P&D Blomberg truck wheel wipers

[Bob]

P&D sells beautifully-detailed equalized brass Blomberg trucks in O scale. Their main disadvantages are that they are not all wheel pickup, and the wheels that do pick up power attempt to transmit it through a sliding, equalized wheel bearing and main bolster. Here is the truck assembly minus wheels and gear boxes. This one is for an EMD F3 (try to find the new wipers...) The GP9 units have a taller black bolster. (Note for those new to the forums: Click on an image to enlarge it. Some can be enlarged further after they open in Postimages, the A&O forum's image server.)



A previous solution I used incorporated phosphor bronze wipers attached by the gearbox screws. These tended to squeak, and 4 wipers were needed per truck.

Some years back Jay Criswell shared an elegant solution for the P&D brass trucks. The insulating pieces were made of black acetyl and required several passes to machine on a mill.

On Since then I've come up with a new design to add all wheel pickup on each truck.

The insulator is sawn from a flat rod of 1/4" styrene procured from McMaster-Carr. Two slots were cut in it to hold wiper assemblies. Each wiper is a strip of 0.005 phosphor bronze soldered to a small piece of 0.008 phosphor bronze for mounting, each with a 4-40 screw.





Tolerances are close. Any concern about shorting to other brass members can be mitigated with thin shims of 0.005 styrene glued to appropriate brass surfaces.





One thing to note: Removing the lower spring plank (is that what it is called?) prior to installation can release 4 small coil springs. Also make sure that the bolster screw is tight (a drop of Loctite wouldn't be a bad idea) so that it doesn't start to unscrew during re-assembly in the locomotive when tightening the bolster screw Esna lock nut.

I didn't mill anything to make this first prototype. I used a Byrnes precision table saw to cut and slot the styrene, and a small drill press to drill the 3 mounting holes. After soldering the PB pickup to the mounting tab, I inserted the soldered bit into the styrene slot and bent the tab to near 90 degrees. A pair of fine pliers finished the bend. It should be self-evident that the wipers still need to be bent to clear the axle gear boxes and contact the back of the wheels.

Now I need to get my, ahem, in gear and make 8 of these for David and 24 for myself.

All the best.
Bob

[Bob]

After considerable deliberation, I decided it was time for a plan B. The first design had a high chance of shorting to the truck frame, and it put the wiper pretty much at axle height. As the wheel rotates, it rubs at right angles to the pickup. That side force is not optimal and could cause the wiper to vibrate and squeak.

Plan B was inspired after a design by Jay Criswell. He mounted two milled blocks to the cast brass part that holds the two side frames together. I modified the idea by making a single block that can be glued to the truck.





The wipers are formed from phosphor bronze wire for the pickups, soldered to shim stock to hold it in place. The shim then screws to the white styrene block.

A custom fixture jig holds each piece of shim stock firmly in place and keeps it from spinning in the drill press, which would be hazardous to fingers. It also precisely locates the hole.







A shallow milled slot next to the black screw head provides a channel for the PB wire and keeps it from rotating (wire not shown.)

Note: Edited to correct terminology. A jig holds a part like a fixture but also guides one or more tools to perform an operation, in this case, accurately locating a drill bit.

[Craig]

Awesome design Bob!!!

[Bob]

Finally a little progress again on P&D Blomberg truck wipers now that the Denver O Scale National Convention has passed. Frankly, a lot of us were joyful but exhausted!

I'm still undecided about the best phosphor-bronze wire diameter for the P&D truck wipers. My first assembly used 0.032 wire and that was way too stiff. Here we see 0.020 wire and that seems better. Any smaller and I'd be concerned about the wear life.

Anyway, I came up with a better way to mount the assembly to the truck. Instead of contemplating the use of evil stuff such as 2-part epoxy, a simple bolt-on solution came to mind.



I added a pair of 2mm screws and washers to the underside of the assembly. This permits disassembly at any time in the future and holds things more securely. In this prototype part, the M2 screws are too close to the edges of the styrene, so the threads can be weak. Next time I'll move those inboard and depend on the edges of the washers to clamp everything down.



Note that this second photo was shot before adding washers on the opposite side. That's why the M2x4 screws are proud of the styrene. Prior to that, M2x3 would be preferred here.

[Bob]

On April 12, 2022 I finally installed the first new wheel wiper on David's F3 and F7 A units. The design changed a bit, and I found a really tiny solder lug to better manage the wires. I also moved the mounting holes farther outboard to avoid potential rubbing against the brass drive tubes. The drive line is all metal but fortunately the worm gears on the axles are Delrin.

Here's a before (original top) and after (lower with drive line installed.) These two trucks were from a dummy unit that had an early Soundtraxx DSX decoder. It normally ran with a Red Caboose GP9 painted in Clinchfield livery.



You can tell which wire was my first bending guinea pig. Now that I have dimensions they are quick and easy to bend without the need for precision jigs. The hardest part was learning how to re-assemble a fully dismantled truck without inventing new words. Some operations are best done with the assembly upside-down.

There's just one major problem with the drive kits. Notice how black the new wheels are? I don't know what they were thinking, but all that black gunk on the tread and wheel backs must come off. It behaves as a nearly perfect insulator. I put an ohmmeter on two points of the same wheel and got an open circuit. Everywhere! None of the pickups had continuity either.

What to do? I don't want to take them apart again, remove the wheels, and skim cut them on my lathe. A little hand sanding with 400 grit paper suggested that the insulating layer may be fairly thin.

I'll attempt fully assembling a drive, then run the chassis upside-down while powered by a DC lab supply. Perhaps the treads can be cleaned with Swiss pattern miniature files, a flat for the majority of the tread and a round needle to clean the critical corner where the flange meets the tread. Next, try to clean the backs with sandpaper. The treads have a lot of grooves from taking rapid cuts with a CNC lathe, so it is not likely that all the black gunk will come off, but all we need are clean high spots.

I don't know when P&D started receiving wheels with this insulating layer. The old dummy wheels from many years ago did not have a thick insulating layer.

[Bob]

Somehow during yesterday's work session one of pieces of tank drive line included in a P&D universal drive kit disappeared. Fortunately I had enough parts at home to rescue the problem.

A lower expanding joint was harvested from an old eBay purchase of a kit 2080K for a Weaver RS3. The shaft was way too short, but I was able to pull off the needed parts without damage using a Palmgren drill press vise in reverse. The tops of the jaws are proud of the vise itself, so I was able to hook the old drive shaft and expand it lengthwise until the expanding joint was released.

It was a simple matter to grab a bit of aluminum rod and turn a drive shaft that closely matched the length of one I have not yet installed in a P&D F3. Here is the part prior to cross-drilling for 1/16" spring pins. I later drove those home first mounting each in a mill chuck after cross-drilling, then driving each home using a 1 ton bench press. Below it is a genuine P&D F3 lower drive shaft that runs through the fuel tank. It uses a steel shaft that has been straight knurled to expand the shaft and engage the Delrin parts.



Should any of my F units fail with cracked lower universal "helmets" I'll turn another drive shaft that barely presses onto the turned part and pin it as well. My inventory of Delrin parts has already aged and achieved full shrinkage. Making them a tight fit then pinning them should work for many years after I'm gone.

[Bob]

Removing the insulating black coating seemed to be successful. Here is a before shot.



After filing the treads.



After using 180 grit sandpaper on the wheel backs. Notice the crusty black flakes that landed on the side frame.



By the way, I found way to make it easy to assemble the P&D universal joints. It can be a real struggle to get the spider in place. Just take a file and make a slight ramp on the end of the helmet prongs. The spider will now be able to spread the helmet and pop in place.