Screw-nut device of rolling bearing type with adjustable clearance or preload

Screw-nut device of rolling bearing type comprising a screw (V) and a nut (E) with multiple threads, of identical pitch. The consecutive threads (I, II) have, on the screw (V) and on the nut (E), intervals (a+.delta., a-.delta.) varying in repetitive manner with a periodicity corresponding to the pitch, changing of the position of assembly of the screw (V) with respect to the nut (E) making it possible, with the same constituent elements, to cause the conditions of contact of the rolling elements (B) with the threads of the nut (E) and of the screw (V) to vary.

BACKGROUND OF THE INVENTION 
The present invention relates to a screw-nut device comprising a screw and 
a nut with multiple threads of identical pitch, and rolling elements 
disposed between the threads of the screw and of the nut. 
Screw-nut devices, whether they are of the type with circulation of balls 
or with satellite rollers, are used in order to convert rotation movements 
into translation movement and vice versa. 
In a screw-nut device of the type with balls or of the type with satellite 
rollers, the rolling elements are in oblique contact with the sides of the 
helicoidal grooves or threads of the screw and of the nut, at an angle 
whose nominal value is generally close to 45.degree. so as to ensure at 
the same time the relative centring of the screw and of the nut and the 
taking up of the mainly axial and occasionally radial loads. 
For an assembly with clearance, the usual design is such that all of the 
contacts of the rolling bearing elements (balls or rollers) with the sides 
of threads of the screw and of the nut are made at an angle having the 
same orientation, for a given direction of the external axial force 
transmitted by the rolling elements, so as to distribute the load over a 
maximum of contact points. 
FIG. 1 of the accompanying drawings shows the contact conditions which are 
established between the balls B and the threads of the nut E and of the 
screw V in a screw-nut device with clearance, under the effect of an axial 
load C. It will be recognised that here all the balls B are in contact at 
an angle .alpha. of the same value and of the same orientation with the 
sides of all the threads. 
In order to increase the precision of such a screw-nut device, it is usual 
to adjust or to reduce to zero the internal clearance of the device or to 
produce an internal preload. 
For this, several systems are currently used. 
1. Monobloc nut and rolling elements of larger diameter 
The use of rolling elements (balls or satellite rollers) of larger diameter 
enables the internal clearance to be eliminated, the rolling elements then 
being in contact with both sides of the threads of the screw and of the 
nut. 
In the case of screw-nut devices with balls, the threads generally have a 
profile constituted by two arcs of a circle so as to maintain angles of 
contact of the order of 45.degree., and each ball is then in contact with 
four sides, as is shown in FIG. 2 of the accompanying drawings. The axes 
of rotation of the balls on themselves thereby find themselves modified in 
comparison with the instantaneous axes of rotation in the case of two 
directly opposed contacts, and this results in increased sliding 
phenomena, creating an increase in the friction torque. The necessary 
torques are then higher and more sensitive to variations in the diameter 
of the screw, and the life of the assembly is reduced because of internal 
frictions contributing to wear. 
For screw-nut devices with satellite rollers, the axis of rotation of the 
rollers does not change, whether the assembly is preloaded or not, but the 
construction of rollers to the precise diameter required in order to 
obtain a given preload is technically and economically more difficult 
because rollers constitute more complex elements than balls. 
2. Reduction of the pitch circle diameter of the nut, by deformation of the 
latter 
Instead of interposing the rolling elements of greater diameter, one may 
proceed to a reduction of the pitch circle diameter of the nut by 
deformation of the nut. The phenomenon is then the same as far as the 
modification of the axis of rotation of the balls is concerned, with in 
addition, for certain embodiments, the disadvantage that the track for 
rolling in the nut is interrupted by a slot, in order to facilitate the 
deformation of the nut. The points of contact with the rolling elements on 
either side of this discontinuity have of course the disadvantage of 
causing a concentration of load accelerating the phenomenon of fatigue at 
these points. 
3. Nut formed of two sections adjustable with respect to one another 
Various known systems exist using nuts formed of two sections with the 
capability of positioning or of adjustment of two sections with respect to 
one another, so that the rolling elements (balls or rollers) are in 
contact with different sides of the threads when one of the two sections 
is compared with the other. 
a) By relative translation between the two sections of nut. 
By adjustment of the axial distance between the two sections of nut, the 
latter maintaining the same relative angular position, it is possible to 
adjust the clearance or to create a preload. 
This relative translation may be produced, for example 
by adjustment of the two opposite surfaces of the two sections, bearing 
directly against one another or onto a rigid or resilient spacer washer, 
by action on the outer surfaces of the two sections of nut enclosed in a 
housing, 
or by any other system of the same principle, the two sections of nut 
maintaining the same relative angular position during the adjustment. 
b) By relative rotation between the two sections of nut. 
The two sections of nut being supported axially in a fixed and opposed 
manner with respect to reference surfaces (outer or inner surfaces 
supported directly or with intermediate spacers), any angular phase 
displacement by rotation of one of the two sections with respect to the 
other amounts to bringing together or separating the opposite sides of the 
threads of the two sections and thus causing a result identical to that of 
the translation described under a). 
There exist various known systems for obtaining this relative angular 
displacement, in a definitive or an adjustable manner, and for maintaining 
the relative angular position of the two sections once the adjustment has 
been made. 
According to the two above-mentioned methods of adjustment, by translation 
or relative rotation of the two sections of nut, the final result is seen 
as a difference between the nominal pitch and the actual distance between 
the threads in the zone of transition between the two active parts of the 
threads borne by the two sections. 
FIGS. 3a and 3b of the accompanying drawings show the conditions of contact 
of the balls B with the threads of the screw V and of the two sections of 
nut E.sub.1 and E.sub.2 in two different relative positions of these two 
sections of nut E.sub.1 and E.sub.2, namely in FIG. 3a with a distance 
D-.DELTA. between the two sections of nut E.sub.1, E.sub.2 and in FIG. 3b 
with a distance d+.DELTA. between the said sections of nut. 
4. Monobloc nut with adiustment of the clearance by central phase 
displacement of the threads of the nut during the machining 
Such a design of the nut makes it possible to dispense with the additional 
machining operations necessary in order to produce the centring and the 
adjustment of positioning of the two sections of nut when these two 
sections are executed in two pieces, as indicated under 3), and therefore 
permits better concentricity while being more economical for single-thread 
assemblies with balls. It also retains, for single-thread assemblies with 
balls, the advantage of an oblique contact limited to two opposed points 
per ball, thus avoiding the disadvantages of the systems with four contact 
points per ball (friction, wear). 
FIGS. 4a and 4b of the accompanying drawings show such a device comprising, 
associated with a single-thread screw V of pitch p, either a monobloc nut 
E.sub.a having a thread of pitch p, except in a central position where the 
gap between two consecutive threads is reduced to p-.DELTA., as shown in 
FIG. 4a, or a monobloc nut E.sub.b on which, in a central position, the 
gap between two consecutive threads of pitch p is increased to p+.DELTA., 
as shown in FIG. 4b. 
Both for the devices having two separate sections of nut in accordance with 
3) and for devices having two monobloc sections of nut in accordance with 
4), the nut must comprise an independent circuit of balls for each of the 
two sections of nut, so as to avoid on the one hand a too large number of 
balls per circuit and on the other hand the irregularities of internal 
friction which would be produced at the passage of the balls, in the 
transition zone between the two sections, from a given angle of contact to 
an angle of contact of opposite direction. 
Moreover, when the advance per revolution (that is to say the pitch) is 
large, the total length of the nut is then increased insofar as the length 
of each section of nut must be at least equal to the pitch (equivalent to 
one turn of balls). This involves technological problems for the 
production of monobloc nuts where rectified nuts are concerned, and 
becomes impossible to produce if one wishes to consider a more economical 
machining by tapping. 
In conclusion, known screw-nut devices with balls or rollers as currently 
used have, when means are provided for adjustment of the conditions of 
contact, particularly in the case of multiple threads, disadvantages 
either of a functional nature or of an economical nature. 
SUMMARY OF THE INVENTION 
The present invention provides a simple and economical solution for 
producing a screw-nut device with rolling elements and with multiple 
threads, particularly with large advance per revolution, providing with 
the same constituent elements (screw, nut, rolling elements), different 
conditions of contact of the rolling elements with the threads of the 
screw and of the nut, this within reduced overall dimensions and 
independently of the process of finishing of the threads of the nut and of 
the screw, while ensuring a low internal friction torque and a reduced 
wear. The invention provides moreover a screwnut nut device with rolling 
elements making it possible to construct, with the same constituent 
elements, an assembly capable of having clearances or preloads of 
different values. Lastly, the invention provides a screwnut device with 
rolling elements and with multiple threads making it possible to produce 
an, assembly adapted in optimal manner to the loads to be borne. 
The screw-nut device with rolling elements and according to the invention 
comprises a screw and a nut with multiple threads of identical pitch, 
rolling elements disposed between the threads of the screw and of the nut, 
and means for adjustment of the conditions of contact of the rolling 
elements with the threads of the screw and of the nut. According to the 
invention, the consecutive threads have, on the screw and on the nut, 
intervals varying in repetitive manner with a periodicity corresponding to 
the pitch of the threads, in such a manner that the changing of the 
position of assembly of the screw with respect to the nut makes it 
possible to cause the conditions of contact of the rolling elements with 
the threads of the screw and of the nut to vary. The main difference, with 
respect to known devices, therefore consists in the fact that the same 
arrangements provided for adjusting the conditions of contact are made 
simultaneously on the nut and on the screw, over the entire length of 
these elements. 
In the simplest case, the screw and the nut have an even number of threads 
and the intervals between the threads have two different values repeated 
in alternate manner. Thus when the screw is associated with the nut so 
that the threads of the screw and of the nut are in phase, the same 
conditions of contact are obtained for all the rolling elements with the 
threads of the screw and of the nut. On the contrary when the nut is 
associated with the screw so that the threads of the screw and of the nut 
are dephased, opposed contacts of the rolling elements with the successive 
threads of the screw and of the nut are obtained. 
The screw and the nut may also have an odd number of threads and the 
intervals between the threads may have two different values repeated in 
alternate manner. In this case, the conditions of contacts of the rolling 
elements with the successive threads of the nut and of the screw are the 
same when the threads are in phase and are opposed when the threads are 
dephased. On the contrary, because of the odd number of threads, the 
association of the screw with the nut with dephasing of the threads 
provides opposed contacts with an orientation privileged in one direction. 
In both cases, depending on whether the threads are in phase or dephased, 
as a function of the diameter of the balls, an axial clearance or a zero 
clearance is obtained when the threads are in phase and a zero clearance 
or an internal preload is obtained when the threads are dephased. 
Other conditions, concerning the number of the threads and the variation of 
the intervals between the threads, permit other noteworthy combinations 
with respect to adjustment of the clearance and of the preload as a 
function of the position of the screw with respect to the nut. 
Referring to FIGS. 5 to 8 of the accompanying drawings, we shall describe 
below in greater detail several illustrative and non-limitative 
embodiments of a screw-nut device with balls according to the invention; 
in the drawings:

On the screw-nut system with balls according to the invention as shown in 
FIGS. 5a and 5b, the nut E and the screw V comprise two same threads I and 
II with two different values of intervals, namely a+.delta. between the 
threads I and II and a-.delta. between the threads II and I. 
According to FIG. 5a, the screw V and the nut E are associated so that the 
threads having the same identification are in phase. In these conditions, 
all the balls B have the same conditions of contact with the threads, and 
this is comparable with a conventional device constructed with an equal 
distance between all of the threads on the screw and on the nut. 
According to FIG. 5b, the threads of screw V and of the nut E are dephased, 
that is to say the threads I of the screw V are associated with the 
threads II of the nut E, and vice versa. In these conditions, the balls B 
have oblique contacts in opposition with the consecutive threads. 
Moreover, if the diameter of the balls B is selected so as to obtain a zero 
clearance in the position shown in FIG. 5a, the position shown in FIG. 5b, 
produced with the same balls, leads to an internal preload equivalent to 
-2.delta.. 
Conversely, for an axial clearance of value 2.delta. in the case of FIG. 
5a, a zero clearance is obtained in the case of FIG. 5b. 
A change of diameter of the balls between the position shown in FIG. 5a and 
the position shown in FIG. 5b makes it possible to augment the possible 
variants of clearance and of prestress. 
In the embodiment shown in FIGS. 6a and 6b, the screw V and the nut E have 
three threads with two different intervals between consecutive threads, 
namely a+.delta. between the threads I and II, a-.delta. between the 
threads II and III and a-.delta. between the threads III and I. 
By associating the screw V with the nut E in the position shown in FIG. 6a, 
in which the threads of the screw are in phase with the threads of the 
nut, an identical contact is obtained for all the balls B, in a manner 
similar to FIG. 5a. 
On the contrary, when the threads of the screw V and of the nut E are 
dephased by one thread shown in FIG. 6b, oblique contacts of alternate 
orientation are obtained for the balls B, with an axial preload of value 
-2.delta.. 
Because of the odd number of threads, a privileged orientation of the 
oblique contacts will be produced automatically, which may be 
advantageously exploited in order to make the screw-nut device bear 
preponderant axial loads in one direction or in the other. 
According to the embodiment of FIGS. 7a to 7d, the screw-nut device 
comprises an even number of threads, namely four threads I, II, III, IV in 
the example shown, with progressive intervals a (between IV and I), 
a+.delta. (between I and II), a+2 .delta. (between II and III) and 
a+3.delta. (between III and IV). 
When threads of the same identification of the screw V and of the nut E are 
in phase, as shown in FIG. 7a oblique contacts of the same orientation are 
obtained for all of the balls B. 
By inverting the screw V in the nut E, alternately opposed contacts are 
obtained, as shown in FIG. 7b, with the following dispositions: 
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interval a a + .delta. 
a + 2.delta. 
a + 3.delta. 
a 
between 
threads 
nut thread IV I II III IV I 
identi- 
fication 
thread II I IV III II I 
identi- 
fication 
screw interval a + .delta. 
a a + 3.delta. 
a + 2.delta. 
a + .delta. 
between 
threads 
Difference of the 
-.delta. 
+.delta. 
-.delta. 
+.delta. 
-.delta. 
intervals between 
nut-screw threads 
______________________________________ 
If, in the case of FIG. 7a, balls B have been selected so as to obtain a 
zero clearance, the same screws, nuts and balls rearranged as shown in 
FIG. 7b will lead to an axial preload of -.delta./2 with the same set of 
balls. 
In the case of FIG. 7c, in addition to the inversion of the screw as shown 
in FIG. 7b, a dephasing by one thread has been performed, and this gives 
the following dispositions: 
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interval be- 
a a + .delta. 
a + 2.delta. 
a + 3.delta. 
a 
tween threads 
nut thread iden- 
IV I II III IV I 
tification 
thread iden- 
I IV III II I IV 
tification 
screw interval be- 
a a + 3.delta. 
a + 2.delta. 
a + .delta. 
a 
tween threads 
Difference of the intervals 
0 -2.delta. 
0 +2.delta. 
0 
between nut-screw threads: 
if the differences are 
0 -2.delta. 
-2.delta. 
0 0 
added cumulatively: 
______________________________________ 
Thus, by recentring the interval III-IV of the nut E with the interval 1-II 
of the screw, an alternation of oblique contacts opposed in pairs is 
obtained, with an axial preload of -.delta.. 
In the position shown in FIG. 7d corresponding to such an additional phase 
displacement that the interval II-III of the nut E is centred with the 
interval IV-I of the screw, contact conditions similar to those of FIG. 7c 
are found again, that is to say an alternation of contacts opposed in 
pairs. 
With different sets of balls B, it is possible to obtain, with a device as 
shown in FIGS. 7a to 7d, the following combinations 
______________________________________ 
FIGS. 
7a 7b 7c or 7d 
______________________________________ 
1st set of balls 
clearance 
preload preload preload 
0 -.delta./2 
-.delta. -.delta. 
2nd set of balls 
clearance 
clearance 
preload preload 
+.delta./2 
0 -.delta./2 -.delta./2 
______________________________________ 
FIGS. 8a to 8d show a screw-nut device on which the screw and the nut have 
an odd number of threads, in this case three threads I, II, III, with 
progressive intervals a, a+.delta., a+2.delta. between the consecutive 
threads. 
When the threads of the same identification of the screw and of the nut are 
in phase, as shown in FIG. 8a, oblique contacts are obtained of the same 
orientation for all of the balls B. 
In order to obtain contacts opposed in an alternate manner, the screw V is 
inverted with respect to the nut E. 
Assuming that the screw-nut device comprises a zero clearance, in the case 
of FIG. 8a, it is possible, after inverting the screw round with respect 
to the nut, to obtain for example a preload -.delta./2 in the case of 
dephasing as shown in FIG. 8b or as shown in FIG. 8c, or a preload 
-.delta. in the case of dephasing as shown in FIG. 8d, with, each time, a 
preponderance of the oblique contacts in one direction. 
Of course the embodiments described above and shown in the accompanying 
drawings have been given solely as indicative and nonlimitative examples 
and many modifications and variants are possible in the context of the 
invention, with respect both to the number of threads and to the number of 
the various values of the intervals between consecutive threads. In all 
cases, the threads on the screw and on the nut are executed so that when 
the threads of the screw and of the nut are in phase, all of the rolling 
elements have oblique contacts of the same orientation with the threads of 
the screw and of the nut, whereas by dephasing of the threads of the screw 
with respect to the nut or by turning the screw round in the nut, contacts 
of directions opposed in an alternate manner are obtained.