Saturday, February 29, 2020


Construction of my turntable and associated track and service facilities continues as a long-term, many-step process.  I previously reported the preparation of the turntable pit:
The next step was construction of the turntable bridge.  

An important reason for my selection of a Diamond Scale turntable was that product most closely matched the Eugene, Oregon turntable.  The prototype turntable still exists and sees limited use in spite of the down-sizing and almost elimination of that once large yard and locomotive service base.  In particular, the Eugene turntable featured a deck girder bridge with a rounded-top turntable arch.  The arch served to convey power to the electric motors for the table.  In the model form, that same arch will serve as part of the track power circuit.

The Diamond Scale turntable bridge is built up around a rotating core consisting of a shaft held by two bearings and a center block.  The primary bridge structure consists of wood girders that straddle the center block and end pieces which serve as mounting points for the turntable drive dolly models.  Bridge ties are glued to the wood bridge girders.  A pair of brass strips are attached near the center of the bridge to support the bridge arch.  One of my project delays involved getting appropriate 00-90 screws to affix the straps with.  In hindsight, I could have substituted pins.  A cosmetic plastic girder set is fixed to the exterior of the wood girders.

Turntable bridge girder built up and mounted on the center block and shaft assembly.  The green wire will bring track power up through the shaft in parallel to a similar circuit using the bridge arch.  Two brass straps straddle the center of the bridge.  These will support the bridge arch.

The turntable bridge ties were laid out on double-stick tape on the template provided by Diamond Scale.

The turntable ties were glued to the bridge girders and weight applied while the glue set.

The bridge dollies have been built up and are awaiting installation.  Note the wire dangling near the center of the turntable bridge at the top of the photo.  That wire is soldered to both of the near-center brass straps to convey power from the bridge arch to one of the bridge rails.  The other rail will receive power from the bridge dollies, including the Tomar wiper seen here.

Bridge dolly mounted on the end of the bridge.  A wire is attached to the wiper at each end and through a mounting screw to the dolly.  These wires feed the "right hand" rail on the bridge when looking toward the control cab.

Aligning the rail on the turntable bridge proved an interesting challenge.  For my first attempt, I carefully measured to find the center of the end blocks.  I then laid the rails using these locations (both ends).  When I tested the bridge, I found turning the bridge 180 degrees did not align with the pair of tracks (approach and roundhouse exit) that I tested with.  This was undesirable.  

The first three roundhouse tracks at Eugene aligned with three approach tracks allowing locomotives to run straight through the turntable into those roundhouse tracks which then exited into the back shop.  Although I do not have space for the back shop, I still wanted to be able to run locomotives straight through over the turntable into the first three roundhouse tracks, regardless of which end of the table faced the roundhouse.

The solution was to use my four feet long straight edge to align the turntable center, ends and the extended tracks.  I was able to get very close, with a slight bit of tolerance created by chamfering the ends of the turntable bridge rails as called for in Diamond Scale's instructions.

Aligning approach and roundhouse tracks with the turntable bridge.  Use of the four-feet straight edge provided center marks that could be used to lay the rail on the turntable bridge.

Once the rails were on the turntable bridge, I could complete its assembly.  This included bridge handrails, walkways, the control cab, and the bridge arch.  I chose to paint the handrails white to match photos of the Eugene roundhouse and turntable shot in the mid to late 1950s.  Before that time, the handrails likely were basic wood, perhaps with some treatment.  By the 1960s, the handrails appear to have weathered and had pieces replaced such that they take on the same grungy appearance as early photos.  Even later, the wood handrails were replaced by pipe stanchions and cable.  The interim period with white handrails was quite striking and fit well with my prime modeling year focus for this area--about 1958 when the new diesel sand facility was built.  Photos taken at the end of steam locomotive operation also show the white handrails.

The turntable arch was built up from a number of white metal castings.  This proved a tedious task, as the various geometric elements precluded laying it over a template for assembly.  I needed to glue each joint with CA and let that joint set before moving on to the next joint.  The couple of days it took me to assemble the bridge arch seemed to summarize my entire experience with the turntable bridge assembly--not difficult, but tedious.  The turntable arch was mounted by soldering it to the underside of the pair of near-center brass straps.  I also soldered the end of the thick magnet wire that completes the center arch circuit from the feed tube at the top of the arch to one of the brass straps.

Mounting the bridge arch by soldering tabs at the base of the arch supports to the undersides of the near-center brass straps.  The magnet wire connected to the feed tube at the top of the arch has been soldered to the strap in the upper right position in this photo.

After a tedious construction effort, the turntable bridge has been installed!

Completed turntable bridge installed.

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