adrian
Flying Squad
My only thought on this is maybe the two beams have become mechanically linked i.e. if both of them were soldered to the brass tube of the pivot point whereas they should be able to pivot independently.
7mm Sandy's Workbench Thread
- Thread starter Sandy Harper
- Start date
My only thought on this is maybe the two beams have become mechanically linked i.e. if both of them were soldered to the brass tube of the pivot point whereas they should be able to pivot independently.
Sandy Harper
Western Thunderer
Adrian, That was my thinking as one fixed axle, with the other two on a compensating beam, has got to give you at least two wheels touching the railhead on one side even if the third wheel is on a piece of dirt.
Sandy
Sandy
simond
Western Thunderer
Hi Sandy,
it’s not an assumption!
simple corollary. If you have a “perfect” four legged stool, and an uneven floor, it’ll rock. Remember every pub table in the country... (I remember going to pubs...) Whereas if you have a three legged stool, it will always touch the ground.
Some kit makers (and I can name a few but I won’t) might be able to make beautiful models, but they don’t understand physics or engineering principles. So them putting an unnecessary and unhelpful part in their kits is not going to convince me otherwise!
And if you need more convincing, look out the paper Brian Clapperton wrote, it’s on his ABC Gears website, I think.
I don’t have a copy of Mike Sherman’s book, but I believe it clearly and thoroughly explains the principles of compensation.
Twin beams are perfectly reasonable, provided the other axle(s) can rock, indeed, you will need to do this on 8-coupled chassis. The beams must be independent. If they’re linked, then you’ll end up with three wheels touching on one side and, worst case, only one on the other.
ATB
Simon
it’s not an assumption!
simple corollary. If you have a “perfect” four legged stool, and an uneven floor, it’ll rock. Remember every pub table in the country... (I remember going to pubs...) Whereas if you have a three legged stool, it will always touch the ground.
Some kit makers (and I can name a few but I won’t) might be able to make beautiful models, but they don’t understand physics or engineering principles. So them putting an unnecessary and unhelpful part in their kits is not going to convince me otherwise!
And if you need more convincing, look out the paper Brian Clapperton wrote, it’s on his ABC Gears website, I think.
I don’t have a copy of Mike Sherman’s book, but I believe it clearly and thoroughly explains the principles of compensation.
Twin beams are perfectly reasonable, provided the other axle(s) can rock, indeed, you will need to do this on 8-coupled chassis. The beams must be independent. If they’re linked, then you’ll end up with three wheels touching on one side and, worst case, only one on the other.
ATB
Simon
Sandy Harper
Western Thunderer
Simon, I'm still trying to get my head around this. Are you saying that to use twin beams on the front two axles effectively, the third axle would also need to be able to rock independently or be compensated (linked) to the middle axle so that all three axles had a degree of movement dependent on each other?
Sandy
Sandy
simond
Western Thunderer
Hi Sandy
you know what they said about pictures and thousands of words...
Firstly, a simple 0-4-0. Arrange that one axle is in fixed bearings and the other can rock side-to-side about the centre-line of the loco.
the Three Legs of the stool are the three red arrows of course.
this is the normal 0-6-0 with a rigidly fixed axle, and a single compensation beam on the other axles. This means that the two axles can rock from side to side, independently, and also, if one goes "up" the other gets pushed "down". This is probably the easier way to do an 0-6-0, and the motor can go on the fixed axle.
ok, this is the other way of doing a 6-wheel chassis. The rocking beams (in black) are independent, and they provide two points, the third point is the middle of the other axle. My 14/48xx is compensated like this, with the motor on the middle axle. The trailing truck is supported centrally.
It may be that this gives better weight distribution than the previous example. Another advantage of this is that it does give smoother running than the simple version. No free lunch, it's extra work of course!
and just in case anyone is building a 2-8-0 or similar - my Finney 47xx has this - Martin was one of the kit makers who did know what he was doing!
Turning back to your question, if you link the two rocking beams so they are not independent, you then have 4 wheels that can only all touch the rails if both chassis and rails are perfect, and of course, that constraint is what you're trying to avoid. You really do not want the beams connected.
In all these images, you have to imagine that the loco presses down on the beams or axles at the red arrows, and the beams press on the top of the axles (or axleboxes), and it does this so that the weight of the loco is shared (literally "equalised" at least in theory) over the wheels. In most cases, the wheels can drop under their own weight, but the aim of the game is to actually press them down onto the track. Equalisation is probably not actually exact, as it depends on the wheelbases of the loco, and the weight distribution - but it will certainly be very much better than a rigid chassis.
Hope this makes it clearer
atb
Simon
you know what they said about pictures and thousands of words...
Firstly, a simple 0-4-0. Arrange that one axle is in fixed bearings and the other can rock side-to-side about the centre-line of the loco.
the Three Legs of the stool are the three red arrows of course.
this is the normal 0-6-0 with a rigidly fixed axle, and a single compensation beam on the other axles. This means that the two axles can rock from side to side, independently, and also, if one goes "up" the other gets pushed "down". This is probably the easier way to do an 0-6-0, and the motor can go on the fixed axle.
ok, this is the other way of doing a 6-wheel chassis. The rocking beams (in black) are independent, and they provide two points, the third point is the middle of the other axle. My 14/48xx is compensated like this, with the motor on the middle axle. The trailing truck is supported centrally.
It may be that this gives better weight distribution than the previous example. Another advantage of this is that it does give smoother running than the simple version. No free lunch, it's extra work of course!
and just in case anyone is building a 2-8-0 or similar - my Finney 47xx has this - Martin was one of the kit makers who did know what he was doing!
Turning back to your question, if you link the two rocking beams so they are not independent, you then have 4 wheels that can only all touch the rails if both chassis and rails are perfect, and of course, that constraint is what you're trying to avoid. You really do not want the beams connected.
In all these images, you have to imagine that the loco presses down on the beams or axles at the red arrows, and the beams press on the top of the axles (or axleboxes), and it does this so that the weight of the loco is shared (literally "equalised" at least in theory) over the wheels. In most cases, the wheels can drop under their own weight, but the aim of the game is to actually press them down onto the track. Equalisation is probably not actually exact, as it depends on the wheelbases of the loco, and the weight distribution - but it will certainly be very much better than a rigid chassis.
Hope this makes it clearer
atb
Simon
Sandy Harper
Western Thunderer
Thanks Simon, That too, is my understanding of the principle. My issue with the third axle, centre pivot, is how is the lateral movement of the footplate is constrained and avoiding it rocking from side to side?
As I said originally this is the first time I have used double beams as my preferred method is springing with a fixed driving axle.
Regards
Sandy
As I said originally this is the first time I have used double beams as my preferred method is springing with a fixed driving axle.
Regards
Sandy
Rob R
Western Thunderer
Sandy,
Lateral movement of the frames, footplate and everything upwards is constrained by the twin rocking beam pivots.
Although those two axles can flex to suit the track, the beam itself (and ergo the chassis etc) stays level.
The single pivot at the other end provides longditudinal stability.
Give it a go (or knock up a cardboard test chassis with wagon wheels) and push it over a bit of wobbly track and all will be revealed.
Rob
Lateral movement of the frames, footplate and everything upwards is constrained by the twin rocking beam pivots.
Although those two axles can flex to suit the track, the beam itself (and ergo the chassis etc) stays level.
The single pivot at the other end provides longditudinal stability.
Give it a go (or knock up a cardboard test chassis with wagon wheels) and push it over a bit of wobbly track and all will be revealed.
Rob
Rob Pulham
Western Thunderer
Hi Sandy,
I have to chip in and say that I love the ugly duckling and can I ask whose kit it is please? I would love one and was planning to scratch build one at some point but if there's a kit available then I may think again.
I have to chip in and say that I love the ugly duckling and can I ask whose kit it is please? I would love one and was planning to scratch build one at some point but if there's a kit available then I may think again.
simond
Western Thunderer
Sandy,
all I can say is that if you do use twin beams, with a fixed axle, you will have a “four legged stool”, and it will rock. That’s not to say it won’t run, but I’m sure it would run better if it only had three legs!
if you cannot fit a single, central beam between the leading & central axles (perhaps working inside motion?), you have a challenge arranging a rocking axle at the rear, if that has the gearbox on it. I recall a build (on here, I think) of a GW loco, possibly a MOK pannier, which had a cage surrounding the gearbox, to which a rocking bearing was fitted. I don’t think I’d want to do that - I’ve tended to fit sprung hornblocks on the last few I’ve built, and that would be my approach.
looking forward to the build anyway
Atb
Simon
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SimonT
Western Thunderer
The 64XX was probably the one on my build thread. Never had a problem with it running on either Aberbeeg or The Back End where the track was deliberately a dog's arse in places. It's now even better as it is my prototype battery and R/C loco using the Protocab kit - the only snag is that it is now silent.
I think that once we get to eight and ten wheelers the three point principle becomes less important as there are more wheels in play and because a lot of us use top contact pick ups that will give a degree of down force to a wheel. My MOK 9F seems to cope despite some criticism of it's suspension.
Simon
I think that once we get to eight and ten wheelers the three point principle becomes less important as there are more wheels in play and because a lot of us use top contact pick ups that will give a degree of down force to a wheel. My MOK 9F seems to cope despite some criticism of it's suspension.
Simon
simond
Western Thunderer
Hi Simon
might well have been your 64xx I remembered.
5 axle locos are going to be a fiddle one way or another. I think if I ever built a 9F I'd spring it. My 47xx is compensated as per my drawing above, as is my 52xx, but my 28 is sprung. They all work so I guess there's little to choose between them.
atb
Simon
might well have been your 64xx I remembered.
5 axle locos are going to be a fiddle one way or another. I think if I ever built a 9F I'd spring it. My 47xx is compensated as per my drawing above, as is my 52xx, but my 28 is sprung. They all work so I guess there's little to choose between them.
atb
Simon
Sandy Harper
Western Thunderer
paulc
Western Thunderer
Can i just say a big thankyou to Simon for explaining how rocking beam type compensation works . I have nearly 30 locos and none of them have rocking beams as i have always sprung them or occasionally used a ridgid chassis but i have a kit coming up that has the option so may give it a go . Keep the info coming guys , its always welcome and stay safe .
Cheers Paul
Cheers Paul
Sandy Harper
Western Thunderer
Basic body work complete.
Sandy Harper
Western Thunderer
A marathon 'lock down' detailing session today. I'm now stuck awaiting a delivery of handrail knobs. Why did the GNR put so many lamp brackets on? 13 in total!!!! I needed to scratch build the coal rails as there was none in the test etch.
Sandy Harper
Western Thunderer
Now ready for a visit to the paint shop!
Sandy Harper
Western Thunderer
Slow but steady progress. Remainder of the castings have arrived, so no excuses now!
