Michael's Werkbank (Bavarian PT2/3)

[Chris Veitch]

Thanks for your explanation of this very interesting kit. I remember being fascinated by Gerard HO narrow gauge kits in the 1980s although they were too expensive for me as a teenager. I had never realised that they were sold in stages, which makes sense now considering how expensive they were at the time.

 

[michael080]

I would assume as it a was produced by Gerard it would have been for the ÖBB 770. Does it have the original KBay.Sts.B coal bunker or the later DRG/DB hopper style and I presume you will be finishing it as DB to match your Br 78.

The kit comes with three variants: low bavarian coal bunkers, high bunker with gangway and high bunker with gangway closed. I am planning to build it with a small wooden extension to the bavarian style bunker.

these are no kits, these are scaled down prototypes

This was a criticism?

not exactly, it was more like an excuse NOT to build one.

I remember being fascinated by Gerard HO narrow gauge kits in the 1980s although they were too expensive for me as a teenager.

same here. The kit was on ebay, it was not cheap, but a lot cheaper compared to the 1980s price.


So here is the first stage, setting up the chassis with footplate.




The frames are made of two layers to get a reasonable strength and the are directly soldered to the baseplate to form a very rigid chassis.




The spacers have some M2 nuts soldered on them that will carry the gear box, the boiler etc.
I love soldering nuts using the RSU, the tip holds them in position all through the soldering process.



the tweezers hold two more nuts while two more are soldered. This is the spacer that will hold the cylinder assembly and the gear box cover in place.

so far so good. Next stage is already on the way...

 

[Chas Levin]

Superb looking kit, very much enjoying seeing you build this!

 

[michael080]

Next stage,

gear box and motor. You may have noticed in the pictures above that the kit has no useable hornblocks in the frame. This stage assembles the gear box that also holds the axles so that no hornblocks are needed.
The gear box is made up of nicely machined brass parts. I have read elsewhere that the original design would be a poor runner with severe gear problems, so I took a closer look.

The motor drives a cardan joint that drives the gear box. The gear box itself is made up of two worm gear sets.




And there was the problem: The worm wheel is spur-toothed, it should have a helical gearing instead. Original gear wheel below.



As you can see, I found a supplier for 0.3mm module helical gear wheel with matching diameter. The drill diameter was only 1.5mm instead of the required 2mm, but that could be fixed by careful reaming. It is too narrow, so I put two wheels together. The construction manual proposed superglue to fix the wheels on the shaft, but I prefer Loctite 603 for this purpose. The narrow wheels were doubled up with a bit of Loctite between them.
The whole inner gear box is rocking around the central shaft to improve pickup.

The complete gearbox assembly is fixed with four screws, the motor is held in place by a plate that mimics the bottom of the water tank between the frames.



It runs like a clockwork.

The axles are slotted, so that the wheel quartering will be a simple thing to do. (Hopefully)


Michael

edit: should have read it before posting

 

[Brian McKenzie]

Michael, I would worry that brass worms meshed with brass gear wheels may not perform well, when the model comes under load. Also, there is a lot of rotating brass, rubbing against brass surfaces supporting the axles and worm shaft.

The latter would benefit with the inclusion of small washers of a slippery plastic, e.g. acetal (Delrin) or P.T.F.E.

It may not be easy to find 'plastic' worms of the size required, or in a different metal. I screwcut a replacement 0.3mod worm (for 2mm dia shaft) recently from bearing grade PEEK plastic, to help avoid this issue of 'brass to brass' friction.


To stop the plastic flexing away from the cutting tool, a 2mm dia shaft was inserted first into the PEEK material, the other end rotating in a brass bush held in the tailstock chuck.


OD of worm is 3.68mm. Shaft centres are at 6.15mm

 

[Yorkshire Dave]

The complete gearbox assembly is fixed with four screws, the motor is held in place by a plate that mimics the bottom of the water tank between the frames.

Nice hidden motor between the frames. I've come across gearboxes like this in Weinert HO kits.

 

[michael080]

Brian, I fully agree that brass on brass is a poor idea. Unfortunately, I can't produce my own worms, I don't have access to a lathe. The other problem is that I had to keep the distance between worm and wheel. The wheels I could get match nicely into the existing frame, so it ssemed to be the best idea forward. Anyway, I guess it is still an improvement compared to the original design.

Here are some pictures of stage 5. I am not really following the proposed sequence.




The cab is has an inner frame with stronger material with half etched overlays on all four sides. The inner frame uses a key and slot pattern.

Inner and outer frame have some 0.5mm holes that can be used to line up both layers before sweating them together.




And this is the final assembly.
The side wall overlays will follow in one of the next stages.



Michael

 

[michael080]

It's already three weeks since my last report, so here is what happened.

The side walls are also doubled up around the inner frame. The have exentsions for the tanks. It takes a littel time to make sure that everything is square and parallel before the side walls are soldered.



The tanks are large enough to put two parallel colmns of 6 supercaps into them.




Plus there is enough space left to fill the tanks with lead. The supercaps are fixed with hotmelt, the lead with epoxy.





This is the final stage for today. Tanks covered and boiler rolled and attached. The smoke box is a bit special. It is made of a heave turned brass core with a flimsy overlay. This gives some extra weight, but it makes solering not easier. You can also see the steam tube cladding, that is so characteristic for this class.




Michael

 

[michael080]

oh my! No news for almost a year.
Here is what my little engine looks like a year after the last posting:



Quite clean as steam engines were at that time, only a lot of oil at the running gear. 70 007 still carried its petroleum lamps in 1930.

Michael

 

[King Crab]

Very sweet loco!
That's a lovely piece of work.

Peter

 

[Chas Levin]

That is absolutely beautiful: terrific job!!

 

[simond]

It’s a truly lovely model, but such a curious prototype.

What was the rationale for having the wheelbase so disposed? It seems far from ideal in many ways - long for a given loco length, driven axles in the way of the ashpan, non-driven axle taking a significant proportion of the potential tractive weight. I suppose the short overhangs minimise throw on curves, and the slide bars can be easily supported at their rear ends, but that seems relatively unimportant.

presumably someone somewhere said “this is ideal, I will build it like this! But why?

 

[michael080]

there have been many opinions why this layout was chosen, up to the idea that it was initially a 2-6-0 loco with an axle removed. Firstly, it seems silly, because the boiler is the heaviest part of the engine, so the driveing axles should be below the boiler. However, the boilers were small and considering the weight of coal and water, the COG may be further back than expected. The engineers at Krauss in Munich were the building Germanies best engines at that time, so they probably understood their business better than I do.

I think it needs to be seen from the point of historical development. There were a few 2-2-2t locos in Austria that performed well on small lines with low performance requirements. They were lightweighted, could run through narrow curves and had easily accesible components.

At that time, Bavaria developed the concept of light fast trains with reduced staff. The PT2/3 was initially developed for these trains and has a door to move to the coaches. The fireman had to take over conductor work. Better performing compared to the 2-2-2 engines, but still cheap. They performed very well with these light trains, not least because they could be turned at the destination to always run smokebox first.

In later years, they were replaced with BR64 engines 2-6-2t on these mainlines and were used on secondary lines where they lasted well in the 1960s. However, they couldn't be turned at the terminal and had to run backwards. This was a big problem and derailments were common.

Michael

 

[Chas Levin]

Interesting historical detail Michael.
I've wondered about the layout before now (not just from your excellent model but from similar RTR locos) and wondered whether it might also have anything to do with terrain, or other permanent way layout issues? Other unconventional loco wheel arrangements I've read about have often been to do with that - negotiating tight curves, or steep gradients...

I must admit though tat I can't immediately see how this particular arrangement would benefit unusual terrain; and my knowledge of German and Austrian geography is pretty poor too. Time for some further reading!

 

[Rob R]

Chas,
A recent Fleabay purchase.


I haven't a clue what it says. I abandoned O level German after the first week and did Geography instead
Are you coming to Doncaster with the GN Soc stand this weekend?
If you are I'll pack the mag with the layout so you take a peek.
Rob

 

[Chas Levin]

Rob, that looks very interesting, doesn't it? Sadly I have to miss Doncaster, family event clashes. I'll be at Princes Risborough Feb 22nd, or Ally Pally a month after that?

 

[Rob R]

Rob, that looks very interesting, doesn't it? Sadly I have to miss Doncaster, family event clashes. I'll be at Princes Risborough Feb 22nd, or Ally Pally a month after that?

Unfortunately both PR and AP are no goes for me. (time, distance, SSMRS AGM etc)
There were still a few issues on fleabay last time I looked or I could run it through the scanner next week.......

 

[Brian McKenzie]

. . . . because the boiler is the heaviest part of the engine, so the driving axles should be below the boiler. However, the boilers were small and considering the weight of coal and water, the COG may be further back than expected.

Looking at the side view of Michael's model, I wondered if a low COG had much to do with the design. There appears to be a substantial body of water stored in the frames between cylinders and driven wheels.

 

[simond]

A large well tank might be a rational explanation.

 

[michael080]

The engine had in fact a water tank in the frame. See the picture below. In total, the loco carried 6000l water and 1100kg coal. Compared with the total weight of the boiler 12600kg (including water), the COG may be not so far away from the driving axle.




I could find a drawing in Andreas Knipping PT2/3 bible "Die Baureihe 70" that specifies the axle load for all three axles. 11.2t for the leading axle and 13.6t each for the driven axles. Obviously not so bad.