Profile picture for user MikeHoulder

Member for

10 years 1 month

Posts: 245

While making sense of the innards of the RAF Browning .303, It struck me that the thousand and one springs in the gun has a very hard life.
Look at this one:


This is a photo from Walter's copy of AP1641C showing the components of the feed slide and pawl which fit into the breech cover.
On the left is the pawl with the pawl leg on its right which has to my great misfortune become almost totally faded away.
Then further to the right is a weedy looking spring, followed by the feed slide itself.

That weedy spring worked 19 times a second to bump the pawl & its leg up and down. What would be the life of such a spring?
500 cycles, 5000 cycles or what?

I can imagine the armourers stripping these guns down and replacing all the springs, perhaps, every week.

Is that so or do these weedy springs have a sterling hidden strength?

Original post
Profile picture for user MerlinPete

Member for

14 years

Posts: 1,270

Interesting point, although the "weedyness" is simply the wire diameter, which has a bearing on the strength of the spring. Spring steel has a very long fatigue life, and tends to be very sensitive to surface imperfections and corrosion rather than use.
Spring making is highly skilled, the sort of thing referred to as a black art.

The valve springs in a Merlin can run continuously at 23 cycles per second, or 25 for take off.


Profile picture for user Denis

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15 years 11 months

Posts: 1,496


Fascinating fact of the week :)

Profile picture for user MikeHoulder

Member for

10 years 1 month

Posts: 245

I called that spring weedy not just because the wire diameter is thin, but also because the coil geometry is not uniform.
It looks like a spring from a discarded mechanical toy which has been squeezed, stretched and bent.

I assume that aero engine valve springs are high precision items with near perfect geometries (Is that so?)

Completely dependent on intuition, my feeling is that geometric non-uniformity produces discontinuity in the force/strength values along the length of the wire.
These discontinuities produce weak points in the spring where perhaps fatigue resistance is greatly reduced.

Hence the pawl spring shown might have a much shorter useful life relative to your Merlin valve springs.

Of course, all this is a distraction from completing the planned work on the Browning and further delays the return to work on the Lancaster (which is where I want to be).

Member for

13 years 5 months

Posts: 9,690

With steel it is possible to have a limitless 'fatigue life' if it is not stressed beyond certain limits; my guess is that springs are designed to work within those limits. That being the case springs should only fail due to a fault in material, manufacture or corrosion.