Big Boy parts: turbo-generators, drivers, spring rigging, pilots, stack deflectors, and cabs

Posted by John E. Bush
on Thursday, August 16, 2018

In Part 4 I’ll continue detailing interesting elements involving the design and construction of the 20 Union Pacific Big Boy locomotives Nos. 4000-4019.

In another of the series “25 things you may not have known about Big Boy locomotives,” the turbo-generators were located in a totally differently location than on previous UP locomotives.  Interestingly, it was decided to mount the generators low on the right side of the locomotive in the space between the last driving wheel and the front of the trailing truck frame.  I have not personally seen any internal correspondence relating to this decision, but it’s pretty clear that the main intention was to improve access to the unit.  Previously, almost universally on Union Pacific, generators were placed either directly atop boilers or on the upper left side of the wrapper sheet not far ahead of the fireman’s side of cabs.  On the Big Boy, a heavy shelf which included a protective shield for the front of the units was mounted to the main frame.  Superheated steam was supplied via piping visible alongside the firebox. 

It is well known that the driving wheel diameter of the 4000s was 68 inches.  Both the railroad and Alco clearly stated that the locomotive was designed for a maximum speed of 80 miles per hour, meaning that the design engineers factored the forces at work at such speed in preparing their calculations relating to frames and running gear.  Imagine yourself standing next to the driving wheels of a Big Boy.  When moving down the track, the revolving weight of main and side rods, the huge crank pins and to a lesser extent the valve gear parts, when coupled with the reciprocating weights of the piston & piston rod, the crosshead and forward portion of the main rod as well as some of the valve gear parts imparted massive strains on the locomotive as well as roadbed.  The balancing of the revolving and reciprocating forces inherent in steam locomotives had been a continuously evolving and advancing science for decades by the time these engines were in the design stage.  An important element of balancing of the forces at work included efforts toward dealing with the “dynamic augment” or the horizontal forces generated by the reciprocating motion.  These forces formed a part of the “waddle” visible in certain locomotives and were destructive not only to the running gear but also roadbed and bridges.  General Steel Castings Co. had recently commenced use of a new method of cross-counterbalancing the main driving wheels of locomotives through use of two pockets cast into the wheel centers which allowed placement of the wheels on either side of the locomotive, the pockets were located 90 degrees ahead and behind the main crank pin.  Their first use had been on the 20 800 class 4-8-4s delivered to Union Pacific in 1937.  In June of 1940 Otto Jabelmann had filed a patent for an improvement in this design of the method for placing weights in the pockets, which was granted on Nov. 11, 1941. It was incorporated into the 4-8-8-4s.  Perhaps a small thing to spend so much time writing, and reading about, but interesting that Jabelmann himself was still involved in advancing the design of details parts of the steam locomotive.  Utilizing highly expensive special lightweight alloy steels throughout the running gear and the very latest in counterbalancing technology, Big Boy was as thoroughly balanced as the very latest in Union Pacific passenger power, which was itself at the forefront of this science.

Significantly, very careful attention was given to the design of the Big Boy’s spring rigging system.  In concert with the previously mentioned use of not one but three of Alco’s Lateral Driving Box Cushioning Devices on both front and rear engines (as had first been used in UP’s second order of 4-8-4s in 1939) this contributed to the designer’s highly effective treatment of counterbalancing.  Controlled lateral motion was also designed into both leading and trailing trucks.  Alco made specific and pointed mention of these systems at every opportunity in the trade press of the day, referring to it as their “lever principle” which resulted in the ability “…to move around curves smoothly with complete absence of the succession of violent guiding oscillations characteristic of many existing steam locomotives.”  Over the service lives of the Big Boy, crew reports would verify that they were known as possessing very good riding tendencies.

The Big Boy included the first use of a new pilot casting patented by the General Steel Castings Co., a system which included use of a horizontally swinging coupler arrangement.  When the coupler was pivoted to the rear the assembly included an opposing section which automatically swung into place and incorporated fully into the design of the pilot, forming a part of the deflecting properties intended by the contours of the design.  When the coupler was not in use, the appearance of the pilot was such that it could easily be assumed no coupler existed and created a pleasing, sleek look.  Elements of the design provided a throwback in appearance reminiscent of pilots of much earlier locomotives.  As with certain other elements I’ve written about so far, we’ll come back to the pilot later.

It was clear that operations at large throttle openings would be required through tunnels along the planned route over which the Big Boys would operate most frequently.  The railroad was aware that some arrangement of exhaust blast control would be required.  Union Pacific was very familiar with such equipment but Big Boy presented a new challenge as a result of the double stack design.  The solution was similar to the system utilized on other UP power, consisting of curved plates which meshed together, raised by air-operated cylinders as locomotives approached tunnels to direct the exhaust blast back along the top of boilers.  With Big Boy, the plates were made of sufficient length to cover both stacks and were hidden behind the shroud so clearly seen in photos.  Photographs of the hoods raised in terminals for testing, or of plates stuck up in partially-raised positions are not uncommon.  Unfortunately, photos of the hoods in actual operation, particularly in tunnels are not known to exist, at least I can’t recall having seen one.

I’ll make one more mention in this part, the Big Boy’s cab.  I have often felt that a great many railroads considered locomotive cabs merely as something required by union agreements, and grudgingly provided them and their related fixtures with the least possible element of excess.  Over the years I have frequently been shocked by photos which reveal enginemen seated in remarkably cramped conditions at throttle & brake or fireman’s controls.  Further to this may be the existence on many roads of the shanty placed on tenders for use by head brakemen, possibly a holdover from the days before the automatic brake in which head brakemen were expected to quickly make their way atop cars for application of brakes when signaled by engineers.  There were some exceptions to this minimalist thinking.  While it can’t be stated that Union Pacific “always” saw to spacious cabs such was most certainly the case the majority of the time and by comparison with many other roads UP’s cabs were larger than most.  It won’t surprise that Big Boy’s cab was among the largest ever used with the exception of truly “all-weather” cabs which were necessarily longer due to the additional area behind enginemen.  When inside the cab of a Big Boy (or any other late UP power for that matter) one feels as if they are in a large room compared with the cabs of some other engines of similar size and era.  Not two seats, not three seats, but four seats were provided with engineer and fireman’s seats adjustable fore & aft as well as vertically.  The cabs were insulated, heated, and fitted with air defrosters at the front windows.  In addition, a Jabelmann-patented system of doors provided more protection from the elements encountered in the Utah and Wyoming winters.

I hope you’ve enjoyed another bit of information and perhaps trivia relating to the 4000’s.  Next time:  The second throttle.

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