Various other forms of link-motion have been devised which share this advantage. It is also shared by a group of equivalent mechanical devices, called by the general name of "radial valve gears," in which only one eccentric is employed but the valve is caused to take a motion capable of being varied in the same way. Of these the most widely used is the Walschaert's gear, very com mon in locomotives, a skeleton diagram of which is shown in fig. 9. There A is a projection moving with the engine piston. The valve V takes motion partly from it, through the rods AB and BV ; but BV has a fulcrum at D, the position of which varies through the movement of the rod DG. That rod is supported by a sus pension rod I, and its end at G can slide in a link HI rocking about a fixed fulcrum at 0. There is one eccentric, CF, set at right angles to the crank CK, and giving motion to the rocking link HI by the rod FH. The block G can be shifted from near H to I by raising the suspension rod J. This reverses the motion, and allows also of notching up to any intermediate position.
To keep a steam engine running at a uniform speed, notwithstanding variations in the demand for power, hand regulation may to some extent serve, but in very many cases an automatic regulator called a governor is provided. Regulation, whether by hand or by automatic governor, may be effected in two ways : the steam may be throttled more or less on its way from the boiler to the engine, so that the pressure of admission is reduced when the speed tends to rise, and increased when the speed tends to fall: or the point of cut-off may be varied, with the result of admitting a greater or less volume of steam in each stroke. In small engines the governor often acts on a throttle valve, but at engines which aim at efficiency it is preferable to govern by alter ing the cut-off. In such engines, to secure in any case a proper amount of expansion, there is an early cut-off, and a small change in its position largely affects the amount of steam admitted and consequently the amount of work done.
Watt's original governor was a pair of balls revolving about a vertical spindle as conical pendulums under the combined effect of their own weight and the "centrifugal force" due to their speed of revolution. Any increase of speed sufficient to overcome the slight friction of the attached gearing made them take up a higher position, and in so doing they partially closed a throttle valve. This went on till, at a slightly greater speed than before, equilib rium was established between the supply of steam and the demand for power. Here, as in all governors, the effect is not to maintain a strictly constant speed, but to prevent the variation, either way, from exceeding narrow limits.
In modern governors the `!centrifugal force" of the balls gen erally acts not simply against their own weight, but against an ex tra load or against the force of a spring.
In many factory and other engines the governor determines the position of cut-off by means of a trigger action of the type already mentioned : the admission valve, which has been opened and held open by a cam turning with the engine shaft, is suddenly discon nected by the release of a trigger, at a point depending on the exact height of the governor balls.
The function of the governor is, generally, to keep the engine running at a nearly uniform rate, measured in turns per minute : the function of the flywheel is to keep the rate nearly uniform within the limits of any one revolu tion. For this purpose it must act as a reservoir of energy, alter nately storing and restoring the excess amounts which result from periodic variations of turning moment in the operation of the piston on the crank. At the dead points the piston's rate of doing work is zero, and it rises to a maximum at an intermediate point the position of which may be much affected by an early cut-off. The flywheel is drawn upon for energy during those parts of the revolution in which the work done by the piston (or pistons) on the engine shaft is less than the work done by the shaft on the mechanism which the engine drives; and it takes up the surplus during the other parts of the revolution. To effect this alternate give and take of energy the flywheel must undergo small varia tions of speed, the magnitude of which may be kept down to any assigned amount by giving the rim a sufficient mass and speed.
The effective effort on the crank is greatly influenced, especially in high-speed engines, by the inertia of the piston, piston-rod and connecting rod. At the beginning of the stroke, when the piston is coming towards the crank-shaft, these reciprocating masses are acquiring motion; from a point near the middle to the end of the stroke they are losing motion. Hence in the early part the effort on the crank is reduced below what is due to the steam-pressure on the piston and in the later part it is increased. In a high-speed engine the forces due to acceleration become so large as seriously to affect questions of design. In direct-acting steam pumps the inertia of pump rods and buckets forms an additional item in the considera tion of reciprocating mass : its effect there is on the whole bene ficial in tending to equalize the force which is exerted on the pump throughout the stroke.
Another aspect of the acceleration of reciprocating parts is its effect on the balance of the engine as a whole. This is specially important in locomotives and marine engines (q.v.). In a locomotive the forces on the engine frame arising from the inertia of the pistons and rods tend to make the engine sway laterally : this is to some extent prevented by placing "balance masses" on the wheels. But such masses, although they may be adjusted nearly to balance the horizontal forces, introduce unbalanced vertical forces, causing what is called hammerblow. In extreme cases these vertical forces might even lift the wheels from the rails : short of that, however, they are objectionable in causing periodic variations in the pressure on the rails, which tend to set bridges into oscillation and may add substantially to the stresses for which the railway engineer must provide.