Chapter XVI: Part XI: The Valve-Gear (3)

QUESTION 237. _What are the proportions of the valves and eccentrics used in the ordinary practice in this country?_

_Answer._ The following report made by a committee of the Master Mechanics’ Association will show the proportions used on thirty-five different railroads, and is a fair indication of the common practice.

===================================================+=======+======+
TABLE | | |
SHOWING THE AMOUNT OF LAP, LEAD AND TRAVEL OF THE | | |
VALVES OF LOCOMOTIVES USED ON 35 OF THE RAILROADS |Outside|Inside|
IN THE UNITED STATES AND CANADA. | Lap | Lap |
---------------------------------------------------+-------+------+
| in. | in. |
For locomotives running express pass. trains 25 use| ⁷⁄₈ | ¹⁄₈ |
„ „ „ „ „ „ 6 „ | ³⁄₄ | ¹⁄₁₆ |
„ „ „ „ „ „ 4 „ | 1¹⁄₄ | ¹⁄₄ |
„ „ „ accom. „ „ 20 „ | ³⁄₄ | ³⁄₈ |
„ „ „ „ „ „ 10 „ | ⁷⁄₈ | ¹⁄₁₆ |
„ „ „ „ „ „ 5 „ | ⁵⁄₈ | ³⁄₁₆ |
„ „ „ heavy freight „ 19 „ | ³⁄₄ | ¹⁄₁₆ |
„ „ „ „ „ „ 11 „ | ⁵⁄₈ | ¹⁄₈ |
„ „ „ „ „ „ 5 „ | ¹⁄₂ | ³⁄₁₆ |
===================================================+=======+======+

===================================================+======+====
TABLE | |Lead
SHOWING THE AMOUNT OF LAP, LEAD AND TRAVEL OF THE |Travel| in
VALVES OF LOCOMOTIVES USED ON 35 OF THE RAILROADS | of |full
IN THE UNITED STATES AND CANADA. | Valve|Gear
---------------------------------------------------+------+----
| in. | in.
For locomotives running express pass. trains 25 use| 5 |¹⁄₁₀
„ „ „ „ „ „ 6 „ | 4³⁄₄ |¹⁄₈
„ „ „ „ „ „ 4 „ | 5 |¹⁄₈
„ „ „ accom. „ „ 20 „ | 5 |¹⁄₁₀
„ „ „ „ „ „ 10 „ | 5¹⁄₂ |¹⁄₁₆
„ „ „ „ „ „ 5 „ | 4¹⁄₂ |¹⁄₈
„ „ „ heavy freight „ 19 „ | 5 |¹⁄₁₀
„ „ „ „ „ „ 11 „ | 4¹⁄₂ |¹⁄₁₆
„ „ „ „ „ „ 5 „ | 4³⁄₄ |¹⁄₁₀
===================================================+======+====

QUESTION 238. _What should be the width of the bridge between the steam and exhaust ports?_

_Answer._ It is usually made about the same thickness as the sides of the cylinder, in order to secure a good casting; but sometimes it is necessary to make it wider, in order to prevent steam from escaping from the steam-chest into the exhaust, which is apt to be the case if a valve has little lap and a long travel.

QUESTION 239. _What determines the width of the exhaust-port?_

_Answer._ The throw of the valve. This will be clear if we refer to fig. 146, which represents a valve with a travel of 5¹⁄₂ in. It will be seen that when it is in the extreme position in which it is shown the width _A_ of the opening of the exhaust-port is very small. If this opening is contracted too much it will of course interfere with the free escape of the exhaust steam. It is therefore best to make the exhaust-port so wide that with the greatest travel of the valve the width of its opening will be either quite or very nearly equal to the width of the steam-port.

_Fig. 146._

Scale ³⁄₁₆ in. = 1 foot.]

Fig. 147.

Scale ³⁄₄ in. = 1 foot.]

QUESTION 240. _Where is the reverse lever located and how is it constructed?_

_Answer._ It is located on the _foot-board_[65] _K′ K′_, as shown in plate II. It consists of a lever _O_, _O_, with the fulcrum at the lower end. The _reverse-rod e k_, which connects the lever with the vertical arm _k_ of the lifting-shaft, is attached above the fulcrum of the reverse lever. Figs. 147 and 148 represent side and end views of the lever on an enlarged scale and with some of the details attached which are omitted on plate II. _C_, _C_ are two curved bars, which in this country are usually called _quadrants_, but in England are called (and more properly) _sectors_. These are placed on each side of the reverse-lever and are fastened to some portion of the engine. They have notches, _n_, _n_, _n_, cut in them to receive the _latch L_, which slides in a clamp, _H_, and holds the reverse-lever in the notches in which it is placed. This latch is operated by a _trigger_, _D_, which is grasped by the locomotive runner when he takes hold of the handle _A_ of the reverse-lever. The trigger works on a pin, _E_, as a fulcrum and is attached to the latch by a rod, _r r_. When the trigger is pressed up against the handle, the latch is raised out of the notches by the rod _r r_, and is pressed into them again by the spring _s_ when the trigger is released. _F_ is a set-screw which presses against a gib, _G_, and is intended to keep the latch tight and prevent the reverse-lever from shaking.

[65] The _foot-board_ _K′ K′_, plates 2 and 3, is a platform for the
locomotive runner and fireman to stand on and is located at the back
end of the engine.

QUESTION 241. _How are the notches in the sector arranged?_

_Answer._ They are usually arranged so that the steam will be cut off at some full number of inches of the stroke when the reverse-lever is in each one of the notches. They are therefore located so that the steam will be cut off at 6, 9, 12, 15, 18 and 21 inches, or at 6, 8, 10, 12, 15, 18 and 21 inches of the stroke. A notch is also placed so as to hold the link in mid-gear. In some cases as many notches as there is room for are put into the sectors. The latter seems to be much the best plan, as it gives more gradations in which the valve-gear can be worked, and it is a matter of no consequence whatever in the working of an engine whether the steam is cut off at some full or some fractional number of inches of the stroke. By referring to fig. 144 it will be seen how very great the difference of the distribution of steam must be, as indicated by the 5th, 6th and 7th motion-curves.

QUESTION 242. _How long should the reverse-lever be?_

_Answer._ The lever should be sufficiently long so that in throwing the link from full gear forward to full gear backward the handle _A_ will move _not less_ than four times the distance that the link is moved. It is much better to give the end of the handle _A_ five or even six times the motion of the link, as there will then be a much easier action in reversing the engine. This will also make it possible to use longer sectors, and give room for more notches.

QUESTION 243. _What provision is made in the reversing gear for overcoming or neutralizing the weight of the link and other parts of the valve-gear?_

_Answer._ Their weight is counterbalanced by the pressure of a spring of some kind. In fig. 103 the two volute springs enclosed in a case, _H_, are used for this purpose. These are compressed by the rod _m_, which is attached to a short arm _l_, on the reverse shaft _A_, when the link is lowered, and consequently the tension of the spring resists the weight of the link when the latter is down or in forward gear. Different kinds of springs are used for this purpose and sometimes are attached to the reverse-lever instead of to the lifting-shaft.

QUESTION 244. _What is meant by “setting” a slide-valve?_

_Answer._ It is to fasten the eccentrics in the right position on the axle and to adjust the length of the eccentric-rods and valve-stem so that the valves will give the required distribution of steam.

QUESTION 245. _How are the valves of a locomotive set?_

_Answer._ After the wheels, axles, main connecting-rods and valve-gear are connected together, put the rocker-arm in its middle position, and lengthen or shorten the valve-stem so that the valve will be in the centre of the valve-face. Then place the crank on the forward centre and the full part of the forward motion eccentric above and that of the backward motion eccentric below the axle, and fasten them to the axle temporarily by tightening up the set-screws. Then throw the link down until the block comes opposite to the end of the eccentric-rod, and turn the wheels,[66] and at the same time, observe whether the travel of the valve is equal to the throw of the eccentric and also whether it travels equally on each side of the centre of the valve-face. If its travel is greater than the throw of the eccentric, raise the link up; if less, lower it down until the two are just equal, and then mark the position for the notches on the sections or quadrants to receive the latch of the reverse-lever. If the valve does not travel equally on each side of the centre of the valve-face, either lengthen or shorten the eccentric-rod, as may be necessary. Repeat this operation for the backward motion, by raising the link up until the block is opposite the end of the lower eccentric-rod. After having done this, go over the whole process again to see whether it is all correct. Now with the crank on the forward centre, and the link in full gear forward, loosen the set-screws in the forward eccentric, and move it around the axle so that the valve will have the required lead and then fasten it again. Now raise the link up into full back gear, and set the backward eccentric in the same way. Then turn the wheels so as to bring the crank on the back centre, and observe whether the lead is correct for the back end of the cylinder. If it is not, lengthen or shorten the valve-stem or eccentric-rod so as to make the lead alike at both ends, and if it is then too much or too little, it can be increased or diminished by moving the eccentrics on the axle.

[66] This can be done by moving the engine on the track, or by
raising it off its wheels, so that the latter can be turned without
moving the former. In some shops a pair of rollers is put in the
track so that by placing the driving wheels on them they can be
turned without any difficulty.

Great care must be taken in setting valves to be sure that the cranks are exactly on the centres or dead points, and it is impossible to set them in that position with sufficient accuracy from the motion of the piston or cross-head, and therefore the centres of the crank-pins should always be set so as to conform to a line drawn through the centre of the cylinder and the axle. When the cylinders are horizontal, it is of course only necessary to place the cranks on a horizontal line drawn through the centre of the axle.

When the valves are set it should also be noticed whether the axle-boxes (whose construction will be explained hereafter) are in the centre of the jaws, and if not they should be moved to the centre by driving wooden wedges between them and the frames, either above or below, as may be required. The position of the boxes has a very material influence on the valve-gear.

If it is intended to lay off the notches on the sectors so as to cut off steam at certain definite points of the stroke, these points should be laid off in the guides from the motion of the cross-head. The latter being placed in any of the required positions at which steam is to be cut off, the reverse-lever should then be moved so that the link will just close the admission port. The lever can then be clamped to the sectors, and the wheels turned so as to show whether its position is correct for each end of the stroke. As before stated it is impossible to get the ordinary link-motion to cut off at exactly the same points at both ends of the cylinder, but a very close approximation can be made by proportioning the different parts properly. As has already been stated, it is believed to be a much better plan to put as many notches in the sectors as possible, than to locate them for certain definite points of the stroke.

In setting the valves of locomotives, care must be taken to turn the wheels _forward_ for the _forward motion_ and _backward_ for the _backward motion_.

After the valves are set the position of the eccentrics on the shaft should be marked, so that in case they become loose on the road they can easily be set again. It is usual, too, to mark the position of the valves with centre-punch marks on the valve-stem and on the stuffing-box of the steam-chest, so that with a gauge made for the purpose the position of the valve can be determined without taking off the steam-chest cover.

In some cases the eccentrics are keyed on, which is done after their position is determined by setting the valves. The ends of the set-screws which are used to fasten the eccentrics should be cup-shaped and case-hardened, so as to hold as securely as possible to the axle when they are screwed down.