IRSE News 150 Nov 09.pdf

NARROW GAUGE POINTS
FROM COAL-FIRED POWER STATIONS
TO POINT MOTORS
Several options for driving the cam plate fore and aft were
considered. The ball screw has already been mentioned and
compact proprietary packages known as linear actuators were
investigated. The clear winner was a type that has been used in
coal-fired power stations for many years to operate the damper
doors.
Negotiations with the manufacturer resulted in a special
version incorporating a phosphor-bronze worm wheel to drive the
screw and the facility to operate the device manually. The
slipping clutch idea couldn’t be included, but another method
presented itself, of which more later. The use of a linear actuator
would also allow the machine to be adapted easily for pneumatic
operation.
Now that the basic design was established the refinements
could be added. Limit switches would detect the position of the
lock dogs. A timer would cut power to the motor circuits if the
points were to jam. Greasing points on the outside of the main
casing were provided, as was the facility to insert a cranking
handle for hand cranking, use of which would isolate the motor.
The machine was accommodated within a one-piece stainless
steel cover with powder-coated finish.
LOST MOTION DRIVE
Some applications of the machine would require switch rail
detection and some would not. For that reason provision was
made for an optional detector to be fitted on a common
alignment plate. A variety of designs could be used, including the
well-known BR998 detector box, with suitably modified slides.
Use of the cam plate results the machine having a fixed throw.
So, how can the points be centred and adjusted? One method
would be to use shims at the stretcher bar attachment points, but
this seemed too fiddly so a lost motion drive was used instead. A
short cylinder is attached to the point stretcher bar and has the
drive rod passing through it. Top hat nuts threaded onto the
drive rod locate within the cylinder such that they allow a degree
of freedom in the drive rod travel, which is slightly greater than
the maximum required points throw.
THE PROTOTYPE
All that remained was to construct a prototype and undertake
testing and trials. It worked well, but it soon became apparent
that the machine was capable of delivering far too much force. In
a dead stall (obstruction of the switch rail) the throw bar could
exert in excess of one tonne on the stock rail. The motor current
rose to 24 Amps - the design limit being 9 Amps - and the screw
thread tried to turn itself inside out. Well, you have to try these
things! Clearly, some form of current limitation would be
required. This could be used as an electrical equivalent of the
slipping clutch used in standard gauge machines. This was
surprisingly difficult, but eventually a suitably proven product was
found in Denmark. The device transformed the machine into a
fully reliable, indestructible and safe piece of signalling
equipment.
4. Schematic view of the point machine.
5. General view of the point machine.
6. Point machine with a BR998 detector box in place.
A feature of the design that came under very close scrutiny
was the drop link attached to the throw bar. This is designed to
deform during a points run-through, preventing all the forces
being transmitted into the machine. In doing so it would allow
some movement of the switch rails. As with a lot of things
though, a compromise has to be found. The link shouldn’t
deform under any loading that the machine itself can impart and
yet it should allow a light vehicle to force through the points
without suffering flange climb out. After a lot of testing and
discussion the link was set to deform under a load of 1.7 tonnes.
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IRSE NEWS | ISSUE 150 | NOVEMBER 2009