Split nuts

A split nut is formed from two substantially identical screw-threaded half nuts, each of which has chamfered ends, preferably at about 60.degree. to the axis of the nut, to engage in a conical counterbore of a washer or workpiece. When the split nut is tightened it remains securely in place. Each half nut has side faces extending in radial planes with complementary locating means symmetrically disposed on either side of the central plane of the half nut taken at right angles to the axis, and the screw-thread has a start at one end only, whereby one of the half nuts is inverted so that the respective complementary locating means interengage to align axially the half nuts.

The present invention relates to split nuts which comprise a plurality of 
segments which cooperate together and replace a conventional one-piece nut 
for use on a stud or bolt. 
Although in smaller sizes, split nuts will be more expensive to 
manufacture, they may be more economic if account is taken of the cost of 
fitting the nut onto a stud or bolt. More particularly split nuts offer a 
great potential advantage in connection with the removal of the nut from 
the bolt. This is particularly the case where the bolt is exposed to the 
risk of damage, corrosion or coating with foreign material which could 
cause a conventional one-piece nut to jam on the thread. However, hitherto 
split nuts have been of a complex design and therefore expensive to 
produce and often expensive to fit. Frequently, the split nut relies on 
captive devices to hold the nut segments together and generally the nut 
segments differ from one another, for example by having left and right 
engagement features. 
In U.S. Pat. No. 3,797,336 segmented nut arrangements are described. One 
embodiment comprises a nut formed in two halves with a circumferential 
spring clip located in a groove to hold the nut halves together and to 
urge them into clamping engagement around a bolt or stud. However, the 
arrangements described rely on the use of special tools which would 
prevent the split nut arrangement being used where restricted access 
occurred. The object of the special tools is to spread the segments of the 
nut so as to disengage the segments from the screw thread so as to permit 
the nut to be rapidly positioned at the location at which it can be 
tightened. A different tool is required when the split nut is to be 
removed after loosening. This patent specification does, however, describe 
the use of a chamfered end face at one end of the split nut to engage a 
corresponding shaped washer so that as the nut is tightened a wedging 
effect clamps the nut segments firmly around the bolt. 
There is, nevertheless, still an unfulfilled demand for a split nut 
arrangement which can be applied easily in most situations, and yet is 
still a strong nut. Furthermore, consistently correct application is 
vital, even if the fitter is giving the operation less than his full 
attention. 
According to one aspect of the present invention, there is provided an 
element which forms half of a split nut assembly, the other half of the 
split nut assembly being a substantially identical element which is 
inverted relative to the first element. The element is thus arcuate in 
shape and extends around an axis which coincides with the axis of the bolt 
on which the split nut is to be fitted. The element has an inner screw 
threaded surface, an outer peripheral surface to be engaged in order to 
rotate the element and respective end faces which are chamfered so that in 
use the chamfered surfaces of two elements forming the split nut engage a 
corresponding surface of a washer or a workpiece so that as the split nut 
is tightened, the elements are urged into clamping engagement around the 
bolt. The element also has first and second side faces extending between 
the inner and outer surfaces with at least one of the side faces having 
location means associated therewith and formed so that in use the location 
means interengage with the location means of the other element of the 
split nut. This causes the elements to be positioned relative to one 
another in the axial direction, and incorrect assembly is prevented. 
To assemble the nut, one of the elements is inverted relative to the other 
element, and the screw threading on the inner surface of the elements is 
such that when two elements are assembled to form the split nut, a screw 
thread is provided leading from one end of the nut to the other. 
Advantageous further features which may be used in association with the 
invention will now be described. 
Preferably both of the side faces of the element have first and second 
corresponding location members, for example the first location member in 
each case comprising a projection and the second location member 
comprising a recess. 
It is preferable that the pair of location members of each of the side 
faces is the same as the other pair and that in each case the first and 
second location members are symmetricaly disposed on either side of a 
central plane which extends through the elements and at right angles to 
the axis of the element. 
So that identical split nut elements can be used, the screw threading is 
such that when the nut is viewed as a true half nut a line on the plane of 
the split at the midheight of the nut element passes substantially through 
the centre of a crest of the thread at one side of the element and on the 
other side of the element passes substantially through the center of the 
root of a thread. 
The side faces are set back from the axial plane of the split nut in order 
to provide a clearance which permits the elements to be clamped tightly 
around the bolt. The chamfer on each end face can be flat so as to lie on 
a cone. Alternatively, a domed or spherical face could be provided and 
although theoretically this may be preferable from a strength point of 
view because of the true rocking motion permitted, it may be more 
convenient in practice simply to adopt a chamfer of conical shape. 
If the cone in each case is considered to have an axis coinciding with the 
axis of the element, then the angle of the conical face to the axis is 
preferably in the range 30.degree. to 75.degree., and an angle of about 
60.degree. is particularly preferred since this permits the split nut to 
be tightened with a relatively small amount of rotation and a very strong 
structure can be provided with a split nut of standard height and without 
making the counterbore in the washer or workpiece excessively deep. 
The invention extends to the combination of two elements to form a split 
nut. This combination may also include a relatively thick washer having a 
countersink corresponding in shape to the chamfer, the washer being thick 
enough to resist the bursting pressures applied when the nut is tightened. 
Use of at least preferred embodiments of the invention permit substantial 
advantages to be obtained over the prior art known to the applicant. The 
element is of simple design and each of the two elements forming the split 
nut can be identical. Strong and readily installed split nuts can be 
provided readily by known manufacturing techniques at a relatively low 
price and manufacture to within very close tolerances is possible. 
Not only can the split nut be lighter than a conventional nut, but the 
split nut embodying the invention may be applied rapidly and, for example, 
the split nut can be tightened with about a quarter of a turn in the case 
of a fairly coarse threaded nut. Furthermore, the design can ensure that 
correct assembly occurs and the axial alignment of the two elements in the 
split nut obviates the risk of one of the elements being a portion of a 
thread displaced in the axial direction. If such an occurrence were 
permitted to happen, although the nut may appear to be tightened it would 
be much less strong than it should be since one of the elements would not 
be correctly seating on the washer or workpiece. 
Furthermore, complex captive devices are obviated and the enormous 
practical operational difficulties of having left and right elements are 
avoided. 
Although it is preferred to provide the side faces in planes extending 
parallel to the axis of the element, this is not essential. For example 
the planes could be at a small angle such as 10.degree. to the axis. 
Should a "stiffnut" be required, i.e. a nut which tends to be more firmly 
retained or locked in position than a conventional nut, then a spring clip 
may readily be mounted around the periphery of the assembled split nut.

Referring first to FIGS. 1 through 3, the element 1 comprises a threaded 
inner surface 2 which in this embodiment is threaded according to British 
Standard Whitworth, an outer peripheral surface 2a having the shape of 
half a hexagon, first and second end surfaces 3 and 4 and side faces 5 and 
6. Each of the end faces 3 and 4 is chamfered to provide a substantially 
half-annular shoulder 7 which in this embodiment is conical, the angle 
between the axis and the cone being 60.degree.. Each of the side faces 5 
and 6 has first and second locating members, which in this embodiment are 
provided respectively by a raised rectangular block 8 and a similarly 
shaped recess 9 in which the block 8 would be a loose fit. The block 8 and 
recess 9 in each case are substantially symmetrically disposed about a 
central plane A-A which is normal to the axis of the element, the recess 
being enlarged to provide a clearance when a block of another element is 
inserted. The plane A-A will intersect the plane of the split at a line 
which if the nut element were projected to be a true half nut would, on 
one side of the element, pass through the center of the crest of the 
screwthread and on the other side pass through the center of the root of 
the thread, the element having a single `start` indicated at 10. 
A split nut is assembled by taking two identical elements and inverting one 
of the elements relative to the other so that at one end of the split nut 
the end face 3 of one element is adjacent the end face 4 of the other 
element and the side faces 5 and side faces 6 confront one another. The 
raised block 8 of each element engages into the opposed recess 9 in the 
other element and thus the elements are axially aligned and motion in a 
direction normal to the axis is also limited. 
In use the workpiece must have a counterbore of suitable shape or 
alternatively a strong thick washer with a suitable counterbore must be 
used. FIG. 4 illustrates the latter alternative in which a strong washer 
12 is provided on a bolt 13, the washer having a conical counterbore 
corresponding to the chamfer 7 provided on the nut elements 1A and 1B. 
An optional spring clip 16 is shown in FIG. 4, this being substantially 
annular with concave confronting end faces for engagement with a tool 
adapted to press the clip open for fitting and removal. The clip 16 can 
greatly increase the resistance to rotation of the nut when tightened. 
The use of powder metallurgy is particularly suitable for manufacturing the 
nut elements since a high degree of accuracy can be obtained by the use of 
a known techniques. Furthermore, the present invention offers the 
advantage of the element being formed readily and economically in a die. 
Sintered steel split nuts, it is believed, can be manufactured to comply 
with required standards for conventional low carbon steel nuts and 
sintered copper infiltrated steel split nuts, it is believed, can be made 
to comply with standards for high strength nuts. 
Where a conically counterbored washer is used, this can readily be made by 
cold forming or powder metallurgy. 
Regarding the economics of use, it has been reported that the cost of 
fitting a conventional nut is about four times the cost of the nut. It is 
expected that split nuts embodying the present invention in smaller sizes 
would be slightly more expensive than equivalent conventional nuts but 
actual experience has shown that fitting is about four times as fast, and 
thus it may be cheaper in many applications to use nuts according to the 
present invention. The invention can be especially useful in many 
instances particularly where demanding service conditions exist. Where it 
is expected that nuts will have to be removed in due course for 
maintenance purposes, the present invention offers considerable time 
saving potential, particularly where there is a high risk of corrosion or 
damage to the screw thread. Such damage could make it difficult or 
impossible to remove a conventional nut without burning or cutting. 
In many engineering situations, accessibility is extremely difficult but 
nuts embodying the present invention can be applied and removed quickly 
because turning the nut through a small angle is all that is necessary. 
Apart from the above advantages, a split nut embodying the present 
invention can be mechanically superior to a conventional nut in that a 
better load distribution is applied to the threads. It has been 
established that with conventional nuts, most of the load is taken on the 
first thread because the pitch of the bolt increases due to tension and 
the pitch of the nut decreases due to compression, and also the nut 
expands away from the bolt under the wedging action of the threads thus 
further effectively decreasing the pitch of the nut on the first few 
threads. Use of the present invention with a conical washer forces the two 
split nut elements firmly onto the bolt and a slight rotation of the two 
elements occurs so that the free end of the nut contracts more than the 
loaded end. According to his theory, this nut rotation causes an effective 
increase in the pitch of the nut so that the load is taken up more evenly. 
The applicant offers the above theoretical explanation as an aid to 
understanding the present invention only and does not wish to be bound by 
the accuracy or completeness of this theory. 
Test have been conducted with embodiments of the invention and it has been 
demonstrated that a strength advantage over equivalent conventional nuts 
is provided. Tests were conducted on three-quarter inch nuts and 
experiments extended to the chamfer angle being 60.degree., 55.degree. , 
and 50.degree.. The nuts embodying the present invention were consistently 
stronger, the extra strength varying from a few per cent up to more than 
twenty per cent. 
It is suggested that the optimum chamfer angle is about 60.degree. (taken 
between the surface of the cone and its axis). Smaller cone angles would 
result in the nut having a greater height in order to accommodate a 
spanner and more rotation would be necessary to tighten and remove the 
nut. Furthermore a stronger washer would be required to accommodate the 
bursting load.