Britannia wheel balance weights

[Deleted]

I'm building the Modelworks 5" Britannia (I have a website at uk.geocities.com/britanniabuilder ) and I've been alerted to the fact that the balance weights on the driving wheels, which consist of steel plates bolted inside and outside the spokes, do not have lead filling in between all the spokes. Does anyone have information on this - would it be standard across all Brits, or are the wheels balanced individually?

Regards, John

[JohnP]

In full size, the wheels were balanced individually (or maybe a pairs on an axle) The exception with BR satandards was those fitted with roller bearing axleboxes, which had "average" weights determined after a few wheels were balanced. This was due to the balancing rig not being able to accept roller bearing axleboxes, so dummy plain ones were used and the wheels had to be pressed off, the axleboxes changed over, and the wheels pressed back on. Having done this a few times they got fed up and used the "average" technique. On some locos the balancing was very poor and as well as rough riding caused failure of the intermediate drawgear (between the engine and tender).
BUT in our sizes, I've never known anybody balance their wheels. Someone will probably shoot me down in flames but it certainly isn't the norm.

Hope that helps.

JohnP

[Deleted]

John, thanks, that's very helpful. I'm surprised that they balanced the wheels in isolation - I would have thought that they would want to balance part of the weight of the coupling rod as well as the crankpin - or maybe they added a dummy weight to the crankpin? I wasn't so much thinking of balancing the model wheels, but more aiming to get a realistic effect. It's also interesting that the balance weights on the leading and trailing axles are not centred diametrically opposite the crankpins, but offset by half a spoke - I can't think why this should be.

Regards, John

[JohnP]

Hi John, You raised a good point about balancing in isolation. I must admit the article I read didn't go into that much detail, but it's certainly true that you'd include part of the connecting rod when balancing an internal combustion engine.
Regarding your question about the balance weights for the coupled wheels, I admit that my first thought was that it only has to balance the coupling rod so should be opposite the crank pin - BUT - There's a complication. The coupling rod is outside the wheel, so even if it is statically balanced, when rotating it will produce a twisting effect. This could be prevented by not fully balancing the coupling rod on its wheel, but adding some extra balancing on the opposite wheel, which would be offset from the crankpin on the second wheel. The same argument would apply when balancing the second wheel, with some offset weight on the first one.

Now this is only what I think might have been done. I have read explanations in the past which I haven't fully understood but maybe it's starting to percolate through..

At least this conversation is getting the number of my postings up!

Regards,

JohnP

[havoc]

This is a rather complicated topic, since you have to balance the up-down component of the forces of the whole motion. It isn't possible to compensate at the same time the back-front motion. So yes, you compensate for the connection rods, but also for the drive rod from the piston. When this is angled -in the dead points- with respect to the connection rods, then you need to offset the balance weights.

[waggy]

Hate to throw a spanner in the works, but I remember reading many years ago in the railway magazine that the Southern Region had turned out of works a Bullied pacific with no balancing whatsoever! They measured the impact in traffic, the results were inconclusive!
In "our" sizes I suspect balancing is more good luck than good judgement. Do we go fast enough to imitate scale speeds? Is our motion heavier than prototype, taking into account material mass, etc?
I think the old adage "if it looks right, it is right" comes into play. Modellers licence? If you want to fill 'em in, fill 'em in!

Waggy.

[havoc]

Is our motion heavier than prototype, taking into account material mass, etc?

Yes. But I do agree with the rest of your remarks. On the other hand, why not?

[greasemonkey]

HI All
I dont think it will matter what you do as the pockets usually filled up with muck and rubbish anyway. As a matter of intrest Phil Atkins has a section on balancing wheels in his book "Britannia, Birth of a Locomotive" In this book Cox is quoted as saying that "Advantage has been taken of the weight and length of the new engines to balance 40% of the weight of the reciprocating parts". Phil then goes on to say that "a portion of the reciprocating weight was the connecting rod, the small end of which was reciprocating and the big end rotating. It was an arbitrary decision (usually taken as one third) regarding how much of its weight was indeed reciprocating and what percentage of the (total) reciprocating weights should be balanced. Three and four cylinder engines are largely self balancing and so could achieve nil hammer blow..... High reciprocating balance made for improved riding but at the cost of increased of hammer blow on the track, which increased as the square of of the rotational speed".
The change in coupling rod section on the Britannia's necessitated a recalculation of the balancing arrangements. The balance weights in the centre coupled wheels of engines from 70025 onwards extended over only 7 spokes instead of the previous 8. Out of intrest which rod style are modelworks supplying, the original fluted or later fish bellied?

that the Southern Region had turned out of works a Bullied pacific with no balancing whatsoever! They measured the impact in traffic, the results were inconclusive!

this is possibly due to the statement mentioned above that 3 and four cylinder engines tended to self balance, however
an Austerity 2-8-0 , a design that had no balancing for reciprocating mass and consequently known to be "rough riders" was used in slipping test on 9th December 1951. "With the rails greased, the engine was allowed to loose its feet with the regulator wide open. The outcome was that the leading pair of driving wheels rose above the rail, due to the unbalanced reciprocating masses". Has anyone got any idea what would happen in a similar situation with as model engine?!!
If you want to try balancing your wheels then put weights on the crank pins equal to what ever percentage of con/coupling rod weight you want to balance for. Let us know how you get on.

Andy

[havoc]

Wouldn't it be possible with modern cad soft to actualy "simulate" this? I know from experience that Solidworks can calculate the mass center of a whole assembly. So you could draw the whole drive train, then calculate the mass center for different axle positions. Add weight and try again. Not automated, but you can do everything as a whole instead of a single axle at a time and guessing the part of connection rod you take into account.

[Deleted]

Thanks for all the feedback. As I mentioned earlier, I wasn't particularly thinking about balancing the wheels, but more about getting a realistic scale model. I've been contacted by a volunteer at Great Central Railways where Oliver Cromwell is being restored and I've asked him if he can find out how the weights are positioned - I'll then take a view on whether to mimic this or not - I suspect it might look rather gap-toothed, and it might in the end look neater to just fill all the gaps.

The coupling rods from Modelworks are the fish-bellied type as currently fitted to OC - I haven't got that kit yet, but I'm told that the amount of curvature is not as much as it should be.

On the subject of balancing, I think that the vibration or hammering effect on a model is relatively less than on the full size locomotive. To keep the numbers simple, and this is stretching my memory of A-level physics to the limit, assume that we have a 1/10th scale model. We would expect to run it at about 1/10th scale speed, ie 9mph instead of 90mph flat out. So the revs per minute of the wheels would be the same on the model and full size locos. If we look at the simple case of a completely unbalanced crankpin, ignoring coupling rods etc, the centrifugal or hammering force would be proportional to (crankpin mass x rpm squared x distance of crankpin from centre of wheel). For the same rpm this means that the hammering force on the real loco would be 10,000 times greater - the crankpin mass being 10 cubed times greater, and the radius being 10 times greater. The static weight on the track is only 1,000 times greater. (Although I do take the point made earlier about masses not being exactly to scale). So the hammering force as a percentage of axle load increases in direct proportion to the scale of the loco. The model would have to go at (square root 10) times scale speed - ie about 30mph - to get the same hammering effect.

I guess this explains why nobody has found the need to balance wheels on models.

[steammadman]

Most wheels @ 4' - 81/2 were ballanced to some degree, at least they were at DONCASTER.
If you look at the subject logically, what would happen if two very badly ballanced wheels were put on the same axle. A VERY VERY ROUGH RIDE. And as any old loco man will tell you loco 's were hard enough riding as it was.
Also why do you ballance your car tyres,?. A nd dont say they are rubber and inflated, you can get the same effect with steel.

[Deleted]

I've now received photos of Oliver Cromwell's wheels, and I've posted one on my website at www.britanniabuilder.co.uk on the kit 8 page. When viewed with the balance plates at the bottom, the leading and trailing wheels have the rearmost gap half-filled and the next gap somewhat less than half filled, and the other two gaps appear to be empty. On the driving wheels, the second and third gaps from the front are completely filled, and the other five gaps appear to be empty. I suspect that the asymmetry on the leading and trailing wheels compensates for the fact that the plates are not exactly opposite the crankpin, but offset to the front by half a spoke (see reply #2 above). The asymmetry on the driving wheels is probably to balance the return crank, which hangs backwards from the crankpin.

[david]

JJ, I have just visited your website and found it most informative since I am building a 5" Britannia. Thank you for that.
Kind regards David

[Deleted]

Hi David,

The comments I made above about the amount of lead in Oliver Cromwell's balance weights based on the photos that someone sent me are roughly correct, but you can visit OC at Loughborough any day to see the exact amounts, or I can check them next time I visit if you like. I've decided just to fill all mine about 3/4 full, which looks realistic and hides the rather large screws between the Modelworks balance plates.

Kind regards, John

[david]

Hello JJ,

Thank you for your kind offer. In the mean time I will follow your lead re the levels of the lead as getting to Loughborough is a wee bit far even at the weekend as I live south east of Melbourne, Australia.
Can you please point me in the right direction of obtaining a full side on colour photograph of Oliver Cromwell. I have looked in magazines etc but they are usually taken at about 30 - 45 degrees angle from the front.
Thanking you in anticipation.

Kind regards, David

[Deleted]

Hi David,

I agree that a day trip to Loughborough would be a bit far (although I once did a day trip to Sydney on business). I'll measure the lead when I next visit, although this may not be for another few weeks. I haven't seen a side-on colour photo of Oliver Cromwell, although I can take one for you when they eventually put it back together - I guess that this will be at least a year away, judging by current rate of progress.

Kind regards, John

[david]

Hello John,

Thank you for your kind offer re the photo and all the best with the wheel balancing project.

Kind regards, David