Loco Surging Question

[haasmarc]

I didn't know which forum to put this in because I don't know if it is a mechanism issue or a decoder issue so feel free to move it if it is in the wrong one.

I am test running a couple of the latest Intermountain SD45-2's.  I had to do a lot of cleaning and polishing on the pick-up points to get them to run decent.  Now that they are at least running and not stalling constantly, I have noticed an issue which is not just with these units.  I have seen this on several other different locos, of different manufacturers, and have never figured out what it is.  At a certain speed range, sometimes in the first couple of speed steps, sometimes in a set of steps higher up, the engine will start oscillating.  It will look like it is bouncing back and forth like a freight car with Micro Trains 1015s.  I can have multiple locos from the same release and some will be smooth as silk through the whole speed range and others will have this bounce at certain spots.  Once you get past the spot, the engine runs smooth again.  Slow down back down into the certain range and it starts bouncing.

Has anyone figured out what causes this?  Is it a mechanism issue, is it a BEMF or some other decoder setting?  Or is it a sketchy decoder?  I run mostly ESU and BLI decoders.

I could try swapping decoders between the units and see if the range moves with the decoder or stays with the engine. but I thought I would see if someone has already figured it out before I go to the trouble.

[peteski]

Can you test them on DC with decoder out of the picture?  Your description though seems ot point to the decoder (BEMF?). But it is also unusual that you can see the same behavior with multiple brands of decoders.  What is BLW decoder?

[haasmarc]

What is BLW decoder?

Sorry, I meant BLI.

[haasmarc]

Another question related to these locos.  Can the Intermountain trucks be swapped out with Atlas or Katos?

[sizemore]

I’ve seen the same thing most recently with the C424 (Soundtraxx), two run perfectly but one unit likes to hunt around throttle setting 14. And have seen it with other brands of locos and decoders.

I don’t think it’s a decoder issue, but moreso a motor signal output at a given voltage and the decoder is responding in kind.

Want to say not all decoders turn off BEMF when in consist, or may use a reduced output of BEMF while in consist. You could turn BEMF off, set a straight 3 point speed curve and then test. With BEMF off you may get a completely different throttle response (typically much higher throttle) and have to find that spot where it may have hunted.

The S.

[haasmarc]

Interesting that you see it in the 424 with the Soundtraxx decoder.  I bought the silver version and put ESU's in mine and haven't seen it in mine.  I've seen it in some Atlas FA's, BLI F7's and others that I can't remember now.

[peteski]

I wonder is since the problem shows up in multiple brands of models and  decoders if it is somehow related to to the dcc system itself as that seems to be the only thing in common. Do you have other locomotives which do not exhibit this problem? For example, put one of the misbehaving locos about a foot away from a normally working loco and temporarily consist them so you can run them from a single throttle. Then run them and observe if the "bouncing" only happens in one loco?

As far as disabling BEMF in advanced consist I think only Digitrax decoders do that.  That only applies to advanced consists (which IIRC is not a default type of consist with digitrax).

[chessie system fan]

Another question related to these locos.  Can the Intermountain trucks be swapped out with Atlas or Katos?

Intermountain used Atlas SD50 trucks on these. So, yes.

[sizemore]

Interesting that you see it in the 424 with the Soundtraxx decoder.  I bought the silver version and put ESU's in mine and haven't seen it in mine.  I've seen it in some Atlas FA's, BLI F7's and others that I can't remember now.

Yeah I didn’t expect to see it either and haven’t dug into it enough. I’ve seen the hunting predominantly in Atlas (split frame) stuff with Digitrax decoders. I have a Bachmann Class J (2k8 release) with Loksound and have the hunting in the upper single digit speeds. Even after auto-tune and tweaking I can’t get it to go away. I think the Achilles heel is the Class J is a 3 pole motor.

If you have JMRI, make a new entry for each SD-45-2 and see if they have close/similar numbers after auto-tune. Assuming the trucks aren’t binding, might indicate there is enough variance in the motors. Alternatively you could probably turn down the “kick” of BEMF and see if it goes away.

The S.

[haasmarc]

Well, doing some more troubleshooting on one of my units.  It definitely has a drive train issue with the rear truck.  I've swapped trucks and the issue is still in the rear.  Feels like not enough gear mesh with the worm.  There is a rhythmic clicking and the wheels actually stop rotating for a split second.  I've taken it apart and nothing stands out.  In the product SD45-2 thread, someone said Intermountain was waiting on a truck redesign and that was what delayed them.  I'm wondering, if the truck was redesigned, if the tolerances on the gear mesh were not kept.  I would like to try and swap in an older version of the truck (maybe an Atlas one) but I do not have any.  I only do more modern stuff in R&N so I do not have a fleet of these that I could rob some trucks.  I do have a modern Atlas SD50, but the trucks are wired making them not an easy swap.  It looks like Atlas is out of the truck assemblies in their parts catalog.  I'll have to see if I can scrounge some up to try.

[garethashenden]

Does the truck roll smoothly on its own out of the locomotive?

[u18b]

Wow....
When I saw "bounce" in the title...  it brought back memories of the infamous "hopping" reported (but never fully documented) from early Atlas SD50s.

Given the description in the original post, maybe bounce is not the best term?

Could it be said the loco is surging and hesitating?

It does sound like a decoder issue as described.

[fcnrwy23]

Your term of "Bounce" is a little vague.
Is the "Movement"...
UP and Down...
OR
Side to Side.. 

And Yes, (as stated before).  The Trucks are Atlas manufacture...

Jerry

[peteski]

I don't know about this particular loco but some older IM locos used Atlas parts. Even the part number on the exploded diagram were identical parts used in Atlas locos.  They probably shared the same manufacturer in China.

I also agree that bouncing is not the best term to use. Surging or hesitating might be better. Maybe @haasmarc could edit his initial post and  change the subject line?

[sizemore]

The parts diagram for the SD-40/45T shows a hexagonal engagement with the flywheel but the worm is ball/cup. Atlas SD7/9’s had ball/cup on the flywheel side which was a poor design, I think even @u18b even wrote an article about it. Got an SD-9 on the bench now I need to switch over to SD-35 driveshafts.

If this is a mechanical issue check the flywheel engagement with the driveshaft, and driveshaft ball/cup from the worm.

The S.