Formed Spring Steel Material

[Buck61]

Hello,
The project I'm working on requires a last minute part added (formed spring, drawing attached) to be used in a closed plastic enclosure. Its intent is to exert force on the far side a stamped metal speaker frame to compress a water seal. I have designed this spring very similar to a visor clip for a garage door remote.)

I need to manufacture the formed spring x .025" thick at the lowest fabrication cost without compromising spring force maintain stiffness, low mechanical creep), can be formed easily, & be finished to protect against corrosion (ZINC plated, etc). Problem is I must avoid added cost of post-forming heat treatment & fixture and other costly secondary operations including expensive plating/finishing. I do need to hold tolerances to +/- .03", though cut strip width must be held to .675 +.015/-.010" since spring will be assembled via *interference fit between two plastic ribs in the plastic housing.
[*No feature or area is available on/in plastic to accept mechanical hardware. I specified VHB tape originally to fasten spring in place which worked great but cannot be used due to serviceability issue.]

That said, the question is would this spring, formed using annealed spring steel, e.g., 1074 or 1095, maintain its spring force stiffness? Part will be secured at its closed end and compressed slightly by another part assembled above the spring (the geometry of the part assembled above the spring cannot adequately back or seal the speaker).

I'm open to other materials that can maintain about 75+% of its spring force. (perhaps a phosphor bronze w/ inexpensive plating??)
BTW, all material and finishes must be RoHS compliant.

Best,
Buck

 

[desertfox]

hi

So what is the current stiffness, deflection, force and stress encountered in your present design?

 

[Buck61]

Hi Desertfox,

Deflection is about .10 inches. Stiffness, force and stress are unknown at this time since the only 'spring steel' prototype I have is a modified visor clip x .025" thick where I managed to bend the lower leg slightly to fit into our device.

 

[desertfox]

Well I cannot see how the spring works ie where it's restrained when it's assembled, now until you work out the stresses and force for the current deflection wherever that deflection is,nobody can answer your question about a different material.
Some post a sketch of the spring showing how it's restrained in the assembly and which part deflects 0.1 inches.
My gut feeling is that your current spring design won't work as it stands.

 

[TVP]

My gut feeling is similar to desertfox. There is a reason why spring steel is hardened, either as part of the cold rolled strip, or as a final, discrete component.

 

[Buck61]

I have attached (2) new views to illustrate more clearly.

Actually, crude as it is, the retainer (spring) prototype worked quite well since the adhesive/foam speaker seal withstood IP55 water hose spray applied from outside of device for 45 seconds without any indication of water ingress. During IP55 testing, the retainer (spring) was fixed to the LSE plastic housing using 3M VHB 4952 x .5" long strip (which we unfortunately can no longer use). The VHB was adhered at the closed end of spring, from tangent of 3.2 mm radius on closed vertical portion, followed the radius down until VHB ended on the underside of the bottom retainer leg closest to the plastic housing.

Assembly sequence:
The speaker is 1st assembled face down and secured with PSA to inside of the LSE plastic housing.

Next the retainer spring is assembled in place via an interference fit, between spring edges & the plastic ribs (edges adjacent to where VHB used to be) at the closed end of the spring. See plan view in new attachment.

The capacitor is then placed inside housing and rests on housing ribs with profile concentric to the cap OD. See to profile and ISO attachments.

The device is closed by attaching and securing the upper enclosure (not shown) to the lower housing with 12 Hi Lo screws. The upper housing also has same concentric rib features to the cap OD (and two 1/2" thick compressible Poron foam blocks) to capture the Cap. The springs retainer's upper (straight) leg is compressed slightly by the cap OD after the upper housing is secured. See to profile attachment.

 

[Buck61]

Here is "Profile" screen shot

http://www.eng-tips.com/viewthread.cfm?qid=362260

 

[Buck61]

Here's "ISO" screen shot.

 

[TVP]

I'm sure that a properly designed and hardened spring will work. Annealed 1074 or 1095, I'm not sure about that.

 

[Buck61]

Hi TVP,

I know and am sure that a heat treated spring will work. The primary reason for my question was to find out if anyone is familiar with a flat spring material(s) that can be cold formed without severely injuring oneself. This is why I asked about about possibly designing a spring with annealed material (No Heat Treat) from widely used spring steels, i.e., 1074 or 1095, and whether there is an alternative to heat treating and associated drawbacks (cost, difficulty holding tolerances, etc.)

I have since found a source for alternative material that is engineered to be cold formed without HT though more expensive:

http://www.precisionsteel.com/spring-steel/special-temper-high-carbon-strip

 

[dgallup]

You could use work hardened (cold rolled, deep drawn, etc.) austenitic stainless steel. It's not necessarily cheep but it makes very good springs and you don't need to secondary corrosion resistance for mild environments. I've had a lot of springs made from 302.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[desertfox]

Hi
From the ISO shot you posted it seems to me that the load acts at about two thirds up the straight leg of the spring.
Now for a rough estimate of the spring stiffness you could treat the straight leg of the spring as a cantilever and work out the stress and deflection in the spring based on say a force of 1N load.
Once you have these figures you can calculate and compare the spring stress with the yield stress for that material which will tell you if the Pringles will remain elastic or not.
Assuming a cantilever in this case will I think give you a higher stiffness than you will get in practice so if the figures come out good in theory then the practice should be okay, my only reservation is that a deflection of 0.1 inches is a large deflection compared with the material section thickness and I think the possibility of the spring yielding is high.
It also would be worthwhile contacting a spring maker who would be able to advise better than I can.

Desertfox