Farm Collector

LOCOMOTIVE VALVE-GEARS COMPENDIA

958 Sheridan Avenue Ypsilanti, Michigan 48197

Valve gears, their design, their operation and their relative
efficiency have been an intriguing study and a serious problem that
have eventuated into a wide variety of inventions. From the time of
the building of the first locomotive, the method employed to
actuate the valves that are required to supply and regulate steam
from the boiler to the cylinders and discharge it after it had
performed its work, were wildly experimental and mechanically
crude.

About 1840 the double-eccentric hook motion had naturally
evolved into a shifting-link action, under the name it has ever
borne—-The Stephenson Link Motion; and due to the ultra
conservatism and a lack of a creative and a progressive incentive
on the part of American locomotive builders, this ancient and
somewhat misfit valve gear remained a vital part of all American
locomotives even as late as several years of the early years of
1900.

However, to remain solvent and to be competitive in reference to
new and accruing more modern methods of freight transportation,
longer freight trains were essential as a matter of expediency and
sound railway economics, which situation, in turn, required more
powerful and larger locomotives for railway operation. In the more
powerful locomotives, the increased diameter of the boiler barrel
left no room underneath for the necessarily larger four eccentrics
that would be required, including the rest of the Stephenson valve
gear. This fact, in conjunction with the added inertia of the
heavier required parts, make it positively impossible to continue
the application of the Stephenson link-motion valve gear for any
further use in large locomotive construction. Thus it was the force
of circumstances that triumphed over the bullheaded and dogmatic
conservatism of American locomotive builders and forced the
discontinuation of further applications of the Stephenson link
motion valve gears on American locomotives! Due to its many years
of successful operation on European locomotives, its comparatively
light weight, its outside convenience for inspection and repairs,
its minimal width factor and its superior steam distribution
qualities; these advantageous qualities in summation, determined
the consideration and the adoption of the Walschaert valve gear as
the only feasible application and practical solution for the
pressing locomotive valve gear problem.

The year circa 1844 was a transitional period in the development
of valve gears. At this time, Egide Walschaert was Master Mechanic
for the Belgian State Railways. Egide Walschaert, it seems, was
very much dissatisfied with the results obtained from the use of
two eccentrics to govern the motion of one main valve. He
visualized the possibilities, from the standpoint of economy, of
further utilizing the expansive power of steam in the cylinders an
accomplishment hitherto had been impossible with the hook motion.
So with the incentive of attaining a greater operational economy as
an objective, he invented the form of valve gear that bears his
name.

It was in 1848 that Walschaert first applied his new valve gear
to a locomotive on a trial basis, and the results turned out
exceedingly favorable just as he had anticipated. His greatly
improved valve gear attracted attention throughout the Continent,
and in due time it came into general use on all the principal
railways of Europe. The mechanism of the Walschaert valve gear is
entirely different than the general run of valve gears in that the
resultant motion of the valve is due to two independent component
motions; one motion is produced by the eccentric-crank-arm which
moves the valve to a position of no lap plus no lead, while the
remainder of the valve movement, lap plus lead, is actuated by the
crosshead.

The principal distinction between the Walschaert and the
Stephenson valve gears, so far as steam distribution is concerned,
is that the Walschaert gear gives constant lead at all cut-offs,
whereas the Stephenson gear varies considerably in this feature.
The only time the valve of a Stephenson gear is operated by a
single eccentric is when it is in full gear forward or full gear
backwards. At all intermediate gear positions, the motion actuating
the valve is not that of a single eccentric, but a combination
motion produced by the main eccentric in gear and a sizeable
increment of motion produced by the other eccentric. Due to the
requirements of lap and lead, the angle of advance, for both
forward and backward gears, the leverage effect of the two
eccentrics of link motion are not in a diametrically opposite
phase. If they were, the link would rock about a stationary point
between the two eccentric-rod connections. Since this is not the
case, any ‘hook-up’ in either forward or backward gear,
produces a change in lead. If the arrangement of the eccentric rods
is of the ‘open-rods’ variety, ‘hookups’ in either
forward or backward gear, will cause an increase in lead;
‘Crossed-rods’ will produce a decrease in lead in the
various ‘hook-up’ positions. ‘Slip’ which occurs
between the slide-block and the link, is another annoying feature
associated with the Stephenson gear a feature that tends to
increase due to wear. However, when in perfect repair with the link
in such a position that the saddle pin is directly over the
slide-block, the slip is comparatively small.

In the Walschaert valve gear, the eccentric-crank-arm, attached
to the main crank, is set at 90 degrees out of phase with the main
crank. The eccentric rod connects with the lower end of a rocking
link that is pivoted at its exact center. Connected to the
link’s slide-block is a ‘radius rod’ with one end, at
the link, extended a short space to the shifting lever, which, with
the customary linkages, terminates at the reverse lever. The
effective radius rod extends from the slide-block to a vertical
lap-and-lead lever. The lower end of this lever, through linkage,
is attached to and controlled by the motion of the crosshead. Since
the motion, due to the displacement of the crosshead, is greater
than the motion produced by the eccentric-crank-arm, the valve stem
is attached at the top, or near the top, of the lap-and-lead lever.
Its exact place of attaching depends on the type of slide-valve
used. The result, of the mechanical set-up of the Walschaert valve
gear, is that the lead remains the same at all points of cut-off
which is a desirable economic factor in the matter of efficient
distribution of steam. The special adaptability and all-around
efficiency of the Walschaert valve gear made its adoption and use
an exclusive necessity and an absolute requirement throughout the
entire era of big steam locomotives!

To complete the discussion of radial locomotive valve gears, the
Baker valve gear possesses sufficient commendable qualities to
justify consideration. Mr. A. D. Baker, a manufacturer of threshing
machinery, was granted a patent in 1903 on a valve gear for use on
traction engines. This traction engine valve gear had recognizable
and unique valve-motion features that were adaptable, with
considerable alterations, for being converted into an effective
locomotive valve gear.

In 1911 a major patent was granted to a newly formed concern
known as The Pilliod Company, which was founded to manufacture
‘The Baker Long Lap Locomotive Valve Gear’. In the
alterations, making the previous traction engine valve gear into a
suitable and efficient locomotive valve gear, the most visable
operating parts of the Walschaert valve gear were copied, such as
outside installation, an eccentric-crank-arm attached to and set 90
degrees out of phase with the main crank which alone furnished a
valve motion with zero angle of advance. This motion alone would
call for an elementary valve without lap and would admit steam for
full stroke. Motion from the crosshead, through proper linkage,
like as in the Walschaert gear, gives the additional travel to the
valve to make up for lap and lead. While the outside visable
feature of the Baker gear are direct copies of comparable
Walschaert features, some of the other parts of the Baker gear are
suggestive of applications contained in the Marshall gear, a
marine-engine gear.

The Baker gear, notwithstanding some of its visual Walschaert
features, it, nevertheless, possessed some individually desirable
qualities. It averted the use of the curved link and avoided any
sliding-friction members in its mechanism. A major claim for this
gear was that it provided an extra long valve travel which
permitted the use of a longer steam lap.

Since the appearance of the Baker gear was pretty much of the
‘Johnny-come-lately’ variety, in the time schedule of the
remaining few years of the steam-locomotive era, its application
was somewhat local and during these remaining few years of service,
it made no perceptible dent in the long and efficient record of the
Walschaert valve gear!

A contributor, in a recent issue of the ALBUM, vehemently claims
the Walschaert valve gear is not a ‘radial type of valve
gear.’ However, not wishing to pursue the argument into tedium,
I’ll quote Prof. Furman’s definition of a radial valve gear
as it appears in his textbook on ‘Valves and Valve Gears’
which is as follows: ‘The characteristic feature of a radial
valve gear is that the resultant motion of the valve is taken from
a vibrating link or rod. In case of the Joy gear, there is not even
one eccentric, but nevertheless a vibrating motion of a link or rod
is obtained.’ End of quote. Therefore, the Walschaert valve
gear, according to this authority, fulfills all requirements to be
classed as ‘a radial type of valve gear’ in spite of any
senseless counter claims.

Some further reverberations spewing from the same source, claims
the Walschaert valve gear is a ‘sloppy gear’ and also a
‘mechanical monstrosity!’ Such inane vaporings can
originate only in one suffering from a thick mental fog, and acting
in a puerile and silly manner comparable to Cervantes’ zany
character, Don Quixote who ‘jousted windmills.’

In big locomotive construction, can anyone imagine a big
Articulated-Mallet locomotive, with wheel arrangement of 2-10-10-2
and rated 6,000 hp being equipped with any valve gear other than a
Walschaert valve gear? In big locomotive construction, the
Walschaert valve gear functioned as the heart and soul of the
engines that propelled the locomotives. To consider any other
equipment functioning in such an essential capacity would be just
as futile and impossible as a hare-brained attempt to obtain
productive results by the loony and impossible operation of
‘THRESHING CHAFF!’

  • Published on Nov 1, 1971
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