Edge distance using key inserts

[cc_aitlin]

Hi,

I'm designing a large and heavy vibration fixture, made out of several machined plates of 1" thick aluminum alloy (total assembly weight approximately ~750 lbs). I've been going back and forth with a coworker on bolt sizes, which is driven by our choice of key insert, which is driven by our available edge distances.

He is set on using ~20+ #8-32s along the side face of this 48"x48"x1" plate in order to keep a hole distance of > 1.5D from the edge, where D is the nominal external thread diameter of the key insert. I'm concerned about the strength of 8-32s in this application, considering the thickness and weight of the whole structure, so I suggested using 12x 1/4-20 inserts where the edge distance would be about 1.14D. It's pretty close but the fixture is definitely overengineered for the situation and all the information I've gathered online has ranged anywhere from 1-2D.

Is it best to use #8 or 1/4-20 inserts in this case?

 

[3DDave]

Best? I don't know. However, the pullout strength in aluminum should typically be higher than the screw strength, so if a #8 screw can take the load the insert should be fine. However, my inclination is that 20 .250 screws is better than 20 .164 screws. Why skip the .190 size?

Are there match reamed solid dowel pins being used to take the shear load? Also the bearing under the screw head is another place to check. It doesn't help if the inserts are fine but the heads of the screws dent their bearing surface and leave the plate loose.

 

[MintJulep]

Without a clue as to the magnitude and directions of loading do you really expect us to give valid advice?

 

[rb1957]

#8s are not usually (IMHO) structural fasteners, and 1.14eD should sort of rule out 1/4s ... wot about #10s ?

How critical is bearing ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[IRstuff]

I'm confused by your "several machined plates" Where are these plates in your image?

Given that a large vibration table typically uses something like a 3/8"-16 fasteners, I wonder why both your fastener choices seem somewhat undersized.



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[desertfox]

The rules of thumb I used when working was that a bolt hole should be positioned between 1 and 1.5 times the screw or bolt diameter away from an plate edge. That rule of thumb was to prevent breakout of the fixing through the edge of the plate and so we had to analyse where the loads where acting and there magnitude, as stated by others without more information we cannot properly advise you.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein