Pulley loads

The allowable pulley loads for standard motors are given in Table 8.10. These are recommended by a few manufacturers for their motors, but they are generally true for other motors also. 

Table B.10 Allowable pulley loads for standard motors...

The flat belt is a friction drive transmitting load through the friction between the belt and the pulley while the V-belt is a positive drive which is flat on one side and has a projection like a geartooth on the other. These... 

These are long centre drives with small slips. The slack side of the belt is kept on the top side to increase the angle of contact with the pulleys by sag on the top side. This is essential for an efficient transfer of load. The recommended maximum power that can be transmitted by one belt of different cross-sectional areas is provided by the belt manufacturer and some ratings are given in Table 8.1. When selecting these drives, the following parameters should be borne in mind ... 

Similarly, for shorter centre distances between the drive and the driven pulleys, the arc of contact will decrease. To ensure a good arc of contact, the centre distance C (Figure 8.10), should be kept as much as possible, otherwise the provision of a jockey pulley, as noted above, will also be necessary. A higher arc of contact will ensure a better grip of the belt on the pulley and hence a smaller slip during transmission of the load. A smaller slip would mean a higher transmission of load and vice versa. 

Determine the width of a heavy-duty double-leather flat belt for transmitting a load of 18.5 kW at 1480 r.p.m. from a squirrel cage motor to be switched directly on line. Diameters of pulleys are 250 mm on the motor side and 200 mm on the load side. The centre distance between the pulley may be considered as 800 mm. 

In Example 8.2 we illustrate a step-by-step procedure to select the most appropriate size and length of belts and pulley sizes to transmit a particular load. 

Bending moment at the motor shaft due to pulley and load... 

Note The shaft deflection should not be more lhan 11% of the air gap between the stator and the rotor. For loads that exert more force and torsional stress on the motor shaft and bearings than is permissible, due to the larger width of pulleys which may shift the... 

Diameter of pulley on motor shaft D1 (mm) Width of pulley on motor shaft W1 (mm) Diameter of pulley on load side D2 (mm) Width of pulley on load side W2 (mm)... 

Brake and pulley method (usually for very small motors). Considerable care needs be taken in the construction and use of the brake and pulley. When conducting this test conditions should be such that a scale pointer remains practically stationary at any given load. Proper cooling, preferably water cooling, should be provided for the pulley. 

Calibrated machine. When brake and pulley or dynamometer methods are not possible, the test motor may be loaded onto a calibrated generator. The efficiency curve of the generator must be available. 

The mechanical seal on the radiator water pump of your car has to work under severe conditions. This seal must resist the pressures and temperatures, corresponding to the velocities of the motor, and the variable operating times. This seal is not a precision seal (it has stamped parts rather than machined components) and the pump is a portable pump. The pump doesn t use a direct coupling but a v-belt pulley with radial loading. The seal must resist many vibrations commencing with the v-belt slapping and whipping. 

The friction forces in a pulley system will never hold the load by themselves, but the effort required can be quite small. In the case of a block and tackle used to lift an engine out of an automobile, the weight of the chain hanging from the last pulley may be enough to keep the engine in place. By wrapping a rope once around a post, a cowboy can hold a raging bull 111 check. 

A fixed pulley is a device for changing the direction of an applied force. A common form is a mounted wheel with a rim around which a rope passes. In a vei y primitive form it could he a vine looped over a tree branch. A pull downward on the rope (vine) results in lifting a load on the other end. Neglecting friction, the mechanical advantage of the single fixed pulley is 1 the load moves the same distance as the applied force. 

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. 

The National Electrical Manufacturers Association (NEMA) has developed a standard that specifies minimum pulley diameters that should be used on electric motors. Since motor bearings are specified using a relatively small maximum overhung load, minimum pulley diameters can have a dramatic effect on motor bearings. The increase in tension that is created by smaller pulleys can transmit a potentially destructive side-load on the bearings. 

Excessive belt whip Excessive belt whip is usually more common on long-center applications (applications where the distance between the two pulley shafts is great). Pulsating loads in the drive system can produce this... 

You may have seen pulleys used in a warehouse to lift heavy loads. Another use for a pulley is on a large construction crane. The cable extends from the object being lifted up to the top of the crane boom, across a pulley, and back down to the electric winch that is used to pull on the cable. In this situation, the pulley again causes a change in direction of the pulling force, from the downward force of the winch that pulls the cable to the upward movement of the object being lifted. 

The revolutions per minute (RPM) (or rotational speed) of a fan can be increased by increasing the size of the motor pulley, which is the grooved wheel on the motor shaft. A small increase in the diameter of this pulley will greatly increase airflow through the cooling bundle. But, according to the affinity or fan laws, doubling the diameter of a pulley increases the driver amp load by 800 percent. That is, driver horsepower increases to the cube (third power) of the fan s speed. 

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