Anti-vibration thread form

An internal V-thread form has a concave following flank for engagement with the crest of an external thread.

BRIEF SUMMARY OF THE INVENTION 
It is the purpose of this invention to provide a vibration resistant thread 
form that may be consistently manufactured to proper size and shape. 
The invention accomplishes this purpose by means of a relatively small 
structural change in a standard internal V-shape symmetrical thread form. 
In one form this change comprises an increase in the flank angle of the 
following flank so that the threads are asymmetric, the modified following 
flank having a larger flank angle than the leading flank. Preferably, the 
modified flank is given a concave curvature; and this, preferably, is 
selected so that the tangents to any point thereof that will be engaged 
under load by the bolt threads define "flank" angles greater than that of 
the leading flank or a standard following flank. In another form both 
leading and following flanks are modified in the same way so that the nut 
is bidirectional, i.e., may be used with either end on top. It is also 
feasible to modify the standard V-shape symmetrical bolt thread form for 
use with this internal thread form by changing at least the following side 
of the thread crest to a convex shape having the same curvature as the 
modified internal thread flank.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is illustrated in connection with a threaded fastener 
assembly 1 for clamping parts 3 and 5 together. The assembly comprises a 
bolt 7 having a head 9 with a bearing surface 11 and a threaded shank 13 
extending through aligned holes 15 in the parts 3 and 5 to thread into a 
nut 17 having a bearing surface 19. When sufficient torque is applied to 
the wrenching surfaces of the bolt and nut, the resulting tensile load in 
the shank 113 is balanced by compression of the bearing surfaces 11 and 19 
against the parts 3 and 5 to clamp them together. It is the purpose of 
this invention to provide a thread form that will resist inadvertent 
unclamping of the parts. 
If standard symmetrical V-shaped threads are used in the threaded assembly 
1, it is likely that the parts 3 and 5 will come loose if subject to 
vibration or alternate transverse loading. This is thought to occur 
because dimensional variations in thread manufacturing produce clearances 
in use that allow relative lateral motion of the bolt and nut to occur in 
the threads. This eventually allows variable loads on the fasteners to 
work them loose. It is believed that looseness may also be the result of 
internal thread bending in combination with variable loads. The thread 
form of this invention combats the tendency to loosen by increasing the 
frictional resistance to loosening and also by modifying the flank 
geometry so that the crest of the bolt threads is, in effect, disposed 
inside of an inwardly tapered hole. Angular tightening motion of the bolt 
therefore powerfully aligns and centers the external threads in the 
internal threads to develop very tight thread contact along the length of 
the helical crest and minimize or eliminate clearances that permit 
relative lateral thread motion which could develop into looseness. Load 
transfer between engaged internal and external threads takes place at 
maximum diameters thereby minimizing bending of the internal threads and 
the tendency for that to contribute to looseness under alternating loads. 
The nut 17 has a helical internal thread configuration 21 which is 
basically a modified standard V-shaped symmetrical thread with a 
60.degree. thread angle and a 30.degree. flank angle. In the embodiment of 
FIGS. 2-6, the standard (or "nominal") thread form is modified so that the 
following flank has a flank angle more than 30.degree. and the thread is 
asymmetric. The thread 21 has a straight leading flank 23 on the 
30.degree. angle and a following flank 25 which is preferably of concave 
curvature as shown in the drawings and on a larger flank angle than its 
nominal straight 30.degree. flank 26. The flank 25 commences at the root 
diameter 27 of the thread form and its curvature is selected, preferably, 
so that tangents to all points on it that may be contacted by the bolt 
thread make flank angles greater than 30.degree.. A preferred way to shape 
the curved surface of flank 25 is to form it on a relatively large radius 
R (FIG. 6) extending from the root diameter with a center lying on a 
perpendicular P to a standard following flank at its minimum diameter. 
Preferably, the radius is at least two times the length of the flank for a 
30.degree. flank angle thread in order to maintain substantial flank 
length. 
The phantom lines in FIG. 3 illustrate the symmetrical nominal following 
flank 26 that would be obtained if the thread 21 were not modified in 
accordance with the invention. It will be seen that it has a smaller flank 
angle than any tangents to the curved flank 25. It will also be seen that 
the larger angle of flank 25 causes it to intersect the adjacent leading 
flank 23 at a larger diameter than in the case of a symmetrical thread 
form, thereby shortening the thread. 
Referring to FIGS. 3-5, the no load condition when the nut and bolt are 
loosely connected together is shown in FIG. 3. When torque is applied, the 
crest 41 of the standard, symmetrical, 30.degree. flank, external 
V-threads 43 of bolt 7 will engage the following flank 25 of the nut 
threads as illustrated in FIG. 4. With the application of more torque to 
the assembly 1, clearances are taken up and some bolt thread deformation 
may occur as the crest 41 moves downwardly along the flank 25 as shown in 
FIG. 5. The flank 25 is inwardly tapered with respect to the crest 45 so 
that this movement tends to align and center the bolt threads on the 
thread axis and provide contact under load along the helical length of the 
crest 41 to resist relative lateral movement of the bolt and nut threads. 
Since the angle of contact is less than the original 30.degree., friction 
is greater as is resistance to lateral shifting. Tangents to surface 25 
make angles greater than the 30.degree. angle of its nominal straight 
flank 26 (FIG. 3) so that the surface 25 is out of contact with the 
following flank 47 of the bolt threads 43 to maintain engagement at 
maximum thread diameters where the hoop strenght of the nut body can take 
the load. Thus, thread strength and vibration resistance are improved as 
compared with the standard V-shaped symmetrical internal thread. 
The internal threads 21 may be formed with thread cutting tools or taps 
that are substantially the same as conventional V-thread forming tools and 
may be tapped in blind holes and formed in large forgings, castings, etc. 
Thus, the nut body 17 shown herein is merely illustrative of a part having 
internal threads embodying the invention. The thread form 21 of this 
invention is substantially as simple to make as standard V-shaped threads 
thereby promoting economy and consistent quality in manufacturing. 
However, the change from the nominal straight flank 26 whereby the line of 
contact with the concave following flank is angularly moved in the 
direction of parallelism with the thread axis provides means to increase 
the thread holding power and improve resistance to loosening under 
oscillating or alternating loads on the parts or threads. 
The threads 21 shown in FIGS. 2-6 are asymmetric in that the leading flank 
23 is straight and the following flank 25 is concave in accordance with 
the invention. The thread 21 is therefore unidirectional since the bolt 
must be inserted from a particular end if the benefits of the invention 
are to be obtained; though it may be noted that if the direction of 
insertion is reversed (e.g. by turning nut 17 upside down), the threaded 
assembly merely reverts to a standard engagement with 30.degree. flanks 23 
and 47 engaging each other under load. It is a simple matter to make the 
threads bidirectional as illustrated in FIG. 8 by nut 17' with threads 21' 
having following flank 25' like flank 25 and a correspondingly curved 
leading flank 23'. Thus, if the threads 21' are inverted (i.e., the nut 
17' used upside down) the leading flank 23' becomes the following flank; 
and, since the two are symmetrical, there is no difference in operation or 
function of the threads. 
It will now be apparent that the invention provides a thread configuration 
that is simple and relatively easy to consistently produce and which 
improves the holding power and vibration resistance of the threads as 
compared with standard straight flank configurations. The thread 
configuration may be regarded as having an actual form (i.e., the threads 
21 or 21') and a nominal form (i.e. the threads shown in phantom lines and 
the standard symmetrical V-shape threads) which the invention has modified 
into the actual form by curvature of the nominal following flanks 26 to 
produce the actual flanks 25, 25', and 23'. Variations in the specific 
details and structures shown may be made without departing from the spirit 
and scope of the invention.