Simpler safety valve?

[paul]

The book I've been referring to whilst making my boiler & fittings shows a safety valve where the 'business end' consists of a ball bearing blocking a 1.5mm hole. The ball is held in place by a small spring that bears down on an inverted 'T' shaped guide rod (I don't know why I'm describing this as it seems to be the norm for valves on little engines!).

The guide rod has a 'cup' to fit over the ball. Does anyone have a suggestion as to how this cup can be made, keeping in mind the ball is 4mm dia and I'm using hand tools and a drill press only (short of whacking the ball into softer metal!)? Would it be possible (or rather, advisable) to dispense with the ball altogether and make the guide rod a simple inverted 'T' shape that covers the hole? To my simple mind the pressure required to lift a flat surface would be no different to that required for a curved surface (whoa...brain tells me that may be wrong since the ball presents a fractionally larger surface area for the pressure to act upon but at these dimensions the difference must be tiny).

Here's the safety valve from my first model (I had to 'adapt' the instructions that came with the kit - the M3 nut on the valve did not fit through the M6 nut that formed the bush! Filing the flats off the smaller nut meant the valve could be removed for servicing/adjusting etc). I wonder whether this very simple arrangement is better than the ball bearing idea?




Sorry about my terminology... I can see you guys cringing already ;D

[chris vine]

Hi,

How to make the little bit which pushes down on the ball? (T shaped is a bit misleading because the bottom bit is round)
You could make the guide rod a simple straight piece of rod, stainless steel is best or brass.
The bit which pushes on the ball could be a larger piece of rod, very short with a hole drilled through it the size of the guide rod. Solder the guide rod into it but not sticking all the way through or even to the end of the hole so that the hole will sit central on the ball. If you wanted you could open out the end of the hole a bit to make a bigger bit to sit on teh ball.

Have fun, great that you are starting a fantastic hobby,
Keep asking questions, everyone asked them at one time!
Chris.

[havoc]

The guide rod has a 'cup' to fit over the ball. Does anyone have a suggestion as to how this cup can be made, keeping in mind the ball is 4mm dia and I'm using hand tools and a drill press only

You could take a piece of brass, drill through and then at one side, use a countersink to make a conical area. Then solder the piece on the stem of the T.
Or search for several tubes that slide into each other, then cut different lenghts and solder these to get a stepped indent for the ball.

Would it be possible (or rather, advisable) to dispense with the ball altogether and make the guide rod a simple inverted 'T' shape that covers the hole? To my simple mind the pressure required to lift a flat surface would be no different to that required for a curved surface

Yes, it could be done, the pressure would be identical. But it will be harder to get it closing. The ball can shift a bit to close the hole, but the flat underside would have to be perfectly perpendicular to the hole or it doesn't close. Or you would have to make a loose flat valve and let the stem of the T press on a single point in the middle. Far more complicated than the ball system.

wonder whether this very simple arrangement is better than the ball bearing idea?

For small boilers I don't think it would make a difference. If you are familiar with making such a valve, then why not continue using it if it fits your model?

[paul]

Thanks Chris and thanks Havoc - several methods there for me to chew over! Maybe I could even adapt a cheese-head type screw of the right size - just countersink the top to fit the ball, turn it upside down and bingo!

For small boilers I don't think it would make a difference. If you are familiar with making such a valve, then why not continue using it if it fits your model?

I want to try something different/a little more complex. It can only go wrong...

[houstonceng]

Paul

Your 'photo is almost the same as the safety valves beloved on early Mamod stem-engines.

It worked for them at low pressure, but it would be a bit of a B*$X£R to set during steam testing.

Andy

[havoc]

Maybe I could even adapt a cheese-head type screw of the right size

You could try a screw with internal hex head. Use the hexagonal ident as a cup.