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Building tenders with 3 axles

frame and wheel suspension

The mainstay of stock in any major railway company is formed by locomotives with a separate tender. In model form this type of loco offers quite some possibilities for the starting modeller. It is amazing how the manufacturers of continental models all seem to avoid the advantages of a good design at all costs. Either by trying to accommodate an oversized motor in a too small loco body or by producing an underpowered motordriven tender with bad pickup due to traction tyres. Often there is ample space for motor, geartrain and weight if the functions are split over both tender and loco chassis. Coupling the locoframe and body electrically will assure that all wheels are available for pickup. With the motor in the tender, the boiler can be massive providing sufficient weight for a succesful model with ample power. The motor can drive the geartrain by a cardan, which can be simple wire loop. Particularly the tender is easy to experiment with, often it has only 3 axles and it doesn't have difficult coupling rods. Thus this can form a nice introduction to learn about splitaxle technique and compensation. With 2 mm shop wheels and components in combination with a second hand tender and loco body this offers many hours of fiNe modelling at relative low costs. Such a loco can lay the basis for further skills to tackle more requiring models. The following gives an introduction on how to exploit the tender as a base for a good running loco.

Contrary to the usual continental practice of a neutral frame the split axle technique uses a electrically live frame of which each side has the polarity of a railside. Thus split axle means a separation of left and right part of the axle each half carrying a wheel and also of the frame of the tender such that each half of the frame has a different polarity. The frame plates are fixed into a unit by using non conducting spacers. This fixing can be by screws in plastic spacers but also by soldering to slotted pieces of double sided copperclad printed circuit board (pcb). I prefer 1mm pcb and material of 0.5 mm for the frameplates. After soldering together 2 strips, 1 edge is cleaned and straightened and dimensions are set out. The frames are cut with a piercing saw and finished by filing.

The splitaxle technique is particularly useful for cardan drive locomotives. In this case reliable pickup of the tender is first order priority and immediately pays off in running properties of your model. The splitaxle technique itself is not better or worse than using contact springs on the wheels.Both methods have the same number of contact crossings namely 2, from rail -> wheel but we replace the wheel -> pick-up spring by axle -> frame. In practice the splitaxle method shows a higher reliability over time as it is less prone to collection of dirt.

my first exercise with a compensated splitaxle tender

Even well layed track is not always flat and small unlevel spots can be an origin of contact problems. Also base boards itself might be warped permanently or just temporarily due un uneven ground surface and adjoining boards may cause uneven joints in the track. Better pickup is realised if wheels can follow the track. This can be the result of springing independent axles but there also exist a somewhat easier approach, especially for small scale models, called compensation. This principle is based on the 3 legged stool where each leg is in firm contact with the ground. If you can arrange the weight to be suspended over a triangular positioned contacts on both rails than the first game is already won.

There do exist various splitaxle tenders on the market, e.g. the Arnold tenders for G8 and the Kato tender on their G10 model. However these are not very well designed. Both use extended axles in vertical slots in a contact frame plate. To enable the wheels to move up and down unassisted by springs or such these slots are oversized and there are no contact springs, thus contact between axle and keeper plate is at most times pretty much unreliabe.

The second step in creating better running properties comes from using flywheels. As soon as your motor is running, a flywheel will preserve its momentum. Thus temporary contact failures will no longer be of major influence. However flywheels only have use in combination with good quality free running motors such as Faulhaber coreless motors. Third measure is a good control of your motor of course. The closer the control sits onto the motor the less interference can interact in the control. A good dcc decoder finetuned on your motor properties will reward you with far better control than any conventional controller can promise that feeds via the track. Particularly adding the decoder to a single tender body already accomodating motor, flywheel and weight required further optimisations in my design. But even in the smaller tenders this looks possible with the new generation small footprint decoders, avoiding the solution I had to follow for the G8 by fitting the decoder to roof of the cabin

The first splitaxle tender I built straight following the booklet "Building an 0-6-0" of the 2 mm Scale Association. The photo above shows that triangular suspension is arranged by using a fixed axle on one end and an axle that can pivot around the centerline on the other end. The third (center) axle is taken out the equation by using vertical slots with a small spring bearing on the axle for contact. The pivoting axle is hang up in its own frame. The wheels pass through elongated holes in the bearing frame and thus cannot fall out. The pivot is a simple piece of wire on the centerline soldered to a spacer. The contact of subframe and inner frame however brings a new uncertainty in the equation unless we use thin litze soldered to bridge this gap. The outer frame is purely cosmetic only bearing the spring detail etc. The accompanying photos show details on construction of such a tender.

splitaxle tender as per 2mm SA booklets.

The drawback of the method is that the inside frame seriously limits the dimensions of the flywheel and that the fixing of inner frame with wheels to the body of the tender is not straightforward. In this case where the outer frame is fixed to the body then the opening in the body also forms a limitation for motor and flywheel. The advantage of the method is that no special tools are required to produce something like this.

Of course the splitaxle principle can also arranged different. Doing away with the inner frame frees up the necessary space. It is also a more prototypical construction. An outer frame is much wider and this allows to accommodate flywheel and motor partly between the frameplates. Larger flywheels are more effective and enlarging the diameter is the best way to do it. The easiest way to create the fixed axle is to turn out a set of wheels with extended axles but using standard muffs. About 1 mm extension of 0.5 mm diameter suffices. We can use tophat bearings drilled through with a 0.5 mm bearing in the frame. The remaining 2 axles can be united in a subframe as a bogie. basic frame

basic form of a tender frame

This photo shows a small splitaxle frame as described. We see a construction of profiled frameplates separated by pcb board. The frontaxle is directly fixed in bearings in the outside frame. The wheels have an extended axle of 0.5 mm. The bearings are 2 tophat bearings drilled with a 0.5 mm hole. The other 2 axles are mounted in an internal framed bogie. The bogie is mounted on central spigot in the form of an M1.4 screw. The screw is mounted in a 2mm bush soldered to underside of the piece of pcb board. On the outside of this between bogie and pcb board there is a small spring made from a piece of N coupling spring.

(Of course in reality tenders mostly carried spoked wheels but as the spokes are not visible at all from normal viewpoint in tenders with these closed tender frame plates I consider that a waste of money.)

These photos clearly show the differences in possible flywheel sizes (10 ∓ 13mm).

Search the motor! Optimising on weight and size of flywheel (14mm) does not leave any space for a decoder.

In my frames the bogie subframe can swivel around a centerpoint.An alternative would be using a double cardan suspension in two planes or to directly pivot the subframe plates on the outer frame in horizontal plane, until now I have not experimented with such variations. For the centerpoint subframe we have the choice to make it rigid or make this into an independent triangular suspension too. The rigid solution only guarantees that at least on each side 1 wheel is in contact with the rails. If we compensate the subframe too we can rely on 4 wheels being in contact, but with again 2 wheels in elongated holes needing springs for contact. In practice a rigid bogie turns out to function well enough and is lots easier to produce. I use small spiral PB contact springs around a pin bearing against the frame plates to make an electrical connection. These are wound around a pin from 42 swg PB wire.

The drawback of this method is that the resulting triangle of your 3 legged chair is smaller than in the first approach by half the wheel distance between the paired wheels. This means that you need a little bit more care to select which axle should be fixed. The centre point of gravity of the tender needs to lie well within the triangle to get stable running properties and preferably this point sits a bit nearer to the fixed axle.

In my latest frame an entirely different approach is by using hornblocks instead of the bogie method. In combination with the outer frame we need 3 sets of extended axles making the method more demanding. The 2 sets of hornblocks sit in straight slots, one axle is fixed as before. The outer axle is able to raise and fall using a central pivot and the middle axle in a slot only able to fall. Both use springs to ensure contact.

Here we see a home made kit for a tender with hornblocks. In this case all wheels have extended axles of 0.8 mm diameter x ~1 mm long. The hornblocks are from phosphor bronze and fit into slots in the frame. There is a overlapping backplane to prevent the hornblocks from falling out.

motor mounts

The photo above with the comparison between frames with flywheels shows the frames fitted with motor. The easiest approach is glueing the motor to a mount in the form a V-slot. Even with 2 component epoxy glue the FH motor bodies are that smooth that they can be pried loose when necessary. However the newer frames show a bit more engineering for the mounts. From heavy pieces of brass rod to delicate thin mounts just fitting in the body within tenths of a millimeter. The elegant way is to fit the motor in a squarish block of about 5 mm thick brass with a 10 mm opening. Normally we find that there is so much strain in a thick piece of brass that after cutting a slot in the top edge the opening closes again leaving enough clamping force for the motor without any further fixing. The block of brass can be soldered or screwed to a piece of 1 mm double sided pcb. This can be the same as that for the bogie fixing.

Using a propietary body with a new frame, buffer beam and weight piece. In this case a tender of prussian origin.
Here we see a 12 mm flywheel but just because I ran out of larger diameter brass stock material

Producing the tender body

The body can be produced in various ways. The easy way out is using a propietary body. However making your own is much more fun. Often tenders are rivetted affairs. Rivets can be produced by etching but also by direct impressing a pin into thin material. The latter method can be seen on the accompanying photo. A broken drill shaft is ground to a very sharp point by using a minidrill and a grinding disc in the main drill or grinding machine. This sharp point is carefully blunted to a round nose using fine waterproof sanding paper. Then using a drill stand rivets can be pressed into a piece of brass. The depth is controlled by the limit checks of the drill stand. Mounting a piece of 0.1 - 0.125 mm brass can be best done with double sided tape on a flat piece of material such as an aluminium plate. This thin layer also caters for taking up the head of the rivet instead of producing a dimple in the aluminium plate. Of course some rows rivets as test on a piece of scrap are necessary before starting out. For nice and equal distanced rivets a crosstable is a very handy accessory, but for wider spaced rivets this not a must have. Using the crosstable up to 3 rivets per mm are possible before they start to fuse into a full line. The crosstable generally gives a more crisp result than etching. Note however that the pattern is pressed from inside to outside thus a mirror sketch with dimensions is helpful.

Producing rivets with a cross table and the resulting sheet for producing a wrapper for the tenderbody

As you can see on the photos the sheet gets a bit distorted in areas with a high density of rivets. This can be helped by flattening the area around the rivets with a cocktail stick wit a blunt point. I made the sheet in two halves. This because of the round corners on the tender. I find it very difficult to make a symmetrical folded U with rounded corners and ending up with the exact width of the U. To avoid this I simply make it in two halves and make it to fit by filing. But you have to accept the butted line, which may be slightly visible in the end. After rivetting the sheet is cut up into 2 sides that are fitted to a block of brass. It needs some careful filing to make the lower edge of rivets sit nicely parallel to the footplate. Alternatively you can directly solder it to the baseplate and fill empty space with a cast block of lead. The bufferbeam is soldered to the base plate. Along the base plate strips of 0.5 mm square brass are soldered to imitate the solebar reinforcements.

parts for the body

assembly of lower body part

coal bunker and toolboxes added

One of the more difficult parts is soldering the flat beading strip that reinforces the top of the tender sides. Keeping such a thin strip straight and to sit nicely needs care. A round beading such as on top of the coal bunker is a bit easier in that respect. After fitting these components the product starts to look like a tender. This is a 20 m³ belgian tender of which some 600 where produced by various firms coupled to whole range of different locomotives. The coalbunker on top was enlarged in the fifties and photos show all kind of constructions to hold more coal, with or without keeping the old steel plates intact, using wood top ups or all steel constructions etc. This is an example of a neat conversion using steel plate. The toplevel with the coalbunker, toolboxes and fill openings is a separate item that fits snugly between the tender sides. The coal bunker will be used to hide the decoder and probably some space will left for some lead + real coal.

text (and loco) to be finished

copyright: Henk Oversloot
date: 11 november 2002