DCC Loco Conversions - 22 - Dublo & Wrenn Brush Holder Extraction

The open frame motors used on many Dublo locos present a different issue when considering conversion to DCC, like many RTR locos the chassis is used as part of the power circuit and one brush or brush holder is connected mechanically and electrically to the chassis. Isolating this can be achieved with a little work especially removal of the original brush holder tube.

Fitting an insulated replacement tube may be performed in multiple ways and actually requires more work than the extraction.

The method I have devised to remove the original tube is quite simple and does not cause damage to the hole in the chassis which can occur when trying to drill it out. What I do is to screw jack it out with the aid of a simply made up tool, a piece of studding, tube, nut and a washer.

Scenario

Dublo locos are built to imperial dimensions, i.e. inches and British screw sizes, luckily materials (e.g. tube) and screws are still available in imperial sizes to suit the US market and are still popular with UK model engineering enthusiasts.

The brush tube bore is nominally of 1/8 inch (3.18mm), which is close enough to the tapping drill size for a 4BA screw of 3mm.

To date I have found there are two ways the bush holder tube may have been made.

1) on an early 2-6-4 tank loco I have found that the tube is formed from sheet with a gap running the length of the tube which is partially closed by compression when inserted into the chassis.

2) the most common, a solid drawn tube, i.e. it has no visible seam or joint and has the one end knurled for cutting into the mazak alloy for a firm grip. They do vary in length, so just make the replacement the same length, length will affect the compression of the brush spring, excessively tight or loose will affect motor performance and brush wear rate.

The two types are illustrated below :-




Extraction Process - Preparation

1) Remove the magnet and pole pieces, if Alnico either attach a steel bar across the magnet via the pole pieces and remove carefully to minimise loss of magnet strength.

Otherwise re-magnetise on reassembly or replace with a NEO magnet, but if it is a NEO just be careful not to let it escape and damage itself against the pole pieces, they are brittle and dislike impacts, eye protection should be used.

2) Remove the tube plugs, springs, brushes and motor armature, with the top bearing inverted or at least horizontal to avoid losing the top ball bearing. Having the bearing at least horizontal while unscrewing will help prevent loss. These tiny ball bearings evaporate like drops of water on a hot sunny day!

3) Use e.g. masking tape to prevent swarf from the extraction process getting into the lower bearing and onto the driving axle gear wheel.

Extraction Process - Extraction

4) This depends on the type of tube. If it is the formed type your luck is in, just inserting the 4BA tap into the tube and it biting will allow the tube to be extracted with a little twisting and wriggling, not too hard though as taps don't bend, they are more likely to snap. Then skip to (11).

If however it is the drawn type more work is required as follows.

Run a first (or taper) 4BA tap into the brush holder as far as you can go, the tube is approximately 0.8 inches long (on e.g. Montrose, 2-6-4T, and probably others according to the Dublo service sheets which share the same part number) so the tap may only just show it's tip at the armature end.





Not a problem, in fact it is useful as the studding can grip the tube better and the brush holder tube is really only a tight fit to the chassis at the plug end.

When cutting brass, and the brass tube used by (Meccano) Hornby is a little on the hard side the cut material comes away as very fine chips and clogs the tap quickly when thread cutting without a lubricant. I preferred to cut this dry to avoid the spread of lubricant and swarf within the loco chassis.

If you can hold the loco chassis such that you are tapping upwards then the swarf drops out onto the bench instead of towards the armature space.

First time I did it with the loco chassis on its wheels. During the progress of cutting the thread I needed to remove the tap completely a couple of times to unclog the tap.

Either way back off frequently to aid swarf clearance, patience is a virtue, a broken tap is a real pain!

5) Run a length of 4BA steel studding into the tube and nip up with fingers, or lock together two nuts as shown below and use a spanner on the outer nut, if it starts to rotate when extracting in the following steps it will just bite further into the partially tapped armature end of the tube. The length I used was 55mm long (what I had to hand).



6) Remove the nuts if used, and add a metal tube with a bore and length large enough to contain the brush holder tube, courtesy of my "that'll be useful some day" (or decades later) drawer.

7) Add a washer and nut.



8) Slowly tighten the nut with a nut spinner or a spanner whilst ensuring the metal tube is aligned correctly so the brush holder tube will be drawn out into it.

9) Once the brush holder tube has been drawn out about 0.3 inch it will become easier as that section is knurled which can be seen in the first image above.

10) Once loose the brush holder, studding and its metal tube, can easily be removed.





11) Job done, chassis hole undamaged, damage is easy when I originally tried drilling one out of a scrap chassis.


Fitting the Replacement Insulated Tube

How the replacement insulated tube is implemented depends on what approach you choose to take.

I will cover some options in the next article (#23) and how I did it.

https://platform1mrc.com/p1mrc/index.php?threads/dcc-loco-conversions-23-dublo-wrenn-brush-holder-replacement.6025/


Discussion always

Jim

Return to Index

 

Hi Keith

I had a brief look at that possibility but the choices of reamer are bewildering and the sites I have looked at so far assume you know their dimensions, I need some guidance on that.

Certainly would be a better hand technique even if a few tapered reamers were required, finishing with a parallel one.

Jim

 

I want to open out the hole in the chassis block from approx 4mm to approx 4.6mm to take the brass tube with a decent thickness heat shrink sleeve.

'Doing me sums', I reckon that I will need up to three taper pin reamers, 3.5, 4 and 4.5mm, bearing in mind I can only use the first 30mm of any of them before hitting the insulated brush tube opposite the hole I am enlarging.

Finishing off with a 3/16 inch parallel reamer as the nearest size for a good fit.

Some 'cheap' ones from fleabay look good enough as I only want to modify 16 locos

To be continued

Jim

 

I have revised this article today in the light of further experience, the choice of reamers worked well and will be covered in the next article.

Jim