FWC, FWP and RSO

[mattslade222]

Hi,

I have been looking at past questions around points maintenance procedures. I don't have any hands on experience apart from in training schools so am struggling to visualize what exactly is being measured and why during the FWC, FWP and RSO measurements taken during point inspections.

Here is my understanding - please correct or confirm if you wouldn't mind.

Residual Switch Opening (RSO) - the gap between the switch blade and the stock rail in the closed position. Shud be nominally around 1.5mm, between the lock stecher bar to the send P-way stretcher bar. The Facing Point Lock test sort of checks this at the point toe, as the 1.5mm should allow lock to engage, where as 3.5mm should fail.

Free Wheel Clearance (FWC) - Is the gap between the stock rail and open switch blade. We measure this for the length of the moveable point and ensure the minimum distance is greater than 50mm (some exceptions apply based on switch type and track speed)

Free Wheel Passage (FWP) - Is the distance from the outside edge of an open switch blade to the inside edge of the closed switch blade. This should be 1387mm for the length of the movable points, which I believe is derived from the track gauge - the FWC.

Switch opening - Is the gap between the open switch blade and the stock rail, for the wheel flange to pass through, this should be 102mm -120mm, usually.

This is based on the an old version of the SMS I found, looking at various parts. I have a few questions:

1) Is my understanding correct!? 
2) At the point toe what is the difference between the Free Wheel Clearance and Switch Opening, are they not the same there?
2) The FWP, what's the impact of this not being set correctly? Does it though off the FWC which could result in a derailment?

[Peter]

1) Is my understanding correct!? 
2) At the point toe what is the difference between the Free Wheel Clearance and Switch Opening, are they not the same there?
2) The FWP, what's the impact of this not being set correctly? Does it though off the FWC which could result in a derailment?

You are basically correct. The FWP is all about the space between the back edge of the wheel that should pass between the open switch rail and its associated stock rail. The issue was about wheels hitting the open switch rail (flange back contact) and then putting stress into stretcher bars and other components because they open switch rail was being forced further open. Yes, at the front, the FWC and Switch Opening are the same measurement and although the standards talk about measuring FWC throughout the length of the switch, the fact that the opening at the front is typically in the range that you give means that there is always a requirement for it to be greater than the FWC min.

The attachment in the following post has the calculation for the values and was the response to the issue of repeat broken stretcher bars potentially caused by inadequate clearance and resultant flange back contact.

Peter