Points changer finished

[Matty F]

I've been making devices to keep tram rail points to one side or the other by means of a spring. Here's the finished points changer:

started with the usual block of steel and cut the end out of it. I made a roller in the lathe, which does a really good job of putting the hole in the middle and making the sides straight. And I got a steel bolt to use as a spindle. Even though I'm going to rivet it in, a rivet is too soft and may bend under the pressure I'm going to put on it. can use the lathe now since the man in charge of the lathe died of a heart attack. Now I'm using all of his machines to make all the things that he said were impossible!

Here is the roller assembly put together. As usual the angle grinder made a nice job polishing it all up. I put grease inside before I rivetted it together:

I made a cam out of another block of steel. And that's the spring that will hold the cam against the roller: that the cam has a sharper slope near the middle. Try and make that on your fancy milling machine! I just ground the curve on a grinder. The curve gives the cam a bit more force to flick the rail over from the middle. If the angle is too steep the cam doesn't work at all. I already tried that! About 45 to 50 degrees is about all that will work.

Here are most of the parts put together. I polished up the cam with the angle grinder too!

a tram runs the "wrong" way through the points the movable rail simply flips to the other side. It needs to be spring loaded so that it stays on that side. When the tram is going the "right" way through the points we wouldn't want the points to move across when half the wheels have gone on one track while the second half go on another track!

The usual type of spring points changer is too wide for the box I need to put it in, and the design is inferior since the maximum force is applied only at the end of its travel. A cam gives maximum force on the movable rail for most of its travel.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Matty F saying something like:

Excellent stuff. Would a more rounded profile on the nose of the cam not be a bit kinder to the roller?

[Matty F]

Yes it probably would. I'll do that next week. I need to adjust it yet since the cam is not quite in the middle of travel.

[Chris J Dixon]

Chris

[Andrew Gabriel]

Just wondering if you considered an over-centre toggle mechanism? I think that would have a similar force profile, but no centre dead spot.

[Matty F]

That sounds interesting. I've had a bit of a Google for one but I can't find a decent picture.

This is another type of points mechanism. But it has almost no force around the centre point, and definitely doesn't work as well as the ones that I have made:

[dennis]

It should be almost impossible to get it to stop in the centre if the joints are free.

[Matty F]

But there is a lot of friction when the rail is moved sideways. The rail rests on a flat steel surface that gets rusty, and there are lots of leaves and dirt and other crud all around, even though they are cleaned out and greased every week. So I can make that rail stop at at almost any point.

[Andrew Gabriel]

,

It surprises me that I can't either. It's the mechanism commonly used in fast make/break toggle switches, bi-metal thermostats, etc, to ensure bi-stable operation -- i.e. the mechanical system has only

2 stable states. Hopefully someone will find a good picture or moving diagram. Even Wikipedia was useless.

Surprisingly (for me at least), the search brought up lots of rail points systems. One is this one

from a very quick read, I think that's doing more than you want in the "Snap to the reverse position when the points are trailed."

means you could get it to stay in the centre position because there's no force applied at that point, i.e. it's tri-stable, not bi-stable.

[Matty F]

,

one

but from a very quick read, I think that's doing more than you want

I reckon the force that it applies on the rail goes like this:

10 side 8 5 2 0 centre 2 5 8 10 side

while the cam method goes like this:

7 side 8 9 10 0 centre 10 9 8 7 side ... which is the effect that I want.

The cam spring exerts more pressure near the middle position. Because I've curved the cam it exerts more pressure near the middle also. Any amount of spring tension will hold the rail at one side or the other. More force is needed near the middle point so that the rail never stops in that position.

But a bistable system would be better.

[Andrew Gabriel]

,

The force profile for over-centre toggle is not static - same position will have different force depending on the state of the bi-stable, and therefore the next direction of state transition.

There are effectively two centre-like positions, offset equally (in your case) from the real centre of travel. If you start from one side, the centre point will be the further one, i.e. past the real centre of travel. As you reach it, the centre point flips to the one you already traveled past, so the force suddenly flips to push towards the side you are moving towards. The force is highest as you approach the flip point. It's never zero, and can be made to change little throughout the travel (although not to remain constant).

So going from side A to side B, with +ve numbers forcing to side A:

+7 side A +8 +9 +10 +11 [centre of travel] +12

-9

-8

-7 side B

and going back in the opposite direction (side B to A)

+7 side A +8 +9

-12

-11 [centre of travel]

-10

-9

-8

-7 side B

[Andrew Gabriel]

,

Thinking about it, this one isn't necessarily quite right. The spring force is most at the centre, but the mechanical force advantage is greatest at the sides and zero at the centre. So depending on the spring characteristics, multiplying these could result in the peak force anywhere except just around the centre, and could even drop slightly before you hit the sides, depending on the spring used.

[Jules]

Keep posts like that coming! Interesting to many, I'm sure - and personally I don't have much time to go looking at blogs for this kind of stuff, so it's nice to see it here on the ng.

Where in NZ is this tramway, btw? (I may have asked that before, but if I did I've gone and forgotton!)

cheers

Jules

[Matty F]

.com>,

I based my estimates on the one in the picture and what happens when I operate it with a track bar. The spring is much larger and heavier than the one I've used with the cam version, and it's compressed to a high extent. When in use it doesn't compress very much more (maybe

20%), so the force it's exerting is roughly similar over the whole travel, except for the mechanical force advantage. When I lever the rail with a track bar, it needs most force at the side. As the rail approaches the middle position it needs very little force to move it, and I can let it stop over about half the total distance of travel because of the friction. I am only concerned with the rail flicking right over and not stopping just after the halfway point. With the cam version, at the middle position the spring is compressed to about half its length, so it is exerting much more force over the middle position. The trams have plenty of force available to push the rail just past the halfway point. If they go fast enough the rail flicks over very suddenly, but they are not allowed to go fast! The worse situation is the points with a simple spring to hold the points to one side only. Each wheel on one side of the tram has to push the rail over, and it flicks back after each wheel. But that is the way it has to be. In normal use the driver does not ever have to get out to change the points, except when deciding which trambarn to go into at night.

[Matty F]

I have lots of weird hobbies. Perhaps you have not seen the nuclear accelerator that I'm trying to restore!

Ah, I don't think I'm at liberty to say! But there are lots of heritage tramways and railways here in NZ. I've discussed making strange objects before and nobody wants to know what they are:

[Clive George]

?

sufficient answer.

[The Medway Handyman]

A new Superhero is born - Angle Grinder Man!

[Tim W]

The Medway Handyman wibbled on Thursday 01 October 2009 19:55

I too am deeply impressed. No silly tile trim wibblings or questions about wiring 3 way lights. Nope - go for something proper and heavy duty like a DIY points spring bias doobrey.

I vote Matty for the 2009 DIY'er's award :)

[Jules]

Now you've got me trying to remember the name of the NZ guy a few years ago who was buying up odds and ends to make his own cruise missile... wasn't you, was it? :-) I think he got pretty far before the gov't leaned on him...

Yeah, there are. I used to bump into a few folk over there into that kind of thing, and lots of ex-railways staff - all with all sorts of interesting tales to tell! :-) Seemed to be a lot more people into trolleybuses than trams, though.

cheers

Jules

[Matty F]

That would be Bruce Simpson. He's gone further ahead than I ever would You have to see this jet powered plane he's flying!

>> Where in NZ is this tramway, btw? (I may have asked that before, but if

Yes I'm interested in trolleybuses too. Hopefully we should get some overhead up to run them. It's tricky running them off tram overhead, but it can be done! There's bound to be something in my volunteer contract about publicity so I won't publicise.