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One thousand two hundred eighty-three pounds is a tremendous weight to be included unnecessarily in rapid-acting machinery; especially when the deflection of one-thirty-second of an inch may change the motion sufficiently to blind one of the events—lead, for instance. And in substituting the Walschaert gear it means cutting off thirty-five (35) per cent of the weight of the old link motion.
But some of the above are fixed, immovable parts. If we want to find the difference in weight of the working parts we should cut out such as the reversing shaft, rocker boxes, link support (Walschaert), etc., and we then find the weight of the Stephenson motion gear to be 3,120 pounds, and that of the Walschaert gear 1,702 pounds—a saving in weight of the parts that thrust and take of 1,418 pounds, amounting to the still more interesting figure of over forty-five (45)
Of the heaviest passenger engines ever built, at this date,—the class shown in Fig. 19-some have Stephenson link, and some the Walschaert gear. The comparative weights of the valve gears of this class, J 2–6–2, are:
Crank pins, main
Stephenson. Walschaert. lbs.
lbs. 390. 365
325 300. 300 270.
160 I 20.
Total, pounds Saving in weight, pounds or 32} per cent.
2,725 . 1,315
Amount per cent saved in weight of the motion parts of gear, nearly 47.
Another type of Classs J 2-6-2 carries the following weights:
Crank pins, main
cric straps Eccentric rods Link ...
Cutting out the non-moving pieces there is a difference in the weight of the working parts of the two gears applied to Class J 2-6-2—of this particular Prairie type of engine-of 1,835 pounds, equalling a saving by the use of the Walschaert valve gear of Forty-eight plus "per cent (48 + %). The motion part of the latter gear, then, is but little more than one-half as heavy as the corresponding part of the common link motion used on other engines of the same class.
The energy consumed in keeping going all of the superficial weight of the Stephenson valve gear on our modern locomotives might just as well be expended in carrying several more tons of train load. Its reciprocating motion at high speeds is destructive. On engines of large size the eccentrics are of such weight, on account of their increased size made necessary by the extraordinary diameter of the axles, that in spite of an expensive amount of lubrication a braking power is exerted, and they will run hot. These wcights of the valve gear make engines hard to reverse or hook up, and there is a certain lack of economy when, in spite of careful counterbalancing of the shifting parts of the valve gear, any engine “handles” hard.
Freight engines run as fast as passenger engines do. By that is meant that a freight engine has smaller wheels, and to make the running time required of the freight trains of to-day the smaller wheels of their engines must make about as many revolutions per minute as do the bigger wheels of the passenger engines to make their time; and for this rapid action of the machinery the lesser weight of the Walschaert gear makes that type of valve motion most desirable. And no change in locomotive design will call for any increase in the number of reciprocating parts of the Walschaert gear, thus saving the weights, and errors in the motion due to vibration and lost travel that are present in common with “transmission bars,” etc.
This latter point is covered in the matter of directness. There are but few motion parts to the Walschaert gear, and they are of the same number for all engines in all classes of service, except that sometimes the connection from combination lever to valve-stem, or from eccentric rod to link, has to be carried over by a sort of rocker-arm arrangement on account of the steam chest being rather far inside from the centre line of the cylinder.
A motive power official will inform us that an engine with the common link motion has a certain amount of lead, in full gear, but he knows that that matter of lead is decidedly uncertain—a variable quantityconsidering the interference of the numerous joints in the gear from eccentric to valve, the slackness from wear of the two eccentrics and their encircling straps, of the many pins and rockers and the vibration of the transmission bar. And this lost motion is further acted upon by the practice on many roads of using piston valves of inside admission with closed “spools,” instead of having the valves open from end to end; at the instant of steam release from the cylinder the exhaust pressure acts directly against but one end of the double piston, and with a power approximating the steam pressure that is admitted to the low-pressure cylinder of a compound engine, forcibly closing in all slackness of the motion from the valve clear back to the eccentric.
In regard to permanence of adjustment, and the effects of wear, the results obtained from the L. S. & M. S. engine proves the superiority of the Walschaert valve gear; the following statement is from the Super