Atlas SD50

[davefoxx]

Dave,
I have a couple of these older SD50's and never had any problems with them, but the next time you have them apart, take a straight edge and lay on the inner face of each frame half and see if they are flat or if there is any warp to them.  This might cause one or two things that might add up to your problems.
If they are warped inwards, they might bind on the truck towers or the pins that hold them in the frame, or it might make the bearings set just out of alignment which would cause drag.

Speaking of bearings, make sure they are set in the frames properly.  If they are not they would be putting pressure on the shaft in one direction more than the other.  If you've ran these for awhile the bearings might also be worn enough so they don't hold the shaft in the proper position.  Since they are that old, the plastic block that surrounds the bearings might have deteriorated or gotten brittle enough to not hold the bearing properly.

At this point I'm only casting lines in the dark, but it might be worth a look.

A word of caution about getting new trucks, I had some older Atlas/KATO Gp35's that I swapped the frames for some newer DCC ready frames not long ago.  Since the old 35 trucks and fuel tanks were painted and weathered, I just swapped out the frames and used the old trucks and tanks.  After I did this, both sets of the old trucks in the new frame were very stiff.  I wound up taking the dremel and two small grinding bits and ground out the areas where tips on the truck towers locked in the frame as well as the areas that surrounded the towers just a bit to widen them out.  Afterwards they swiveled just fine.  It was almost like the new frames had closer tolerances than the older frames did.

Not sure if any of this will help, but it's worth a shot at this point.

Allen,

I will check the frame halves.  I was wondering about whether it would make any difference when I tightened the frame screws, so (in case I was binding up the mechanism).  There was no change.  I will check to see if the frame halves are straight, though, since that will likely cause problems even with new trucks.

I think the bearings are okay.  I checked them last night when I lubed them.  These locomotives have very little run time (for obvious reasons), so I doubt wear is a concern.  I wish I had some spare thrust washers, though, because there is a little more end play in the worm than I would like.  I don't know if that makes a difference, though.

Thanks,
DFF

[Chris333]

Ron talks about the top worm gear in the tucks being loose here:
http://forum.atlasrr.com/forum/pop_printer_friendly.asp?ARCHIVE=true&TOPIC_ID=42265

[wcfn100]

Ron talks about the top worm gear in the tucks being loose here:
http://forum.atlasrr.com/forum/pop_printer_friendly.asp?ARCHIVE=true&TOPIC_ID=42265

I am also satisfied with the running characteristics of my six-axle Atlas locomotives, which include SD35s and SD50s.

lol

Jason

[Chris333]

Found this on the trucks:
http://mysite.mweb.co.za/residents/grela/atlaskatotruck.html

I don't own one.

[daniel_leavitt2000]

The SD50/60 are infamous for this. The consensus seems to be the the center axle tends to hop and bind on curves. A possible solution to this is to swap out the wheel sets with lo profile sets from Kato. The other source of problems is binding of the straight toothed main gear to the worm. I have had some success sanding the sides of the gear near the teeth slightly thinner. Thicker grease on the worm also seems to help.

[davefoxx]

Ron talks about the top worm gear in the tucks being loose here:
http://forum.atlasrr.com/forum/pop_printer_friendly.asp?ARCHIVE=true&TOPIC_ID=42265

Yes, sir.  I had an SD35 that had that problem.  I replaced the truck, and the locomotive is like new again.  I don't think that's a problem on my SD50s.  I'll double check, though.

DFF

[davefoxx]

lol

Jason

Yeah, if only I had those SD50s back.  I sold them several years ago when I did an era and locale change and look what it got me when I returned to modeling the SBD.  I had to buy the model locomotives again, and they run like crap.   

Besides, I'm not referring to the axle-hopping issue here.  This is some sort of binding, causing the locomotive to slow in the curves.

DFF

[davefoxx]

I have had some success sanding the sides of the gear near the teeth slightly thinner.

I have considered this, because if it is the gear lash when the trucks turn, it could be relieved.  I just feared that I couldn't sand the worm gear without creating a mess of flash at the edges of the teeth.

DFF

[daniel_leavitt2000]

Use a toothbrush and toothpaste after. It will clean up the flash.

[davefoxx]

Use a toothbrush and toothpaste after. It will clean up the flash.

Interesting!  Great tip.  If I still experience the slowing with the new trucks I ordered this afternoon, I'll give that shot.  Plus, I'll have extra worm gears from the old trucks, if I screw it up.

DFF

[John]

Yeah, if only I had those SD50s back.  I sold them several years ago when I did an era and locale change and look what it got me when I returned to modeling the SBD.  I had to buy the model locomotives again, and they run like crap.   

Besides, I'm not referring to the axle-hopping issue here.  This is some sort of binding, causing the locomotive to slow in the curves.

DFF

take the trucks completely apart .. clean and relube .. you put one of the gears in wrong IMHO

[Chris333]

I used a nylon brush in a Dremel to clean flash off NWSL plastic gears, guess it's pretty much the same as using a toothbrush though.

I saw you didn't have the wheel hop, but figured the problems could be related.

[Kev1340]

For what it's worth, the only time I've had issues with any loco 'binding' on curves had been when I had inadvertently over tightened the frame screws. This puts friction on the trucks preventing them from turning freely.

I have several SD50's and SD60's with 'hopping' centre axles, but I've never found that to cause binding on curves.

Cheers,

Kev

[davefoxx]

take the trucks completely apart .. clean and relube .. you put one of the gears in wrong IMHO

John,

I didn't take the gears out, so I couldn't have put them in wrong.  Besides, the trucks roll freely, so that can't be the problem.

DFF

[davefoxx]

I checked the frame halves of the bad SD50, and they are straight.

I checked the trucks for the small fragile pins that support the frame.  They are all intact.

I checked the worm gears, and this locomotive does have one that is loose.  It is clear that the gear tower halves are bowed outwards, so that the worm gear is loose and could pop free.  However, although that is a problem, it doesn't explain why a truck swap failed to make the locomotive run better.

I hope that new trucks will fix the problem.  The only other link in the driveline is the bearings and driveshafts, because I swapped out the motors last night, which didn't improve this situation.  I'm not sure how the bearings or driveshafts would be the culprit in only curved track, but they'll get closer inspection if all else fails.

DFF