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Post by orienteer on Dec 26, 2015 17:06:39 GMT
I've been rather surprised to hear evidence of wheel flats on some S stock lately (on the Met).
Admittedly it's nothing like as loud as it used to be on A60 stock, but I would have thought the use of electric braking down to around 10mph would have precluded wheels sliding.
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North End
Beneath Newington Causeway
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Post by North End on Dec 26, 2015 18:12:39 GMT
I've been rather surprised to hear evidence of wheel flats on some S stock lately (on the Met). Admittedly it's nothing like as loud as it used to be on A60 stock, but I would have thought the use of electric braking down to around 10mph would have precluded wheels sliding. Like with ABS in cars, WSP doesn't prevent wheelslide, it only mitigates it by momentarily throwing off the brakes on affected wheelsets. It can't do miracles, so if conditions are bad enough the wheels will still lock up. So whilst the modern trains are less likely to pick up flats, they're not immune from it. I remember a 95 stock overran Totteridge a few years ago, the railhead conditions were the worst possible combination of first train, downhill gradient, autumn, light rain and a windy night. Despite the WSP all working correctly, the train overran by some distance and every single wheel was flatted beyond repair. Despite WSP generally getting more advanced over time, it can't do the impossible. |
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Post by domh245 on Dec 26, 2015 19:39:28 GMT
You may have noticed that in the event of wheelslip, the electrical braking does drop off completely (actually - can someone in the know confirm if it is the regenerative stopping, or both the regenerative and rheostatic) and switches over to the disk braking only - It's quite easy to spot, you'll start braking into a station and then suddenly the motor and inverter noises come to an abrupt stop, and if you are seated near to one of the saloon brake gauges you'll see it shoot up
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Colin
Advisor
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Post by Colin on Dec 27, 2015 16:12:44 GMT
It may also be possible that drivers are trying to do the job of wheel slide protection rather than staying in a braking position and allowing Wheel slide protection to do its thing....
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Post by orienteer on Dec 31, 2015 16:03:49 GMT
With electric braking, the wheels cannot lock completely, because as the wheels slow down the current generated drops to zero and thus there is no braking effect. That's why I don't understand how flats develop. The friction brakes come on as the speed reduces to about 10mph to provide the final braking to a standstill. Is it possible to develop flats in the last 10mph?
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Post by 100andthirty on Dec 31, 2015 16:21:35 GMT
Good WSP relies on controlling the brakes on each axle individually. This is best achieved with the friction brake. Current traction packages generally control four motors. This means that if one axle detects slide, then all four axles need to have their brake effort reduced which, at best, delivers less braking effort than can be achieved with the per axle friction brake control. Hence, S stock dumps the dynamic brake (rheostatic or regenerative) and applies the friction brake if WSP is detected (not sure whether this is per car or per train but I am sure t697 will confirm).
All that said, WSP can only make the best of whatever adhesion is available. Sanding helps increase adhesion, but I don't think this has yet been commissioned. Also if adhesion is poor enough, the WSP might eventually 'time out' and let the wheel lock, leading to flats. Finally, this last autumn season has been very difficult with many parts of the country seeing sudden massive dumps of leaves in wet conditions - ideal for poor adhesion/flats.
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Post by philthetube on Dec 31, 2015 16:50:41 GMT
On slippery tracks when an A stock slid the driver should have thrown off the brake and re applied it, to avoid flats, however because overrunning a station, or worse, having a spad would cause issues. Flats on an A stock train are not attributable to a particular driver. On S stock flats should not occur during normal braking, I seem to remember from somewhere that wsp drops out after 1 min of sliding, but I cannot imagine that ever happening. However if an S stock is sliding using an emergency brake overrides the wsp and applies a full friction brake, hence flats.
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Post by t697 on Dec 31, 2015 16:51:23 GMT
Yes the S stock goes to friction braking on all cars once you get any WSP activity.
Sanding still not in use on S stock this season. But the Sandite trains on the Met have done a pretty reasonable job. As a Met passenger I've not noticed very many flats on S stock this autumn, despite plenty of leaf fall. However there was a lot of tree felling and cutting back this summer along the line.
Sensible driving helps too of course.
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Deleted
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Post by Deleted on Dec 31, 2015 18:24:22 GMT
On slippery tracks when an A stock slid the driver should have thrown off the brake and re applied it, to avoid flats, however because overrunning a station, or worse, having a spad would cause issues. Flats on an A stock train are not attributable to a particular driver. On S stock flats should not occur during normal braking, I seem to remember from somewhere that wsp drops out after 1 min of sliding, but I cannot imagine that ever happening. However if an S stock is sliding using an emergency brake overrides the wsp and applies a full friction brake, hence flats. Drivers generally just motored for a second or 2 on A/59/62 stocks to get the wheels moving again as the wheels locked something that can't be done on modern stocks. |
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Post by t697 on Dec 31, 2015 19:24:54 GMT
On slippery tracks when an A stock slid the driver should have thrown off the brake and re applied it, to avoid flats, however because overrunning a station, or worse, having a spad would cause issues. Flats on an A stock train are not attributable to a particular driver. On S stock flats should not occur during normal braking, I seem to remember from somewhere that wsp drops out after 1 min of sliding, but I cannot imagine that ever happening. However if an S stock is sliding using an emergency brake overrides the wsp and applies a full friction brake, hence flats. S stock Emergency Brake doesn't override the WSP control. But the occasional flat shows that even this good modern WSP is not able to prevent every single flat. |
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Colin
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Post by Colin on Jan 1, 2016 4:06:11 GMT
Is it possible to develop flats in the last 10mph? Absoloutely! Metal wheels on metal rails.....and the actual contact area is quite small....coupled with a lubricant such as rain equals great difficulty in maintaining grip at any speed. |
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Post by stapler on Jan 1, 2016 9:42:11 GMT
At which depots can flats be dealt with?
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Colin
Advisor
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Post by Colin on Jan 1, 2016 13:28:36 GMT
It's not the only facility LU has, but we have a wheel lathe road at Upminster depot.
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Deleted
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Post by Deleted on Jan 1, 2016 16:49:21 GMT
There is one at Neasden as well?
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neilw
now that's what I call a garden railway
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Post by neilw on Jan 7, 2016 16:14:19 GMT
orienteer. Theoretically you are right, of course, but we see exactly the same effect at Crich with our trams. Using the rheostatic brake coming down the hill, if the railhead conditions are horrible, the wheels will lock, everything goes quiet, and the controller is returned to off before re-applying. I agree with you, you would think that as the wheels slow the resulting braking force must drop to zero and so they would never lock. I will ask around ......
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Post by jacko1 on Feb 29, 2016 20:23:35 GMT
not quite in the same cat as wheel slip and picking your wheels up on poor adhesion rails,but on lu lines,is there a method of turning sets as on a confined route with the units hardly ever turning,how do they prevent uneven wheel wear? main line stock tends to get turned quite a bit in service,and certain trains like on heathrow express get loco hauled around a angle to reverse the formation every now again. just wondered what they do on lu.
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Post by philthetube on Mar 1, 2016 8:56:45 GMT
Many main line stocks are not normally turned, nearly all long distance London services have the first class located at the London end.
All underground lines with the exception of the Vic, Bakerloo and the W&C (as far as I know) have a train turning option somewhere. Met on the north curve at Watford, Central in the Fairlop area, District at triangle sidings, Northern at Kennington and the Circle, no explanation necessary.
I imagine that the only place where it would really matter would be on the Circle line.
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Post by countryman on Mar 1, 2016 9:52:52 GMT
I remember reading somewhere that Circle line trains were regularly turned by running to Aldgate East. As some Circle line trains come into service from Barking it would be easy to send some via Tower Hill and others via Liverpool Street. This would even things out.
I wondered why the Eurotunnel terminals were set out the way they are, with a clockwise loop at Folkestone and anticlockwise at Calais. Then I realised that the reason was at least partly to even out wheel wear!
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Post by norbitonflyer on Mar 1, 2016 10:05:27 GMT
Many main line stocks are not normally turned, nearly all long distance London services have the first class located at the London end. They can get back to front though, for example cross country services by calling/not calling at Gloucester, or by the route taken between Bromsgrove and New Street. Some ECML services eneter Newcastle by the High level Bridge instead of the King Edward Bridge. MML services to Sheffield can get reversed at Nottingham. Occasionally GWML services get turned round in the Bristol Parkway area. If the opportunity arises, they try to turn them round again as soon as possible, e.g using the Greenford loop, or by sending them out the "wrong" end of Newcastle Central. I did read of one occasion when a push pull (74+TC) set ran in service round the Kingston Loop in order to get the loco at the right end without using up an extra path. It was a failure to turn an HST on the Swansea triangle, to put a cab with working AWS at the front, that led to the Southall disaster. All underground lines with the exception of the Vic, Bakerloo and the W&C (as far as I know) have a train turning option somewhere. You've missed out the Jubilee. District stock could be turned on the triangle, but no service trains are scheduled to do so, which is why the D stock can be "handed". Someone posted a few days ago about a D stock driver taking a "wrong stick" at Gloucester Road and having to reverse at HSK, leaving the train back to front when it got to Earls Court. I understand it was sent back via HSK on the very next return journey in order to get it back the right way round again. |
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Post by John Tuthill on Mar 1, 2016 10:44:14 GMT
Many main line stocks are not normally turned, nearly all long distance London services have the first class located at the London end. They can get back to front though, for example cross country services by calling/not calling at Gloucester, or by the route taken between Bromsgrove and New Street. Some ECML services eneter Newcastle by the High level Bridge instead of the King Edward Bridge. MML services to Sheffield can get reversed at Nottingham. Occasionally GWML services get turned round in the Bristol Parkway area. If the opportunity arises, they try to turn them round again as soon as possible, e.g using the Greenford loop, or by sending them out the "wrong" end of Newcastle Central. I did read of one occasion when a push pull (74+TC) set ran in service round the Kingston Loop in order to get the loco at the right end without using up an extra path. It was a failure to turn an HST on the Swansea triangle, to put a cab with working AWS at the front, that led to the Southall disaster. All underground lines with the exception of the Vic, Bakerloo and the W&C (as far as I know) have a train turning option somewhere. You've missed out the Jubilee. District stock could be turned on the triangle, but no service trains are scheduled to do so, which is why the D stock can be "handed". Someone posted a few days ago about a D stock driver taking a "wrong stick" at Gloucester Road and having to reverse at HSK, leaving the train back to front when it got to Earls Court. I understand it was sent back via HSK on the very next return journey in order to get it back the right way round again. Piers Connor in his excellent book on the '1938 Stock' mentions that when stock was transferred from the Northern to the Piccadilly, when it was found to be 'the wrong way around' it was taken via the triangle to reverse it |
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castlebar
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Post by castlebar on Mar 1, 2016 21:41:35 GMT
@ Jtuthill
Not quite but near enough.
It was stock the Northern had borrowed from the Picc. Because of the Kennington loop, some stock had been turned whilst the Northern had it and it was returned to the Picc wrong ended. There was no Heathrow loop in those days, so it had to be turned either via the High Street triangle, or, "one drive" around the Circle and then back to Northfields. The High Street option was chosen.
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