'Vintage' piston 1st-stage main springs, is there a danger of metal fatigue/spring snapping in two?

[Bryan@Vintage Double Hose]

I just don't remember enough about it to give you an accurate answer on that....sorry.

 

[scubafanatic]

All springs will eventually either sag and/or work harden and become a bit stiffer... generally speaking a catastrophic failure usually needs to propagate from a stress riser like a rust pit or deep nick. Sagging is far more common IMO.

I was overhauling a Sea Hornet Mk V which had not been rinsed off properly and then stored for many years... the spring recess was a mass of rusticles and the spring came out in two pieces, having failed at a deep rust pit near the ground and closed end

For regs that still have widely available new parts (like Atomic) I routinely replace the first stage springs if they have sagged more than 1mm from new height. At $17 each it is a cheap insurance policy!
Check this thread:
Main spring sag...

With a vintage MK 5 if you can't get them brand new from Scubapro or vintage NOS I would look at each one very closely for rust pits or nicks before re-using.

Springs (and pistons, etc) long ago ceased being available from Scubapro, although a while back I picked up a few NOS springs off E-Bay, so for the most part such parts are only available by cannibalizing other sacrificial units for parts. My concern is the fear of some sort of 'hidden' metal fatigue in what otherwise appear to be good condition springs.

 

[scubafanatic]

like @Fibonacci said, it's not age, it's use that will cause them to stiffen. Breaking is not something I'd worry about

So 'stiffening' won't ultimately lead to excessive brittleness and potential snapping in two ?

 

[Fibonacci]

So 'stiffening' won't ultimately lead to excessive brittleness and potential snapping in two ?

It could but as I said earlier:

... a catastrophic failure usually needs to propagate from a stress riser like a rust pit or deep nick.
Sagging is far more common IMO.
​

Here is an example of a G250 second stage mainspring that suffered a helical failure propogating from a single deep rust pit:

 

[tbone1004]

So 'stiffening' won't ultimately lead to excessive brittleness and potential snapping in two ?

eventually, but we're talking hundreds of thousands of cycles before you will find something measurable because they don't move a whole lot, mm's not inches and as a percentage of total spring length it's pretty small so any fatigue is going to take decades to show up to anything that would be problematic. They'll weaken over time as they are constantly under load, but that's different

 

[Fibonacci]

@tbone1004 Yes, auto cylinder head valve springs last reliably for many millions of cycles, with far greater travel vs SCUBA reg mainsprings. However, they operate in a constantly refreshed oil bath not the extremes of salt water immersion and intermittent cleaning that a regulator first stage has to endure.

Typically auto valve springs have a series of specifications that can be checked during overhaul with a Rimac style machine:
http://www.mwamachine.com/productline/brochures/RIMAC Brochure Spring Testers.pdf

• Free height
• Installed height
• Pressure at installed height
• Pressure at maximum travel (ie fully compressed within design travel envelope)

For some reason I've always struggled a bit with, SCUBA equipment manufacturers don't seem to list any of these specifications in their service manuals.
Perhaps it doesn't matter... but given the consequences of a failed spring at depth underwater I'm still surprised!

 

[tbone1004]

@Fibonacci a valve on an engine cycles on average 2000 times per minute. A human breathes call it 10 times per minute, say 20x/minute with power inflator use. Let's say a diver logs 5000 dives average 100 minutes or 1,000,000 cycles of the regulator. That is equivalent to 500 minutes of that car running or a single decent road trip. A car may do 200,000 miles and average or 400,000,000 cycles of that valve and valve springs aren't exactly something that fails.
I don't think measuring any of that is relevant for a regulator since we can easily adjust for change in spring pressure, we inspect on a yearly basis and part of that includes checking the spring and making sure it will hold a stable IP. The adjustment range of the screws is not very large either so if the spring starts failing it won't hold IP properly and then you replace. Doesn't happen often.

 

[Couv]

A few years ago another member of this board sent me a couple of new MK5 springs to make a comparison to springs that had been in use for a while. I took the newest and oldest MK5s I had both in terms of age and use for the experiment. I swapped out the springs into each position and cycled them plenty of times. There was no difference in IP between the new, old, and middle aged springs.

 

[Eric Sedletzky]

Not to get off topic, but could this be a possible topic for debate in regards to Chinese made piston regs (or springs) and other piston regs with known quality standards and verifiable stainless specs used for the main springs?
Just curious because in my experience with other Chinese stainless products, I found the material is very unpredictable and some of it is utter garbage.
I will only use American or German stainless steel now.
I wonder if there is a way to find out the origin of modern main springs used by various manufacturers of piston regulators?

 

[scubafanatic]

Not to get off topic, but could this be a possible topic for debate in regards to Chinese made piston regs (or springs) and other piston regs with known quality standards and verifiable stainless specs used for the main springs?
Just curious because in my experience with other Chinese stainless products, I found the material is very unpredictable and some of it is utter garbage.
I will only use American or German stainless steel now.
I wonder if there is a way to find out the origin of modern main springs used by various manufacturers of piston regulators?

Thank goodness my Mk 5s predate the Chinese infestation of our supplier chains.