Thumping in Drive Gear

[woliners]

My 2-10-2 Loco (Spectrum) makes a "thumping" noise as it moves on the tracks. It also seems to jerk or limp a little bit in rhythm to the thumping. None of my other Loco's do that so I know the track's OK.  I can't see anything that would explain the noise as it moves on the track.  It's hard to troubleshoot when I run it on it's back (in a repair rack) because the thumping disappears and the wheel movement appears smooth as far as I can tell. 

I've disassembled the drive gear and, to my untrained eye, I cannot find anything obvious. No apparent or uneven wear of axles or  axle journals.  All the linkage appears well aligned, lubricated, and moves smoothly without interference. i.e. nothing bent or hitting anything else.  Wheels pairs are parallel and properly spaced and the left and right side connecting pins are all offset correctly (relative to each other) and there's no slippage of wheels on axles. Drive and worm gears don't appear worn or damaged and are well lubed. And finally, the motor runs smoothly.

I understand it's a precision piece of machinery and, as such, beyond my skill set to completely analyze.  Another option for me is to replace the Drive Gear assembly. They're going for $25.00+shipping from the Bachmann parts store as opposed to $45.00+(who knows what) to send it in for repair.

So what am I missing?

Bill

[jonathan]

Hmmm,

Did you open up the motor assembly, or run it while still sitting in the frame halves?

Assuming your drive train/valve gear assembly works fine per your post... I'm guessing the thumping and limping comes from something caught inside the motor assembly.  Even a split belt can cause those symptoms.  When upside-down, the pressure is relieved from the motor assemby, which makes the noise go away.

Just to help us, has your 2-10-2 always done this, or is this a new symptom since purchase?

Thanks.

Regards,

Jonathan

[woliners]

The frame isn't designed in halves, it just one complete chassis. There's no drive belt either, just a worm gear, idle gear and drive gear config. The motor and idle gear are mounted on top of a single chassis assembly.

I've run it with progressing assembly: first the motor by itself (unmounted and then mounted with no load), then added the idle gear, then added only the geared wheel pair, and then added one pair of wheels at a time (with side rods) and then added main drive rods and then the peripheral valve gear rods........nothing sticks out.  But I think you are right, running it upside down relieves whatever is causing the noise. And the more rolling stock I add, the more pronounced it is.

If worse comes to worse, the the Bachmann parts catalog has the entire chassis assembly for $65.00 or just the lower portion (no motor or worm gear) for $25.00 or I guess I could just send it in for $45.00.  I was hoping there was some common issue that other people have encountered (and fixed). 

The noise started after running it for quite some time, however, I bought it used on ebay so the warranty won't cover it.