No 21 Backhead Drawing

This backhead drawing for No 21 was prepared by David Fletcher.  The final details resulted from a series of emails between Fletch and Keith Christenson.  No original backhead drawings are known to exist.  So these drawings are the best estimates of two men that have spent a significant amount of time studying existing photos and drawings.  Both have extensive knowledge of the operation of steam engines.


Click image for a PDF of the original CAD Drawing.


Click image for a PDF of the original CAD Drawing.





Logic Behind Placement of Key Components

What follows is a listing of the key components of the backhead and cab area of NPC No 21.  The logic behind the placement and design of each component is given.

  • Engineers Seat – On the right hand side of the locomotive when facing out the front of the cab.  See photo on Backhead Detail page showing Bill Thomas in this position.  Both engineer and fireman seats were fold up seats.
  • Fireman’s Seat – On the left side of the locomotive when facing out the front of the cab.  As second photo on the Backhead Detail page shows the fireman in this position.  Note the placement of the oil burner controls to the right of the fireman.
  • Burner Vent & Controls – Shows vents on front of the drum admitting air to the oil burners.  Earlier Backhead drawing on the Backhead Detail page also shows openings on side of drum.  It is likely there were holes in both locations as this was the only air supply to the oil burners.  Later photos of NPC No 21 show that the headlight was moved up and a hole was placed in the front of No 21 supplying air through a cowling to the drum openings.  The two levers on the drawing operate the dampers – controlling the extent to which the front and side openings admit air.
  • Oil Burner Controls – These controls which face the fireman control the flow of oil to the burners.  A side view of these controls is clearly shown on the photo of the firemen on the Backhead Detail page.  A schematic of these controls is also shown on the Backhead Detail page.
  • Air Brake Stands – While only one is shown, there were probably two brake stands, the direct air engine brake stand and the automatic air train brake stands.  They were likely cleated off the cab walls.  They would have been cleated to the front cab wall or to the side wall to the side of the engineer.  We feel the front cab wall is the more likely location.  The Backhead Detail page shows a Westinghouse Brake photo on the left and an engine brake photo on the right.  Note that brake gauges would have been placed in front of the engineer on the front cab wall.
  • Lifting Feedwater Injectors – Mounted on the upper slope of the boiler sides.  The injectors were probably “lever action” as was common locomotive practice. The water and steam settings were pre-set and the engineman pulled the lever to start the feed.  A photo on the Backhead Details page shows a feedwater injector just behind Bill Thomas’ head. 
  • Johnson Bar – Is secured to the floor to the left of the engineer’s seat.
  • Hydrostatic Lubricator – Two are shown, one on each side of the backhead.  They would have been similar in design to the photo on the Backhead Detail page.  Globe valves shown would allow steam to be shut off from the lubricators in case of breakage and when the lubricators were to be filled with steam oil.  Oil is fed to the cylinders via a line going to the cylinder covers.
  • Dual Water Glasses – This is not obvious from the photo but it is likely they were positioned 2 feet in front of the backhead so as to be visible to the engineer and fireman by looking sideways.  Note that given the slant of the boiler, it was not necessary that the cab side of the steam drum contain water.  As a result, the upper and lower ends of the water glasses in all likelihood extended from just below the top of the boiler to the lower part of the steam drum.  The only reason the water glasses would extend higher on the steam drum would be to check whether water was too high, causing a risk of priming the engine’s cylinders with water.  Lines feeding the water tubes would be braced with brackets attached to the boiler or possibly the top of the cab.  Globe valves at the top and bottom of the water glasses would allow the water to be shut off in the event of breakage of the glass.  Globe valves would also be placed there the pipes exited the boiler and steam cylinder so the flow should be shut off in the event of pipe breakage.  The preferred water level on this engine is 2 inches above the inner corrugated drum casing.  This keeps spots in the boiler from overheating.  Too much water in the steam drum reduces the steam generating capacity of the engine.
  • Oil Lamps – Lamps with vertical slits would be mounted to throw light on the water glasses.  They are shown to the inside of the glasses in the drawing.  The drawing shows them fixed to the pipe work and off straps attached to the backhead.
  • Turret – Comes off the middle of the steam drum backhead.  It supplies steam to the two hydrostatic lubricators, the compressor lubricator and the oil injector.
  • Throttle Valve – Was at the junction of the dry tubes.  It controlled the flow of steam to the cylinders.
  • Throttle – Earlier drawings on the Backhead Detail page show the throttle swinging directly off the throttle valve.  To reach the throttle, the engineer would need to reach behind his head and around the steam pipe.  In practice, the throttle would have been in front of the engineer’s seat in front of the Johnson bar.  The above drawing assumes it was suspended from the ceiling with a linkage back to the throttle valve.
  • Compressor Lubricator – Shown on the left side of the backhead, this lubricator uses steam pressure to feed oil to the air compressor mounted on that side of the engine.  While shown above the entry door (for clarity) it would have been mounted to the right of the entry door behind the throttle.
  • Steam Dry Lines to Cylinders – Earlier drawings on the Backhead Detail page show these lines in front of the backhead.  Yet these lines don’t show on the photos.  Furthermore, they would block access to the hand holes.  So in drafting this plan, they were curled around the inside outer edge of the boiler.
  • Water Tube Hand Holes – These appear to be 40 circles with a bolt in the middle on the drawing.  While they are appear to be circles, it is likely they were elliptical in shape.  They are actuall drawin that way with a vertical orientation.  As the drawing on the Backhead Detail page shows, the flanges were on the inside of the outer boiler casing.  If elliptical in shape, they could be removed through the hole in the outer casing.  The dots between the hand holes are the backhead stay bolts.  In one of the photos on the Backhead Detail page, you can see the brackets and bolts that held the hand hole covers in place.
  • Steam Pressure Gauge – is not shown on the drawing.  Given his seating position, it is likely the gauge was mounted to the front cab wall in front of the fireman.
  • Tri-cocks – Are not shown in this drawing.  Given their position above the crew’s head, the lack of space, the fact they were being phased out and the unusual characteristics of this boiler, it is unlikely this boiler had tri-cocks.
  • Entry doors – In all likelihood, these were not doors but openings.  They are drawn that way.  To exit the engine, both the fireman and engineer would need to lift their seats and exit through these openings.  Some NPC crews were concerned about the lack of collision protection provided by the cab forward design compounded with the time it took to exit.  
  • Cylinders – This engine was built with parts from wrecked Baldwin 4-4-0s.  Cylinder dimensions would have been consistent with Baldwin 4-4-0 design.  However, they had been upgraded with Bill Thomas’ patented American Balance Valve.

Fletch, and Keith — thank you for all the time and expertise that went into these drawings.

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