Thread Engagement

“Designate a failure point, or the system will find one for you.”

Whenever we design a fastened joint, it’s preferable for the fasteners to fail before the part. This way only one fastener needs to be replaced in the case of failure instead of remaking and replacing and entire part or assembly, as well as all the fasteners needed to put them together. When bolts are used in tapped holes, the bolt yield strength should give before the threads strip out. To ensure this, we When the time comes to select a fastener length, first we have to consider the female or mating fastener. For loose fasteners like nuts and threaded inserts, select a bolt that fills all the threads of the nut.

• Screws need a minimum thread engagement to reach full strength, but additional threads are an unnecessary expense.  
• General rule (for grade 5 bolts) is 1 to 1.5 D for steel and 2 to 2.5 D for Aluminum 
• This would hint, don’t uses screws that are much wider than sheet metal thickness, and only use fine thread screws. 

• For anything other than steel, consider using helicoils 
• Never tap plastic 

Brass Tacks