Derailing SD7

[yankee flyer]

 Hello All.

I'm unsure what anyone can do to help me on this problem but with all my other questions I have received help on this forum.
I have a SD7 that derails on one and only one turnout, every once in a while. It wants to turn off when I want it to go straight. None of my other locos have any trouble with this turnout, and I have a pair of ALCO PAs with six axles.Even the new 2-6-6-2 sails through.
The main line comes out of a left hand 21" radius curve, as soon as the rails straighten out there is a right turnout . Ever so often the SD7 wants to take the right hand turnout.
The trucks turn freely in all axis And the switch blades lay solidly again the rails. The turnout is a # 6 Atlas. I can not determine if it is picking the points are just jumping the switch. 

Lee

[selector]

With what you describe, it is sure to be a problem with the gauge of the axle that causes the detour.  That is not to say the points couldn't be tuned to lie better against their stock rails...maybe, maybe not in this one instance....but if all other items go through the turnout fine, it kind of points to this one exceptional engine, and the item that determines which way the engine goes is the axle that does not conform to the configuration of the turnout.

Do you have an NMRA gauge?

When I reply to posts like yours, I try to be thorough.  It could also be the approach to the turnout.  I had one engine that kept derailing when it entered the diverging route, went through the frog and was to exit at the points end.  I couldn't figure it out until I used something with a good planar side and placed it on the rails approaching the turnout from the diverging route. I found a dip there that twisted the engine sufficiently that it lifted the leading driver axle up and over the frog....instant derailment.  So, you might want to check how planar, or matching to the general area's grade, your turnout approach is at the points end.  Place something with a smooth surface down on the rails and shine a light behind it toward your eyes.  You'll soon know if you have a substantial dip.

-Crandell

[Heartland Division CB&Q]

I have two orignal production Life Like Proto 2000 SD-7's. Is that what you have? Later production runs of Proto 2000 SD-7's and SD-9's do not have that problem.  

My two original SD-7's will derail where no other engine will do that. I checked wheel gauge and so on with the engines. I suspect the trucks are just a little too snug and need to be able to rock slightly, but I do not know that for sure.

I have reworked the track and replaced turnouts. After a lot of track work, the engines stay on the rails.

 

[dgwinup]

Three-axle trucks are often prone to picking the points of a turnout when coming out of a curve.  With three axles, there is a lot of force on the outside rail of a curve.  When the loco comes out of the curve, the truck needs to straighten out to relieve the outward force.  You may not have enough straight track before the turnout to allow the truck to fully straighten out.

Is the 21" radius curve also on an upward grade leading to a level turnout?  This will compound the problem.  Not only will there be outward force on the trucks, there could be a slight hump where the curve joins the tangent.  The front axle of the truck will actually lift up off the rails, allowing the outward force from the curve to cause the front wheel to derail on the same side as the outside of the curve.

Start by checking the gauge of the wheels on the loco, then the gauge of the turnout.  If these are correct, the problem is in the geometry of the track and turnout.  You will have to re-align them to eliminate the problem.

Hope this helps.

Darrell, quiet...for now

[yankee flyer]

 

 

dgwinup

Is the 21" radius curve also on an upward grade leading to a level turnout? 

Track is on level ground.

dgwinup

could be a slight hump where the curve joins the tangent. 

There seems to be a consensus, that there is a dip or bump in the track ,and I will check that out as soon as I get time. I had another place were water got to a seam in the plywood and raised a little bump.
I'm playing lumber jack in my 1/4 acre back yard today But I would bet you guys are right,

Thanks 

Lee

 

[yankee flyer]

By George I think I've got it! 

It looks as though I had a small dip in the track just before the turnout. Actually there may have been two problems. These Atlas turnouts are not very tight at the pivot and as the loco drives through from the point end,  the blade can lay over at the pivot so that the wheel flange meets the blunt end of the fixed rail. This does not help the loco stay on track. The six axle locos can now go through at run 8.

Another problem solved by the crew on this forum. Thanks 

Lee

[BerkshireSteam]

So the turnout is directly after the curve? Could that be the problem? I would move the turnout away from the end/begining of the curve at least the distance of your longest rail car.

[bogp40]

As GARRY mentions earlier in the replies, the trucks pivot freely but have no side play. I experienced this same problem with 2 of my "first run" P2K SD7s. Upon contacting Lifelike, in Balimore them, I was told that the trouble stems from the fingers on the worm gear cover were creating a tight truck not allowing any side play. New worm covers were set to me, however, I solved the problem by lightly sanding the tabs of the cover, always checking as not to remove too much material. This should solve the derailing problems. Do use care  not to remove too much from those tabs. I found that dragging the fingers across fine sandpaper to be the best method.