BELT, ROPE, AND CHAIN GEARING 369 
the belt, or the desired pressure may be obtained by means of an 
adjusting screw. 
The use of a jockey pulley also permits of two pulleys, differing con- 
"siderably i in diameter, being placed much closer together by increasing 
the are of contact of the belt on the smaller pulley. 
314. Power Transmitted by Belts.—When a belt is transmitting 
wer from one pulley AB (Fig. 569) to another CD, the motion being 
in the direction of the arrows, the tension in the por- 
i, tion BD is greater than the tension in the portion 
_ AC. BD is called the tight side, and AC the slack 
side of the belt. Let 'T, = the tension on the tight 
side, and T,=the tension on the slack side, then 
4 T,-T, =P is the driving force at the rim of the 
eS pulley CD. If V is the velocity of the belt in feet 
per minute, v the velocity in feet per second, and H the horse-power trans- 
PV Fv 
mitted, then H = 35000 = B50 
The ratio of T, to T, when the belt is on the point of slipping was 
discussed in Art. 243, p. ‘27 7. For most practical cases T, may be taken 
equal to 2T 
If bis abe breadth of the belt, and ¢ its thickness, both in inches, and 
f the working stress in lbs. per square inch, then T, = Uéf, and if T,=xT,, 
where 2 is a fraction, P=(1—)T, =(1—n)dtf.. Hence atks n)bifo 
550 
For leather belts, / is generally from 200 to 350. 
3 315. Effect of Centrifugal Tension on Power Transmitted by 
__ Belts.—In Art. 92, p. 76, it was shown that a thin hoop revolving has 
a tensile stress in it due to centrifugal force, and the demonstration there 
given is applicable to a belt running on a pulley. If /, is the stress on a 
belt, in Ibs. per square inch, due to centrifugal force, w, the weight of 
a cubic inch of the belt in Ibs., » its velocity in feet per second, then 
Onn. v2 2 iat 2 ; 
i= — —"”’ where w is the weight in Ibs. of a portion of the belt 
Fia. 569. 
——., where P is in lbs. 
12 Suen long and 1 square inch in section. For leather, w may be 
taken at 0-4 Ib. 
The total tension on the tight side of the belt is now T, + dt/, =dt/, 
and T,=t(f—j,). The total tension on the slack side is T, + d#f,, and 
P=, -T,=(1—n)T, =04(1 —n)(f—f,) = o4(1 - 0) (s-22). 
_ Pv _dt(1—n) 
Hence H = 550 ~ — B50 ( Sv- =) this is of the form H = av — ev’, 
bt(1 — n) =n) f bt(1 - i . 
where a= “550 and CO" BE 
To find when H is a maximum, TH = a — Bcc, and H will be a 
maximum when Buo, that is, when 3cv? =a, or v= nN oa , putting in 
§ the values of aandc. H is a maximum when v= a 2 
2a 
