Scratch Build O Gauge Drewry 0-4-0 153HP Diesel Shunter

[Durley]

Hello

 

Having mentioned this build on another thread, I’ve started this topic to document my build of a Drewry diesel shunter in O gauge, intended for use on a small shunting layout I am slowly developing. My model of the Drewry is based on the General Arrangement in the Drewry Car Company catalogue from the 1950s. I’m not modelling any particular prototype loco, but photos of the now preserved Elizabeth on the Alderney railway have been a useful reference.

 

This class of loco is available as an O gauge kit from Connoisseur Models. Having built a few of Jim’s kits in the past, I’m sure this would make up into a great model but I fancied trying a few different ideas and, having recently acquired a 3D printer and CNC milling machine, I thought it would be an ideal candidate for a scratch build using these new toys.

 

My intention is as follows:

- Brass/nickel silver chassis cut on the CNC with Continuous Springy Beam (CSB) suspension, using cutouts in the frames as the horn block guides.

- 40:1 Roxey Mouldings gearbox and Highlevel Kits 1626 coreless motor. Although intended for 4mm scale use, I’ve found these motors to be plenty powerful enough for use in small to medium size locos in O gauge.

- Slaters wheels and crankpins.

- Hornby HM7000 Bluetooth DCC control (made possible in O gauge by the smaller motor ensuring stall current is under the rating of the chip), powered by an onboard LiPo battery with battery management system (BMS) to prevent over discharge.

- DCC sound using whatever is the closest profile from Hornby, currently a Class 08.

- 3D printed cab, bonnet and details.

- Removable bonnet to provide access to the battery for charging/swapping.

All design work will be in Fusion360, using its inbuilt manufacturing features to produce STL files for the printer and NC files for the CNC machine.

 

Some progress already is shown on this thread: 

 

Edited November 19, 2023 by Durley

[Durley]

CSB Suspension

 

The principles of CSB suspension are meticulously documented here: http://www.clag.org.uk/beam-annex3.html. CSB offers a number of benefits, combing both springing and compensation in one, giving improved ride and wheel to rail contact when compared with a rigid, independently sprung or compensated chassis. My loco is battery powered, so doesn’t rely on wheel to track contact for electrical supply. However, CSB does ensure close to even weight distribution across the wheels which gives optimum traction, something that is important for a small loco intended to pull heavy loads.

 

I have built few locos in 4mm scale with CSB suspension using Highlevel Kits hornblocks, CSB tags and drilling jig, which all work well. Unfortunately nothing similar is available for use in O gauge, so this was where I started the project by designing some parts in Fusion360.

 

Holes for the CSB pivot points in the chassis need to be positioned with high accuracy. Small inaccuracies in the position of these mounts can have a significant impact on the functioning of the CSB system, leading to uneven weight distribution. That is why in 4mm scale I use the Highlevel Kits jig to position the holes. Using the CNC means I can have the machine drill these holes without the need for a jig, with precision of 0.1mm or better.

 

I laid out my hole positions in Fusion360. Vertical positions need to be set based on the distance between the axle and the springy beam, allowing for deflection of the beam when loaded. I designed in 0.75mm deflection to hopefully achieve the correct ride height when the chassis is carrying the full weight of the loco. Horizontal positions are easy to determine for an 0-4-0, with one hole centralised in the middle of the wheelbase, and the outer holes equidistant from the centre holes. I had to move these slightly outboard from the optimum position (as detailed on the CLAG site) to avoid a clash with the brake hangers on  the rear axle.

 

To connect the hornblocks to the springy beam, I designed a simple part the could be milled on the CNC to fit over a standard Slaters brass hornblock. The beam passes through a hole in a small tab the is bent at 90 degrees, using a 1mm milled pocket as a fold line, in a similar way to a half etched fold on an etched kit. The hole in the tab is 6mm above the centre of the axle, the holes in the chassis to mount the beam pivots being 5.25mm above the centre of the axles, thereby allowing for the 0.75mm deflection.

 

To avoid the need to mount separate hornblock guides, I decided to use slots in the frames to act as the guides, something only possible because of the accuracy afforded by the CNC machine. I did this by making an inner and outer frame, the inner frame has a narrow opening into which the hornblock is slotted, this being a loose fit. The hornblock is held accurately in position and prevented from rotating by the opening in the outer frame. After a bit of trial and error, I found an opening width that was 0.15mm wider than the Slaters hornblocks gave a nice sliding fit. The pivot mounting holes where drilled into both inner and outer frames, which then acted as a register to accurately align both parts together, using the mounts (re-purposed handrail knobs) as alignment pins.

 

The spring beam itself is a metal guitar string. The exact gauge will be determined when the loco is close to complete and I know the all up weight. For now I have used a 22 gauge string.

 

Hopefully these pictures explain what I’ve attempted to describe.

 

[Ruston]

This looks to be an interesting project. Is all the extra work involved in fitting CSB really worth it for something with only 4 wheels? What's the advantage over a simple 3-point compensation that would do the same job of keeping the wheels in contact with the rails for traction?

[Durley]

Hi Ruston (it feels weird typing that as Ruston is my username on most forums!)

Good question, I’d say it does improve the ride somewhat as the loco tends to glide over track imperfections with CSB suspension, rather than the sudden jerky movement when the fixed axle on a 0-4-0 encounters a bump on a three point suspended loco. However my main motivation with this build was to try out some ideas and push the capabilities of the CNC as a learning exercise for future builds.

[Durley]

Slow progress due to work commitments but I have managed to make the cranks and some spacers for the jackshaft drive. These are designed to fit on a Slaters square ended axle which will then set the quartering. I’ll 3D print a cover to represent the crank and counterweight of the prototype, which will fit over this functional crank.

Edited December 21, 2023 by Durley

[Durley]

Jackshaft drive assembled and torque reaction post from brass rod added to the gearbox which sits between two brass tubes, allowing the driven axle to move up and down on the suspension.

 

Test run with sound enabled on the HM7000 DCC decoder using a LiPo battery as the power source. It’s the 08 sound profile so not particularly prototypical sounding but the closest currently available from Hornby.

 

 

 

[Durley]

A bit more progress to share. I’ve been drawing (in Fusion360) and printing in resin (on my Creality Halot Mage) the details for the chassis including footsteps, air cylinder, brake shoes & hangers, sandboxes and the channel guard iron. I’ve milled the coupling hooks and printed the extended hook plate that was a feature of these locomotives.

 

I found some whitemetal buffer housings in my bits box,  left over from a kit of a BR Class 03, so have used these in place of 3D printing my own as I’m sure they’ll be more robust in metal rather than resin.

 

Final clean up still to be done on the prints but I’m quite pleased with the results, especially the mesh guard for the rear steps which is just about fine enough to look convincing whilst retaining some strength.

[Nova Scotian]

With the jackshaft fixed, is there enough slop for when the axles deflect in the hornbeams? Or do you deal with it another way?

[Ruston]

The jackshaft crank looks strange without a balance weight. Is there a particular reason that you've made it that shape?

[Durley]

On 28/12/2023 at 23:07, Nova Scotian said:

With the jackshaft fixed, is there enough slop for when the axles deflect in the hornbeams? Or do you deal with it another way?

I’ve elongated the holes in the connecting rod to give circa +/- 0.3mm movement fore and aft, which appears to be sufficient to allow the suspension to work through the range of travel.

13 hours ago, Ruston said:

The jackshaft crank looks strange without a balance weight. Is there a particular reason that you've made it that shape?

I’m still to my draw up and make the 3D printed cover to sit on top of the jackshaft crank, that will include a representation of the balance weight. That will probably be todays task!

[Captain Kernow]

On 20/11/2023 at 18:25, Ruston said:

What's the advantage over a simple 3-point compensation that would do the same job of keeping the wheels in contact with the rails for traction?

I was thinking the same thing...

 

And don't you have to have a PhD in Physics and Maths to understand CSBs?

 

[Ruston]

4 hours ago, Captain Kernow said:

And don't you have to have a PhD in Physics and Maths to understand CSBs?

Having had a very quick skim through the website that Durley linked to I'd say so! Way beyond my pay grade for sure. Best to work on the KISS principle.

[Captain Kernow]

21 minutes ago, Ruston said:

Having had a very quick skim through the website that Durley linked to I'd say so! Way beyond my pay grade for sure. Best to work on the KISS principle.

What really, really annoys me about the whole CSB thing is how the relatively few protagonists who started

it all (seemingly all in the S4 Society) make out that it's so, sooo simple and that 'anyone can understand it!'

 

No, not so!!

 

I once attended a lecture for an hour or so at a Scaleforum on the subject. I stayed until the end, although afterwards I wondered why I'd bothered. The facts went in one ear and out the other straight away and 5 minutes after I'd left the lecture room, I'd already forgotten all the information!

 

(With apologies to the OP, none of this is aimed at him!).

 

[Durley]

On 29/12/2023 at 18:34, Ruston said:

The jackshaft crank looks strange without a balance weight. Is there a particular reason that you've made it that shape?

Scale crank now added from a 3D print. I managed to make it a sufficiently tight fit over the functional metal crank that it is push fit and retained by the crankpin bush. Allows me to easily disassemble for painting and any subsequent maintenance.

[Durley]

It’s starting to look like a Drewry shunter now. I’d been having problems getting the cab and bonnet to print properly on my 3D printer which I thought was my error but turned out to be a bug in the slicer software I was using that was randomly inverting some layers meaning it was trying to print the voids and not the model! Anyway, I updated the software and the problem went away.

 

I’m happy with the cab but will be making some modifications to the bonnet as I’ve made the sides a bit too thin causing the print to distort slightly, which is why the bonnet sides are angled where they butt up to the cab. I also want to refine some of the details. I tend to find it takes me 2 or 3 test prints on larger items before I get a result I’m content with.

[Barclay]

On 30/12/2023 at 19:20, Captain Kernow said:

What really, really annoys me about the whole CSB thing is how the relatively few protagonists who started

it all (seemingly all in the S4 Society) make out that it's so, sooo simple and that 'anyone can understand it!'

 

No, not so!!

 

I once attended a lecture for an hour or so at a Scaleforum on the subject. I stayed until the end, although afterwards I wondered why I'd bothered. The facts went in one ear and out the other straight away and 5 minutes after I'd left the lecture room, I'd already forgotten all the information!

 

(With apologies to the OP, none of this is aimed at him!).

 

I keep feeling that I should have a go at it, probably shamed by those same few protagonists on the S4 Forum, who make you feel like a 1970's throwback for using compensation. However I still fail to understand how I can set up a chassis for a loco. that I haven't yet built, but the weight, and weight distribution of which, I must presumably have to know in advance - my modelling doesn't work like that I'm afraid!

[Durley]

I do agree that the CSB approach appears complicated, mainly because it has generally been explained in terms of ‘beam theory’ using pseudo-scientific language. In reality, all the hard work has been done by others to develop the spreadsheet tools that are free to download on the CLAG site, making the process relatively straightforward for 4, 6 and 8 coupled locos, so covering the vast majority of prototypes.

 

Firstly, only two things are needed to establish the positions of the fulcrum points (i.e. where to drill the holes in the chassis for the handrail knobs that the spring wire passes through), that is:

1. the lateral distance between wheel centres (wheelbase) and

2. the height of the beam above (or below) the wheel centres.

 

The wheelbase can be entered in the relevant spreadsheet tool from the CLAG site. Selecting auto calculate will give offsets (i.e. the lateral distance from each wheel centre to its adjacent fulcrum points) which achieves (as close as possible) an even weight distribution across all coupled wheels.
 

The spreadsheets do allow fulcrum positions to be manually altered if, for instance they are beyond the end of the chassis, clashing with a frame spacer or brake hanger mount, etc. This then recalculates the other fulcrum positions accordingly. It can also recalculate positions for an off centre weight distribution, but as Barclay points out, generally you won’t know the weight distribution until the model is complete.

 

The vertical position of the fulcrum points is dictated by whatever method is used to connect the hornblocks to the spring wire, minus an allowance for deflection of the spring. In 4mm scale there are various proprietary solutions e.g. HighLevel, London Road Models, which use PE ‘tags’ soldered to the hornblocks. I have used the HighLevel ones myself on some 4mm scale builds, which provide for 3 different heights, which is useful so the beam height can be selected based on the geometry of your specific chassis. In 4mm scale, a beam deflection of about 0.5mm is commonly used. For 7mm scale, I have used 0.75mm.

 

With fulcrum points determined and corresponding holes drilled in the chassis, it can then be assembled with hornblocks on all coupled axles, using jig axles and the coupling rods to align everything, as you would with a compensated chassis. It is important that the hornblocks use the same datum as was used to drill the fulcrum positions so that the correct offsets are achieved, HighLevel produces a jig to help with this which work with its hornblocks.

 

Once the model is built (but no ballast weight added) it can be set on the tracks to see if it sits level and at the correct buffer height. Adding ballast weights at either end (e.g. in the smokebox or bunker of a tank engine) will normally be sufficient to get the model sitting level. Ride height can be adjusted either by adding (or removing) weight centred at the middle of the coupled wheelbase and/or by fitting thinner or thicker spring wires which will deflect less or more for a given weight. I just do this by trial and error. I have a range of steel guitar strings from gauge 8 to 24, with 10 to 14 gauge usually being used for 4mm scale locos.

 

Is CSB worth the effort? I think so but as with everything it is down to personal preference. The main advantages are:

1. Smooth ride as the loco is properly suspended on the springs. It does even damp out some oscillations from any wobbly wheels

2. Compensation keeping the loco level over track irregularities

3. Coupled wheels maintain better contact with the track, good for electrical pickup

4. Even weight distribution so driven wheels will slip at the same time maximising traction (traction is always limited by the first wheel to slip which will be the least loaded one if weight distribution is uneven)

5. Hornblocks are retained by a single spring wire which is less fiddly than dealing with multiple hornblock retainers.

 

 

 

[Durley]

I’ve been making slow progress but have been remiss I’m not posting an update recently.
 

I’ve been painting today and now have body and chassis mostly painted. Buffer beams, rods and crank still to be painted red and other details still to finish off. I’m going to do the wheels in body colour blue.

 

I’ll be using the loco as a shunter for a small  dairy layout. Express Dairies never had a Drewry to my knowledge (it’s probably a little big for that role in reality) so I’ve had to take some artistic licence with the colour scheme. I may yet add some lining in white.

 

While the paints were out, I also painted the driver.

 

 

[JimRead]

Hello Durley and Hello Ruston,

 

Great build there looks superb.

 

I was at a show in Goole a few years ago, lovely venue and great food. A bloke came along and sat down, I always had a couple of chairs in front of my micro. Turns out he drove an 0-4-0 in Hull and recounted tales of it squeeling  around the tight curves and listing through the points. He said if he had a mug of tea then he would hold it in his hands. and not leave it on the control panel. I could tell that he loves the job.

 

Cheers - Jim

 

[Durley]

A bit of lining added to finish off the livery. Blue is darker in reality than it appears in the photos.

 

Getting close to completion.