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B Spring Steel Metal Strip in Eyeglasses question

  1. Nov 28, 2017 #1
    Hello.. (see 3 colored pictures below to illustrate the question)

    I have a pair of eyeglasses with screwless spring hinges that uses spring steel metal strip (see full pictures below). I’d like to understand something about it. When the temples are fully opened.. the spring metal strip are in bend position (see first picture).. when I close the temples.. the spring metal strip are more flat (see last picture).. what I’d like to know is.. how many times or how many cycles between closing and opening the temples can the spring steel metal flat strip just crack or fracture? Anyone seen other eyeglasses like this? It's branded Thomo (not the expensive Ray Ban). I know my mistake was not choosing just the simple screw type hinges without worrying someday the spring metal strip can just fracture from fatigue. Usually how many cycles before this fatigues, any ideas? Thank you.

    temple open.. spring steel metal strip bend


    following shows temple closed.. spring steel metal string flat

    How many times of closing and opening the temples before the spring steel metal strips cracks?
    For the math wiz.. how do you compute it?
  2. jcsd
  3. Nov 28, 2017 #2
    In order to model that, we would need to know exactly what alloys are being used - and we would need an ambient temperature value.
    As a matter of practicality, even the manufacturer would determine the cycle life by actually exercising several devices. Of course and unfortunately, that is a destructive test.
    I would not guess that the spring itself is the weak link. I would be more concerned about the junction between the two J shaped pieces - the one that pushes the spring and the other the meets the frame.
  4. Nov 28, 2017 #3


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    That's clever design - I like that .

    The spring is more likely eventually to break away from the fixing point rather than to break .

    Fatigue life of the spring could be estimated but is likely to be at minimum tens of thousands of ops .
  5. Nov 28, 2017 #4
    Thanks. But note the spring is not like that of a car suspension or that spiral thing that you commonly see. It’s just a flat strip of steel bend at an angle. When you have a metal strip and you bend it even a few dozen times. It should already break. So what makes so called spring steel strip more resistant? And whenever you bend it, does it break a few atoms from the reservoir of billions of atoms that after many ops or cycle, only a few atoms left and it finally breaks? How is the metallurgy of it?
  6. Nov 28, 2017 #5


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    It's a matter of choosing the appropriate alloy of steel. They have been making steel of all kinds for hundreds of years and not all springs are helical. The secret is not to bend the strip past its elastic limit. The actual displacement in your glasses spring is not actually much. When they are old and you have a new pair, take the spring out and go at it with two pairs of pliers. It will bend or snap when displaced by much more than the hinge subjects it to.
  7. Nov 28, 2017 #6
    Guys. If the strip is pure steel or iron.. can't it have larger elastic limit? how come creating alloy can create more elasticity? what is in the alloy that can make the atoms and molecules flexible enough? Also I'm worried it can rust easily since I always clean my spectables with water. Are spring alloys automatically more resistant to rust.. why? Thanks for those who helped!
  8. Nov 29, 2017 #7
  9. Nov 29, 2017 #8


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    This is what metallurgy is all about. This link could be a way into finding what you want. This link is a bit elementary but could help. Read around.
  10. Nov 29, 2017 #9
    Ok.. thanks guys.. but isn’t it screw based eyewear hinges are most reliable as you can rotate the temple thousands of times.. so why would anyone wants to have spring based metal strip as the rotating mechanism in the temple? What would be the advantage of this? Anyone with eyewear with spring metal strips too?
  11. Nov 30, 2017 #10


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    The spring in your glasses isn't too dissimilar to old fashioned car leaf springs. They work for millions of cycles.

    Some conventional hinges also have a spring that causes them to "lock" in the open and closed position. The design you have appears to simplify things eliminating the need for a screw. Ive not seen it before but it looks quite elegant. My kids frequently loose the screw on their glasses.
  12. Nov 30, 2017 #11


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    You can make springs out of stainless steel.
  13. Nov 30, 2017 #12


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    I have had dozens of pairs of specs since childhood and I would say that screw based hinges are far from reliable. They tend to get loose and floppy and you have to peen the end of the thread to stop it turning. Sometimes they totally unscrew and then you have to find a piece of temporary wire to go through until you can get a suitable screw. This clip arrangement, with a spring, strikes me as being ideal. It's easy to assemble and also to disassemble if you have a suitable small press or vice.
    Don't knock 'em. It looks me like a good bit of engineering. The pin and hook can be made as strong as you want and they are full diameter, unlike threaded parts.
  14. Nov 30, 2017 #13
    Here are more accurate angles of the spring strips showing the closing and opening of the temples:
    (opened temples)


    (closed temples)


    You can see the angle changes is significant.

    I’m familiar with the concept of yield stress and strain back when I was studying the structure design of engineers of my house. But it was mostly in the axial in that the rebars either get compressed or pulled (as in the top and bottom section of a concrete beam). I didn’t study about the effect of bending of the rebars therefore I can’t compute the stresses and strain of my eyeglasses string sections. As review (go to the last paragraph if you are so familiar with this). I know as wiki https://en.wikipedia.org/wiki/Yield_(engineering) described that “A yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.” But there is also a concept of Fatigue https://en.wikipedia.org/wiki/Fatigue_(material) “In materials science, fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.”

    Furthermore I read about fatigue limit https://en.wikipedia.org/wiki/Fatigue_limit that “Typical values of the limit (Se) for steels are 1/2 the ultimate tensile strength, to a maximum of 290 MPa (42 ksi). For iron, aluminium, and copper alloys, Se is typically 0.4 times the ultimate tensile strength. Maximum typical values for irons are 170 MPa (24 ksi), aluminums 130 MPa (19 ksi), and coppers 97 MPa (14 ksi).

    Going back to my spectacles( see the above pictures where the angles of opening and closing the temple is clearly shown). When the temple is closed and open. How do you compute the stress induced in the spring strip? I know how to compute for stress in rebars but it is either pulled or push.. but not bending… For bending in the spring strip in the spectacles. Do you estimate is it less than ½ the ultimate tensile strength or more? This is important to know because whenever I go downstair to eat dinner.. I always closed the temple of my old glasses before holding and carrying them downstair (the spectacles mentioned in this thread are reading glasses or eating glasses used for viewing plate details (such as fish bones)). But now I’m concerned whether to close the temple when bringing them down or hold the spectacles without closing the temple as I don't want my hinges to fail after 5 years. Thank you.
  15. Nov 30, 2017 #14


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    How many times are you planning to open and close your specs? The valve springs in your car are compressed thousands of times a minute as you are driving. 'They' say that you should replace valve springs on high performance cars every few thousand miles but that's just because they creep a little and are less stiff. Do they ever break?
    PS you are lucky if you can expect your specs to have the right prescription for you in 5 years. You will have moved on to an even more cool pair by then. :smile:
  16. Nov 30, 2017 #15
    I fold the temples 6 times a day.. but now thinking of limited it to 2 times a day depending on the analysis of the stressed and fatigue performance. I went back to the small optical shop to see if they have other brands with such spring hinges. I saw another one branded Lightec.

    See. http://morel-france.com/morel-lightec/



    I need another spectacles for distant vision. I plan to get screw hinges but for the one I already got. I really need to compute the bending stress.

    In structural engineering. I’m only familiar with axial loads in the rebars. For example: http://richardson.eng.ua.edu/Former...es/Review_of_Stress_Strain_due_to_Flexure.pdf


    Some formulas.

    σ = P/A gives the stress (psi) for a member of a certain area loaded axially.
    Δ = PL/AE gives change in length of a member when loaded axially.
    For a given cross section, A, the failure load is equal to σ*A
    Also, by algebra: Δ = σL/E

    Even in flexure of beam. The top and bottom rebars are computed axially.. not on the individual rebars bending too..

    For bending stresses in the rebars or hinges spring steel strips.. how do you compute? Anyone got an idea?
    Last edited: Nov 30, 2017
  17. Dec 1, 2017 #16


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    @bluecap : The topic of strength of structures is interesting and hard at the same time and I approve highly of your interest. But why choose these specs to lose sleep over? There are many more vital structures that can let you down with much more dire results. For instance the steel hinges on outside gates and doors. If they are not lubricated fairly regularly, they can very gradually seize up and, instead of the hinge doing all the flexing, the edge of the hinge can take the strain. It just feels a bit stiff (mechanical advantage masks the problem) but every time it flexes the hinge and fatigues it. They eventually really do fail and the door falls off. :eek:
  18. Dec 1, 2017 #17


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    Ok . Let's model the spring . Post a dimensioned drawing of the hinge assembly
  19. Dec 1, 2017 #18


    Ok, the above gives the dimensions of the spring strip including the thickness and angle from opening to closing of temple. Assuming

    the materials are:

    1. Low alloy manganese
    2. Medium-carbon steel
    3. High-carbon steel with very high yield

    Compute the stress of each when the end of the spring is pushed to 4 degrees above horizontal. Since we don’t know the exact composition of it.. then let’s use 3 computations for each material.

    I’d like to know if the stress is less than or more than one half of the yield stress (elastic to plastic deformation) and tensile stress (cracking or fracture stress) to estimate the fatigue performance. I don’t know how to compute for bending stress of metal strip so please do the entire computations. Note this is for personal knowledge and not a homework. Thank you!
  20. Dec 1, 2017 #19
    Doesn't that approx. 1mm x 2mm attaching point on the plastic arm bother you? I would think that tiny piece of plastic would fail before the spring steel would...
  21. Dec 1, 2017 #20

    No. The plastic arm is glued to another metal support which is glued to the spring strip as shown above. But the arm as a whole is quite big compared to thin frame spring strip like the following brand.http://morel-france.com/morel-lightec/ with the description:

    "An exclusive flexible hinge without screws, which gives a glimpse of a thin line of metal on the temple: this is the Lightec signature.


    So a separate question is one would give more resistance.. bigger spring steel strip or smaller one like the above.
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