486 APPLIED MECHANICS 
sufficient velocity of water the top of the wheel requires to be abot 
2 feet below the head race. Hence for high falls the overshot wheel 1 
of large diameter. Wheels over 70 feet in diameter have been used. — 
The velocity of the buckets is from 3 to 6 feet per second, or abou 
half the velocity of the entering water. The efficiency of overshot wat er. 
wheels is from 70 to 85 per cent. when well designed and prope 
constructed. It is interesting to notice that the hydraulic eficency of 
the overshot wheel is greater at lower loads when the buckets carry le 
water, because then the buckets do not begin to empty until a g 
part of the descent has been made. 
422. Breast Wheels.—The feature which gives its name to 
breast wheel is the casing, apron, curb, or breast between the boa 
and tail race, which enables the 
buckets to retain the water for a 
greater portion of the fall. This 
breast fits as close to the wheel as 
is consistent with security from 
actual contact. Wheels with breasts 
are also termed high-breast, breast, 
and low-breast wheels, according as 
the water is delivered to the wheel 
above, at, or below the middle level 
of the wheel. Fig. 782 shows a 
high-breast wheel as made by Fair- 
bairn. The regulating sluice and 
its seat are curved, so as to fit close 
to the wheel. The water passes over 
the top of the sluice through guide 4 
passages designed to deliver the water to the buckets without shod c. 
The sluice is operated by a rack and pinion under the control of the 
governor. v7 
The power is taken from the wheel by a pinion gearing with a large’ 
internal toothed wheel attached to the rim of the wheel, as shown. ee 
position of the pinion is such that the downward thrust, due to the 
weight of the water in the buckets, is taken by the pinion without being | 
transmitted to the axle of the wheel, and no torque is carried by the 
arms, which have only to carry the weight of the wheel. The arms are 
comparative slender rods, and are in tension like the spokes of a bieyele 
wheel. ; 
The buckets are of iron, and it will be observed that they stand out 
a little way from what is called the sole of the wheel, permitting a free — 
circulation of air over the water in the buckets, which facilitates a 
discharge of the water from them when they reach the lower end of t 
breast. With this arrangement for the admission of air to the buckets, 
the latter are said to be “ ventilated.” 
The high-breast wheel, like the overshot wheel, acts almost entirely. 
by the weight of the water, and its efficiency is about the same. ¥ 
In low-breast wheels the water acts on the wheel partly by impuls 
and partly by weight. 
Breast and low-breast wheels have efficiencies varying from 50 to 8 
per cent., being greater for large than for small wheels. ‘i 
a 
Q 
a * 
ono oe 
Fig. 782. 
