Points and how they work

I mean I know how electric ones work e.g Mainline br but I am asking how/when the compressed air points were introduced.

There is also hydrolic points which are very popular with NR.
I dont know the date for when air points were introduced but they are very slowly being phased out for electric and hydrolic points.
The air main is classed as a pressure vessel and as such as to meet the same standards as a working boiler.
There is 100's of air work points operated across LU and as such will not be changed overnight to more modern systems.
Even electric points are not any better as they have a higher failure rate compared to there air operated counterparts, as i type this the District is suspended between Edgware Rd and High St Ken from a electric point motor failure at Edgware Rd.
Try Googling S63 Point Machine, HW1000, Surelock's, they are the electric versions which LU currently use for points.
Most points not all mind only drive one switch rail and the other switch rail is connected to the other one by a strecther bar. Depends on the type but in principle they all work the same they detected, unlock,throw,lock,detected

Has anyone any idea why electric point motors should be unreliable?

They have been around for long enough and there are enough of them for ultra-reliable versions to have been designed and built.

Apart from the Surelock point machine the others were never designed for Metro systems and not for the harsh environment they actually have to work in. Points on LU are thrown in some places a lot more then NR and "should" last 10 years or 1,000,000 cycles.

Compressed air systems generally wear less (and have a mechanical fail safe), but with the modern systems that are coming out, compressed air points are being made redundant. LUL seems to be replacing air points with the electronic kind.

What fail safe? If your referring to the groundlock aka WL these are only fitted to facing passenger moves

Air systems have a tendacy to creep (move very so slightly) if there is no air on the points the WL stops this but having a addition lock. Most points require 2" to unlock before they throw.

Electric and hydrolic points dont suffer from this

Air but been silenced

Hi.

We too use either air or electric operated Points here in Sydney. The main reason that air points are chosen is that they are way faster in changing than electric operated ones. So if the traffic density is high then air operated points are the way to go. All facing points wether they're air or electric operated have some form of facing point lock to ensure that the points cannot move under the passage of a train. In yards etc where passengers are not conveyed you can do without them. Cost, distance from a compressor etc all come into the final decision.

Bye for now,

Stephen.

Most LUL depots are still hand worked points apart from Northumberland Pk,Neasden,Upminster & Stratford Market.

The short answer, but not really why we have a WL.

Air operated points have stored energy keeping them in the last called for position.
The D block on the point auxillary valve will keep them called in that position, for example reverse.
A blown fuse to the Normal point auxillary would prevent them from normalising.
So though you are calling for them normal, the point auxillary valve is still calling them reverse.
Now what if they were binding on the drive or the lock before you called them reverse to begin with?
The points would be lying normal and detected. The signalling system after trying them reverse and failing would select for them to go normal.
Now the system has them selected normal and detected normal, but the D block is still reverse due to the blown fuse.
A train on the points would cause the points to unlock and throw under the train.
The WL or ground lock will prevent this from happening as it requires a secondary souce of power to unlock it but not to lock it.

You didn't understand my response.
The lever is normal. The lever is not trying to move so track locking wouldn't affect it.
The D block is still reverse because the fuse to the N valve has blown.
Also if they are binding then the points don't move at all.
Think about a set of 4'. If the drive rod has expanded in the heat then it can force the butterfly so hard against the roller the escapement wont move at all.
The only reason detection is lost is because the WL picks or the other end has gone reverse.
But if there is no WL then normal detection would remain.
So if there is no WL and the lever is normal and the points are normal but the D block is reverse the points are still trying to go reverse.
So a signal can clear and a train can go on to the points.
This will then cause the binding failure to become undone and the points to throw.
If its a facing move then the train will derail.
That is why the signalling standards state a WL is required for air operated points on facing moves.
Yes WL's are on trailing points too but are not required.
And yes we are looking at 2 separate failures occurring.
But designers back then thought about this kind of scenario.

Euston on the mainline from 1960ish until the remodelling in the early part of this century (sorry age and wine wearies the memory) had this set up.
They even had pneumatic clamplocks!

I apologise I did read it wrong in the first place