Atlas SD-35

[carmelmodelrr]

I have a pair of Atlas SD-35's (DC).  While these are weak pulling locos, each of the SD-35's runs nicely when run by itself (no consist).  However, while one is marginally satisfactory, when I hook up the other one (solo) to almost any number of cars (3 or more), this SD-35's lights shine brightly, the wheels spin -- and it won't pull anything.  I've replaced the gear cases, the wheel sets, the truck frames and the motor--all with no improved results.  I'm at a loss to determine what is causing this.  Any ideas?

Thanks.

Dick Wroblewski
carmelmodelrr

[mecgp7]

Dick,
Is the shell sitting too low so that the pilots actually hit the rails? They could be lifting the chassis just enough to prevent traction. Does the loco run OK without the shell? Will it push the same cars without the shell?

[davefoxx]

Dick,
Is the shell sitting too low so that the pilots actually hit the rails? They could be lifting the chassis just enough to prevent traction. Does the loco run OK without the shell? Will it push the same cars without the shell?

This is what I was thinking.  I've noticed that I have to be careful putting the shell back on my some of my Atlas locomotives, because you can push the shell on far enough that the pilot will hit the rails.  Usually when I push the shells down, I give a little tug upwards.  You can feel the shell fit back into the detents on the frame.

DFF

[peteski]

Are you positive that the wheels are actually spinning in place, or you are simply hearing the motor running, assuming that the wheels are spinning?

[jagged ben]

Are you positive that the wheels are actually spinning in place, or you are simply hearing the motor running, assuming that the wheels are spinning?

It's pretty difficult for an N scale loco motor to spin if the wheels don't. 

[wcfn100]

It's pretty difficult for an N scale loco motor to spin if the wheels don't.

All it takes is for the universal joint to crack.

Jason

[u18b]

Dick.  You said they run fine by themselves.

Then the key question is:  do they run the same SPEED?

Put them on the track next to each other but NOT coupled.

Run them slow, faster, front, backwards.

Do they run about the same speed.

I'm predicting one is much faster than the other.  In that situation, an MU setup is pointless.  The faster loco is DRAGGING (or pushing) the slower one and traction is actually LOWER than even one sometimes.

[carmelmodelrr]

In answer to the comments so far:

The wheels definitely move--the loco runs solo and when pulling a tiny number of cars. 

One reason I replaced the motor was that the old motor was significantly slower than its "mate".  Both now are about the same speed (some variation, but nothing much).

I checked the height of the shell on the body and it seems to be sitting right where it should be.  I can feel the "click" when I replace the body.

I did try running the loco pulling about 6 or 7 cars on a wider radius track and there was more pull.  It definitely pulled 6 or 7 cars with some regularity, but it was far from acceptable (to me) for a 6 axle loco.  Furthermore, I had to really jack up the speed to get past the spinning at first.

I think I've covered most of the comments to date.

Dick Wroblewski
carmelmodelrr

[peteski]

Dick,
see http://www.atlasrr.com/pdf/N%20EMD%20SD35%20DIESEL%20LOCO%20B.pdf . If one or both universal ball joints at the ends of the worm shafts split, they would still allow the wheels to turn but they would start slipping under load. The loco would run but it would have very little pulling power.

[nkalanaga]

If you don't want to take the locos apart to check the universals, you might try this: 

Find a metal weight that will sit on the loco without falling off.  A square block or bar would be best.  See how many cars the unit pulls without the weight, and make sure the wheels really are spinning when it stalls.  You might take a Sharpie and make a mark on the side of one wheel in each truck to have something to look at.

If the wheels AREN'T turning when the unit is stalled, it's the drive train.  If they do, put the weight on the loco, and see if it can pull more.  If it can, the only problems left are lack of weight (you haven't carved the frame or anything, have you?) or too-slippery wheel treads.   Weight I can't help with, as that would probably need a new frame casting.  The wheels could have oil or grease on them, but that should hurt pickup, causing rough running.  You might clean them with alcohol or track cleaner to be sure.

If the trucks don't fit right, or the pilots are too low, the extra weight will actually make things worse, by pushing the body down, which should be very noticeable.  Since you've replaced the trucks, I doubt that they're the problem, although I've seen some with damaged latches that would sit too far into the loco, or would actually push the loco UP under power, disengaging the worm and worm gear.

The original motor problem sounds like you got two runs, the old "high speed" motor, and the new "scale speed". 

"I did try running the loco pulling about 6 or 7 cars on a wider radius track and there was more pull.  It definitely pulled 6 or 7 cars with some regularity, but it was far from acceptable (to me) for a 6 axle loco.  Furthermore, I had to really jack up the speed to get past the spinning at first."

That sounds to me like two issues.  First, binding in the worm/worm gear due to the sharp angle on tight curves.  I've seen that before.  Second, slick wheels, where the unit slips due to higher train drag when starting.  Once the cars are moving, they need less tractive effort to keep moving, and the unit can handle them.  It also takes more effort to pull cars on sharp curves, and you may simply have too tight a curve for long trains and 6-axle locos.  That would also be prototypical.


The marks on the wheels?  Some railroads used to paint white marks on tender and passenger car wheels to make sticking brakes easier to find.

[johnh35]

I tend to agree with the slipping under load school of thought. I had a similar problem with an SD-7 and noticed that while the wheels on one truck were spinning, the other truck's wheels were not. It was a cracked universal. With that said, i changed the flywheels to the type with a female hex from a donor engine and went with the older Atlas/Kato driveshafts. I wish they would get rid of those universals. If you do find the universal joint to be bad, be aware that they are designed to be installed in on direction (I.D. bevel toward the motor).

Edit: Forgot that you have an SD-35. I don't think they have the donut with ears (universal), do they?

[carmelmodelrr]

I've taken the latest suggestions and added weight to the loco.  That helped considerably.  One additional question, though.  If I add lead wherever I can on the body, is lead an electrical conductor?  Stated differently, do I need to be concerned about separating the lead on the left and right sides of the loco?

Additionally, I denatured-alcohol-cleaned the wheels and ran a bright boy over my track (which I almost never do because the track appears clean).  That, too seemed to help.

Thank you for all your suggestions so far.  (BTW, this loco does not have the donut with ears universal).

Dick Wroblewski
carmelmodelrr

[C855B]

...  One additional question, though.  If I add lead wherever I can on the body, is lead an electrical conductor?  ...

Yes. Maybe not as good as copper, but conductive nonetheless. Think electronics solder, which is roughly 40% lead.

[carmelmodelrr]

One final question.  The SD-35 does not have traction tires.  To improve "pull", would anyone consider putting Bull Frog Snot on the wheels of one axle?  Since the wheels don't have the groove for traction tires, I'm not sure if I should consider this. 

Thanks again.

Dick Wroblewski
carmelmodelrr

[peteski]

Dick,
this problem has me really stumped. The SD-35 should be able very easily to pull more than 7 cars on a level track (ore even on some grade).  Would you be willing to send this loco to one of the experienced loco-teardown modelers (like me) to see if we can figure out what is wrong with that model?