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DR class BR 52 19-20

Improving the wheels

Typically for the seventies this model has excessively oversize pizza cutter flanges. To suit the available space nonetheless the wheels are generally too small. Of course my fingers were itching to do something about that.

Leading truck

The flange problem was especially conspicuous on the leading truck. First of all most BR 52s that were built during WW2 were fitted with a disc wheel. So I wanted to do away with the spoke wheel anyway. And while doing so I wanted to have the RP25 profile on the flanges. The prototype's leading wheel had a diameter of 850 mm, 9.77 mm in HO. I happened to have a wheel set of a suitable size.

For comparison

Here are the results of the measurements.

The outline of the spoked wheel is indicated in dark blue.

The new disc wheels are in yellow.

 

As you can see the new wheel is substantially larger (and only marginally smaller than the prototype), yet including the flanges the new wheel is still smaller than the original pizza cutter so it will fit under the existing truck frame without the need for modification.
The difference in flange size is apparent.
The one thing I did not like about the disc wheel is that the recess between the centre boss and the rim of the wheel tread was too curvy. I wanted it to be a more 90 degree angle with a small radius in the corner.
So I made a fixture in which the wheel was concentrically seated. It was secured with an M2 bolt

With a custom ground cutting tool I turned the recess flat with only a marginal radius in the corners

The difference is very visible but very hard to capture on a photo
The axle holes in the wheels were a bit too large for the 2 mm axle. I made bushes to bridge the gap. 2.35 mm outer diameter and 2.0 mm inner diameter, so a wall thickness of just about 0.175 mm!
Well, it fits nicely under the mainframe

Tender wheels
The tender wheels were turned to RP25 in a very similar fashion as the leading truck's wheels. So, no need to go into very much detail about that. Just a few pictures to compare before and after.
The results won't be spectacularly visible but hey, I know it's there!
One thing of note is that I removed the rubber adhesion bands before turning and placed new-bought ones instead.

Drivers

As said wheels tend to be too small and spaced too far apart to accommodate for the pizza cutter style flanges. So when I set out to modify the driver wheels first thing to do was measuring the status quo:
  • The drivers should be 1400 mm / 87 = 16.1.
  • I measured 16.1. Spot on!
  • The distance between all pairs of drivers is 1650 mm / 87 = 19.0
  • I measured 18.7 mm but it is an inaccurate measurement.
  • The total distance over all five drivers should be 4*1650 + 2*700 = 8000 mm = 92.0 mm.
  • I measured 92.6.

I call that a win!

Intermezzo

Turning the wheels to RP25 norm brings a considerable risk.

  • I am not overly experienced in lathe work.
  • The wheel consists of a steel tyre on a plastic centre, so melting is a danger
  • Pressing the wheel on and off the axle also has the risk of breaking up the plastic wheel centre.

So I wanted spare parts, just in case. I found them in the form of ..... a second BR52. At only €40 from eBay I had a good stock pile of spare wheels and other parts. Moreover this one came with the original box, which would have cost me €20 for the box alone.
First thing to do was press one wheel carefully off the axle

Next I turned a mandrel from brass which served to support the steel tyre during turning and to centre the axle.

This mandrel was adapted in later stages to facilitate the turning as can be seen from the right hand picture where an extra recess has appeared. The mandrel consists of two parts, the left hand side in the chuck and the right hand side which is pressed up hard against the wheel rim with the live centre of the tail stock.
First the wheel flange is roughly turned down to the approximate height with a conventional lathe tool (left). This is to minimise the use of the (rather expensive) special RP25 form tool. Once the flange has almost reached the norm height this special form tool comes in to play (right). From both sides the flange is reduced to the desired height and width and at the same time formed in the correct shape. Note that the lathe's carriage is positioned at a slight angle (3 degrees) to match the wheel tread's angle. A slight sliver is taken off the wheel tread to smooth its rather rough surface.
Spot the difference!
The middle driver posed a special challenge as it had a far larger crankpin then the other drivers.
So I quickly milled an extra slot to accommodate for that crankpin.

After chemically blackening they all looked like this.

When reassembling the wheel sets I pressed only one wheel back on its axle as it was, so with a very tight fit. I reamed the wheel centre of the other wheel so it would just slide on the axle with a close but not overly tight fit. This will allow the wheel to adjust itself during quartering. Very important however is now not to overshoot the axle size as the wheel will then wobble about and will be useless.

The reamed wheel of the middle axle is mounted on the axle with a tiny amount of epoxy glue. The middle axle is then approximately quartered. It is advanced at the right, that means: if the left crankpin is top dead centre, the right hand crankpin points towards the cylinder. It does not need to be exactly 90 degrees but the closer the better. The middle axle is left to cure over night.

Quartering

Next thing to do was making a solid coupling rod from brass with the correct crankpin spacing

I drilled five holes, four of 1.9 mm and the middle one of 2.8 mm. The latter needed a little reaming to fit on the larger crankpin of the middle driver. Accuracy is paramount here, the better you hit your dimensions here, the better the wheels will be quartered. Hence I mounted the two strips of brass, soldered together, on the cross-table.

I also made a brass plate of 14,5 mm wide which would serve as a gauge to space the wheel correctly. Again accuracy is paramount. I filed the plate by hand and the width varies between 14.51 and 14.55 (1.5 thou difference for our imperialists). Close enough.

The axles are installed in the frame. One "coupling rod" is offered up to the wheels that are fixed hard on their axles and the four crankpins are pushed in. When the preparations were done carefully there will be very little play between all the axles. Next the "loose" wheels are offered up to the axles with a tiny amount of epoxy glue in the axle hole. The second rod is installed and the crankpins are pushed in as well. The brass gauge plate is used to space the wheels correctly on the axles. Then the whole assembly is carefully placed on a piece of rail and rolled forwards and backwards. That is where the magic happens. The middle axle is already quartered and via the coupling rods the "loose" wheels on the other four axles are forced into the same angle and they are "automatically" quartered. When you have done the preparations carefully this is a doddle. After this run check the back-to-back distance with the brass plate gauge and if necessary do a second run. Once happy let the glue set over night.

The next day I installed the real coupling rods and did a test run with a fully quartered frame:

 

As the original coupling rods have larger holes by quite a margin the frame runs absolutely without any form of binding

End result

I reassembled the locomotive and made a few photos with the original loco. The difference is very visible