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Baldwin Mallet vs. Other Articulated Locomotives

Author's Note:  By the turn of the 20th century, Baldwin had performed the research into the work done by some of the most innovative locomotive engineers in the 19th century, Robert Fairlie, William Mason, and Anatole Mallet.  As the previous Evolution chapter in this series on Baldwin Record 65 shows, Baldwin had experimented with multi-truck articulated design with a Baldwin Fairlie design as well as its first articulated locomotive.  The stage was set for it's first really successful Mallet, those built in 1906 for the Great Northern Railway.

By the turn of the century in 1900, the Great Northern was one of the major US cross country freight haulers.  The GN faced with two major issues that strained the capabilities of its heavy Mikados.  There was the Cascade range in western Washington.  And there was the extremely heavy freight loads involved in hauling iron ore drags from Northern Minnesota to the great lakes ore docks in Allouez, Wisconsin. 

The solution would have to be a longer heavier locomotive with more boiler capacity and the greater tractive effort exerted by a larger number of drivers.  However, the engine would need to round curves.  So Anatole Mallet's articulation principal was very appealing to GN management.  They were also attracted by the potential for improved thermal efficiency from Mallet's approach to compounding.

In 1906, Baldwin delivered an order of five 2-6-6-2 Mallets to the GN.  Lionel Weiner in "Articulated Locomotives" refers to Mallets as semi-articulated locomotives.  That is because the rear set of drivers are fixed to the boiler as in a traditional locomotive.  But the front drivers rotate along a vertical axis allowing the drivers to move into a curve.   

The following drawing compares the articulation approach taken in the Fairlie and Mallet types.

In the top Mallet design, articulation occurs at a hinge point between the front and rear six coupled driver blocks.  In the bottom Fairlie design, articulation is accomplished by allowing the six coupled driver blocks to rotate on their hinge points placed at mid block.

It is interesting to note that virtually all Mallet models implemented in the model railroad arena use the Fairlie approach to implementing articulation.  For reasons discussed in Baldwin Record 65, the Fairlie design is able to handle shorter radius curves, common in model railroad implementations.  What follows is a continuation of a paraphrasing of the material in Baldwin Record 65.

Baldwin points out that in both designs, pressure provided by the cylinders to the wrist pins tends to push the pins in the direction of the pressure to rotate the driving wheels to which the pins are attached.  At the same time, the cylinders and fames to which they are attached tend to move in the opposite direction.  In both designs, the pins on one side are 90 degrees in advance of the other.  As a result the truck tends to want to move in the direction of the pressure.  This causes the engine to want to move in opposite directions twice for each driver revolution, once to the left, and once to the right.  

Given the heavy loading pressure of the boiler over the pivot points, this movement is resisted by friction and no movement about the hinge points until the engine has moved sufficiently into the curve that slack in its drive train in journal boxes, rails to wheel flanges, wedges, gibs, and frames has been taken up.  With given weights per axle, and equal rigid wheel bases, this resistance is greater in the Mallet type than with the Fairlie type because of the longer average radius through which the resistance acts.  Baldwin concludes that with the greater resistance to turning in response to these lateral forces, the Mallet is more stable transversely.  This is important in that if the resistance is not sufficient, the engine will move from side to side, causing the moving parts to wear rapidly until clearances are abnormal, and side motion is too excessive for safety and economy of maintenance.

Author's note: In his book, 'American Narrow Gauge Railroads', Hilton points out that Mason Bogies had a reputation for constantly hunting for their own curvature.  His comments are another way of pointing out the same issue raised by Baldwin.  Mason Bogies were criticized for their excessive flange wear and for their tendency to derail, especially in situations where speed across curvy trackage was required.  The North Pacific Coast, a narrow gauge line that was an early adopter of Mason Bogies ultimately restricted them to branch lines where speeds were low and their tractive force advantages and ability to run well in either directions could be taken advantage, while minimizing their disadvantages.  On the other hand the Baldwin GN Mallets were designed for higher speed main line runs.  The Mallet design overcame the limitations of the Fairlie design allowing the Mallet articulated locomotives to become the primary long-haul freight haulers to the end of steam. 

In Mallet locomotives, no high pressure steam is carried in flexible pipes, overcoming a second disadvantage of the Fairlie design.  This improvement is particularly important in cold climates where leakage causes clouds of condensed steam, obstructing the view of the engineer operating the locomotive.  By creating a compound locomotive with low pressure steam being used in the movable front cylinders, Baldwin limited the flexile pipes needed in his design to those needed to carry low pressure steam.

Author's Note:  The above top elevation drawing of the 1906 GN Baldwin Mallets clearly show the hinge point between front and rear six coupled trucks at the midpoint of the rear cylinders.  Note that the front and rear cylinders have significantly different diameters, the rear pair being those carrying high pressure steam and the front pair larger in order to handle the increased volume of the expanded low pressure steam.

Baldwin claimed a number of advantages associated with compounding.  Reduced fuel costs were accomplished by getting maximum propulsion out of the use of steam, once at high pressure to drive the rear driver set, and again at low pressure to drive the front driver set.  

In addition, Baldwin felt the high and low pressure combination reduced driver slippage.  Articulated locomotives, essentially having two engines fed by independent steam pipes need to exert equal tractive power if slippage is to be eliminated.  When tractive forces are unequal, one driver set will tend to lose adhesion and slip.  This is a particular problem on grades as slippage could cause the engine to stall before the loss in pulling power can be recovered.

Because both the front and rear engines depend on the same source of steam, a compound locomotive automatically counters slippage on one set of drivers.  Should slippage occur on the rear high pressure drivers, its exhaust would fill the receiving pipe to the front drivers faster than could be relieved by the low pressure engine, causing back pressure on the high pressure pistons.  this back pressure on the high pressure piston would prevent further slippage.

Should the low pressure engine slip, it would exhaust the contents of the receiver until the reduced pressure feeding the low pressure cylinders was reduced sufficiently to stop the slippage.  The only slippage that could continue would be slippage with both sets of drivers.  This slippage could be countered by the engineer using the same techniques used in ordinary locomotives.

As compared to the Fairlie design, the Mallet simplified matters by substituting one large boiler for two smaller ones.  This reduced the number of parts needed to support the design and reduced the number of fires that needed to be tended from two to one.  Mason's articulated locomotives only had one boiler but also only had one set of drivers, cutting the the tractive effort in half for a given engine weight.  

Author's note: In addition to having two boilers to tend, the Fairlie fireman had the additional disadvantage of having much more limited fuel carrying capacity as the fuel needed to be carried at the midpoint of the engine.  On the other hand Mallets could carry long tenders behind their fire boxes making them much more practical for long distance freight hauling.

Baldwin points out that because the Fairlie and Mason designs rotate the driver blocks on a pivot in the center of those blocks, these engines are more stable on sharp curves than the Mallet with a single hinge point between the two sets of trucks.  Because the Mallet boiler is fixed to the rear driver trucks, the front trucks must move out from underneath the front boiler in order to handle curves.  This causes the center of gravity to move at the bearing support at the front of the engine off the center line between the tracks.  The sharper the curve, the greater the boiler overhang, and the greater the center of gravity shift.

As the following drawing indicates, this problem is compounded by the fact the boiler on a Mallet must sit significantly higher than on a conventional locomotive in order to allow the front driver block to move under the boiler as it deals with curves, magnifying the instability caused by the center of gravity shift.  

Baldwin concluded that this was not much of a problem on main line railroads with their more widely separated tracks (4' 8 1/2 inches) and their wider curve radius.  But it would limit the use of Mallets on narrow gauge railroads with their reduced track separation and tighter curves.

The next section compares Mallets vs. Conventional locomotives.

(c) 2007 Iron Horse 1:29