Sensible thinking by Northern and NR signallers today

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142094

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Was on 2N41 15.32 Nunthorpe to Hexham service today (co-incidentially on 142094) between Middlesbrough and Newcastle. Soon as we left Middlesbrough it was obvious the engine in the leading carriage had packed in and we were struggling to get to Thornaby. In times past it would have been terminated there and then, forcing an hour wait. Not so today - the train ran to Eaglescliffe to reverse, allowing the carriage with the engine still working to be at the front, and managed to get back to Newcastle around 30 down. Much better than being an hour late and good to see some common-snese thinking from both Northern and the signallers.
 
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Nym

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Only today?

There's one running round from Newton Heath with only two working engine mounts, so when it's idling it's shaking itself loose and trying to escape from the frame...(!)
 

Nym

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Would pulling and not pushing make much difference?
Yes, but at 4am I'll be dammed if I can explain why.

OK, now I've had some coffee...

Think about the rolling resistance and what happens when you push something into a corner rather than pulling it round the corner, when you're pulling a carriage behind you the yaw against the flange is damped by virtue of the force applied to the drawbar, when pushing this is an open loop unstable scenario and the stability is provided by the high levels of damping when at high levels of yaw, that cause a lot of noise and heat generation, wasting energy being generated by the one remaining engine, converting this into peak damped pendular motion of the leading, un-powered carriage. Due to the nature of how a pacer chassis is set up, there is little bult in yaw damping that would be able to stabalise the system against the flanges without making use of the flange friction to contain the pendular yaw, this would be handled by Yaw dampers or bogie construction on other vehicles, as the system peaks at much lower levels with higher levels of damping within the internals before coming into the flange to remove energy from the pendular motion, it does not cause as much of a problem for bogied stock, even less for those fitted with yaw dampers.
When being pulled this is less of a problem as the force being applied as a result of the torque onto the track is normal to the direction of the flange impact, not causing internal damping of yaw, but instead 'pulling it into line'.

There's one reason for you...
 
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embers25

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Yes, but at 4am I'll be dammed if I can explain why.

OK, now I've had some coffee...

Think about the rolling resistance and what happens when you push something into a corner rather than pulling it round the corner, when you're pulling a carriage behind you the yaw against the flange is damped by virtue of the force applied to the drawbar, when pushing this is an open loop unstable scenario and the stability is provided by the high levels of damping when at high levels of yaw, that cause a lot of noise and heat generation, wasting energy being generated by the one remaining engine, converting this into peak damped pendular motion of the leading, un-powered carriage. Due to the nature of how a pacer chassis is set up, there is little bult in yaw damping that would be able to stabalise the system against the flanges without making use of the flange friction to contain the pendular yaw, this would be handled by Yaw dampers or bogie construction on other vehicles, as the system peaks at much lower levels with higher levels of damping within the internals before coming into the flange to remove energy from the pendular motion, it does not cause as much of a problem for bogied stock, even less for those fitted with yaw dampers.
When being pulled this is less of a problem as the force being applied as a result of the torque onto the track is normal to the direction of the flange impact, not causing internal damping of yaw, but instead 'pulling it into line'.

There's one reason for you...
Wow very impressive for 3am!!!
 

142094

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A slightly easier explanation could be like the flat trolleys at B&Q - when you push it it is much harder, but when you pull it it does what you want.
 

John55

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Yes, but at 4am I'll be dammed if I can explain why.

OK, now I've had some coffee...

Think about the rolling resistance and what happens when you push something into a corner rather than pulling it round the corner, when you're pulling a carriage behind you the yaw against the flange is damped by virtue of the force applied to the drawbar, when pushing this is an open loop unstable scenario and the stability is provided by the high levels of damping when at high levels of yaw, that cause a lot of noise and heat generation, wasting energy being generated by the one remaining engine, converting this into peak damped pendular motion of the leading, un-powered carriage. Due to the nature of how a pacer chassis is set up, there is little bult in yaw damping that would be able to stabalise the system against the flanges without making use of the flange friction to contain the pendular yaw, this would be handled by Yaw dampers or bogie construction on other vehicles, as the system peaks at much lower levels with higher levels of damping within the internals before coming into the flange to remove energy from the pendular motion, it does not cause as much of a problem for bogied stock, even less for those fitted with yaw dampers.

When being pulled this is less of a problem as the force being applied as a result of the torque onto the track is normal to the direction of the flange impact, not causing internal damping of yaw, but instead 'pulling it into line'.

There's one reason for you...
If I read the above correctly an unpowered front car of a Pacer when pushed is "open loop unstable" and only damped by the by the friction between the flange and the rail?

How is it the last time I was on an unpowered leading car of a Pacer on plain straight track and gentle curves with no flange contact the train stayed on the track and there was no hunting whatsoever, and all at 60 - 70 mph downhill?
 

Phil6219

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I was on a pacer on Saturday (142054 I think - it was an ex Merseyrail one but had northern seat covers on the MT seats).

The engine in the rear car (WC car) packed in when passing Crows Nest box, the lead car was working as the train was crammed and the engine spluttered to life around Daisy Hill before conking out before clearing the platform. It then sprung to life again at Hag Fold and seemed ok when I jumped off at Atherton.

Phil 8-)
 

Clip

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A slightly easier explanation could be like the flat trolleys at B&Q - when you push it it is much harder, but when you pull it it does what you want.
Isnt that because the trolleys have fixed front wheels and moveable ones to steer at the back therefore making it easier to pull then push it?
 

142094

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Isnt that because the trolleys have fixed front wheels and moveable ones to steer at the back therefore making it easier to pull then push it?
Probably, although I'm not physicist, so I'm hoping what Nym explained is something similar to the trolley analogy.
 

Nym

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If I read the above correctly an unpowered front car of a Pacer when pushed is "open loop unstable" and only damped by the by the friction between the flange and the rail?

How is it the last time I was on an unpowered leading car of a Pacer on plain straight track and gentle curves with no flange contact the train stayed on the track and there was no hunting whatsoever, and all at 60 - 70 mph downhill?
Because you were on plain track heading downhill...
 

Nym

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The movement within the wheel profile in combination with the force pushing the unit makes the yaw oscillations "open loop unstable" without the 'bumpstops' of the flanges on the rails.
 

John55

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The movement within the wheel profile in combination with the force pushing the unit makes the yaw oscillations "open loop unstable" without the 'bumpstops' of the flanges on the rails.
In normal running the flanges don't touch the rail so how can they damp an "open loop unstable" condition?
 
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