What Causes That Loss of Power in Loco After Direction Change?

[tehachapifan]

This question, pertaining primarily to diesel models (as that's all I run), comes to mind every so often but I've never really came up with a good answer on my own. I'm talking about that complete loss of track power in a loco immediately following a direction change, after the loco has moved maybe a half inch to an inch or so. Sure, probably some crud on the wheels or track is playing a part, but why does it seem to mostly become an issue immediately after a direction change (and not necessarily because it's when the loco is moving the slowest). I've observed several occasions where a loco can creep along with nary a hiccup but, as soon as the direction is changed, power loss upon moving the other way. Then, a finger nudge gets it going again into a flawless creep until the direction is changed again. I do have a theory as to why this might happen with (some of?) the older Kato-made drives that did not have contact strips, where I believe the worms may sort of climb over the truck gears when they begin rotating the other way. This climbing to the other side (due to torque, I think) may cause the truck contacts to briefly separate from the frame. However, on locos with contact strips, this particular scenario shouldn't happen. Thoughts? Has this been discussed before?

[peteski]

Seems to me to be some sort of electrical pickup problem.
 I think you are right on the money with the older Kato engines, but the trucks in the newer locos (with springy pickup strips) also have some for/aft play, and they likely shift slightly on direction change. Then the sideframe pickup prongs also shift slightly on the pickup strips, possibly hitting some dirt or oxidation.  I like to polish the tops of those prongs, and the area on the pickup strips where those prongs contact.

[MK]

My WAG would be gear backlash.  Maybe not tight enough tolerance in the gear train so that when things spin in the other direction, they jam up, overcoming the torque of the motor.  Then Joe Human comes along to give it a nudge, enough to overcome the jam up.

[conrad]

I've had/have the exact same problem with my Kato and Atlas Diesels.  Drove me nuts as I thought that maybe the commutator was dirty, but, I solved it by wire brushing the wheels with the good old Minitrix wheel cleaner.

I don't think it's gear binding as I've got locos to move with a gentle poke on the side of one truck with a toothpick.  Also, Peteski is right, polish the the pickups.  I use a pencil eraser.

My opinion is that flywheels do really help with dirt spots when moving but not when they stop for reversing.  Maybe watch the loco's headlight when moving to see if there is flicker.  I also suggest looking very carefully, with magnification, at the wheels.  Dirt, dirt, dirt, the enemy of electrical contact.

Conrad

[tehachapifan]

Thanks for the replies thus far!

To be clear, this is an electrical pickup issue as it's a complete loss of electrical power that I'm talking about. Not to say that a mechanical issue is not somehow a contributing factor. And, yes, I'm very particular about wheel cleanliness. Sometimes this will happen with freshly-cleaned wheels and track. I am also aware of other contact points that need cleaning, such as axle cups and contact strips.

I didn't want this thread to be about needing to clean contact points per se, but rather about identifying the culprit that causes the majority of the power loss stalls or hiccups to occur immediately following a direction change. I think peteski might be on the right track with the truck contacts possibly shifting to a different contact point on the contact strips upon direction change. I bet that is a factor.

Peteski's example got me thinking...perhaps it might also be the wheelsets shifting from one side of the trucks to the other, causing a brief loss of connection to the axle cups? I have noticed that many/most/all(?) locos will sometimes/often/always(?) travel along the track in an ever-so-slight crabbing position (more noticeable on the shorter locos). This indicates to me that the gear torque thing I mentioned earlier is perhaps indeed occurring, which may shove the wheelsets from one side of the truck to the other (there always seems to be a little lateral play with wheelsets in the trucks, it seems). Maybe there's not a complete loss of contact with all axle points and cups, but enough to cause a problem if the axle cups are not completely clean. Thoughts?

...maybe the last example is the reason one mfr (Kato?) went away from the axle cup concept in favor of the thru-the-hole design?

[peteski]

But the through-hole axle doesn't have any better pickup (the hole is quite a bit larger than the axle). And there is more friction.  Also IMO the problem you describe is not common (I have never heard of anybody other than you having this problem on a regular basis).

What is even more puzzling is that all the contact points are redundant.  There are 2 truck-to-pickup strip contacts for each side, and then either 4 or 6 wheel to sideframe (and also track to wheel) pickup points, spending if this is a 4- or 6-axle loco.  Unless the contact areas are severely neglected (and dirty), there should be plenty of redundancy to keep the model going.

If you have one loco with most severe example of the problem, take it fully apart and clean and lube all those contact points, then see if that solves the problem.

[tehachapifan]

Thanks, peteski.

I wouldn't say I have a significant problem with this, though I don't disassemble locos to the point of cleaning out axle cups as often as I perhaps should or as others may do. It just occurs to me from time-to-time that, when a loco is entering a point in time when it probably needs a thorough cleaning, the first sign of it is a loss of track power/pickup upon direction change. It was this that made me curious as to the mechanism behind it. I think we've identified some possibilities here.

[peteski]

Just don't put off the maintenance for too long. 


Great thing about the screw-held split-frame loco design is that you can pop the shell, fuel tank, slightly loosen the screws and the trucks just fall out for servicing.  Then each truck also pops apart easily.  The springy contacts can also be easily removed/polished.  You can even lube the worm shaft without disassembly (you need a hypodermic tube oil applicator like Labelle oil bottles have.

So, there really aren't many excuses for not cleaning the electrical contact points.  You only need to fully disassemble the loco when it comes the time to lube the motor.

[tehachapifan]

...So, there really aren't many excuses for not cleaning the electrical contact points.  You only need to fully disassemble the loco when it comes the time to lube the motor.

True. I guess, for me, disassembling a loco is more about messing up the often-tedious job of getting all the wires for lights, etc., properly tucked away when (re)installing the shell. This being magnified more if the loco is an SP unit (with lots of lights).

[woodone]

While I have seen this problem (stalling) after a direction change . I don’t understand pick-up issue not being a a loss of power?
If your loco does not pick up power from the rails you have a complete loss of power. When you nudge it ,it starts to run.
There are several things that will cause this. First is- the motor must overcome friction- this takes more current, it also has to over come any other friction losses, all which take more current. So if there is a poor connection the the electrical path the motor will not run.
Motor bearings might be dry. Might have some effect? But all goes back to a poor or no electrical connection between the track and the motor. There might be more that just one problem, they add up to a loco not running.
I know most of us have had a loco that would not start- nudge it add off it goes and does not give you another problem for days, some times weeks. WHAT CHANGED?
Electrical pick up— don’t forget electricity will take the path of least resistance- so even though your loco has several points of contact there is but one carrying the load. If you have 6 wheels touching the track I doubt that each of the wheels is carrying 33 and 1/3 of the current-.
I am a bit puzzled about Kato having through holes on their wheel contact strips.
 I would think that would be like putting on a break on each axle point, plus the wear factor would really go up.
I have had many Kato’s apart and yet to see this type a axle contact.
For trouble shooting- if you can get your loco to stop and then not run( Murphy’s law will step in here) take a very sharp needle with a test lead hooked to it from one rail and very gently touch a wheel on the same side of your test lead and I will guess the loco will start.
I am thinking poor contact some place in the electrical path. JMO

[Mike C]

   @woodone   I believe that this is a very well worn pick-up . I think the holes are from the axle point wearing away the contact strips ........Mike

[peteski]

Woodone, the photo showing holes in the bearing cups was just an extreme example of what can happen when model is ran for extended period of time with no maintenance/lubrication.   It was a loco of one of my NTRAK club members - he neglected servicing for a very long time (and ran the loco for hundreds of hours).  He finally gave it to me for a tuneup, and that was what I found.  I just posted it here as a warning of what can happen when a model is not maintained properly.

As I understand the total loss of power tehachapifan's locos experience is purely contact-related. His locos are DCC and I believe that the decoder loses power (headlights, motor, and sound all cut out).  At that point the motor does not have any power to overcome any friction or to move enough to reestablish electrical contact.  But as I mentioned before, I'm really surprised that this happens when the model has multiple redundant electrical contact areas.  I think those models need a thorough tuneup of all the electrical contact areas (not just wheel cleaning).  I  think tehachapifan neglects to do that due to the difficulty with taking the models apart due to a rats-nest of wires inside.

[tehachapifan]

peteski has it correct, though I wouldn't categorize my locos as neglected per se. They are very well cared for but I will admit I don't clean axle cups very often. However, I did recently clean some axle cups on one loco and, yeah, they were very dirty. Those pointy Tamiya Q-tip thingies work really good for that, incidentally.

[peteski]

peteski has it correct, though I wouldn't categorize my locos as neglected per se. They are very well cared for but I will admit I don't clean axle cups very often. However, I did recently clean some axle cups on one loco and, yeah, they were very dirty. Those pointy Tamiya Q-tip thingies work really good for that, incidentally.

I didn't say that your locos as a whole are neglected. Maybe I should have phrased it a bit differently.

"I think those models need a thorough tuneup of all the electrical contact areas (not just wheel cleaning).  I  think tehachapifan neglects chooses not to do that due to the difficulty with taking the models apart due to a rats-nest of wires inside."

How about taking one of the offenders and giving it a good electrical cleaning. That means taking the trucks off, taking them apart and cleaning/lubricating them), also polish the truck contacts which touch the pickup strips, and also polishing the surface of the pickup strips where the truck contacts are contacting. Also clean the wheels.  I'll be curious whether that takes care of the problem.

Speaking of wheel cleaning, how exactly do you clean them?

[tehachapifan]

I really didn't want this thread to turn into a loco cleaning thread or in some way indicate that I don't know how to properly clean a loco. My fault if I didn't make this clear. What my question was intending to ask is this...

If a loco is getting to a point where wheels and electrical contact points are perhaps needing to be cleaned, why is one of the very first signs (or sometimes the only sign) a complete loss of electrical pickup immediately following a direction change (forward to reverse or vice versa), where the loco travels a half inch to an inch or so before losing pickup?

Hope this is clearer. We were on the right track with what I was looking for when talking about parts moving/shifting when changing directions.