MP35N Sring Design

[mlandry]

Hello,

I am new to forum so thanks in advance for any help.

I need to put together a consulting team quickly for a very specific spring design for an orthopaedic implant. Can you guys help me find the right people?

The design requirements are established. My envelope is 10mm OD X 30mm long (free length). The solid height must be 24mm or less. The working height must be 26mm with a force of 40 lbs (k=254 lbs/in). My material choices are limited because this is a long term implant. I can use 316L stainless, titanium (several alloys), MP35N, or Elgiloy.

My best spring to date is a spring made from .087” dia MP35N wire (cold-worked 45%, Aged to 255ksi). After cold winding, the spring is shot peened. Note, the spring index for this is low, 3.5, and I know this is not ideal but it is the only way I can seem to get the proper spring rate.

This spring would be perfect except the fatigue life is only about 1MM cycles and I need 10MM cycles.

I am confident that with the right wire, wire treatment, coiling and post treatments I can get the increases in Fatigue life I need. I just don’t have the time to figure it out so I am willing to hire outside help to expedite the process. Any ideas who I should contact?

Thanks,

Michael

 

[israelkk]

When you say cycles, do you mean from free length (30mm) to working length (26mm)? Of some other first working load and deflection?

http://israelkk.googlepages.com/home

 

[TVP]

There are a number of things to investigate for improving fatigue life while maintaining the design envelope, such as multiple peening operations, the set removal operation, surface finishing after peening, etc. The first place I would contact is Associated Spring (

http://www.asbg.com).

They have expertise designing and manufacturing springs for every conceivable end use.

 

[mlandry]

Yes, a cycle is from the free length to the working length (30mm - 26mm) which would be from a load of 0 to a load of 40 lbs. I would like to maintain some minimum load on the spring at all times to improve the fatique life but the application will not permit it.

 

[israelkk]

When you say the fatigue life is only about 1MM cycles do you mean calculated or tested? I made a quick calculation and I could not get over 5,000,000 yet. However, real spring may live much less. You have to design your spring for the minimum material properties and not typical. When you test an actual spring you have to know its actual tensile stregth compared to the minimum the spec allows and adjust the fatige life accordingly. If a tested spring survives 10,000,000 cycles and the actual strength of the wire is over the minimum allowed by the spec then your design will not meet the requirements yet.

 

[sbozy25]

Talk to Atlantic Spring

They are located in New Jersey and often work with titanium springs. You can contact them at 908.788.5800 If you ask for John King, I believe he is still the engineering manager, he should be able to help you out.