The Link Motion and Reversing Gear

VOL. II. LOCOMOTIVE LINK MOTION. PLATE XXXI.

The link motion of an American locomotive is shown in Figs. 3329 and 3330. In Fig. 3329 it is shown in full gear for the forward gear, or in other words, so as to place the engine in full power for going ahead.

The meaning of the term full power is that, with the link motion in full gear, the steam follows the piston throughout very nearly the full stroke.

In Fig. 3331 the link motion is shown in mid gear, in which position the engine is at its least power, the cut off occurring at its earliest point, and in Fig. 3332 it is shown in full gear for the backward motion.

Referring to Fig. 3329 for the full gear forward, the reversing gear proper consists of the reversing lever, the segment, the reach rod, the tumbling shaft, and its counterbalance rod and spring; while the link motion proper consists of the eccentrics and their rods, the link, the link block or die, the suspension link s, the rock shaft and the rod p p. These, however, are terms applied for shop purposes, so as to distribute the work in sections to different men, it being obvious that a complete link motion includes the reversing gear, the eccentrics, the link and its block, the rock shaft, the rod p p, and the valve and its spindle or stem. This mechanism, as a whole, may also be called, and is sometimes called, the valve gear, because it is the mechanism or gear that operates the slide valve.

The link motion may be moved from full gear forward to full gear backward or to any intermediate position, whether the engine is running or at rest, but is, when the engine is running, harder to move from full gear forward toward back gear, and easier to move from full gear backward toward mid and forward gears, which occurs because of the friction of the eccentrics in the straps, and it follows that this will be the case to a greater extent in proportion as the revolutions of the eccentrics are increased.

If in a properly constructed link motion we move the link from full gear forward to mid gear when the engine is standing still, and watch the valve, we shall find that the lead or opening at f gradually increases; and if we then move it from mid gear to full gear backward, the lead will gradually decrease and finally become the same as it was in full gear forward. The construction of the parts is as follows:

Referring to Fig. 3329 (full gear forward), the segment is fixed in position and the reversing lever is pivoted at its lower end. r r is a bell crank, which is pivoted to the reversing lever and to which the latch rod is pivoted at its upper end. The spring acts on the end of r r, and thus forces the tongue of the latch into the notches on the sector as soon as the tongue comes fair with the notch and r r is released from the hand pressure. As the reversing lever is moved over from full gear forward, the reach rod moves the tumbling shaft, whose lower arm i (through the medium of the suspension link s) lifts the link and brings the centre of the saddle pin nearer to the centre of the pin in the link block, which reduces the amount of motion given to the lower arm (b, Fig. 3331) of the rock shaft, and therefore reduces the amount of valve travel, thus causing the point of cut-off to occur earlier in the piston stroke.

The weight of the eccentric rods, the link, suspension link s, and the tumbling shaft arm i, is counterbalanced by the counterbalance spring in the box s s, whose rod attaches to the lug g on the tumbling shaft. To regulate the proper amount of counterbalancing, the nuts at m are provided, these nuts regulating the amount of tension on the spring s s.

The forward eccentric e is that which operates the valve when the link motion is in full gear forward, as in Fig. 3329, and the backward eccentric is that which moves the valve when the link motion is in back gear, as in Fig. 3332.

This occurs because it is the eccentric rod that is in line or nearest in line with the link block that has the most effect in moving the valve. When the link is in full gear, the motion of the valve is almost the same as though there was no link motion and the eccentric rod was attached direct to the rod p p, the difference being so slight as to have no practical importance. This will be seen by supposing that we were to loosen the backward eccentric f upon the shaft and revolve it around the shaft by hand, in which case it would swing the lower end of the link backward and forward with the centre of the link block as a pivot or centre of motion, the forward eccentric rod rising and falling a trifle only, and therefore moving the rock shaft to a very slight amount.

Let it now be noted that the suspension link not only sustains the weight of the link and eccentric rods, but also compels the centre of the saddle pin to swing (as the link is moved by the eccentrics) in an arc of a circle of which the centre is the upper end of the suspension link. Suppose, therefore, that the backward eccentric rod was to break, or was taken off and the engine could still run forward, but no motion would be given to the valve, if the link was placed in mid gear, because in that case the forward eccentric rod would simply swing the link on the centre of the link block as a pivot. Now, suppose the forward eccentric rod was to break or be taken off, and the engine may be made to go ahead by setting the backward eccentric fair with the forward eccentric and connecting its rod to the upper end of the link.

Similarly, if the engine was running with the smoke stack toward the train and the link motion in backward gear, and the backward eccentric rod was to break, we may take it off, shift the forward eccentric so that it comes fair or stands in line with the backward eccentric and connect its rod to the lower end of the eccentric and with the link motion in backward gear, the engine would still haul the train.

If the reach rod was to break, the tumbling shaft could be held in position by loosening the cap bolts of the tumbling shaft journal and putting between the cap and the tumbling shaft journal a piece of metal, which, on bolting up the cap screws again, would firmly grip the shaft and prevent it from moving.

Setting the Slide Valves of a Locomotive.—The principles of designing, and the action of D valves, such as are used upon locomotives, have been so thoroughly explained with reference to stationary engines, that there is no need to repeat them in connection with the locomotive, and we may proceed to explain how to set the valves of a locomotive. In doing this, there are two distinct operations, one of which is to place the crank alternately exactly on its respective dead centres, and the other is to set the position of the eccentrics, and get the eccentric rod of the proper length. These two operations comprise all that require to be done to set the valves, under ordinary and workmanlike conditions; hence we may proceed at once to explain the operation.

The first thing to be done is to put the crank pin on a dead centre, and it does not matter which one.

In Fig. 3333 it is supposed that the piston is to be at the head end of the cylinder when the crank is on its corresponding dead centre.

The first thing to do is to put the reversing gear in full gear forward, so as to set the forward eccentric, and see if its rod is of proper length.

The next thing to do is to move the wheel so that the crank pin is nearly on the dead centre, and then take a tram (such as shown in the figure), pointed at each end, and mark on the splash plate, or any other convenient place, a centre punch dot in which the point b of the tram can rest. Next, from the centre of the axle as a centre, mark arcs or portions of circles a, a. This being done, point b of the tram is rested in the centre punch dot before referred to, and with the other end a line c is marked, a straight edge is then rested against the ends e e of the cross head, and a line d is marked on the guide bar, this line being exactly even or fair with the end e e of the cross head.

We then move the wheel in the direction of the arrow, and as soon as we begin to do so, the cross head will move to the left and away from the line d on the guide bar. But as soon as the crank pin has passed its dead centre, the cross head will begin to move to the right, and as soon as the end e e comes again exactly in line with the line d marked on the guide bar, we must stop moving the wheel, and again resting the point b of the tram in the centre punch mark before mentioned, we move its other end so as to mark a second line, which will be the line or arc f.

The next thing to do is to mark a fine centre punch dot, where c and f cross the arc or line a, and then find the point g midway between f and c, and mark a fine centre punch mark there. This being done, we must move the wheel back into the position it occupies in the figure, and then slowly move it in the direction of the arrow, until with the end b of the tram resting in the centre punch dot, the other end of the tram will fall dead into the centre punch dot at g, at which time the crank pin will be exactly on the dead centre.

During this part of the process we have nothing to do with anything except getting the crank pin on the dead centre, but there is one point that requires further explanation, as follows:

In this operation we have first put the crank on one side of the dead centre and then put it to the same amount on the other side of the dead centre, both being improper positions; but by finding the mean or mid position between the two, we have found the proper position. In doing so, however, we have moved the wheel, the wheel has moved the connecting rod, and the connecting rod has moved the piston. But in the actual running of the engine, this order of things will be reversed; for the steam will move the piston, the piston will move the connecting rod, and the connecting rod will move the crank and therefore the wheel.

The difference between the two operations is this: Suppose there is lost motion or play between the connecting rod brasses and the crank pin, or between the connecting rod brasses and the cross head pin, and then if we move the wheel in the direction denoted by the arrow, we take up this lost motion, so that if the tram was fair with the centre punch at g and steam was admitted to the piston, then there would be no lost motion to take up, and as soon as the piston moved the crank pin would move. But if we moved the wheel in the opposite direction to that denoted by the arrow, then we are placing any lost motion there may be in the opposite direction, and if steam were turned on, the piston and connecting rod might move before the crank and wheel moved.

In which direction the wheel should be moved while placing the crank on the dead centre depends upon the condition of the engine, as will be explained presently, the assumption being at present that the engine is in thorough good order, in which case the wheel should (while placing the crank on the dead centre) be moved in the direction of the arrow in the figure.

The object is under all conditions to bring the working surfaces to bear (while setting the valve) in the same way as they will bear when the engine is actually at work.

Having placed the crank on the dead centre, and thus completed the first operation in valve setting, we may turn our attention to the second, viz., correcting the lengths of the eccentric rods and setting the valve lead. Almost all writers who have dealt with this part of the subject have fallen into a very serious error, inasmuch as they began the operation by what they call squaring the valve. This means so adjusting the length of the eccentric rod that the valve will travel an equal distance each way from its mid position, so that if the engine wheel is revolved and the extreme positions of the valve marked by a line, these lines will measure equally from the edges of the steam ports, or, what is the same thing, from the centre of the cylinder exhaust port. This procedure is entirely erroneous, because, on account of the angularity of the eccentric rod, the valve cannot, if equal lead is to be given to the valve, travel equally beyond the two steam ports, and if the eccentric rods are so adjusted for length as to square the valve, they are made wrong.

The valve lead, and the lead only, it is that determines the length of the eccentric rods. Suppose that, as is generally the case, the lead is to be equal, or, in other words, that there is to be as much valve lead when the piston is at one end of the cylinder as there is when it is at the other, and if we make the eccentric rods of such a length that the valve travels equally on each side of the steam port, there will be less lead at the head end port than there is at the crank end port. The proper method, therefore, is (as soon as the crank is on the dead centre and the link in full gear, as in Fig. 3334) to set the eccentric so as to give the desired amount of lead, and then give the wheel a half revolution, the lower end of the tram falling into the centre punch dot at s, when the crank pin will be on its other dead centre and ready for the lead to be measured again. If the lead is equal at each end, one eccentric rod is of the right length, and all we have to do is to set the eccentric so that the right amount of lead is given.

We now turn our attention to the backward eccentric and its rod, putting the reversing lever in full gear for the backward motion, and putting the crank on the respective dead centres, and testing the lead for both ports as before, and when the required amount of valve lead is given the valve setting is complete.

In some practice the wheel is blocked up on the pedestal guides while setting the valves, but a more correct method is to let the engine rest on the rails and push it back and forth with a crowbar to revolve the wheels when putting the crank pin on the dead centre. The best thing to measure the lead with is a wooden or leaden wedge having but a slight degree of taper, as say ⁄16 or ⁄4 inch in a length of four inches. We have in this example of valve setting supposed the parts to be of the proper dimensions, as they would be in a new engine or in an engine that had been running and merely had a new valve or a new eccentric put in.

But suppose the notches were not cut in the sector, and we have then to mark them off while setting the valves. All the difference that this makes to the operation is that we must clamp the reversing lever to the sector while setting the valve, taking care to so clamp it that there is the same space between the top end of the link block and the end of the link slot in the full forward gear as there is between the bottom end of the link block and the end of the link slot when the engine is in full backward gear. In this connection it is, however, to be remarked that when the link is in full gear, either forward or backward, and the crank is on the dead centre, the link block is not at the end of its motion toward the end of the link slot; hence it is a good plan to move the wheels around and to so regulate the length of the reach rod and the position of the reversing lever on the sector, that when the link block is at the highest point in the link slot for the forward gear and at the lowest point in the link slot for the backward gear, it comes an equal distance from the end of the link slot.

The setting for an Allen valve is the same as that for an ordinary one, but in determining the amount of the lead it is to be borne in mind that it is virtually twice as much as it measures at the port because there are two openings for the steam. This will be seen from Fig. 3335, in which the valve is open to the amount of the lead at f. But the steam also enters at e, and passes through the port in the valve and into steam port a.

We have now to call attention to the fact, that the eccentric rods, when properly connected, are, in an American locomotive, crossed when the piston is at the crank end of the cylinder. In Fig. 3334, the piston is at the head end of the cylinder, and the rods are open. In Fig. 3336, however, the crank pin is supposed to be at b, and the eccentric rods are crossed, f being the forward and e the backward eccentric.