Thread Interchangeability
Consider you are designing an assembly with fasteners you readily have available in your toolbox. After sorting through your hardware, you realize you have a mixed bag of UN, UNJ, and UNR thread forms, and want to ensure there will not be any material interferences when threading different bolts and nuts together.
- Will your UNJ bolts assemble into UN nuts?
- Will your UNJ nuts assemble into UN bolts?
- Will UN bolts assemble into UNR nuts?
Expand Hint
The UNR thread is a modified version of a standard UN thread. The single difference is a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. When first introduced decades ago, it was necessary to specify UNR (rounded root) threads. Today, all fasteners that are roll threaded should have a UNR thread because thread rolling dies with rounded crests are now the standard method for manufacturing most threads.
Hint 2
UNJ thread is a thread form having root radius limits of 0.150 to 0.180 times the thread pitch. With these enlarged radii, minor diameters of external thread increase and intrude beyond the basic profile of the UN and UNR thread forms. Consequently, to offset the possibility of interference between mating threads, the minor diameters of the UNJ internal threads had to be increased. 3A/3B thread tolerances are the standard for UNJ threads. UNJ threads are now the standard for aerospace fasteners and have some usage in highly specialized industrial applications. UNJ bolts are like UNR, but the curve of the thread root is gentler which requires that it be shallower. In fact, the thread root is so shallow that the bolt thread cannot mate with a UN nut, so there is a UNJ nut specification as well.
The UNR thread is a modified version of a standard UN thread. The single difference is a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. When first introduced decades ago, it was necessary to specify UNR (rounded root) threads. Today, all fasteners that are roll threaded should have a UNR thread because thread rolling dies with rounded crests are now the standard method for manufacturing most threads.
UNJ thread is a thread form having root radius limits of 0.150 to 0.180 times the thread pitch. With these enlarged radii, minor diameters of external thread increase and intrude beyond the basic profile of the UN and UNR thread forms. Consequently, to offset the possibility of interference between mating threads, the minor diameters of the UNJ internal threads had to be increased. 3A/3B thread tolerances are the standard for UNJ threads. UNJ threads are now the standard for aerospace fasteners and have some usage in highly specialized industrial applications. UNJ bolts are like UNR, but the curve of the thread root is gentler which requires that it be shallower. In fact, the thread root is so shallow that the bolt thread cannot mate with a UN nut, so there is a UNJ nut specification as well.
UN nuts will not assemble with UNJ bolts. UNJ bolts will only assemble with UNJ nuts. UN bolts will assemble with UNJ nuts. UN and UNR threads are interchangeable.
UN nuts will not assemble with UNJ bolts. UNJ bolts will only assemble with UNJ nuts. UN bolts will assemble with UNJ nuts. UN and UNR threads are interchangeable.
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