The Steam or Pressure Gauge (Fig. 193) is a circular brass case with a dial in front, somewhat similar to a clock; within the case is a small curved tube made so as to be somewhat elastic, this tube is in direct communication with the steam in the boiler. A somewhat peculiar action is relied upon, which is, that as the pressure of steam is exerted within this curved tube, it tends to straighten it, and this, by a simple arrangement of wheels, causes the pointer to move round the dial, which is provided with figures round its edge showing the pressure in pounds (to the square inch) that is exerted in the boiler as the indicator points to them.
194. SAFETY VALVE. (SECTION)
The utility of the safety valve (Fig. 194) is obvious. They are invariably made so that by means of a weight or other device they can be regulated to blow off at whatever pressure the engineer dictates, the pressure being indicated by the pressure gauge.
Fig. 195 represents a cylinder with piston inside; the cylinder should be encased with wood or some non-conducting material, or be provided with an outer iron casing or “jacket,” the space between the casing and the cylinder being converted into a steam chamber. Whatever method is adopted, the object is to keep the cylinder from losing its heat and to prevent condensation. The “ports” are openings through which the steam passes; by means of “slide valves” the steam is alternately admitted and expelled from each of these, so that the opening which serves to admit steam on the instroke serves as an exit for the steam on the outstroke, and the slide valves are worked from the main crank shaft by valve gear and eccentrics.
195. Cylinder, with piston inside.
The Valve Gear is the arrangement of rods that connect the eccentrics with the slide valves.
The Piston consists of a circular disc of metal made to most accurately fit the interior of the cylinder; to this is connected a rod as shown, called the piston rod, which is in direct communication with the crank. It will be seen that when the steam is admitted into one end of the cylinder the pressure causes the piston to travel towards the other end; when the piston reaches a certain point (called the dead point) the slide valve shifts and the inflow of steam is changed to the other end, and this causes the piston to travel back again, and so it continues; an instroke and outstroke give one revolution to the crank and flywheel.