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Post by 100andthirty on Oct 13, 2010 19:20:47 GMT
the shoebeams are connected to the axleboxes via the brakcets that crack. the wheelsets follow the irregularities in the track and it is these irregularities that input vertical, lateral and torsional movements into the shoebeams/brackets. These movements haven't changed with the new bogies. It's the frame that flexes and accommodates the track irregularities better without having to have very soft primary suspension; the wheel movements haven't changed.
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Post by tubeprune on Oct 13, 2010 19:37:35 GMT
the shoebeams are connected to the axleboxes via the brakcets that crack. the wheelsets follow the irregularities in the track and it is these irregularities that input vertical, lateral and torsional movements into the shoebeams/brackets. These movements haven't changed with the new bogies. It's the frame that flexes and accommodates the track irregularities better without having to have very soft primary suspension; the wheel movements haven't changed. I was thinking of the differences in lateral motion between the two axles. If the frame for the leading axle is moving in a different way from that of the trailing axle, surely the motion imparted to the axle ends will vary compared with that of a rigid frame? |
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Post by singaporesam on Oct 14, 2010 13:57:19 GMT
The purpose of the design was to accommodate severe twist and lozenging , and the Bogie guys in Derby did a lot of work on this. Maybe there is some kind of excessive anti-phase yawing of the wheelsets that is causing cyclic compression and tension in the shoebeam.
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Post by tubeprune on Oct 14, 2010 18:22:17 GMT
The purpose of the design was to accommodate severe twist and lozenging , and the Bogie guys in Derby did a lot of work on this. Maybe there is some kind of excessive anti-phase yawing of the wheelsets that is causing cyclic compression and tension in the shoebeam. Exactly! I couldn't have put it better myself! |
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SE13
In memoriam
RIP 23-Oct-2013
Glorious Gooner
Posts: 9,737
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Post by SE13 on Oct 14, 2010 18:45:51 GMT
Going back to previous answers, strike action and a work to rule will result in the same problem as we have on buses... They simply can't be taken out on service unless they've had their 21 day service, which results in a shortage of traffic.
Drivers can't complain, they just sit around and drink copious amounts of coffee while being paid full wages.
Passengers will suffer, but as always, it's the drivers that take the brunt of the deal. It ain't my fault I can't take ***** bus out, it needs a service, so yes I had to drop the last run because I simply didn't have a bus to drive.
There are times when I wonder about the mentality of passengers.... We can only do what we can with the equipment we are given. If it isn't there we can't use it. We can't fly the things.
In short, if it's late or doesn't run, then there's a damn good reason behind it, and it isn't our fault.
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Post by singaporesam on Oct 15, 2010 16:32:15 GMT
the wheel movements haven't changed. The range and type of wheel movements that an individual wheelset is able to make will not have changed . However the manner in which the two wheelsets in a bogie are moving relative to one another will definitely be different. |
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Post by t697 on Oct 15, 2010 21:11:12 GMT
The range and type of wheel movements that an individual wheelset is able to make will not have changed . However the manner in which the two wheelsets in a bogie are moving relative to one another will definitely be different. Assuming the wheels weren't completely unloading in the original bogies, then any difference is only to the extent that the difference in bogie frame stiffness alters the deflection of the actual trackform where it isn't properly supported by the ballast and formation surely? I'd predict that difference to be of second order to the general effect of normal vibration, track irregularities and so on. D78 stock is 30 years old now, some components need exchange. Similar components generally have this sort of trouble at about the same time. It will be interesting to see whether later computer FEA assessed designs meet their predicted lives. I think they will generally do better, time will tell. |
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Post by singaporesam on Oct 16, 2010 6:44:27 GMT
Not quite, the flexible joint would permit much larger in phase (parrelogramming) and anti-phase yawing movements of the axles than would have been permitted in the original bogie. If the brackets are steel then any problems due to normal forces would have been found earlier , once this type of component gets past 6-7 years life you can pretty much say that they're good forever from a fatigue perspective. However if you change the loading then thats a different story altogether.
As for FEA assessment there are two views on this, one is the one you've noted i.e. that better prediction allows the component to be better designed and assessed to meet the required life, the other is that it brings an end to conservative design rules and in doing so makes reliability and safety dependent on the competence and modelling skills of the stress Engineers.
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Post by t697 on Oct 16, 2010 8:23:09 GMT
I can see the scope for parallelogram movement with the D78 bogie, not so sure about antiparalleling since the side frame holds the two axleboxes for that side and the primary suspension is unchanged.
However, similar fatigue issues arise with similar components on the older plate frame bogies which have fairly flexible frames so without a careful comparison to the original fairly rigid D78 bogies it is difficult to be sure that deterioration has been accelerated. Perhaps someone on the design team would comment.
Good point on getting FEA modelling right.
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