pendulum support springs

[gerry maddenParticipant @gerrymadden53711]

I have over the last few months ploughed through R J Matthys book on pendulum design. For most of it he seems to be suggesting Be-Cu is the best all-round material for suspension springs. Then, right at the end he suddenly seems to change his mind and tells us to go for Al-Bronze!

Other sources suggest that the Al-bronze has poor fatigue resistance and is not easily solderable. If true these don’t seem to make it such a great choice, overall. I guess it’s all about what parameter we all rate highest.

Anyone care to comment on their experiences?

Gerry

[roy entwistleParticipant @royentwistle24699]

For the majority of clocks that I have repaired I have used feeler gauges

Roy

[bernard towersParticipant @bernardtowers37738]

I use feeler guage as well

[John HaineParticipant @johnhaine32865]

I generally use CuBe, originally on Matthys’ recommendation and because I have a lifetime’s supply of it!  (Happy to share a bit.) I’ve never had any fracture. Feeler gauge is also often used. Best is NiSpan-C which is an alloy formulated for zero tempco of elasticity with temperature.

[Michael GilliganParticipant @michaelgilligan61133]

A couple of comments on the Matthys book, if I may …

These will probably be obvious to the small group of current participants, but might be useful to newcomers.

1. The book is clearly compiled from years of work and does not “flow” particularly well … I found it helpful to skim through everything and then go back to study individual chapters at leisure.
2. The high-spot for me was his discussion of ‘Solid one-piece suspension springs’ [CHAPTER 18 and its Appendix] … But it’s all good stuff !!

 

MichaelG.

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[gerry maddenParticipant @gerrymadden53711]

Ok, then I think I will run with the CuBe material for now.

To try to move the zone flexure in the springs away from the support junctions (a source of long-term instability) I was planning to profile the edges of the springs to make them narrower around the middle of their length.  Milling would be a bit crude for this and grinding not so easily accomplished, so I was thinking of water-jet or even laser cutting. Has anyone tried this and is it do-able on this material ?

Michael, yes agree with your comments on the book. It is a little chaotic, but nevertheless, a very useful tome.

Gerry

[John HaineParticipant @johnhaine32865]

Actually the spring does not bend on the clamping lines. It is constrained to be tangential to the rod at the bottom and the chops at the top, and the effective hinge point is slightly below the top.  Not worth profiling the shape.

[duncan webster 1Participant @duncanwebster1]

Has any clever person calculated the bending stress in a support spring? I guess it’s a lot less than the static load from holding the bob up, but it would be nice to know it’s within the fatigue endurance limit

[John HaineParticipant @johnhaine32865]

There might be a calculation in Matthys’ book? I think you just apply the standard beam bending equation to find the strain, how the fatigue bit works I don’t know.

[gerry maddenParticipant @gerrymadden53711]

Yes, Matthys does go into the mathematics of the basic stresses (tensile and bending) in the spring from the swing and bob-weight point of view. He also points out that the junctions between spring and support blocks are important for long term stability but in a more qualitative way. That’s understandable though. Under basic bending beam theory, these junctions are considered ‘simple’. In reality they are anything but. These junctions are a myriad of tensions, compressions, stress concentrations and slips, all of which will conspire to cause microscopic wear over time.  It’s this that’s the root of the long-term stability issue.

Matthys suggests soldering these joints. Whilst this would undoubtedly help, I don’t feel it’s the best solution. Solder is soft, highly plastic and has no fatigue resistance. So looking under the microscope again, such a junction is still likely to deteriorate.

As I see it, the better thing to do is to try to minimise the stresses at the junctions. The most effective way to do this is add more material at these points. My plan is therefore to make a suspension spring that looks something like the sketch below. I probably need to work on the proportions of the spring a bit, but right now my challenge is to think of a way to make nice good quality cuts in the thin sheet. I suppose might need to encase the spring in plastic or something similar for the machining process and then dissolve it away afterwards.

Gerry

 

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