speed control on grades / train developed jerky movement on down grade

it's only noticeable at slow speeds (lower than 40 smph)

Maybe the other 3 budd cars are pushing the front engine, Does it happen  with the engine alone?

 If so I don't know what the fix would be... you can't tie a hand brake on the  trailing budd.

Hard to believe the other cars are causing this, else any locomotive would have similar issues on a grade.  I'd start by looking at mechanical issues.

Does it happen with engine only?...I've run across this before but before the days of cruise. (surging).  Sort of like a lash-up where the trailing unit  runs faster than the lead and wants to push the lead unit. Of course the lead unit is having no p[art of being pushed and fights back. Are the dummy budds acting like the trailing unit on the steep downhill grade??

Thanks for the responses.  It behaves the same with or without the consist. I'll pop the shell after work and run it stripped to see if I can identify the problem. If I find anything I'll report back. I appreciate the help. 

Check the truck motor screws, if they are loose, it will do all sorts of odd things.  If you have to top off, also make SURE the motors are not wiggling at all in respect to the actual truck body.  The two motor mount screws and the single truck screw are frequent offenders.

So much for my surging theory. It definitely  seems to be an engine problem. Maybe turn the fly-wheels by hand and check for anything binding. ( Iron filing in gears etc)   Are all the traction tires in place? 

This is actually a fairly common problem on downhill grades.  At least in other scales.  It has to do with excessive play (sometimes called "gear lash") in the worm/worm gear mating.  If there is enough play, the worm will thrust away from the worm gear, disengaging temporarily, then settle back in and engage again.  Hence the jerking.

I have no idea how this works with truck mounted motors, as opposed to the typical smaller scale single motor horizontal drives.  But I believe even truck mounted motors transfer power via a worm gear, so it seems like a plausible consideration.

The typical solution involves adding thrust washers to take some of the excess play out of the drive line.  

Jim

My Lionel Veranda Turbine woudl do the same thing at certain times and it was the truck going over uneven track causing the flywheel to hit the handrail that was passing through the body.

Sounds like a speed sensor to me but I have no idea how to trouble shoot an MTH engine. Hopefully someone will know a procedure for this.

gunrunnerjohn posted:

Check the truck motor screws, if they are loose, it will do all sorts of odd things.  If you have to top off, also make SURE the motors are not wiggling at all in respect to the actual truck body.  The two motor mount screws and the single truck screw are frequent offenders.

Would be interesting to know if same behavior in reverse going down the previously uphill grade.  Also if you face the engine the other direction does it jerk in forward going down the previously uphill grade.  I realize that "other" grade may not be the same 3% but you get the idea of what's being asked.

Do you have a conventional-mode transformer with Whistle/Bell buttons?  If so you can operate in conventional with speed-control turned OFF.  This simple procedure takes the digital tachometer out of the equation making the engine run proportional to track voltage alone.  This engine will speed up and slow down going up/down grades and around curves but does the jerki-ness changes for the same approximate speeds in forward and reverse?

big train posted:

This is actually a fairly common problem on downhill grades.  At least in other scales.  It has to do with excessive play (sometimes called "gear lash") in the worm/worm gear mating.  If there is enough play, the worm will thrust away from the worm gear, disengaging temporarily, then settle back in and engage again.  Hence the jerking.

I have no idea how this works with truck mounted motors, as opposed to the typical smaller scale single motor horizontal drives.  But I believe even truck mounted motors transfer power via a worm gear, so it seems like a plausible consideration.

The typical solution involves adding thrust washers to take some of the excess play out of the drive line.  

Jim

Yes it's called bucking, and it sometimes rears its ugly head.  It can be exacerbated by closed-loop speed control systems like PS3, especially when combined with self-locking worm gears.  The system may sense the train speeding up and abruptly cut the power.  Depending on the timing and gear lash, this may cause a jerk or lurch.

Some might recall that brand 'L' struggled with a lurching problem in its early speed control efforts.  Many recent products from that other brand have back-drivable gears (the wheels can turn the motor, with some friction) and are much less susceptible to this phenomenon...

I'd eliminate the obvious mechanical possibilities before attacking the speed control, etc.  I've seen them run fine in one direction and lousy in the other, and simply tightening the motor truck screw or the mount screws was all it took to fix them.  That's an easy thing to check before chasing other causes.  I also hand rotate the flywheel in both directions for a full rev of the drive wheels on each truck to see if there is any binding.

Not quite a direct connection to this discussion but related on some level. Some running conventional locos may even find it useful.   Forty plus years ago, long before TMCC I built a layout that had very little level track. It was either going up or going down except for some reverse loops that were flat. You simply had to ride the throttle continuously. Too little throttle and trains would stall going up hill too much and they ran off curves on downhill grades.  One day I was over in Dads shop when he was cleaning up. I went over thinking he would throw out some really good tinker toys.  One of his discarded treasures was a box with 15-20 mercury switches inside.   When I got home, sitting next to my work bench and going through my booty I hit on an idea.  I had a prewar 201 switcher that I had put a string of  back to back diodes in series with the commutator because the thing would start running at a lower voltage than the e-unit took to cycle and once the e-unit drum finally came into position the loco would lurch forward. So the diodes raised the amount of track voltage needed to run the motor to more than needed to cycle the e-unit and once the e-unit was engaged the throttle could be further increased and the loco would ease away. While sitting on my work bench stool the idea of using one of dads mercury switches to jumper around the diodes when the loco was going up a grade just popped into my head. So, the next evening I figured a way to mount the switch in the loco on a piece of spring brass with an adjustment screw for fine tuning.  To be honest, the system does not work if you try and back up a hill, but Lionel trains did not back up hills very well to begin with at that time.  In it's essence you hook between 3 and 5 diodes together in a string then make a second string like the first. Now wire them up parallel  + on the first string to - on the second string ,  connected at both ends. You can accomplish the same thing twisting the diodes together back to back in pairs and hook 3-5 of those pairs together. Next you will take your mercury  switch connections and hook one to each end of your string of diodes. Then disconnect one of the wires going to the motor brushes, doesn't matter which one and connect your string of diodes and your mercury switch between the motor brush and the wire you took off the brush guide.  You want the contact end of the mercury switch toward the rear of the loco so when the loco starts up hill the mercury rolls to the rear and closes the  circuit , thus jumpering around your diodes. I found that it worked best if the glass tub when the loco was sitting on level track was tilted down toward the front of the loco enough to keep the mercury from flowing to the contacts upon start up.  Another trick I found was to rotate the glass tube so one of the contacts was higher than the other this made sure that a small surge of mercury that only contacted the lower conductor did not complete the circuit No jackrabbit starts either. J

Neat, but clearly not related to Command Control trains with Cruise technology G

Ted Sowirka posted:

big train posted:

This is actually a fairly common problem on downhill grades.  At least in other scales.  It has to do with excessive play (sometimes called "gear lash") in the worm/worm gear mating.  If there is enough play, the worm will thrust away from the worm gear, disengaging temporarily, then settle back in and engage again.  Hence the jerking.

I have no idea how this works with truck mounted motors, as opposed to the typical smaller scale single motor horizontal drives.  But I believe even truck mounted motors transfer power via a worm gear, so it seems like a plausible consideration.

The typical solution involves adding thrust washers to take some of the excess play out of the drive line.  

Jim

Yes it's called bucking, and it sometimes rears its ugly head.  It can be exacerbated by closed-loop speed control systems like PS3, especially when combined with self-locking worm gears.  The system may sense the train speeding up and abruptly cut the power.  Depending on the timing and gear lash, this may cause a jerk or lurch.

Some might recall that brand 'L' struggled with a lurching problem in its early speed control efforts.  Many recent products from that other brand have back-drivable gears (the wheels can turn the motor, with some friction) and are much less susceptible to this phenomenon...

I had the same problem with a Lionel 7795 0-8-0 from a starter set which had no speed control of any type. As near as I could figure, going down a grade unloads the motor, which causes it to speed up. But the train also speeds up due to gravity, so if the train starts running faster than the motor wants, the gearbox can't accomodate the reverse power flow and friction takes over.

Or so it seems to me.

PLCProf posted:

if the train starts running faster than the motor wants, the gearbox can't accomodate the reverse power flow and friction takes over.

 ...And that's why back-drivable gears are a worthwhile improvement.  A little friction from the worm is unavoidable, but self-locking gears, an undersized flywheel, and a less-than-ideal damping interval for the speed control feedback loop will cause the phenomenon described by the original poster. 

Ted Sowirka posted:

PLCProf posted:

if the train starts running faster than the motor wants, the gearbox can't accomodate the reverse power flow and friction takes over.

 ...And that's why back-drivable gears are a worthwhile improvement.  A little friction from the worm is unavoidable, but self-locking gears, an undersized flywheel, and a less-than-ideal damping interval for the speed control feedback loop will cause the phenomenon described by the original poster. 

On a semi-related tangent-

A series motor, like the pull-mor, has a very high no-load speed, limited only by motor friction and losses. So a pull-mor driving a worm gear can almost always run fast enough to prevent "gear lock" in the worm drive. A PM motor, on the other hand, has a definite no load speed set by the voltage, so it can not "accelerate its way" out of gear lock.