Modern Machine-Shop Practice Part 253

The point at which the cut off will occur, therefore, is determined by the position of the cam _n'_, because if _n'_ is out of the way, the end _g_ of the latch link will not meet it, the latch link will not disengage from the latch block _e_, and the cut off would be effected by the lap of the valve, and independently of the dash pot. As in Fig. 3379 the parts are shown in the positions they occupy at the instant the cut off is to occur, therefore the cam _n'_ has just tripped the latch link, and the end of _e_ has just left contact with the end _y_ of the recess _w_ in the latch link _u'_.

The point in the stroke at which the tripping of _u'_ from _e_ will occur and effect the cut off is determined by the governor, because _d'_ is connected to the governor through the rod G'. In proportion as the governor b.a.l.l.s rise, _d'_ is moved from left to right, and the end of cam _n'_ meets _g_ earlier, or, vice versa, in proportion as the governor b.a.l.l.s fall, the arm _d'_ is moved to the left, _g_ will meet the end of cam _n'_ later, and the point of cut off will be prolonged.

[Ill.u.s.tration: _VOL. II._ =THE CORLISS VALVE GEAR.= _PLATE x.x.xIV._

Fig. 3381.]

We now come to the means employed to close the valve quickly and without shock when the latch block is released from the latch link. Referring then to the crank end of the cylinder, the latch block for that valve is carried upon arm _x_, to which is attached the rod R from the dash pot piston (the arm corresponding to _x_, but at the head end being shown removed to expose the latch block to view). We may now turn again to the head end of the cylinder, rod R' corresponding to rod R at the other end, and it is seen that R' connects to a dash pot piston _p'_ having a stepped diameter, the lower half fitting into bore H', and the upper half fitting into a bore H. The piston _p'_ fits the bore H' and fills it when the rod R' is at the bottom of the stroke, hence as _p'_ is raised there is a vacuum in H that acts to cause _p'_, and therefore R'

and _x_, to fall quickly and close the valve the instant the latch block is released from the latch link. To prevent the descent of rod R' and piston _p'_ from ending in a blow, a cushion of air is given in H by the following construction:

[Ill.u.s.tration: Fig. 3380.]

At S and S' are valves, threaded to screw and unscrew, the ends forming a valve for a seat entering H.

As the rod R' and its piston _p'_ descend, the air in H finds exit through a hole at _h_ until that hole is closed by the piston _p'_ covering it, after which the remaining air in H can only find exit through the opening left by the end of the valve S', and this amount of opening is so regulated by the adjustment of S' that a certain amount of air cushion is given, which prevents _p'_ from coming to rest with a blow. The head of valve S' is milled or knurled, and a spring _t'_ fits, at its end, into the milled indentation, thus holding it in its adjusted position. The under surface of the upper part of _p'_ is covered by a leather disc, while the part that fits in H' is kept air-tight by a leather-cupped packing.

The connection of the cam arms _d_ and _d'_ with the governor is shown in Figs. 3381 and 3382, in which the parts are shown in the position they would occupy when the crank is on the dead centre and the piston at the crank end of the cylinder. The rod G' connects the cam arm _d'_ with the upper end of lever A, which is connected to the governor and vibrates on its centre as the governor acts upon it.

[Ill.u.s.tration: Fig. 3382.]

Now suppose the speed to begin to diminish, and the governor b.a.l.l.s to fall, and the direction in which A will move will be for its lower end to move to the right, thus moving _d_ to the right and carrying its cam away from the end of the latch link, which will therefore continue to open the port for a longer period of admission. Or, referring to Fig.

3381, it is plain that, if the governor b.a.l.l.s were to lower from a reduced governor speed, G' would move to the left and cam _n'_ would be moved away from contact with the end _g_ of the catch link, which, not being tripped, the admission would continue. On the other hand, suppose the governor b.a.l.l.s to rise from an increase of governor speed, and _d'_ (Fig. 3379) would be moved to the right, and the cam _n'_ meeting _g_ earlier, correspondingly hastening the cut off.

The governor is driven by a belt from a pulley on the crank shaft to the pulley W, Fig. 3381, whose shaft conveys motion to the governor spindle through the medium of a pair of bevel pinions in which _v_ represents (referring again to Fig. 3378) the steam or admission valve for the crank end port, and _v_^{1} that for the head end port, while _v_^{2} is the exhaust valve for the crank end, and _v_^{2} that for the head end of the cylinder. All four valves are shown in the positions they would occupy when the crank was on the dead centre and the piston at the crank end of the cylinder, hence the valve positions shown correspond to the positions the parts of the valve motion occupy in Fig. 3381.

The faces of the valves are obviously arcs of circles of which the axes of the shafts _s_, _s'_ are the respective centres. Valve _v_ has opened its port to the amount of the lead, which in this cla.s.s of engine varies usually from 1/32 to about 1/16 inch. As separate exhaust valves are employed, the point of release, and (as the same valve edge that effects the release also effects the compression) therefore that of the compression, may be regulated at will by adjusting the lengths of the rods F, F', Fig. 3379, which have at one end a right and at the other a left hand screw, so that by turning back the check nuts and then revolving the rods their lengths will be altered.

Similarly the amount of admission lead may be adjusted by an adjustment of the lengths of rods C, C', which also have right and left hand screws. Referring now to the admission valve _v_, it is seen that its operating rod C is at a right angle to bell crank _r_, _r_, hence the amount of valve motion will not be diminished to any appreciable extent by reason of the wrist plate end of rod C moving in an arc of a circle, and the point of attachment of rod C to the wrist plate is such that, during the admission, the valve practically gives as quick an opening as though rod C continued at a right angle to _r_. But, if we turn to valve _v'_, which has closed its port and covers it to the amount of the lap, we find that bell crank _r'_ and its operating rod C' are in such positions with relation to the wrist plate, that the motion of the latter will have but little effect in moving the bell crank _r'_. This is an especial feature of the Corliss valve motion and is of importance for the following reasons:

The lap of the valve (which corresponds to the lap of a plain D slide valve) is usually, in this cla.s.s of engine, such as to cut off the steam at about 7/8 stroke, but the adjustment of the cam position is usually so made that, from the action of the governor, the latest point of cut off will occur when the piston has made 5/8 of its stroke, the range of cut off being from this to an admission equal to the amount of the lead.

As the eccentric is fixed upon the shaft, the speed at which the valve opens the port for the admission is the same for all corresponding piston positions. Thus suppose the piston has moved an inch from the end of the stroke, and the valve speed will be the same, whether the cut off in that stroke is to occur at quarter stroke or half stroke, and as the valve continues to open the port until it is tripped, therefore, at the moment it is tripped, the direction of valve motion must be suddenly reversed.

As the duty of its reversal falls upon the dash pot, it is desirable to make this duty as light as possible, which is accomplished by the wrist motion, which acts to reduce the valve motion after the port is opened a certain amount for the admission.

We have, therefore, that during the earlier part of the admission, the port opening is quick because of the eccentric throw being a maximum, while during the later part of the port opening, this rapid motion is offset or modified by the wrist motion, thus lessening the duty of the dash pot and enabling it to promptly close the valve.

The range of governor action, so far as the governor itself is concerned, is obviously a constant amount, because a certain amount of rise and fall of the governor b.a.l.l.s will move the cams a given amount.

But the range of cut off may be varied as follows: At Z, Z', are adjustment nuts, by means of which the lengths of rods G, G' may be varied.

Lengthening rod G obviously moves arm _d_ and its cam _n_ further from the end of the latch link _u_, and therefore prolongs the admission period.

Shortening the rod G' causes cam _n'_ to move around and away from the leg _g_ of the latch link, and prolongs the admission.

The adjustment of the lengths of G and G' may therefore be employed for two purposes; first, to prolong the point of cut off, and maintain the speed when the engine is overloaded, or to hasten the point of cut off for a given engine speed, and thus adjust the engine for a lighter load.

HIGH SPEED AUTOMATIC CUT OFF ENGINES.

What are termed high speed engines are those whose pistons run at a velocity of more than about 600 feet per minute, some making as high as 800 or 900 feet in regular work. High speed engines are usually provided with an automatic cut off, and a majority of them vary the point of cut off, by means of shifting the eccentric across the shaft, so as to reduce the eccentric throw, and therefore the valve travel. This causes the valve to cut off the steam earlier.

The eccentric, instead of being fixed upon the crank shaft, has an elongated bore, and is hung on an arm that is pivoted at its other end after the manner of a pendulum. This arm is called the eccentric hanger.

A wheel governor is usually employed to shift the eccentric across the shaft. In some cases, however, two valves are employed, one effecting the admission, the release, and the compression, and the other the cut off.

When two valves are employed, the lead, the point of cut off, the point of release, and the point of compression may be maintained equal for all points of cut off; whereas, when a single valve is employed, the lead, the point of release, and the compression will vary with the point of cut off, or, in other words, will be different for every different point of cut off.

The general principles upon which a wheel governor is constructed is, that two weights or weighted levers in moving outwards from the engine shaft, from the action of centrifugal force, move or rather shift the eccentric across the shaft, reducing its throw, and therefore by reducing the travel of the valve hasten the point of cut off and reduce the power of the engine.

In the governor of the Buckeye engine, the centrifugal force may be varied by increasing or diminishing the distance of the weights from the pivots of the arms on which they swing.

[Ill.u.s.tration: Fig. 3382_a_.]

This is shown in Fig. 3382_a_, in which it is seen that the weights A are adjustable along the arms _a_, _a_. The points of attachment _d_, _d_ of the springs to the weight arms are also adjustable.

When reversing is done, by shifting the eccentric across the shaft, the lead cannot be kept equal, but will, if the eccentric is swung from a pivot that is on the line of centres, when the crank is on a dead centre, be greater at the head end than at the crank end of the cylinder. The discrepancy may, however, be equalized by swinging the eccentric from a pivot that is not on the line of centres at a time the crank is on a dead centre.

But this equalization will only exist at some one point in the eccentric position, or in other words, if the eccentric is shifted across the crank shaft, simply to reverse the engine, and not to vary the point of cut-off, it will naturally be moved, in reversing the engine across the shaft, to a given and constant amount, and in this case, the pivot on which its hanger is hung may be so located with reference to the line of centres and the crank (the latter being on a dead centre when the point of suspension of the eccentric hanger is found) that the lead is equal for both the backward and forward gears.

But if the eccentric is shifted across the shaft to vary the point of cut off as well as to reverse the direction of engine revolution, the lead cannot be kept equal.

It is better, in this case, to so locate the point of eccentric hanger suspension as to let the lead be the most at the head end cylinder port, because the piston travels fastest at that end of the cylinder, and therefore requires more lead, in order to cushion the piston.

[Ill.u.s.tration: Fig. 3383.]

A construction for shifting the eccentric across the shaft is shown in Fig. 3383, in which D, D is a disc, having at _b_ a pivot for the eccentric hanger. The amount the throw line of the eccentric must be shifted to reverse from full gear forwards to full gear backwards is from the line _b_ _x_ to line _b_ _x'_, and the shifting is done by two racks F and J, having teeth at an angle of 45 to their lengths. F is fast to the eccentric, and J is carried in a sleeve that slides along the shaft, and sliding it moves the eccentric across the shaft by reason of the teeth of one rack being at a right angle to those of the other.

It is obvious that the eccentric may be moved around the shaft in place of across it, the distance its throw line requires to be moved being the same in either case.

To shift an eccentric so as to reverse the direction of engine revolution, all that is necessary is to place the crank on either dead centre and measure the amount of valve lead. Then loosen the eccentric from the crank shaft, and while the crank is stationary, move it around upon the shaft until it has opened the port full, and nearly closed it again, leaving it open to the same amount as it was before the eccentric was moved, or in other words, open to the amount of the lead.

[Ill.u.s.tration: Fig. 3384.]

Fig. 3384 represents a side elevation of a high speed wheel governor engine, designed and constructed by the Straight Line Engine Company of Syracuse, New York, the construction of the governor being shown in Fig.

3385, in which R is the eccentric rod, the eccentric being carried in a lever strap pivoted at A, and connected at B to two links C and D, the former of which connects to the spring E, and the latter to the weighted lever F. The centrifugal force generated by the weighted end of F endeavors to move the eccentric inwards, and thus reduce its throw, which reduces the valve travel and hastens the point of cut off.

On the other hand, the tension of the spring E acts to move the eccentric in the opposite direction, and maintain the full throw of the eccentric and maximum point of cut off. These two forces are so calculated in the design and proportion of the parts that under a maximum load the engine will run at its proper speed, while, if the load decreases, the action of F will hasten the point of cut off enough to allow for the decreased engine load, and thus keep the engine still going at the same speed.

[Ill.u.s.tration: Fig. 3385.]

Other novel and interesting details in the construction of this engine are as follows:

The two arms forming the frame are cast with and run in straight lines from the cylinder to the two main bearings, and rest upon these self-adjusting points of support.

There are two fly wheels, both between the main bearings, and one of which carries the governor so that the centre of the valve is brought in line with the centre of the eccentric.

[Ill.u.s.tration: Fig. 3386.]