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Minimum Threads Protruding Beyond Nut

ANSI ISO Hardware Design Guide

A minimum thread protrusion length is given to ensure that all the threads are engaged. This is required to ensure a full-strength fastener. Equipment component drawings should specify the type, length, and size of bolt, stud, bolt-stud, etc. to be used to obtain the proper thread protrusion. If not specified on the drawings, threaded fasteners of commercially stocked lengths should be used. Threaded fasteners, when installed and tightened, should protrude a distance of at least one thread beyond the top of the nut or plastic insert.

Industry Rule-of- Thumb

The simple rule is that two full threads should  protrude above the face of the nut. There are no identiļ¬ able problems with this  method as it ensures that all the threads in the nut are engaged. There may, however be situations where this simply cannot be maintained because of assembly problems

Excessive protrusion should be avoided, particularly when necessary clearances, accessibility, and safety are important. Thread protrusion is considered excessive if it could cause damage to machinery or harm to personnel. Where practicable, the number of threads protruding should not exceed five. In no case should thread protrusion exceed ten threads. In the case of a stud, excessive thread protrusion may indicate that the stud has not been properly driven in the blind hole. In self-locking nuts where the distance from the top of the nut to the locking element (plastic insert) is equal to or greater than the chamfer, the bolt or stud end may be flush with the top of the nut.

For existing or reused fasteners where the thread protrusion exceeds 10 threads, verification should be made that the proper length fastener was installed. For existing installations utilizing standard nuts, acceptable minimum thread protrusion would be where the male thread, below any unthreaded chamfer or crown, is flush with the top of the nut. Washers should not be added to reduce protrusion except as specifically required by equipment component drawings or technical manuals.

075 -7.5.2 MAXIMUM THREAD PROTRUSION. 

There is no maximum thread protrusion from the standpoint
of the function of the fastener. Obviously, too much is a waste of material and weight. There is also a safety problem from long, sharp, threaded rods sticking out from equipment, and protruding threads can get in the way of other parts of the equipment. A good rule to follow is to always use the shortest standard length fastener that gives a minimum one thread protrusion. Small and/or short bolts come in 1/8- or 1/4-inch increments. As the fastener gets larger and longer the increments change to 1/2 inch. Up to a five-thread protrusion from a fastener is certainly reasonable, you shouldn’t take a hacksaw to it for that; but if you have ten or more threads, you could probably have done a better job of selecting the length. There is no requirement to reduce the length of protrusion of existing fasteners before reinstalling them. There is also no reason you cannot cut off excess protrusion if it is obviously too long or in the way. However, you cannot use washers solely to reduce thread protrusion.

The equations are:

total length = clamp length + protrusion
protrusion for standard parts = nut height + 2P (P is thread pitch, so "2P" effectively is 2 threads)
protrusion for lock nuts = nut height + 3P

Minimum Thread Protrusion per. MSFC-STD-486B Chart

Threads per Inch Protrusion
64 .031
56 .035
48 .042
40 .050
36 .056
32 .062
28 .071
24 .083
20 .100
18 .111
16 .125
14 .143
13 .154
12 .167
11 .182
10 .200
9 .222
8 .250
7

.286

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