Brass Tenshodo SD24 Total Rebuild

Rebuilding a brass Tenshodo SD24

Part 1

The HO scale SD24 brass diesel locomotive by Tenshodo is a difficult beast to tame.  The goal of this project is to rebuild it and keep as much of the original mechanism as possible - but not the motor.  This locomotive is going to be powered by a rare earth super strong magnet 24 volt motor.

There are 3 mechanical challenges to overcome.  First is the linkage from the motor to the gear tower.  The original design uses a rubber hose.  This is a totally sound concept and the original hose can be replaced by a slightly more flexible silicone hose.  The silicone hose is very cheap and a single roll will probably last a lifetime.  The shaft on the gear tower is one size bigger that the shaft on the motor, but the hose does not have a problem with that.  If you have a locomotive with a worn out hose, the size I used has an outside diameter of 4mm and inside of 1mm.  I also tried a hose with 3mm outside diameter and found it to be too flexible, but still usable.  There is another option and that is to use standard splines and couplings like those found in an Athearn blue box locomotive.  The motor has a much smaller shaft than the Athearn, but since I can 3D print these parts that shouldn't be a problem.  We'll see if it's necessary after everything else is mechanically sound.

The second challenge is the power pickup.  The trucks pickup power from one side only - and then only from 2 of the 3 axles.  The middle axle floats and doesn't contribute to the pickup all the time.  It's not a dead axle, but it's not normally load bearing in order to prevent skidding type derailments.  With little to no weight on it most of the time it shouldn't be considered a reliable source of power, but....  The first upgrade that I am trying is the phosphor bronze wire trick.  If I can use a piece of phosphor bronze wire that wipes the top of all 3 wheels and then use a wire to my power collection board the reliability will improve dramatically.  If that works, it may be possible do it to the insulated side wheels giving me power pickup from all wheels - that would be huge.

Challenge 3 is the drive shaft connecting the 2 trucks on the underside.  This shaft tends to disconnect frequently.  I have successfully test a small silicone hose sleeve on the end that gives the trouble.  This seems to work so far, but the locomotive has yet to complete the obstacle course of tight switches and curves that I setup.  The goal here is to negotiate 18" radius curves and #4 crossovers without derailing.

The first step in the whole procedure is to make sure the trucks have reliable movement.  Gear binding seems to be a serious problem.  One of the worm gears needed sharpening on the drive shaft on the front truck so far, but is still suspect.  One of the axle gears had broken loose.  A single drop of super glue fixed that gear in place very tight, but now we have to see if the tolerance between axle and worm will mesh....

Several hours later.... the binding gear problem was solved.  The gear in question was examined much more closelier, yes closelier.  The gear was split.  To remove it the axle was heated with the big soldering gun and the insulated wheel comes right off.  My first thought was to replace it.  I found a plastic replacement that I could have used if I put one on each geared axle in both trucks.  Like I've said many times, shaft sizes and gears are not special.  When a gear is cut, it's cut using a tool that is a standard size.  The gear cutting set is like a machinist drill bit set in that the sizes are all indexed.  This is why you can almost always find a matching gear to anything that needs it, as long as you know where to look.  For now, I've just removed the old gear and someday I'll run across another wheelset that I can replace that axle.  Unless this project turns out to be much better than expected - then I'll replace all 4 gears from my plastic replacements.

Tomorrow's job will be seeing how smooth I can get it to run before I add the extra power pickup.

The phosphor bronze wire totally did the job.  The multimeter shows full voltage through the wires.  Now that power available, we can test it.  On the road test first we checked to see if the hoses used on the drive shafts both topside and underneath would hold.  The good news is that they work so well that they will be retained.  The hoses have done an excellent job.  The crossover test was successful and without difficulty we went over a large crossover that has long unpowered frogs.  We were able to make several times without derailing or stalling.  Next we tried a string of #4 switches that are from a mix of manufacturers.  Rail is brass, yellow brass, steel, nickel silver and we made it through without stalls or derailments.  Then we moved on to the 18" radius test and that was no problem.  Feeling extra brave, we went through a spaghetti bowl of very sharp turnouts and then down to the 15" radius that crosses open space over a concrete floor.  We went through the 15" with some grinding sounds, but stayed on the track both fast and slow.  I'm calling this rebuild a success so far.

Really digging into the locomotive and its inner workings has helped me step up my game to a new level.  For a couple of years I've considered this SD24 a piece of junk - and for good reason.  I haven't been able to find any good comments from people who have them or any stories of rebuilding them.  I know they are out there and hope this story will bring out other information.  Now that the locomotive is in very good working condition I can move into paint and decals.  I have something exciting in mind....

Now time for some pictures....

And that's a wrap.