need to design spring for electrical contacts

[SurfaceID]

Hello,

I am developing a product that I need to use an electrical spring contact for. We are limited in size, and do not know the engineering rules for designing this sort of spring contact. Does anyone here have experience with this sort of spring?

I need 2 contacts that carry approximately: 20 volts and 5 amps. When the product is mounted it will be under constant load.

overall size and thickness requirement:
- Spring contacts have to fit within a cylinder with diameter of 45mm and height of 3.5mm. Contact points must protrude 2mm out of 1.2mm thick plastic.
- Flat contact pads are imbedded in 1.5mm of plastic which connect to wire in plastic casing with an interior of 45mm x 15mm x 2.5mm.
- Contacts must deflect approximately 2mm

I have something mocked up in 3d, but realize I do not know what material, size, thickness, plating etc to use. I will be eternally grateful for any input you may have to help us reach our goal.

 

[desertfox]

I think you need to post some sketches because I have not a clue what you are trying to describe, are the springs meant to be compression, tension, torsion or flat leaf type springs?
Are the contacts made from a conducting material like bronze which could also be used as a leaf spring?
Please read your post and pretend you know nothing about the product you've describe and then think how would you answer those questions.

 

[SurfaceID]

Yes I will post some sketches early tomorrow... sorry I do realize it is hard to describe with words... CAD is at the office tho.

It will be an electrical contact leaf spring, with very limited space requirements.

 

[TVP]

Nickel-plated stainless steel, Type 301, is a common material. Here is some information on the subject that you should find useful:

http://professional.duracell.com/downloads/batterycontacts.pdf

http://datasheet.octopart.com/SMTU1225-LF-Renata-datasheet-12521146.pdf

 

[SurfaceID]

Hi Again,

Here are some images showing the scenario I am trying to design for... Trouble is I know I am guessing at the sizes. Main constraints are that we have to stay inside the circle for the spring contacts, and that the location of the contact point is fixed.

 

[desertfox]

Hi SurfaceID

Thanks for the sketches there are very good.
Now what you need is a leaf type spring, possibly Phosphor bronze, the problem is that you need to know how much cross sectional area you need to carry the current of 5 amps and at the same time the temperature at which the joint will reach carrying continuous current.
I seem to remember a pressure of 10-15N/mm^2 was about the norm for busbar joints so that might be a good starting point to determine what force you need for the contact area you have currently.
The stiffness of the spring will be determined mainly from the gauge size and if you wish to do a 3mm deflection you need to ensure that the contact doesn't become over stressed and take a permanent set, if it does then some of the contact pressure in the joint will be lost.
I'll try and find my book on spring design by a guy called Berry, in this book he details a strain energy method of calculating springs very similar to yours by a graphical method, the only problem I'm not home till Friday.
You could search online for his book, alternatively approach a spring manufacturer for help.

 

[dgallup]

Totally off subject of your springs, but it's generally not considered good connector design to have your energized contacts exposed like that. Even though the voltage is low its way to easy to short across those pads causing sparks & other hazzards.

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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.

 

[SurfaceID]

Hi Dgallup, Good point. We have met with CSA and they have approved exposed pads with a 'type 2' power supply. Basically it shuts off automatically in the event of any short.

Thank you all for your input.