8 Coupled CSB Question

[Mark Tatlow]

At ExpoEM yesterday, i fell into conversation with a gentleman who was struggling to get Will's CSB calculating programme to give him answers.

That was solved fairly easily, but I was less certain about his next question. He has read on the CLAG site that for 6 and 8 coupled locos, it may be better to lighten the load on the centre axles slightly to prevent any tendancy to rock on them.

He was querying whether this was true (and I do recall that there has been conversation of this nature) and if this is desired, how might the programme assist in giving some revised fulcrum positions?

Can anyone assist?

[garethashenden]

Wouldn't it be a case of adjusting the positions of the fulcrums until the outer axles have a higher percentage of the load than the inners? When I have used the spreadsheets before there's been an element of try it and see involved.

[billbedford]

Porpoising, i.e. rocking of the centre axles, is a theoretical possibility, but it's not likely to visible if the axles are more or less equally weighted.

[Mark Tatlow]

Gents,

It is not the principal I was looking for, it was whether slacking off the weight on the centre axles was sensible and how do you achieve it on the spreadsheet I was asking.

[billbedford]

No, and, making the centre spans longer or the outer spans shorter.

[Will L]

At ExpoEM yesterday, i fell into conversation with a gentleman who was struggling to get Will's CSB calculating programme to give him answers.

That was solved fairly easily, but I was less certain about his next question. He has read on the CLAG site that for 6 and 8 coupled locos, it may be better to lighten the load on the centre axles slightly to prevent any tendancy to rock on them.

He was querying whether this was true (and I do recall that there has been conversation of this nature) and if this is desired, how might the programme assist in giving some revised fulcrum positions?

Can anyone assist?

This advice was related primarily to 6 coupled chassis. I understand why it was given, I may even have given it myself, but these days I doubt it the danger of such oscillation about a centre axle is worth worrying about. An 8 coupled chassis doesn't have the same theoretical susceptibility in the first place.

When you are faced with the every day observation that anything which is suspended from un-damped springs will tend to oscillate on those springs, you have to wonder about the observable fact that CSB locos, or for that mater, the current generation of sprung 4 wheel wagons, don't. In playing with various ways of springing 4 wheel vehicles, I actually did managed to produce a wagon which does exhibit this sort of oscillation, but most systems didn't and behave as if the springs are effectively damped.

The reason why, I decided, was because there is usually friction between the thing which carries the axle bearing and a fixed element to the vehicle chassis. My conclusion is that quite small amounts of friction of this sort is quite enough to dampen down the system sufficiently to prevent any oscillation. For wagons this is usually the bearing carrier rubbing against the W iron, in locos its the axle blocks moving in the horn blocks. Clearly if the middle axle was significantly harder than the outer pair, undesirables effects are possible, but not so much in terms of oscillation, as in effects like the front of the loco lifting significantly when picking up a heavy train.

Therefore I would now say that, when we are aiming for equal weight on each axle, any small errors either way will not produce undesirable motions in the chassis. So the guidance should be to get as close to equal distribution of weight as is easily possible but don't be bother is you can't get the numbers out of the spread sheet exactly the same. It is probably more important to get the weight distribution on the first and last axle the same than getting any particular weight on the middle axles(s).

[Mark Tatlow]

Thanks Will, I think that was probably the answer that was being looked for.

[Alan Turner]

Gents,

It is not the principal I was looking for, it was whether slacking off the weight on the centre axles was sensible and how do you achieve it on the spreadsheet I was asking.

which spread sheet are you using? If it is mine then the fulcrum points are altered using the scroll boxes.

regards

Alan