Re: Asking the experts - Material selection for tongs

I am designing a machine with a pair of tongs that will be used to
> stretch a grommet. It's very similar to the nose of long, thin needle
> nose pliers. The individual tongs are designed as follows:
>
> - 1.6" in length, protruding from a block of metal (the base will be
> connected to a gripper, and the base and tong are one piece, 2 of > which are required).
> - Each tong is a semi circle, so together they make a complete circle > when closed.
> - From the base of the tong (the top of the block) up, here is what it > looks like:
> -- .115 radius section protrudes up for .86"
> -- 3 deg. taper from .86" to about 1"
> -- .107 radius (the radius at the top of the taper) from 1" to > about 1.37"
> -- A conical nose with a 20 deg. taper at the end (from 1.37" to > 1.6")
> -- Each sharp edge broken (radiused)
> -- .0625 radius at base of tong
>
> I will be loading this thing about 30 times per minute with the
> following cycle:
>
> - Apply ~25 lbs (each tong) distributed along the tong, with 25% of
> the total force applied in the first half of the tong and 75% applied
> in the second have of the tong, which would be the tip end (this is
> the act of opening the tongs to stretch the grommet. The force
> deflects the tongs towards each other.
>
> - Hold for .5 seconds while inserting a .110" object between the > tongs.
>
> - Grip the .110" object with ~20 lbs of total force (10 lbs per tong).
> This will most likely cause the bottom of the tongs to touch, or
> almost touch, while the tips of the tongs are separated by the .110"
> distance, therefore they are declected away from each other.
>
> Now, to make the picture a little clearer, the .110" object is a lead
> wire that I am pulling through the grommet (that's the reason for > stretching it).
>
> On to the reason for polling the experts:
>
> As you can see, this is a tough application. I need to balance
> stiffness and resistance to breaking and fatigue. In essence, I am
> bending this thing back and forth like one might do to break a
> paperclip!
>
> I cannot make the tongs beefier because of geometric issues with the
> grommet. So the dimensions are fixed. The only variable is the
> material. If it is too flexible, the tip of the tongs will not open
> enough or perhaps not apply the 20 lbs of force when closing before
> reaching the maximum deflection (when the bottom of the tongs touch).
> If it is too stiff and brittle, it will break. What material do you
> guys recommend? I need this to cycle at least 1,000,000 times before > failing.
>
> Thanks in advance for any help. I have some ideas, and have gotten
> some from colleagues. We are thinking perhaps 1095 or 4140 steel,
> hardened to about 42 RC. I have tried with Crucible CPM 10V material,
> but it was too brittle (really a terrible material selection, I didn't
> realize the forces were so high). It failed after about 20,000 cycles. >
> My guess is that there is some material out there that will perform
> really well in this application, but do not know what that material > is!
>
> TIA.
>
> - Luther
Hi,
for your highly stressed application
may I suggest that you discuss the applicability
of Ultra High Strength Steel, with the technical
support of manufacturers like the following:
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Best regards,
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Reply to
Elia Levi
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