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GWR Small Metro Tank (2)


Ian Smith

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The chassis has moved on a little since the previous post, a gearbox has been milled and the profile of the chassis has been milled/filed to shape.

blogentry-12089-0-97548100-1407955395_thumb.jpg

 

The gearbox has a peg at the rear right corner which locates in a corresponding hole in the main chassis block, and a 12BA bolt secures the gearbox to the block at the front. Cutouts in the main chassis (and side frame) also locate the gearbox (although the one in the side frame is generous to prevent any shorts between the frame and the main chassis block when the gearbox is in place) - The gear box has extension cheeks that locate in these cutouts which also provides the layshaft of the worm wheel and first stage of the gear reduction.

 

Before I can complete the chassis, I need to determine how best to mount the motor. Initially, I had intended to have it as a separate item within the boiler, connecting to the worm shaft via a peg and bar arrangement which would hopefully cater for any slight mis-alignment between motor and worm shaft. A discussion with John Russell at the last Midland Area Group meeting has made me re-think this approach, and I am now considering mounting the motor on the gearbox plate making the motor gearbox a single unit. For this to work, I need to plug the countersunk hole in the gearbox plate and then drill and tap the fixing in the plate and open out the tapped hole in the chassis block to allow the gearbox to be secured from underneath.

 

However, before I go down this route, I need to progress the body a little so that I can see how the boiler will fit in place around a secured gearbox/motor assembly. I had previously built the boiler/smokebox, and the footplate/valances/buffer beams, (http://www.rmweb.co.uk/community/index.php?/blog/1009/entry-8625-2mmfs-gwr-metro-tank-part-1/)but to ascertain the boiler position on the footplate I need the side tanks. The tanks on the loco that I am building had quite pronounced rivets that I really felt needed to be reproduced, so I drew up the side tanks (complete with rivet positions) in Inkscape, the printed version being Pritt stuck to 0.006" nickel silver sheet, and the rivets formed with a home made "drop-rivetter" :

blogentry-12089-0-20988600-1407955417_thumb.jpg

 

A tank top has been fretted/filed out of 0.010" nickel silver sheet around which the sides will be soldered. Before attaching the tank top part, some 1.5mm square rod was filed to the cab opening profile, and a piece of 0.009" guitar string soldered into a rebate filed vertically to represent the distinctive cab handrail found on this class :

blogentry-12089-0-13909000-1407955428_thumb.jpg

blogentry-12089-0-93046400-1407955438_thumb.jpg

 

In the next few days I will hopefully assemble the side tanks around the tank top, and attach this to the footplate which will finally allow the boiler assembly to be cut to shape to fit around the tanks which should then allow the goal of determining how best to fit the motor to be worked out!

 

There's nothing quite like winging it is there? :O

 

Ian

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The chassis looks great Ian.

 

I'm sure that you're on the case with the body too... although I must admit that you lost me a bit there. I'm sure it'll become clear when it goes together.

 

Regards, Andy

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The chassis looks great Ian.

 

I'm sure that you're on the case with the body too... although I must admit that you lost me a bit there. I'm sure it'll become clear when it goes together.

 

Regards, Andy

Andy,

Sorry I lost you :-)  I wonder if it started with my "Peg and Bar" connection between motor and gearbox?  If so, this is what I originally intended to do :

Peg And Bar

 
The idea was that the motor would be secured within the boiler (at the smokebox end pointing backwards), and the boiler/smokebox will be a separate body component bolted on, and the motor would be physically disconnected from the gearbox, the "Peg and Bar" making the connection as the motor spins.  
 
John suggested that I could make the connection permanent since the gearbox itself is separate from the chassis block, and only secure (from underneath) to the block when the boiler is in place.
 

By the way, the engine being modelled is this one which might help clarify things if you're not familiar with kettles ;-)

Metro Tank

 

Another great post Ian, I've been looking forward to the next instalment. 

 

Jerry

Jerry,

Thank you.  Behind the scenes I've been turning up buffers, chimney, dome and safety valve too so once I get going the progress might be a bit swifter than my saddle tank :-)

 

Ian
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If you've got the space to do so, I think keeping the motor and worm shafts physically disconnected, as you originally intended, is a good principle. The peg and bar arrangement is tolerant to misalignments and deflection in the axes of the motor and worm shafts, and, depending on the peg and bar axial initial positions, allows the motor to start 'off-load'. If the motor itself can be mounted on or in something resilient, the peg and bar connection will help reduce noise transmission. (I've used the arrangement with some success, although I used a twin-peg and twin-bar mating for greater symmetry and balance, the pegs sticking out of a flywheel on the motor shaft.) I might have a diagram somewhere.

 

I guess your decision hinges on how easy/difficult the ergonomics of securing the gearbox to the chassis block will be, and whether this will be made more or less difficult if the motor was attached to the gearbox.

 

615 is gorgeous.

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Will significant misalignment of a peg and bar drive be noticeable as a variable velocity within each revolution at the wheels? 

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If you've got the space to do so, I think keeping the motor and worm shafts physically disconnected, as you originally intended, is a good principle. The peg and bar arrangement is tolerant to misalignments and deflection in the axes of the motor and worm shafts, and, depending on the peg and bar axial initial positions, allows the motor to start 'off-load'. If the motor itself can be mounted on or in something resilient, the peg and bar connection will help reduce noise transmission. (I've used the arrangement with some success, although I used a twin-peg and twin-bar mating for greater symmetry and balance, the pegs sticking out of a flywheel on the motor shaft.) I might have a diagram somewhere.

 

I guess your decision hinges on how easy/difficult the ergonomics of securing the gearbox to the chassis block will be, and whether this will be made more or less difficult if the motor was attached to the gearbox.

 

615 is gorgeous.

Miss P,

It was my intention to fit a flywheel to the motor shaft, and have the peg poking out of that - I only drew it as I did to help Andy's understanding :-)  

I agree that 615 is gorgeous - the Small Metros are one of my personal favourites, and when finished in full Edwardian splendour I think they are particularly attractive.  I only hope that I can do her justice!! 

 

Will significant misalignment of a peg and bar drive be noticeable as a variable velocity within each revolution at the wheels? 

Rich,

That was one of my worries.  However, if my calculations (and drawing chassis/motor to scale, and cutting out the parts) is correct, the difference in the axes of the motor and the worm will be negligible (I hope!)  Although I do need to get the body to a point where I can site the boiler (and fit the motor within it) so that I can check these axes!

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I know that problem because I started building a metro once. (I got about as far as you have.) It was halted abruptly when I realised that I had produced a body with small tanks and a 'later' cab as a result of working from several different drawings. The problem: I was trying to build a 455 class engine in early 1920s guise! There isn't much space for error with that boiler and I had planned to 'solve' the motor problem by building the body such that the front section slid forwards for removal, rather than lifting upwards, and the bunker/cab floor attached separately. This to allow the motor to be attached semi permanently to the chassis. It was a shame that the cab is the wrong one because I got the side sheets wrapping around and up the arc of the roof really well too. 

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Here's my twin-pin 'flyclutch' arrangement between a 2mm motor shaft and a 3/32" Exactoscale gearbox shaft. (In 4mm scale - I used 0.7mm wire for the spigots both on the flywheel and the collar.)
 

flyclutch

 

 

 

Will significant misalignment of a peg and bar drive be noticeable as a variable velocity within each revolution at the wheels?
 
Any misalignment between the input and output shafts will result in a sinusoidal rotation output. If the misalignment, either angular (as in a conventional spigot and slot cardan) or axial, is small, the amplitude of the sinusoid will be very small, and will be undetectable (at the wheels) after a normal ratio final drive box. The amplitude of the sinusoid will be far less than that given by cogging of non-skew-wound motor armatures at low speed.
 
The practical advantage is that the mechanical disconnection of the shafts enables the motor and its flywheel to be easily inserted/detached without faffing about.
 
Whether the arrangement will be suitable for Iain's very constrained Metro situation is another matter of course.
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Some interesting ideas along with the neat work. I do like the gearbox idea. In 7mm I became used to the availability of high quality motor gearbox combinations. However if the motor could be easily removeable that would be a big help in the tweaking I seem to need to get good running. I agree the effects of any misalignment should not produce any problems with variable speed at the wheels. It is basically a very simple dog clutch.

Very impressive,

Don

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