Minimum track radius

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chaswarr
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Minimum track radius

Post by chaswarr » Tue Feb 06, 2007 10:09 pm

Dear all. I am currently working on a 5 inch gauge model of a LNWR 6 wheel carriage of the
1890's vintage using a works drawing that I got from the LNWR Society archive.

The carriage has 2 conventional axles but the third is of a radial design which provides
a steering function as well as lateral displacement of the axle to allow negotiation
of the track.

In order to calculate the amount of lateral displacement necessary to avoid derailment
I need to know the mimimum track radius likely to be encountered on a typical 5 inch
gauge track. I will be most grateful if someone could provide the information I seek.

Interestingly the prototype drawing I have does not show any brake gear fitted. Quite
what happened when the cariage was parked in a siding I do not know. Chocking seems
somewhat hit and miss as a way of preventing the vehicle rolling away! However this
lack of brake shoes does allow me to cheat by building lateral float into the conventional
axles.

Many thanks.

P.S. The carriage has Mansell wheels so the article by Dave Noble in Turnout 41 describing
them has proved most useful.Thanks again.

Charles Warr
Charles Warr
Romford
Frits
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Minimal curve

Post by Frits » Sat Feb 10, 2007 7:40 am

Hello Charles,

If you can provide me with the maximum lateral displacement of your middle axle and the distance between the outer axles, I can calculate your minimum curve. 8)

Just let me know.

Frits
Build it perfekt, but keep it simpel.
chaswarr
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Re: Minimal curve

Post by chaswarr » Mon Feb 12, 2007 9:48 pm

Frits wrote:Hello Charles,

If you can provide me with the maximum lateral displacement of your middle axle and the distance between the outer axles, I can calculate your minimum curve. 8)

Just let me know.

Frits

Hello Frits.

Thanks for the offer of help.


The displacable axle on my LNWR carriage is the outside axle not the centre one. Distance between axles is 297mm. The maximum lateral movement of the displacable axle is 6mm.

Hope you can make something from this.

Cheers.

[/quote]
Charles Warr
Romford
Frits
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Minimum radius

Post by Frits » Tue Feb 13, 2007 9:59 pm

Hi Charles,

Just curious, but did I understood you well. Your outer axles are the one's who can be moved lateral? :? :?

If that's the case, I wonder how you kept your carriage streight on the track? :oops:
Anyway, to be sure in my calculation, the displacement of your moveble axle is 6 mm out of the track center in one direction or is that the total displacement (so 3 left and 3 to the right) and I suppose the 297 mm is the distance between the middle axle and one of the outer axles.

Excuse me if my English isn't perfect, I'm Dutch. (but not blond :D )

Please confirm my assumptions.

best regards

Frits
Build it perfekt, but keep it simpel.
chaswarr
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Post by chaswarr » Wed Feb 14, 2007 8:49 pm

Hi Frits.

Thanks for your interest.

Only one of the 3 axles is displaceable and it is one of the outer axles. The other two cannot move laterally. The centre distance between each axle is 297mm therefore total wheelbase is 2 x 297 = 594mm. The lateral displacement of the moveable axle is 6mm each side of its straight ahead running position.

Unfortunately my carriage is not yet built. But I don't want to make something which will not run on a typical 5 inch railway - hence my request for the minimum track radius likely to be encountered.

I chose a 6 wheel carriage in the naive hope it would be less complicated to build as a novice's project. Now I am not so sure it was a good idea. Nevertheless I have now drawn the upper structure using a works drawing and have nearly completed the underframe drawing.

Bye for now

Charles.
Charles Warr
Romford
Frits
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Radius

Post by Frits » Thu Feb 15, 2007 9:49 am

Hello Charles,

Well here are the results of my calculation. Let me explain what I did.
In my 3D CAD TurboCad I set out 3 points on a straight line with a distance of 297 mm each. Then I shift one outer point 6 mm out of the line.
Now my computer draws a "3 point circle" through these points.
This results in the theoretical radius of... hold on....... 29,4 m. :cry: :cry:

But it will be better if I calculate the margin between the wheels and the track too. For instance 1+1 mm. Now the radius comes to 17,6 m.
Still a little large for most of the clubtracks.

Let's see what the outcome is if I say the margin is 2+2 mm.
This gives a radius of 12,6 m.

Just for your information I also calculate the radius in case your middle axle was the moveable axle.
Then the theoretical radius is 7,4 m., which will be in practice 5,8 m. :shock: :shock:
If you couples the lateral moveable middle axle with crossbars to the turnable outer axles your carriage can run through almost any curve. 8)

Image

I hope I did solve your problem. Maybe you can show us a picture of your work?

Best regards

Frits
Last edited by Frits on Wed Feb 21, 2007 7:12 pm, edited 1 time in total.
Build it perfekt, but keep it simpel.
davet
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Post by davet » Fri Feb 16, 2007 1:00 am

If the one end wheel set is described as "Radial" the usual interpretation is that it is guided to slide in radial axle box guides, as in Webb's "Radial tanks". The radius can be approximated by a suitable angle and achieve a similar effect. Choice of the radius is up to the individual in this case perhaps the midpoint between the fixed axles would be a suitable centre position. Your drawing may specify it, in which case we need to know where it is to determine the minimum radius the vehicle will negotiate.

The recommended minimum radius for GL5 is about 30ft.

To check the movement required at the swing one could assume that the fixed wheels are on the tangent track/ curve track junction and the radial set in the radius. once in the curve, the carriage will act like a 4 wheel wagon but with the radial axle helping to pull the vehicle into the curve.

This will only work in one direction of travel, which seems peculiar but then it is a Webb design.

Dave T.
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Post by chaswarr » Mon Feb 19, 2007 12:30 pm

Hello again. The radial axle is carried in a frame which can move laterally thru an arc. It is not a turntable design. On the prototype the arc radius is 7 feet - axle spacing is 11 feet.

I have done a drawing with the radial axle displaced to its maximum ie the wheel is in contact with the main underframe. I have then constructed an arc thru the contact points of the wheels and rail. The arc radius is 29 feet so hopefully the carriage should run on a typical GL5 track. The wheel treads have been drawn to the GL5 standards.

Its interesting what you write about the radial axle helping to pull the vehicle into the curve but which will only work in one direction. The design was developed in the later 42ft carriages (mine is 32ft 3 axle) with 4 axles. The inner pair were fixed but the outer pair were of the radial type - so in this case the steering function would work in both direction.

Don't know why the LNWR should use this design as it seems more complicated than bogies. It may be Webb eccentricity but it may be an attempt to avoid royalties which the Victorians hated paying.

I'd be happy to share the drawing but I don't know how to upload it to the bulletin board.
Charles Warr
Romford
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Post by davet » Tue Feb 20, 2007 1:16 am

Charles,

More thoughts on your carriage. It could be that Francis Webb intended to run these carriages as pairs, close coupled, with the radial wheel sets at either end. This may then allow the lead carriage not only to move more smoothly into the curve but also nudge its mate to a similar accomadation.

Another consideration is the minimum turnout that such a vehicle will go through. If you intend to run the vehicle at a GL5 event you should check the size( Number) of the smallest turn out on the track and then check the clearance through it.

The displacement at the centre of a ridged wheel base is 'd' is:-

d=R(1-cosa) where R is curve radius
a=sin^-1(c/2R)
c is fixed wheel centres (chordal distance)

For the Webb arangement the displcement 'D' at the end of the chord is
D=(2Rsin((b-a)/2)^2-C^2)^.5
Where b=sin^-1(1.5C/R)
C= centre distance between each wheel set.

The pivot of the wheel set at 7ft seems to imply that the wheels will not sit tangentially to the track, (and the axle centre line will not go through the centre of the curve radius), but will be skewed. It may lead to some interesting tracking and wear patterns. Your wheels will need a very smooth profile to avoid shaving the rail head on the curves.

Strange man, Mr Webb. I suspect that the prototypes were not a screaming success and were superseded by bogie vehicles.

Dave T.
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Six wheeled vehicle

Post by Eddyg » Tue Feb 20, 2007 11:18 pm

Charles,

The forum is getting just a bit technical. Your vehicle will be OK down to at least 30ft radius. My six wheeler has similar axle spacing to yours and has side play on the centre axle of less than 2mm, It is quite comfortable on the main line at Gilling, although some of the remote crossings in the sidings give it some trouble, it needs a bit of extra push to negotiate them. Your 6mm at one end is I suppose equivalent to 3mm in the middle, so you'll be all right.

Eddie
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Post by davet » Wed Feb 21, 2007 3:42 am

To technical eh?

For a 30ft radius, a rough calculation shows that the offset from the centre line will be about 8mm. The angle of swing of the pony truck will be about 2.75degrees. Turnouts are a bigger problem it would seem.


Crow-baring vehicles through a turnout may be prototypical for the LNWR but most railways expected their rolling stock to roll smoothly through any track formation.

Davet.
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Minimum radius

Post by Eddyg » Sun Feb 25, 2007 10:33 pm

Charles, Dave,

A bit more technical then. The offset at the centre of a 2ft chord on a 30ft radius is 0.2" or a fraction over 5mm. Theoretically my six wheeler won't negotiate such a tight curve. How then does it manage? The flange clearance (check GL5 standards) is 1/8" which gives the other 3mm needed, and if the track is well designed the gauge will be widened on curves a sharp as 30ft radius by at least 1/16" and perhaps more.

The same points that my vehicle has to be "crow barred" through also give my loco a problem and I know it is capable of negotiating the tightest curve on one of my local tracks which is 27' 11" radius. Points are a different case as you say Dave.

So keep on with your project Charles and let's see some pictures in Turnout in due course.

Eddie
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Post by davet » Mon Feb 26, 2007 12:48 am

Charles, Eddy,

In full size practice of course several fudges were resorted to including thin flanges, extra movement in all the axle boxes, (must have given a rough ride), as well as track gauge widening, particularly through low number turnouts.

The accuracy of full size drawings to what ran on the rails is debatable in my experience, because the works staff, probably with the consent of the drawing office, frequently made unrecorded modifications to improve performamce and/or ease manufacture. The point being. that we are just as entitled to make small adjustments in our models to give good running. Just dont tell any body and most will remian in ignorance of the fudge.

Davet
chaswarr
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Post by chaswarr » Wed Feb 28, 2007 5:20 pm

Davet, Eddie. Thanks for your insights into the problem of getting long wheelbase vehicles through the curves. It's reassuring to know that a similar vehicle to the one I plan will run on the track at Gilling. The CAD predicts that the minimum radius that my vehicle will negotiate is 29 feet and this is without taking the 1/8 inch flange clearance or other wheezes like axle side play and flange thinning into account.

You have given me the confidence to get on with construction and I have placed an order with the laser cutter for the various stretchers, soleplates and reinforcements etc for the underframe.

Thanks again
Charles Warr
Romford
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