416 KINEMATICS OF MACHINERY.
Evrard's Chamber-wheel Gear, Plate XXXV. Fig. 2.
In the Belgian department of the 1867 Paris Exhibition there was exhibited a ventilator of Evrard's which, although very indif- ferently constructed, was yet in itself remarkable, and deserves notice here. Fig. 2, PL XXXV. shows its general construction.
In the Belgian section of the Vienna Exhibition also, one of the same machines was shown arranged as a water-pump. It is essentially a two-toothed chamber-wheel train in which the wheels are not similar in form, although they still, as in the former cases, revolve with equal angular velocities. The wheel a has two spaces falling entirely within its pitch circle, while the wheel b has two teeth lying entirely beyond its pitch circle. The teeth of a are very similar in form to those of Eoot's machine PI. XXXIV. 2, they lie, however, entirely within the pitch circle, the corresponding spaces of b therefore are entirely outside its pitch circle. The curve m I o is the curve described by the point n of b relatively to the wheel a, it is therefore a curtate epitrochoid corresponding to the rolling of the two equal circles of radius r. The curve p n is the common epicycloid (in this special case a cardioid), described by the point o of the wheel a relatively to the wheel b. Contact ceases at o at the instant that m and n come into gear. In order that this may take place, the angle m I o must be made equal to the angle subtended by the foot of the tooth on b, that is to double the angle marked a.
The spaces of both wheels carry the fluid from below upwards as they turn in the direction of the arrows. The greater part of the contents of the spaces of a, namely that represented by the opening m I o, is, however, returned again between the wheels. For each revolution, therefore, the volume of fluid passed upwards through the machine is a little less than the volume of the tooth-ring cylinder. The contact of the two wheels has the excellent pecu- liarity that the tops of the teeth of a roll upon the bottom of the spaces of b without sliding. The ventilator exhibited at Paris, so