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Example 3
A 200-hp (150-kW), 1,800-rpm hollow-shaft motor has an outside shaft diameter of 3-in (76-mm) and inside diameter of 2-in (51-mm). To determine if this shaft size is sufficient to transmit the required torque, solve the following equation for P:
For this example, P must be greater than 200-hp to ensure that the shaft will be large enough to handle the torque of the motor.
80 x P/1800 = (34 - 24)/3
P = 1702-hp
Theoretically, this shaft is capable of transmitting 1,700-hp, so it is more than sufficient for the 200-hp requirement.
Example 4
The amount of torque that a hollow shaft can transmit depends on the thickness of the wall between its inside and outside diameters. A thinner wall cannot handle as much torque as a thicker one. The 3-in shaft in Example 3 was capable of transmitting 1,700-hp and had a wall .5-in thick: (3 - 2)/2 = .5-in. How much horsepower could a 3-in shaft transmit if the wall were only .25-in thick?
The effect of a thinner wall is dramatic. The shaft with the .25-in wall can carry less than 20 percent of the torque of the shaft with .5-in wall.
Conclusion
Engineers tend to design using an ample safety factor, and older equipment in particular was over-designed even by today's standards. Of course, that is one reason many of us appreciate older machinery; it was darned reliable!
Keep in mind that adding a keyway to an existing shaft weakens the shaft. Likewise, increasing the bore diameter of a hollow-shaft reduces the torque capacity. Consider modifying a shaft only with good engineering support. Even then, remember that the greater the consequence of failure, the more generous the safety factor should be. After all, who wants to board an elevator that was designed and built with no safety factor?
Chuck Yung is a technical support specialist at the Electrical Apparatus Service Association (EASA), and Cyndi Nyberg is a former EASA technical support specialist. EASA is an international trade association of more than 2,100 firms in 50 countries that sell and service electrical, electronic and mechanical apparatus. EASA, St. Louis, MO, 314-993-2220, Fax: 314-993-1269, www.easa.com.
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