ensure no doubling up unless the trip finger is struck by the tripper. This play in the link corresponds to the clearance of ordinary trip gears. A leather pad on the link A deadens the noise when the gear attains its extreme position and the link falls on the carrier rail.

The method by which all plucking action on the governor due to the tripping lever striking the finger is avoided is very ingenious. The trip lever is hinged from the sliding collar, and is provided with stops on its upper end, which, whilst allowing the lever a little free play, prevent it swinging out of the way of the finger. The effect of M and N striking is not to slide the collar along the spindle and thus pluck the governor, but to cause the trip lever to bind on the under side of the sliding collar.

It will be convenient to conclude this chapter with a few remarks on dashpots.

Dashpots. The vacuum dashpot shown by Fig. 108, rests upon a cast-iron plate secured to the engine foundations. The dashpot

Fig 08.

D

Vacuum Dashpot.

rod connects to the pin A, and lifts the plunger B, thus forming a vacuum between it and the casing C. Tightness is obtained by the leather ring D secured in position by a circular nut. The small valve E is free to move in one direction only, and allows air to escape from the vacuum chamber into the atmosphere. F is the

cushion chamber, the object of which is to prevent the dashpot closing violently. The cushion part of the plunger is provided with a leather pad which falls on a pad fastened to the bottom of the cushion chamber. When the piston descends, the small valve G regulates the escape of air from the cushion chamber with the greatest nicety, and renders the action practically noiseless.

The diagram of the spring dashpot is self-explanatory, and it only remains to say that the valve A regulates the cushioning of the piston in the manner already described.

A vacuum dashpot is not suitable for a double eccentric gear, because the small movement of the valve lever from engaging position to admission point does not raise the plunger sufficient to produce a good vacuum. There must be some clearance in the vacuum chamber, and therefore the degree of rarity of the atmosphere therein is proportional to the lift. In double eccentric gears the valve movement for early cut-off is very small, and is insufficient to secure prompt action of the dashpots; but with single eccentric gears, the dwelling and lap angles must always be worked through before steam opens, so that even at the earliest cut-off the dashpot lever lifts a considerable amount. With a spring dashpot, an initial tension or compression can be given, which will close the valves promptly at all grades of expansion.

CHAPTER IV.

SINGLE ECCENTRIC GEARS WITH LARGE RANGE OF TRIP,

CONTENTS.-Frikart Single Eccentric Gear-Valve Diagram for Frikart Gear -Farcot Gear-Features of Cylinder-Wheelock Gear-Conclusion.

It remains to illustrate and describe several gears which form exceptions to the rule that tripping can only take place when the eccentric is moving in the opening direction.

Frikart Single Eccentric Gear.-The first gear to notice is that invented by Frikart, and made under license by Messrs. Greenwood & Batley, Leeds. The peculiarities of this gear permit of

both steam and exhaust valves being operated by one eccentric, whilst giving a range of trip up to 75 per cent., or even later. Another advantage is that in consequence of engagement being positive-that is, not relying upon springs to bring the catches into gear the motion is adapted to speeds quite unsuitable for ordinary trip gears.

In order to understand the action of the trip itself, it will be necessary to consider the arrangement of the rods and levers. In the diagrammatic sketch, Fig. 110, A is the crank shaft on which is keyed the eccentric, giving motion to the wrist plate through the swing lever G. The wrist plate communicates its motion to the

exhaust valves in the usual manner; but the steam valves derive their motion through the medium of several levers. The detail of

the trip is shown by Figs. 111 and 112. The two levers A and B oscillate upon the valve spindle, the wrist-plate lever A being loose, and the dashpot lever B keyed on. Swinging on the upper end of A is the tripping lever and catch C, which, at certain periods,

Fig. 111.

Fig. 112.

Frikart Single Eccentric Gear.

engages with the dashpot lever. C is so constructed, that when the centre of the pin D coincides with the centre line of the valve spindle, the dashpot lever is free to descend and cut off steam. Fig. 113 shows the gear in another position, where the centre of the trip lever pin is not coincident with the centre of the valve spindle; and where the dashpot lever is being raised because of the engagement of the trip lever with it.

Reverting now to Fig. 110, the motion of the trip levers will be understood. From a suitable point on the eccentric-rod, the lever N receives motion. N in turn operates the levers K by means of the threearmed lever T. The governor connects to T by means of swinging links, and by its position determines the inclination of the three-armed lever. The latter has a constant motion from the eccentric-rod, so

Fig 113.

that the governor does not regulate the amount, but simply the position of that motion. The levers C are thus seen to be under the influence of three movements. In the first place they are carried round by the wrist-plate levers; the second motion is derived from the levers K; whilst the governor, as before stated,

regulates the position of the lever T, and, consequently, the time at which the trip lever pin concides with the centre of the valve spindle.

The lever A (Figs. 111 and 112) is so formed that in case of the dashpot sticking, the lever B will be forced to descend by coming in contact with the projecting part of the wrist-plate lever. With this arrangement there is no fear of the steam valve remaining open throughout the stroke, even if the dashpots fail to act.

Valve Diagram for Frikart Gear.-The motion of levers K is equivalent to that which would be derived from an eccentric set at right angles to the eccentric A, Figs. 111 and 112, and remembering this, it is easy to construct a diagram showing the action of

Fig 114.

B

the trip levers very clearly. In Fig. 114, the position of the eccentric is given by the line A B. CD is the lap circle, and cut-off is therefore at A E. So far, the diagram is similar in every respect to the diagram for a slide valve, with the same lap and angle of advance. At right angles to A B, draw the line F G, equal to the travel of the trip levers due to the levers K; and on FG describe the circles as shown. When the governor is revolving in its normal plane, the coincidence of D (Figs. 111 and 112) with the valve spindle occurs when the crank is at A B, and cut-off is at this point. Suppose the governor to fall and move the centres of the trip-rods a certain amount. With this amount as radius, describe an arc cutting the polar circles at H. Produce AH,