Steam Locomotive Construction and Maintenance/Chapter IX

by E. L. Ahrons

CHAPTER IX

SETTING THE VALVES

Object of Valve-Setting. The movement of the slide valve with the definitions of “lap,” “ lead,” and “travel” of the valve, together with an explanation of the positions of the piston at which the steam is cut off, the exhaust opens, and compression of the steam begins, are explained in The Steam Railway Locomotive and in Steam Engine Valves and Valve Gears.^[1] The distribution of the steam, and the location of the essential points of cut-off, release, etc., are settled in the drawing office when the engine is designed, but in the erection of the valve gear small inequalities and differences occur. The object of valve setting is to rectify these differences, more especially with a view to producing as nearly as possible an equal distribution of the steam at the front and the back ends of the cylinders. The rectifications are made by slight alterations to the lengths of the valve spindles or eccentric rods.

It should, however, be pointed out that it is impossible to obtain exactly equal distribution of steam at the two ends of the cylinder, owing to the effect of the angular obliquity of the connecting rod. If the connecting rod moved bodily in a direction parallel to the stroke of the piston the point of cut-off of the steam would be the same at both ends of the cylinder, but owing to the sloping position which the rod necessarily occupies, the point of cut-off is later on the “out stroke,” when the piston is moving towards the crank axle, than on the “instroke,” when it is moving away from the axle. For example, the “valve card” of a certain locomotive shows that when in “full gear” the piston moved 18¾ ins. on the out stroke (the total stroke being 24 ins.) before steam was cut-off, but on the return or inward stroke the cut-off occurred when the piston had travelled only 16¼ ins.

Systems of Valve-Setting. It is possible to set the valves on three different systems, each being dependent upon one of the three variables which affect the steam distribution. These are—
1. Equalizing the lead at the front and back.
2. Equalizing the port opening at front and back.
3. Equalizing the point of cut-off at the front and back.
The third of these, which involves unequal laps, is rarely adopted in locomotives and need not be further considered. Of the two others the system of equalizing the leads is more usual, as this gives a more nearly equal compression of the steam at the two ends of the stroke.

Bumping Marks and Clearance. If the piston were allowed to travel far enough at each end of the cylinder, it would “bump” the cylinder covers. The engine is so designed that this does not take place, and a space of about ¼ to ⅜ in., known as the “clearance,” is allowed between the piston and the cylinder cover at each end of the stroke. Since the movement of the piston and of the crosshead connected to the piston rod are exactly the same, all measurements of piston stroke may be taken from the crosshead relatively to the slide bars. The “bumping marks” are scribed on the slide bars at each end before the connecting rod is put up, by pushing the piston by hand to each end as far as it will go until it strikes the cylinder covers, and marking the positions of one end of the crosshead slide block on the bars. When the connecting rods and motion have been put up, and the wheels revolved as described below, the terminal positions of the slide blocks should be distant from the bumping marks at each end by the amount of the clearance.

To set the valves it is found more convenient to turn the wheels by means of the following simple apparatus than to move the engine backwards and forwards on a length of line outside the shop. Two brackets are fixed beside the rails, one on each side of the pit over which the engine is built. These brackets have bearings in which rollers are fitted, and the engine is lifted slightly so that the circumferences of the driving wheels rest upon the rollers. The rollers are revolved by means of a shaft which lies across the pit, the shaft being provided with a ratchet or four long lever arms, the latter being pulled round by a man stationed in the pit during the valve setting process. The rollers cause the driving wheels to revolve, only a small portion of a revolution being necessary at one movement during valve-setting. The wheels must be turned in the proper directions when setting the valves for forward or back gear.

Setting the Valves. Before the eccentric rods are coupled up to the expansion links, the points at which the valve closes the ports to steam are marked upon the valve spindles. A very thin piece of tin A₁ (Fig. 43–I) is held in the front port, and the valve with valve rod is moved carefully against it. The valve spindle S is then marked at a₁ outside the steam chest by means of the trammel T, the straight pointed end of which is placed in a centre “pop” made in the back of the steam chest cover, whilst the right-angled end scribes the mark a₁ on the valve spindle. A small “pop” is made with a sharp punch to show the position of this mark. Similarly the point a₂ at which the valve closes the port at A₂ at the other end (Fig. 43–II), is marked, by transferring the tin to the latter port and pushing the valve in the opposite direction against it. The two “pops” a₁ and a₂ on the spindle show the positions of the latter when the steam is cut off, since these are the points at which the valve closes the ports.

The eccentric rods are then coupled up, and the “dead centres,” i.e., the exact points at

Fig. 43.—Marking the Valve Spindle for Valve Setting.

which the piston reaches the two ends of its stroke, have to be found. It may be noted here that there are four dead centres, two for each side of the engine. As the valve-setting for one side is the same as for the other, it will be sufficient to describe one side only. The driving wheel (Fig. 44) is turned in the direction of the arrow for forward motion until the slide block D is nearly, but not quite, at the front end of its stroke as shown in the illustration. The crank will then occupy the position shown at C₁. At the extreme end of the stroke the edge of the slide-block would reach the dotted line A. A centre punch mark P is made on the side of the slide block, and another similar mark S on the side of the slide bar. A pair of compasses is then set to the distance PS. A centre punch mark is made on the engine frame at Q near the driving wheel, and a mark m₂ is made by a pair of compasses or a trammel on the tyre. The wheel is then slowly revolved so that the slide block D reaches the end of its stroke A, and then begins to move in the opposite direction towards the crank. A man in the pit keeps the compasses set to PS with one point in S and as soon as the other point enters the pop mark P the rotation is stopped. With the trammel having one point in Q, a second mark m₂ is made on the tyre, the first mark having moved on to the position m₁, as the crank has moved round from C₁ to C₂. The distance m₁m₂ on the tyre is divided equally, and a “pop” is made in the mark M, the centre

Fig. 44.—Finding the Dead Centres.

line between m₁ and m₂. The wheel is then moved back to the position C₁ of the crank, and again moved forward until the trammel with one leg in Q exactly reaches the pop M with its other pointed end. The point M will now occupy the position which m₂ has in the illustration, and the position of the crank will now be at F, the edge of the slide block being at the end of the stroke A. This is the front dead centre. The back dead centre B is found similarly by turning the wheel through half a revolution, and two further dead centres are also marked for the crank on the opposite side of the engine.

It may be added that the above somewhat elaborate method is necessary, because it is practically impossible to find the dead centre sufficiently accurately by merely watching the crosshead and marking the point when the latter begins to reverse its direction of motion. The motion of the crosshead near dead centre is very slow, and the crank will move through a small angle without apparently moving the crosshead.

The engine having been set on front dead centre with the crank at F (Fig. 44), by setting the wheel trammel in the pops Q and M, the front port should be open a slight amount A₁ B₁ (Fig. 43–III). This amount is the lead at the front end, and is marked with the valve trammel on the valve spindle at b₁. The distance a₁ b₁ is therefore equal to A₁ B₁. These marks on the spindle are also shown in Fig. 43–IV, and a similar mark b₂ is also made for the lead at the opposite end, when the crank is on back dead centre in the position B (Fig. 44). The lead at the back end will therefore be equal to the distance between the marks a₂b₂ on the spindle (Fig. 43–IV). In some works the leads are measured, not by marking the valve spindle, but by pressing thin wedges into the space A₁ B₁ between the edge of the valve and that of the port, so that these edges leave marks on the wedges which can be measured.

If the front lead a₁b₁ is equal to that a₂b₂ at the back the valves are properly set “to equal leads,” and no further adjustment is necessary. If, however, the lead a₁b₁ in fore gear at the front is say ¹⁄₁₆ in, whilst a₂b₂ at the back is say ³⁄₁₆ in., the valve requires adjusting. To do this the fore gear eccentric rod is taken down and “jumped” or shortened in the smiths’ shop by ¹⁄₁₆ in. This increases the opening A₁ B₁ or the distance a₁b₁ at the front end from ¹⁄₁₆ to ⅛ in., and decreases a₂b₂ from ³⁄₁₆ in. to ⅛ in. so that the leads are made equal. Similarly if the lead at the back end be smaller than that at the front, the eccentric rod has to be lengthened slightly. Some engines are arranged with valve spindles having a screw and nut, which can be adjusted to lengthen or shorten each spindle instead of altering the length of the eccentric rod.

The maximum port openings, when the valve is at the ends of its stroke, are also noted and marked on the valve spindle. The valve spindle is blackened by soot from a lamp, so that the marks made by the trammel may be seen easily. As the ends of the valve travel are approached, when the wheels are rotated, the trammel T is placed as shown in the pop in the steam chest cover (Fig. 43–I) and the vertical point is passed and repassed across the valve spindle as the latter moves until the latter reverses its direction of movement. The limiting position gives the line c₁ (Fig. 43–IV) marked on the spindle, and the distance a₁c₁ is the maximum port opening at the front end. Similarly a₂c₂ is the port opening at the back end. When valves are set to equal port openings instead of equal leads, the adjustments are such that a₁c₁ is made equal to a₂c₂. In this case there is a slight difference in the leads at the front and back ends.

It may be noted that as the reversing lever is “notched up” when running, in order to expand the steam in the cylinders, the lead increases with a Stephenson link motion having “open” eccentric rods.^[2] For instance an engine, which has the valves set to equal leads of ⅛ in. when in full fore gear, may have a lead of ¼ in. when fully notched up, and moreover the link motion may be so suspended that the leads at both ends are not equal when the reversing lever is in this latter position. It is therefore usual for the valves to be set to equal leads, when the reversing gear is placed in the notch or position in which the engine runs most of the time, instead of setting them with the lever in full fore-gear.

The valves are also tried for “back-gear,” in which the engine runs tender first. In the case, however, of tender engines the running done tender first is comparatively small in amount and the setting of the valves in back-gear is sometimes left, unless the inequalities between the front and back of the cylinders is very pronounced, in which case some adjustment is made to the back eccentric rod which may involve a slight alteration of the steam distribution in fore-gear.

Finally, when the valves have been set, the positions of the crosshead, which are equivalent to those of the piston, are measured on the slide bars corresponding to the four points: (1) steam cut-off; (2) exhaust opens; (3) exhaust closes; and (4) compression begins. Owing to the obliquity of the connecting rod these positions are different for the outward and inward strokes of the piston. A record of these measurements is kept for every engine.

1. Both in the same series as this volume.
2. For further particulars of Stephenson’s link motion with open and crossed rods, and the effect of the latter on the lead, see ''Steam Engine Valves and Valve Gears'' in this series of primers.