4-40 or 6-32 control arms?

[Jeff Hughes]

Nothing is ever simple. fatigue life of steel is directly proportional to tensile strength. Fatigue life is also dependent on surface residual stress. So the best thing you could use is a hardened bolt with threads rolled after heat treat (good compressive residual stresses). Like Dean says, finer pitch has a larger root diameter. The trouble is, the cheap hardware stuff we buy is generally rolled before heat treat, so it has crappy residual stress (tensile on the surface). So what I try to do is go large (6-32 or 6-40) and try to find a bolt with a long  unthreaded length (the threads act like stress concentrators).
Jeff

  ----- Original Message ----- 
  From: Bill Glaze 
  To: discussion at nsrca.org 
  Sent: Monday, February 14, 2005 12:47 PM
  Subject: Re: 4-40 or 6-32 control arms?


  ED:
  A few years ago, there was quite a dialogue about this.  One of the participants was Dick Hanson, a man highly respected in Pattern and other disciplines.  The subject came up about a couple of individuals who were looking for extra strength on aileron and elevator actuator bolts.  Dick said, specifically, do not get hardened bolts or anything exotic.  He said such material won't stand up under vibration nearly as well as common rolled thread material, or, for that matter, some of the standard Home Depot or Lowe's bolts right off the peg board.  He said that the ductility is what prolonged their life; that he had never had one break.  I believed him then, and still do.

  Bill Glaze

  Rcmaster199 at aol.com wrote: 
    In a message dated 2/14/2005 12:22:29 AM Eastern Standard Time, ed_alt at hotmail.com writes:
      I've been using 6-32 bolts to make the control horn arms for my 2M setups so far.  It seems like that might be overkill and that 4-40 would suffice.  Is it OK to go lighter like this?

      Thanks
      Ed A.
    Ed I wouldn't do that especially on ailerons. I have had 4-40's snap off like twigs on 90 sized models before. The vibration kills them.

    Matt
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