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firstly...why the need for a 'double-framed' locomotive?

surely the actual steam engine chassis layout really isn't relevant......in this case, with the motor in the tender?

however.......for a scottish loco, which perhaps combines both outside and inside loco frames, what about the Allen-designed Goods engines found throughout the Highland Railway system..from the later 1800's, right up to LMS days?

these were of a design type known as the 'CREWE' type....ie although outside cylinders were employed, the cylinder slide bars were combined with an outside frame, leading to a particularly unique appearance.

for a simple model, this design lends itself to simplistic valve gear.......since slide bars, etc are hidden from view.

http://www.lochgormkits.co.uk/html/page_4_locomotives.html

shows a HR Duke class.....but the 2-4-0 Allen Goods engines were what I was particularly thinking of.

http://www.ambaile.org.uk/en/item/photogra...4663&zoom=2

shows a typical example of a very early HR engine....later ones followed a similar design pattern, but more 'updated' fittings, etc...

http://www.ambaile.org.uk/en/item/photogra...4281&zoom=2

again as above.....

regarding tender-mounted motors?

The ideal method is to step-down the driveline from the motor, to lower the driveshaft line.......this reduces the torque reaction effect.also might place the driveshaft below the cab floor.

around curves there ought to be few issues.....after all, this is the sort of drive associated with better quality diesel mechanisms?

the secret is the placing of either the universal joints, or the flexibility of the driveline.

It IS a widely used system [in the past] when small motors were harder to come by.

I also suggest looking at the tender-loco connection.........cantilevering the tender body [and motor] off the back of the loco chassis increases adhesive weight. [ie, the front of the tender chassis is separate from the main tender body, which is supported at its rear on the tender chassis, and at its front, off the loco coupling.]

Your main problem will be the gearbox arrangement.

are you using the O-4-o chassis in some way?

or just a spare motor from one?

whatever, try to maintain as straight a driveline as possible, motor-to- engine gearbox..
 

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I'd suggest they're both much the same?

the material used for the flexible drive tube needs to be able to resist twisting on either shaft....hence the suggestion of 'pure silicon tube'??

rubber might tend to slip after a while.

an idea might be to get both driveshafts [one in loco, 't'uther in tender]....to end roughly above the coupler pivot points....this will eliminate an unwanted driveshaft 'swing' on curves.

the tube then connects both ends......I doubt there will be much of a lengthening effect on bends.

also, if using a tube driveshaft.....permanently couple the engine and tender!! big pain if shaft separates.

if using ball and socket u/j's, then permanency is less of an issue. [these can be had very cheaply, especialy if one hunts out old Mehano diesels at swapsies.] I've made some out of brass tube, and an input shaft from wire, formed into a tee piece.

I've also used coiled spring as found in car/bike speedo cables.....however, on one old electric boxcab I made, [one truck driven, motor at other end]...the spring would wind up as the motor revs rose........when the shaft gripped enough, the wheels spun and it would take off like a rocket...be warned, much fun can be had experimenting.

NorthWest Shortline make/sell nice neoprene U/J's.......
 

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in addition, this sort of set-up is actually used quite successfully in the proprietary world.....in most decent diesel models???

all that is being created is effectively a diesel chassis.....with a gear tower of sorts, a driveshaft and u'j's....[ a diesel needs these for truck swing]....and a seperately-mounted motor.

I dont see too many modellers longing for the old days of motor bogies?

Rivarossi used [possibly still do] a similar sort of system to power their UP Big Boy and mallets.

they dont have two motors , each powering a separate engine unit...but had one [big] motor linked to two gear towers by driveshafts and u/j [or flexible joints]....

as Richard says, if the driveline can be ''engineered'' to lie as low as possible, it would be almost invisible.

but, even if not, then a slim driveshaft, even going in to the loco through its firehole, is hardly noticeable.

But if appearances ARE a worry......just take a low down, silhoette view of any of those much-vaunted steamers from Hornby/Bachmann?

See the tender drawbar?

none too realistic, is it?

yet, no-one really worries?

......and if one is onto constructing one's own locos , then at this stage of one's modelling career, one is less likely to have retained set-track curves?

so, as with an expensive etched kit, one considers one's minimum radius??

in the past, such drive systems have been plagued by a lack of easily-used flexible material.....at least, one that didn't have to be replaced at every service......now we have aircraft fuel line, and its ''problem solved?''

also, as Richard says......it allows the use of a decent, meaty motor in a slender loco......rather than something akin to a swiss watch?
 

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just to add to comments regarding pros/cons of tender mounted motors....[as distinct from tender drive...which is something else entirely].....I happened to re-read an article in the last MRJ..[no 183]...a description of a loco kit build...namely, a Dave Bradwell 7mm scale kit for an LNER K1...all singin', all dancin'.....it uses a tender mounted motor, driving a loco gearbox via a driveshaft.

reasons for this drive layout.....[especially considering it's a 7mm scale loco, not exactly small, either]..is the ability to get a decent sized motor, plus a couple of hefty flywheels into what is effectively a wasted space, yet an intrinsic part of the loco.
The flywheels are of such a size that electronic 'mass' really isn't needed.

incidentally..the article states that Branchlines sell a line in U/J's and driveshafts....

the gearbox is a standard High Level item.....and with this set-up, there is one other advantage, and that is, the gearbox can be ''fully floating''...ie with the axle, the axles can then be sprung....all of them.
this is hard to arrange with an integral motor/gearbox unit.....

because space considerations are less, step-down gearing can also be arranged in the tender........
 

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QUOTE That's a tangent to the original thread,

not really...simply pointing out what folk find acceptable....since a major argument seems to be one of appearances of the driveshaft?

A simple step-down gearset on the end of the motor shaft would take a driveshaft below footplate level.....

adhesion is an issue...thes old locos were able to pull in real life....otherwise they would have been replaced toot sweet.... or re-built.

Motors tend to be very light in weight, and motor size needs careful thought...as does gearbox layout,if aiming for a built-in power unit.

with a tender-mounted motor size is less of an issue......it also fills a wased space, and allows decent heavy metals to be used within the loco...metals which can be shaped to fit available spaces.....

an issue which needs to be addressed is , how to transfer electrical power from loco chassis to tender?
 

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I suggest collection current from the loco wheels as well as tender wheels........there will be much more weight in the loco for better contact.

In that loco build article, MRJ last month....the builder devised two sprung contacts....much like those pickups for the wheels.......but sprung to bear on PHB pads under the tender front.....two inverted U-shaped bits helped keep the sprung contacts in line with their relevant contact pads, when cornering[??}....the sprung contacts are under the loco cab........this idea is to allow easy separation of loco from tender..........I have in the past used tiny plastic microplugs...nobbut a few milimtres square.....
 
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