A Catechism of the Steam Engine

by C. E. John Bourne

652. Q.--How is the thrust of the screw shaft received?

A.--The thrust of the screw shaft is received upon 7 collars, each 1 inch thick, and with 1 inch of projection above the shaft. The plummer block for receiving the thrust of the shaft is shown in fig. 55, and the coupling to enable the screw propeller to be disconnected from the engine, so that it may revolve freely when the vessel is under sail, is shown in fig. 56. When it is required to disengage the propeller from the engine, the pins passing through the opposite eyes shown fig. 56, are withdrawn by means of screws provided for that purpose, and the propeller and the engine are thenceforth independent of one another.

653. Q.--Will you describe the arrangement of the valve gearing?

A.--The end of the screw shaft, after emerging from the bearing beside the disc, is reduced to a diameter of 4 inches, and is prolonged for 4-1/2 inches to give attachment to the cam or curved plate which gives motion to the expansion valve. This plate is 3-1/2 inches thick, and a stud 3-1/2 inches diameter is fixed in the plate at a distance of 5 inches from the centre of the shaft. To this stud an arm is attached which extends to a distance of 2 inches from the centre of the shaft in the opposite direction, and the end of this arm carries a pin of 2-1/2 inches diameter. From the pin most remote from the centre of the shaft, a rod 2-1/2 inches broad and 1 inch thick extends to the upper end of the link of the link motion; and from the pin least remote from the centre of the shaft, a similar rod extends to the lower end of the link of the link motion. This link, which is represented in fig. 57, is 2-1/4 inches broad, 1 inch thick, and is capable of being raised or lowered 25 inches in all. In the open part of the link is a brass block, which, by raising or lowering the link, takes either the position in which it is represented at the centre of the link, or a position at either end of it. Through the hole in the brass block a pin passes to attach the brass to the end of a lever fixed on the valve shaft; so that whatever motion is imparted to the brass block is communicated to the valve through the medium of this lever. If the brass block be set in the middle of the link, no motion is communicated to it, and the valve being consequently kept stationary and covering both ports, the engine stops. If the link be lowered until the brass block comes to the upper end of the link, the valve receives the motion of the eccentric for going ahead, and the engine moves ahead; whereas if the link be raised until the brass block comes to the lower end of the link, the valve receives the motion of the backing eccentric, and the engine moves astern. Instead of eccentrics, however, pins at the end of the shaft are employed in this engine, the arrangement partaking of the nature of a double crank; but the backing pin has less throw than the going ahead pin, whereby the efficient length of the link for going ahead is increased; and the operation of backing, which does not require to be performed at the highest rate of speed, is sufficiently accommodated by about half the throw being given to the valve that is given in going ahead. A valve shaft extends across the end of the cylinder with two levers standing up, which engage horizontal side rods extending from a small cross head on the end of the valve rod. A lever extends downwards from the end of the valve shaft, which is connected by a pin to the brass block within the link; and the link is moved up or down by the starting handle, which, by means of a spring bolt shooting into a quadrant, holds the starting handle at any position in which it may be set.

654. Q.--What is the diameter and pitch of the screw propeller?

A.--The diameter is 7 feet and the pitch 14 feet. The propeller is Holm's conchoidal propeller. Its diameter is smaller than is advisable, being limited by the draught of water of the vessel; and the vessel was required to have a small draught of water to go over a bar. This engine makes, under favorable circumstances, 100 strokes per minute. The speed of piston with this number of strokes is 700 feet per minute, and the engine works steadily at this speed, the shock and tremor arising from the arrested momentum of the moving parts being taken away by the counterbalance applied at the discs.

LOCOMOTIVE ENGINE.

655. Q.--Will you describe the principal features of a modern locomotive engine?

A.--I will take for this purpose the locomotive Snake, constructed by John V. Gooch for the London and South Western Railway, as an example of a modern locomotive of good construction, adapted for the narrow gauge. The length of the wheel base of this engine is 12 feet 8-1/2 inches. There are two cylinders, each 14¼ inches diameter and 21 inches stroke. The total weight of the engine is 19 tons; and this weight is so distributed on the wheels as to throw 8 tons on the leading wheels, 6 tons on the driving wheels, and 5 tons on the hind wheels. The engine is made with outside cylinders, and the cylinders are raised somewhat out of the horizontal line to enable them better to clear the leading wheels.