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Spring to pre-load a single ball bearing onto a polyester film

Spring to pre-load a single ball bearing onto a polyester film

Spring to pre-load a single ball bearing onto a polyester film

(OP)
I would like to ask this forum's advice on a problem.  I have a rectanglar brass slide, 5/8" x 5/8" x 3".  The slide fits into a matching cavity with a tolerance of 0.002".  The slide is free to move along its long (3") axis for slightly less than 2 inches of travel; being constrained at both ends.  One wall of the cavity is lined with a polyester film and metal-spray material that forms a capacitive position probe.  The slide needs to contain a single 1/16" (1.59mm) ball bearing, spring-loaded, to act as a "stylus" and signal the slide's position by depressing the polyester material with a force of 0.34 to 0.45 lbf (1.5 to 2.0 N).  The force needs to be maintained within this range for a minimum deflection span of 0.005", but 0.010" would be very nice.  At installation/assembly time, the ball must be depressed into the slide by an additional 0.020" to pass over a lip on the edge of the polyester film.  

I have two remaining volumes within the slide that can be used for the spring and ball bearing stylus assembly.  More volume is available, but would require a re-design of the other equipment installed in the slide.  The two volumes are:  8mm x 8mm x 4mm (spring displacement in 4mm axis), and
40mm x 8mm x 2.5mm (spring displacement in 2.5mm axis).

From a wear perspective, it would be very good if the ball bearing rolled on the polyester, rather than dragging.  

I have looked at Bellville Discs but cannot achieve the relatively low spring force I need.  I have also considered a strip flat spring, mounted in a groove along the 40mm axis mentioned above, but my mounting and ball-retaining efforts became very complicated.  

I'm an EE, and out of my depth.  My knowledge of spring design starts and stops at Hooke's Law.  If someone would be so kind as to point me in the right direction for the spring selection, possibly including: "Don't use a ball bearing & spring, use a frammis" I will be most grateful!  

I apologize for being so long-winded; thank you very much for reading and for your advice.  

David

RE: Spring to pre-load a single ball bearing onto a polyester film

You may want to check out wave springs such as those offered by Smalley.  Use the following link for more info:

http://www.smalley.com/default.asp

RE: Spring to pre-load a single ball bearing onto a polyester film

(OP)
Wonderful!  I did not know about wave springs, but they are just the thing for this application.  

Thank you very much for the excellent suggestion!

David

RE: Spring to pre-load a single ball bearing onto a polyester film

Glad to be of assistance, David.  Good luck with your application!

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