USE OF HORSEPOWER TABLES 27 



of the table that the belt should be taken down and re- 

 tightened when its total tension has fallen to 215 Ib. 



To find the horsepower that the belt will transmit when it 

 is on the smallest step of the machine cone, we ascertain 

 from the velocity table its velocity, which in this case is that 

 of an i8-in. pulley at no r.p.m. or 518.3 ft. per minute. 

 This can be considered as 500 ft. per minute. The arc of 

 contact will be the same as before, and as before can be 

 taken as 180 degrees. We may then read the horsepower 

 directly from the horsepower table as 2.27 horsepower per 

 square inch of cross-sectional area of belt. Multiplying this 

 figure by the area of the belt in question, 1.719, we obtain 

 as the horsepower transmitted 3.9. 



Example II. A 25-horsepower motor is located in the 

 roof trusses of a machine shop and drives the shafting for a 

 gioup of machines. The motor has a speed of noo r.p.m., 

 the motor pulley being 9 in. diameter and 9 in. face. The 

 motor is frequently called on to carry an overload of 10 per 

 cent, or a total load of 27.5 horsepower. The center line 

 distance between the motor and the line shaft is 13 ft. 6 in. 

 The pulley on the line shaft is 30 in. diameter. The present 

 belt is 8 in. wide and ^ in. thick. Is it heavy enough for 

 the loads, and if not, what size of belt should be used? At 

 what tensions should the belt be put up and retightened? 



As the belt is in an inaccessible position, the figures in that 

 portion of the horsepower table headed " Countershaft 

 Belts " will apply. From the table of arcs of contact we 

 find the arc of the belt on the smaller of the two pulleys 

 to be 172 degrees, and the answers to our problem will there- 

 fore be found in that section of the horsepower table en- 

 titled " 170 Degrees." From the velocity table the velocity 

 of the belt is ascertained to be 2591.3 ft. per minute. Inas- 

 much as the arc of contact is actually larger than 1 70 degrees, 

 we can assume the velocity to be slightly less than it really 

 is and read the horsepower directly from the table as 6.66, 



