(08-02-2011, 01:27 AM)Saraswathi Wrote: Hi,
Q1) Can anyone explain the types of point machines used on the main line and underground of UK, when & why?
Q2) Occassions where we use trailable, non-trailable and High thrust point machines?
Q3) What is the difference between Facing Point Lock (FPL) and Ground Lock (GL) and when do we adopt them?
As an initial response to part of the question, the two most common point machines used in the UK over the last 40+ years are
1. the Alstom (previously GEC- General Signal) HW2000 series, and
2. the Invensys (previously Westinghouse) style 63.
Both have "Facing Point Locks" internal to the machine and therefore called "combined machines" [the very earliest machines worked more similarly to mechanical points with one drive to move the rails and another drive to operate the FPL in the 4ft (i.e between the rails) thus bolting the lock strtcher into place].
The other point Operating Equipment [POE] that we use include:
a) clamplocks (now re-engineered as the In Bearer Clamp Lock), and
b) The High Performance Switch System
These are the only types that have the high thrust needed for long switches of heavy rail section and are the types to be used for most major relaying work nowadays.
There are also a range of other mechanisms, both ancient and modern, but for pure "point machines" the HW is dominant, the 63 having fallen a bit out of favour, but still used in quantity.
I think I have some manufacturer's info on each that I'll upload later if I can.
For now though I have put the wiring diagram for an HW; it is a 4 wire machine with separate pairs of cores used to rotate the motor in each direction. You will note the contacts that operate to cut off the motor from the supply near the end of its motion and also the electromagnetic clutch- the motor free wheels gradually to a stop after its feed is disconnected, whereas the machine itself has already stopped moving. Note the cotact next to the motor which is disconnected as soon as the manual operation handle is being inserted and remains broken until the handle is fully withdrawn and a reset plunger operated to switch the machine back to power operation again.
In the middle of the page are the detection contacts- a 4 wire circuit is double cut through either the Normal or Reverse contacts as applicable.
Clamplocks were introduced in late 1970s I think. British Rail felt point machines too expensive and developed the clamplock, much of which could be made in railway engineering workshops, but with manufacture of hydraulic power packs by specialist suppliers. Although perfectly ok in many applications and indeed better than point machines in some specific cases (notably when track not precisely to gauge), they can't cope very well with track twist and contaminants like coal dust. Reliability can be variable; if the rails are drilled accurately, correctly installed and maintained they are fine but if rails not quite correct then will probably always be a bit iffy. Also in heavily used applications they don't last as long as machines before needing renewal. They do have some advantages besides their price; for example easier to transport to site, don't need extended P'Way bearers etc and for a long time were the only means of driving long high speed switches, good for switch diamonds etc. However in many applications the saving in CAP-EX is offset by higher OP-EX, particularly as the cost of "track access" for maintenance is viewed somewhat differently in today's railway.
Therefore the tendency was only to use them on the less intensely used railway and in the specific situations when they had ther advantages.
After a time in the shadows have now made a comeback, reborn as the IBCL, helped a bit by the HPSS not yet having proved itself as good as had been initially hoped. One has to live with equipment for 10+ years sometimes to see the various problems gradually manifest themselves......so it is a bit like a pendulum and it may swing back in 5 years time!
Perhaps someone from the Underground can explain their various air driven mechanisms and their modified clamplock; indeed the Invensys Surelock (which is an upgrade from the style 63 but not used on Network Rail at all at present)
