Hymek First Aid
In our previous installment, I'd got hold of some spare worm wheels from BR Lines and installed them in the loco but this had not cured the uneven running which I still thought was caused by the wormwheel skipping on the worm (hence the trashed original wormwheel)
We have AbRail coming up on Saturday, so I finally got back around to looking at the problem... with visions of needing to make new worm shaft bearings or some such nasty job.
Further investigations and testing with just one wormwheel installed at a time confirmed that the wormwheel mesh was indeed the issue. On one bogie it was intermittent (hence the jerkiness). On the other bogie the motor was free to spin with no drive imparted to the bogie at all.
Now there is some play in the worm shaft bearings but I could not believe that this was enough to cause the problem. With the thing stripped down and laid on its side I checked out the worm mesh... and realised the cause of the problem.
To cut a long story short, the phosphor bronze strips that pick up power from the bogies had too much 'spring' in them, probably a misguided attempt on my part to ensure good pickup from the bogies. This meant that the weight of the loco was being carried on these strips and that the bogies were being pushed to the bottom of the travel allowed by the pivots.
The fix was to bend the strips much more carefully so that they still contacted the bogie pickups but without enough 'spring' to carry the weight of the loco. Roughly speaking they need to be fairly level in the chassis when the bogies are removed. With this setup the loco weight is taken by the top surface of the bogie pivot, so the bogies sit as high as possible in the body and the worm mesh is as tight as the pivots will permit. The photo shows one pair of the adjusted strips.
There still seems to be some audible 'chatter' so perhaps I might check out the mesh more closely. Filing the pivot tops would close up the mesh but would not be reversible if I get it wrong, so I think I'll probably leave things alone for now.
The other slight problem in November was that one half of the chassis block had snapped in two. Remember this?
Solution? Mill out a slot in the chassis, make some threaded holes and fit a metal 'splint'? Err... no.
Here is my precision engineering fix for this ...