CPR HALF DECK TURNTABLE IN HO

A working locomotive turntable is always interesting and appealing.  We are happy to say that ours is finally in working order after years of manual semi-operation.  It was built so long ago that some of the details of its construction may be lost in the mists of time, but the main points will be addressed here.  The particular method of construction is a little out of the ordinary, a wood lathe being the major tool used to achieve accuracy.  Care was taken to align the pit hole, the seat for the centre bearing and the turntable shaft. 

Pit Construction:  The model pit was built of 6 segments of white oak, glued up with splines or woodworking "biscuits" between the segments.  It was turned on a wood lathe and a clearance hole bored for the shaft.  A seat for the ball bearing was cut into the underside of the pit.  The wood has remained very stable for about 30 years.  In the photo you can see the joints of the segments near the centre hole.  In shaping the pit, we worked off dimensions which we had taken in the field at the site of the former Brookmere railyard.  The owners of the property at the time were nicely accommodating and made the roundhouse area accessible.  Note the coping timbers under the rails on the perimeter of the pit.  These are detailed in the prototype drawing presented in the previous post.

Pit Rail and Wiring:  During the turning process, a recess for the pit rail was cut into the appropriate place which made fitting the rail in place quite straightforward.  After installing the completed pit into the layout, gaps were cut in the pit rail for isolating the two halves of the rail due to their opposing polarity.  This is called a split ring pick-up and is sufficient for d.c. systems.  With DCC it was found necessary to cut a second pair of gaps to completely isolate the bridge pick-ups for a second or two during rotation.  DCC rails are always hot.  The minor drawback with the split ring system is that sound and light are turned off for a second as the bridge is momentarily isolated on the dead section of the pit rail.  We can live with that.  A more complicated system of wiring the bridge is to use the pit ring for one wire and a centre shaft take off for the other.  This method works fine with d.c.train control systems but for DCC it is necessary to wire in a reversing circuit breaker. 

Bridge Base:  Here is the underside of the bridge showing the wood base, electrical pick ups, shaft and mounting plate.  The wood base for the bridge was also turned on the lathe as a disc and a 3/8" hole bored through and true.  When demounted from the lathe's face plate, it was sawn to width with the 3/8" shaft hole kept on centre.  A piece of aluminum plate (1/8") was prepared for the base, a centre hole bored and screw holes bored for mounting to the wood base.   During mounting, the centre holes in the mounting plate and the one in the wood base were carefully aligned to keep the assembly centred.  The brass shim (.002") under the aluminum mounting plate was necessary for final truing up of the table to the pit and engine leads.

Turntable Shaft: As mentioned, a recess for a ball bearing had been cut into the dead centre of the pit on the underside while being turned.  The bearing was trued up and glued in place with Urethane Glue.  The bearing takes a 3/8" shaft made of cold rolled steel.  The shaft does not actually touch the hole seen in the pit photo, passing clear through to the bearing located in the bottom of the pit.  The top end of the shaft had a shoulder cut into it for fitting hard to the aluminum mounting plate.  This was done on a metal lathe by my friend Mike, a MMR.  He also drilled and tapped the top end for a machine screw and fabricated a solid washer.  A "flat" was filed on the bottom of the shaft to take a set screw.  This set screw is part of the coupling for the drive system that we will reference below.

Bridge Wiring:  In the photo above, can be seen the brass stock glued to the wood base.  Pick-up shoes (by Tomar) are soldered to this brass stock and two wires run to the appropriate Bridge rails.

Bridge Deck: The ties and timbers were applied to the wood base according to the specifications in the prototype plans (see previous post).  The walkway planks have a removable middle section for access to the machine screw and washer for future maintenance purposes.  Rails were laid and wires soldered to them.  Care was taken to install the rails in exact position relative to the centre point of the shaft and the centre line of the table.  Each of the approach tracks were then laid so that both ends of the table track were able to line up with it.  Some minor adjustment was required with each approach track and as well as the bridge rails till every rail lined up every time.

Bridge Girders are resin castings from masters made by friend, Brian Pate of Klondike Mines Railway fame. http://www3.telus.net/KMR/  He has two of the 70 footers on the CPR portion of his layout at Sicamous and Arrowhead.  Thank you Brian.  Nicely done and much appreciated.

One detail still to be modeled is the hoses that were connected to the piping to supply air to the motor from a locomotive.  This is seen in the Lawrence photo from1954.  There are a few other minor additional details that could yet be added to the bridge and pit.

Model Bridge Motor: From the prototype photos and the one appearing in the Hal Riegger book, the various dimensions for the model were calculated starting with the wheel and sprockets.  The toothed sprocket was reckoned to be 33" in diameter as was the sprocket paired up with the drive wheel.  Two wheels from the Walthers' Old Time Coal Conveyor were used for the sprockets, cutting out half of the spokes: (Walthers Part # 933-3520).  Glued to a Tichy plastic 33" wheelset as the drive wheel, it was a good match to the prototype. Judge for yourself.  The model drive wheel is suspended above the pit rail so that it does not touch it.  The brass rods simulating the prototype braces were enough to hold the motor rigidly.  When in motion, the external sprocket does not rotate as the prototype did but if one of the readers figures out a way to do it we would be most interested to hear about it.


Turntable Drive:  Initially, this model was a "finger-strong" turntable.  Now it has a wonderful drive system made by New York Railway Supply: http://www.nyrs.com/  It works perfectly but for quite a while it did not.  This was due to a technical inexactitude by, yours truly, the builder and blogger.  Now corrected, the turntable does what it is supposed to do.

Control panel:  Using components from New York Railway Supply, a custom panel was fabricated.  The pointer of the rotary switch simply indicates which of the 7 tracks the crew wants the Turntable to line up with.  It does not power the track.  The toggle switch indicates which end of the Bridge will line up with the chosen track.  The 5 engine storage tracks have gaps cut in one of the rails to allow the engines to be isolated from the electrical power when not in use.  Pressing one of the momentary-on push buttons will power the appropriate track, thereby passing current to the locomotive to enable it to exit the house under command of the engineman.

One interesting note: Happily, our model Bridge lines up with both the engine house track #2 and the main engine lead regardless of which track the head is oriented to.  A long time CPR shop man told us that the actual Brookmere Bridge would only line up with both tracks in one orientation!  Rotating it 180 degrees would result in the bridge rails lining up at one end only.  So, if your bridge has this problem there is a precedent for it.



Some days we just have to watch the engines taking a spin on the turntable.  Great stuff.

Coquihalla Man