Looking for a six-coupled locomotive

[tom_tom_go]

This did make me chuckle



Although you can't beat:

[artfull dodger]

Also keep in mind, if you pile on the extra weight, your going to induce more wear into the motion. Dont know about Accy's but Roundhouse uses unbushed rods and valve gear, so any extra weight will add to the work they do and wear them quicker. I think the power to weight ratio is spot on with the Roundhouse engines. You need to be able to slip the drivers at some point so you dont overstress other pieces of the motion. On an unsprung engine, if your track work is a bit dodgy, an engine can lift the front drivers clear of the rail if the middle and rear axles are supporting the engine and the track dips down at a joint, this might be problematic when going thru a curve. Roundhouses just run....well like a Roundhouse. Smooth, dependable and they pull well. I can easily pull more freight wagons, all LGB, and the lenght of the train looks to be about right for a light railway goods train. Mike

[Boustrophedon]

Assuming classical friction (actually a very good assumption for unlubricated steel on steel) then neither the number of points of contact nor the area of contact matter. Only two things matter; load pushing the surfaces together and the coefficient of friction. So the number of driven wheels doesn't make a difference.

Of course a real engineer working on real trains has to observer maximum rail loads and wheel wear, so may wish for more wheels. But every wheel that supports the loco weight but is not driven relieves the driven wheels of load and hence traction.

Lady Ann has four driven wheels (2 not touching), Silver lady six, but are otherwise pretty much the same with much the same weight, so one would expect pretty much the same tractive force.
But Silver lady shares that effort between 6 wheels so all the bearing loads are going to be only 2/3 of those in Lady Ann. Got to be better?

I own neither so I will leave real world experiments to others.

[Boustrophedon]

Extract from my copy of the "LOCOMOTIVE ENGINEERS POCKETBOOK" (dated 1926)

Right...  N = Number of coupled axles, T = Tractive Force, L = Load on coupled Axles.

Taken from the above book:

N= 5T
    2240L

for those that don't like algebra, (sorry if I have over simplified this) this means:

N = (5xT)
    (2240xL)

By multiplying both sides by (2240xL) this can be rewritten as:

N x 2240 x L = 5 xT

Dividing both sides by 5 gives:

N x 448 x L = T

or

N x L x 448 = T

Now assuming no leading or trailing axles ie 0-?-0
NxL (number of coupled axles x Load on coupled axles) = Loco Weight.

Therefore:

(Loco Weight in Tons) x448 = Tractive force in Tons     (assuming the coefficient of friction is 0.2)((taken from book above))

Therefore assuming the weight of the loco does not change it does not matter if the centre drivers of a Lady Ann touch or not the Tractive force is the same  ;)

Your algebra is better than that in the book, They have confused pounds and tons somewhere along the line.

The correct equation should be:

LxN/5=T  (all loads in tons)

The 5 being 1/coeff of friction which is taken as 0.2

Giving T= 0.2xLxN since as you say LxN is the total vehicle weight then:

Tractive effort =0.2 x Loco Weight. Which is what you should expect.

Your equation: (Loco Weight in Tons) x448 = Tractive force in Tons Pounds force

Gives the tractive effort in pounds. Your conclusion is correct though.  :D

[GTB]

Assuming classical friction.........

<snip>

I own neither so I will leave real world experiments to others.

I did some experiments on drawbar pull last year and the results matched what would be predicted by the Coulomb model of friction, so it is near enough for practical purposes.

In garden scale I got a co-efficient of friction for steel wheels on brass rail of ~0.2. These little steam dragons have a habit of spreading oil far and wide and that result is close to the quoted static co-efficient of friction for greasy surfaces.

It would be interesting to know if the Coulomb model still applies to very small scales. I know it still applies in HO, as I've done the tests in the past. I've seen a working steam locomotive built to a scale of 1:240 running on a circular track, but it was running light. I can't imagine it developed a lot of drawbar pull, although it hauled itself around as well as any electric model does in that scale.

My reservation with sprung Roundhouse chassis is that the coupling rods are articulated by using a lap joint in the rods on the centre crank pin. The individual bearing surfaces are very narrow on that pin and with reciprocating forces there is no guarantee that both halves of the joint are sharing the bearing load equally all the time. I've seen this sort of joint wear very quickly in HO in the past. I have one sprung Roundhouse loco and one unsprung, so I guess I'll find out eventually.

Graeme

[Boustrophedon]

Assuming classical friction.........

<snip>

I own neither so I will leave real world experiments to others.

I did some experiments on drawbar pull last year and the results matched what would be predicted by the Coulomb model of friction, so it is near enough for practical purposes.

In garden scale I got a co-efficient of friction for steel wheels on brass rail of ~0.2. These little steam dragons have a habit of spreading oil far and wide and that result is close to the quoted static co-efficient of friction for greasy surfaces.

It would be interesting to know if the Coulomb model still applies to very small scales. I know it still applies in HO, as I've done the tests in the past. I've seen a working steam locomotive built to a scale of 1:240 running on a circular track, but it was running light. I can't imagine it developed a lot of drawbar pull, although it hauled itself around as well as any electric model does in that scale.

My reservation with sprung Roundhouse chassis is that the coupling rods are articulated by using a lap joint in the rods on the centre crank pin. The individual bearing surfaces are very narrow on that pin and with reciprocating forces there is no guarantee that both halves of the joint are sharing the bearing load equally all the time. I've seen this sort of joint wear very quickly in HO in the past. I have one sprung Roundhouse loco and one unsprung, so I guess I'll find out eventually.

Graeme

I was assuming an articulated joint not on the crank pin as per full sized. Doing it that way doubles the load on each bearing surface on each rod on the centre pin. Poor little crank pin.

[Keith S]

Graeme, did you SEE one of A.Sherwood's little live steam engines? DO you have any pictures or more information than the internet search provides? I've wanted to see more of those engines for years ever since I found out about them. Apparently there was a video?

[funandtrains]

The Accucraft 0-6-2 Joan is just arriving in the shops if you want a real model.

[GTB]

Graeme, did you SEE one of A.Sherwood's little live steam engines?

Yes, I saw him running them at an AMRA Exhibition in Sydney back in the late '70s. See is a relative term. Even that far back I had trouble clearly seeing something that small.

Arthur Sherwood was a Professor in Mechanical Engineering at Sydney Uni. and his field was kinematics. He also designed and made orthopaedic appliances and instruments.

There's not much published about his model railway interests, the '60s and '70s articles are about all I've seen and they are on this net site.

http://www.zen98812.zen.co.uk/aasherwood.html

I haven't seen a video of any of his models, but there may one around. That far back we'd probably be talking Super 8, rather than video, so it would be pretty grainy. I'd have thought it would be dubbed to video and on Youtube by now if one existed.

Some of his models survive on display in the Engineering Dept. at Sydney Uni. I believe.

Regards,
Graeme

[Keith S]

I wish I'd been able to see that. I thought while I was searching the internet I came across a reference to a video/movie about him in the Australian film archives. I didn't get anywhere trying to find anything on their site though.