sheave pilch diameters. From the geometry ol Figure 3-22, 6 is always less than or equal to 180° or n radians. The lower guideline for 6 is approximately 150°. Below this value there will be increasing tension and slip, which will result in decretised life of the V belts. This limit on 6 imposes a lower limit on the center distance and thus a practical limit on the speed ratio attainable is a given V-belt design. 

The type of sprocket to be used depends entirely upon local or load conditions. The steel plate without hubs is the cheapest and is furnished for bolting to suitable hubs or flanges. The cast-iron or steel type fitted with hubs is made for direct mounting upon shaft and is fastened in place by either keys or setscrews or a combination of both. The split type is almost a necessity when the hub is mounted on a shaft with other pulleys or sheaves. Its construction facilitates installation and removal, but because of its extra cost it is usually not recommended except when solid hubs cannot be installed. The double-duty sprockets are made with steel rims or plates that may be removed or replaced without disturbing the hub, shaft, bearing, etc. Plates and hubs can be obtained either solid or split. They are particularly adapted for jobs requiring changing of drive ratios or where replacements must be made quickly. 

Figure 4-79. Relative service for various D d ratios for sheaves [11]. ...

Belt-drive ratios may be calculated if the pitch diameters (see Figure 44.5) of the sheaves are known. This coefficient, which is used to determine the driven speed given... 

In order to provide a higher torque to the screw, a belt sheave system was installed between the motor and the gearbox. This sheave system had a reduction ratio of 1.95, reducing the maximum screw speed from 125 to 65 rpm. Sheaving... 

D/d = Diameter ratio of friction wheel (head sheave, deflect sheave) and wire rope. 

Double-wrapped hoist drum/sheave configuration to provide a high level of friction, allowing T1/T2 ratios of up to 2.7... 

Belt drives use adjustable sheaves. The axial distance between the sheaves can be varied this changes the effective pitch at which the belt contacts the sheave. This, in turn, changes the transmission ratio. The speed is usually varied by a vernier screw mechanism, which is hand cranked or activated electrically. Belt drives are used up to 100 hp. The largest speed ratio is about 10 1, and a maximum speed is typically 4000 rpm. Belt drives have a reasonable efficiency, tolerate shock leads, and provide optimum smoothness in a mechanical drive. Disadvantages are heat generation, possibility of slippage, and relatively poor speed control. In addition, belt drives are subject to wear and, thus, are maintenance-intensive belts generally have to be replaced every 2000 hours. 

Within certain limits, V belts are well suited to drives that must run at varying input or output speeds. These are common in the air-moving industry and are referred to as variable-pitch drives. These drives must incorporate special sheaves. Speed ratio on these drives is controlled by moving one sheave sidewall relative to the fixed sidewall so that the belt can ride at different pitch diameters. Variable-pitch drives using a single variable-pitch sheave and classical cross-section belts will yield only about 1.4 1 overall speed variation. 

V-belt drives are widely used in industry and commercial applications. V-belts are utilized to transfer energy from a driver to the driven and usually transfer one speed ratio to another through the use of different sheave sizes. 

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