I recently purchased a three new Athearn RTR SD40-2s. I noticed one of them rocks a lot when reversing directions, or starting up. I tried shimming the area where the worm gear covers but that didn’t help much. The trucks aren’t warped, but I noticed one of them has a lot of play in the axle bearing area. I placed in on a window and a mirror (no plate glass) and was able to rock it a lot without the wheels coming off the glass…
I checked my other locos and have noticed some others have the same problem. The only other one I noticed was an atlas U33C.
Is it possible to shim the axle bearings on an Athearn to have less play without losing electrical contact for that truck? Or should I follow the advice of a MRR club member and use a high momentum when I install a decoder?
The issue is a tight gear train and a motor with lots of torque. Your best bet is to polish/file your gears, and replace your motor with a step-down type. Or…buy Kato units…they have much better drives and low torque motors. David B
Shimming the worm cover made no difference at all? The play is usually from a worn of poor fit of the “fingers” of the cover. The other area to inspect is the top of truck to frame bolster. The vertical play in bronze bearings willnot cause a rocking, the engine weight keeps them seated in the slots.
Try switching out a worm cover from another unit to see if it is a problem w/ that cover. If not there is a problem w/ the bolster bearing area either on the truck or the frame. Never seen wear or defect there an any BB engine, just excess grease affecting electrical PU.
My first run pair of P2K SD7s actually would derail as the worm cover fingers were so tight, there was no side play at all. Sanding the fingers until one truck had side play fixed the trouble. You have exactly the oposite happening.
David, Since your post I’ve cleaned/filed the gears, and am running the loco with some abrasive polish in the trucks. Seems to be reducing the rocking.
Bob, The shimming helped some. Just not enough. The bottom of the bolster is bare (not painted or whatever like the others). But that doesn’t really make since if these are new locos.
I’ll leave it running for a while. Then tomorrow I’ll remove the polish and report back with my findings.
Glad a little break-in is working. The absolute worst torque rock I ever saw was on the first Walther’s trainline units- OMG, My LS6 454 wouldn’t torque “rock” as much in the engine compartment when Power braking in drive. Of coarse cracked those secondaries and the wheels just smoked at standstill!
Bob’s observation is an important one, particularly with six-wheel trucks. Side play can’t be entirely eliminated or you’ll have problems staying on the rails through curves and turnouts. Most of my Athearn C-trucked units have a certain “chatter” rolling through turnouts, which seems to be the truck using this slack to keep on track. Most of my Proto SDs don’t have this rather obvious noise, but they nonetheless move those axles back and forth when negotiating track.
Katos do have their charms, but the SD-40s, in particular had their own set of problems with contacts. Certain details are rather behind the curve and there’s little to get excited about in HO scale. Nonetheless, they are good runners in general. But you ordianrily don’t go out and buy another loco to solve your problems with a particualr loco. At least my budget doesn’t support solutions like that.[;)]
With the shell off, does the chassis rock side to side excessively on the trucks ? If so, the problem is the mating surface where the chassis rests on the truck itself. The metal plate the chassis rests on may be slightly deformed allowing to much play.
One should be almost dead flat and the other should have a slight curve in it side to side to allow for changes in track. My guess is that both of them have a slight curve in them and neither of them are holding the chassis rigid. Even with the worm cover removed, you should be able to tap on the chassis and not have it wobble side to side - if it does, keep adjusting that truck plate the chassis rests on.
It ultimately should be set up like your freight cars - one truck should be relatively free to move side to side and end to end, the other truck should only be able to rotate - otherwise, they’ll wobble all over the place.
Of course an LS6 won’t rock; it’s a Chevy, and won’t generate enough torque to rock. And are you sure it’s not oil smoke. Sorry, I just couldn’t resist. [(-D]
I have found older Athearn units in the past where the sheet metal bolster rest has a very slight burr on the edge where it was stamped out. It may not rock in one direction but the other when it becomes the “forward” direction, changes the pivoting point and causes rocking.
With the trucks off, run your finger on the bolster rest and see if there is a burr then lightly file it away without getting the burrs into the gears and creating another issue for yourself.