Patent Description:
More in detail, the invention relates to the formation by rolling of a thread of a mechanical torque transmission element such as, for example, a nut for a screw and nut pair.

The main known methods of making threaded mechanical torque transmission elements are milling, rolling and tapping.

On a workpiece, milling and tapping cause removal of part of the material, generally at the grooves of the thread to be formed. On the other hand, rolling is a deformation process that causes the material of the workpiece to be displaced in a direction of depression at the grooves and in direction of elevation at the crests of the thread to be formed.

Milling affords significantly more accurate results, with machining tolerances as little as less than one hundredth.

Tapping is an alternative process from low-quality removal of the contact and precision surfaces, with a cost impact that is halfway between rolling and milling.

Conversely, rolling is a significantly faster and more cost-effective process.

One of the problems of known rolling, which make it less preferable than milling or tapping in precision applications, is the formation of threads with double crests.

In detail, a torque transmission element, such as a nut, is rolled by means of a rolling tool known as "roll tap".

The rolling tool has its own thread which is designed to operate on the workpiece to deform its surface and obtain the desired thread thereon.

At least a first part of the rolling thread extends on a tapered surface of the rolling tool, often followed by a part of the thread on a cylindrical surface, to progressively press against the workpiece as the rolling tool moves forward with a helical motion. This will form a groove where the crests of the rolling thread contact the surface to be machined, the groove becoming deeper and wider as the rolling tool moves forward. In other words, the material of the workpiece is spread apart, and is depressed where it contacts the crest of the rolling thread and elevated between two successive crests of the rolling thread.

The depression of the workpiece material forms the grooves of the thread of the final resulting torque transmission element, whereas the elevation of the material forms the crests.

Nevertheless, material elevation is accentuated at the two sides of the rolling thread, i.e. the parts that connect each crest with the adjacent grooves.

However, elevation is less accentuated at the center of the grooves of the rolling thread.

Therefore, the thread resulting from known rolling processes does not have a compact crest like that of milled threads, e.g. trapezoidal threads, but a crest portion with a central sag on a centerline of the crest, all along the thread. Thus, the crest has two distinct top portions that are elevated with respect to the centerline, at both sides of the centerline.

<CIT> discloses a pair of fastening elements, such as a screw and a nut, which are adapted to provide a static connection, but are not suitable for dynamic torque transmission. In the screw, each turn of the thread is intentionally formed with a double crest using a special tool. Once the screw production process has been completed, the nut is tightened upon the screw, and presses the two crests against each other. This will cause them to be elastically deformed and constantly press against the thread of the nut, thereby ensuring a steady fastening effect.

This double crest with its sag constitutes a weakness point in the thread, which is deemed to be unacceptable in precision applications in which the torque transmission element is designed to be exposed to high stresses.

A further weakness point in the thread, common to both rolled and milled threads, is in the first and last turns, which have a reduced thickness and hence less resistance to axial stresses.

The object of the invention is to obviate the above discussed prior art problems.

This and other objects, which will become apparent from the following detailed description, are fulfilled by a mechanical torque transmission element as defined in any of the appended claims.

The present invention can reduce the weakness points of the thread of a mechanical torque transmission element, while maintaining low manufacturing costs.

Namely, the present invention can provide threaded mechanical torque transmission elements, with the throughput and costs of rolling, but with significantly reduced technical drawbacks as compared to milled threads.

More in detail, the present invention can provide rolled torque transmission elements whose threads have no double crests.

According to the invention, during rolling, the two elevated portions of the crest portion are deformed so as to be elevated with respect to the centerline of the crest, and close up to mutual contact above the centerline.

This will advantageously provide a unitary crest. As a result of this process, a few elements will remain to distinguish a thread obtained with the inventive techniques from a thread obtained by milling. This will show that the mechanical performances of the thread of the invention are also comparable to that of the milled threads, despite the use of a fast and cost-effective rolling process.

The central sag of the crest found in the prior art is substantially eliminated, or at most a shallow material depression remains along the centerline, which is in any case much shallower than prior art sags.

A possible way to distinguish the thread of the invention from a milled thread is to cut the mechanical element to expose a section of the thread. In this section a physical interface surface may be recognized, i.e. a surface of discontinuity in the thread material, where the two elevated portions have contacted each other, above the centerline. On the contrary, in a milled thread, the thread material has no surface of discontinuity therein.

A channel may be also found below this interface surface, level with the original centerline of the crest portion. The channel and the interface surface represent the visible traces of the sag that is only temporarily formed during rolling before closing up.

In preferred embodiments, in addition to the elimination of the double crests, the weakened part of the first and/or last turns of the thread is also eliminated by milling.

Finally, a heat treatment in an oxide-free environment prevents undesired residues from building up in the thread grooves, which would expose the thread to wear after a small number of working cycles.

Further characteristics and advantages of the invention will be recognizable by a skilled person from the following detailed description of exemplary embodiments of the invention.

The accompanying figures show, by way of example and without limitation, examples that will be useful to understand some embodiments of the invention, and in particular:.

The figures show a mechanical torque transmission element, designated by numeral <NUM>, such as a nut.

Without prejudice to the general scope of the invention, reference will be made hereinafter to a hollow torque transmission element, the thread being formed on the inner surface of the torque transmission element, for example to an internally threaded nut. In other embodiments (not shown), the thread might be formed on both inner and outer surfaces. In further, not claimed, embodiments (not shown) the torque transmission element may be solid, such as a screw, with the thread formed on the outer surface.

Torque transmission elements <NUM> intended for dynamic coupling are of particular interest herein, with complementary elements moving relative to each other, namely for the purpose of torque transmission, and not for static fastening, with the elements intended to remain stationary once fastened.

In light of the above, the torque transmission element <NUM> comprises a body, preferably formed of one piece, which has a through hole <NUM> in the embodiment of the figures. However, in other embodiments, the following may also apply to a blind hole.

The hole <NUM> extends along an axis of extension X-X which here coincides with a center axis.

The hole <NUM> has a circular cross-section, as taken on a sectional plane perpendicular to the center axis X-X. The hole <NUM> also has an inner surface <NUM>, facing the center axis X-X.

A thread <NUM> is formed on a surface of the body which here is the inner surface <NUM> facing the center axis X-X.

The thread <NUM> extends helically and comprises a plurality of successive turns <NUM>. The turns <NUM> also extend helically between two end turns 5a, 5b, i.e. the turns <NUM> that define the two ends of the thread <NUM>.

The end turns 5a, 5b are spaced apart in the direction of the center axis X-X.

Each turn <NUM> comprises a crest portion <NUM>.

The crest portion <NUM> projects to a given height from the inner surface <NUM> of the hole <NUM> in a radial direction Y-Y, i.e. toward the center axis X-X.

Each turn <NUM> covers a single turn of the helix, but is only ideally separated from the subsequent turns <NUM>, with no physical discontinuity between their respective crests <NUM>. However, with the exception of the contact point between successive turns <NUM>, the pairs of successive turns <NUM> are spaced apart in the direction of the center axis X-X.

Then, the thread <NUM> defines respective groove portions <NUM>, which are situated, in the direction of the center axis X-X, between the crest portions <NUM> of each pair of successive turns <NUM>.

Each turn <NUM> of the thread <NUM> also has two flanks <NUM> which connect its crest portion <NUM> with the adjacent groove portions <NUM>.

The thread <NUM> is of essentially trapezoidal type. This means that the crest portions <NUM> and the groove portions <NUM> are essentially perpendicular to the radial direction Y-Y, and that the flank <NUM> of each turn <NUM> converge toward the crest <NUM> portion of the turn <NUM>.

In a perfectly trapezoidal thread <NUM>, the crest portion <NUM> of each turn <NUM> is connected to the adjacent flanks <NUM> at respective convex corners, while each flank <NUM> is connected to the adjacent groove portion <NUM> at a respective concave corner.

In particular, an axial section of the turn <NUM> does not have curved portions, whereby in particular the flanks <NUM> are straight.

However, in the present invention, rounded connections, with no sharp corners, are also admitted between the crest portions <NUM>, the flanks <NUM> and the groove portions <NUM>.

More in detail, and according to the invention, the trapezoidal threads of interest are defined by ISO <NUM>:<NUM>.

Here, as in other sections of the disclosure, reference is made to an axial section of the thread <NUM> or of the entire torque transmission element <NUM>. An axial section refers to a section taken along a sectional plane that crosses the center axis X-X.

Therefore, here the section plane extends in the axial and radial directions X-X, Y-Y.

Various advantageous features that can be implemented in at least one turn <NUM> of the thread <NUM> will now be described. It shall be understood that these characteristics will not be necessarily reproduced in all the turns <NUM>. For example, different characteristics may be implemented in the end turns 5a, 5b. In any case, the characteristics described for a single turn <NUM> can be deemed to apply to all or most of the turns <NUM>.

The thread <NUM> is formed by a rolling process by means of a rolling tool, preferably only one rolling tool. Therefore, as discussed with reference to the prior art, the crest <NUM> of the turn <NUM> has two elevated portions <NUM>. Each elevated portion <NUM> is adjacent to a respective flank <NUM> of the helix <NUM>.

An ideal centerline <NUM> separates the two elevated portions <NUM>. Each of the elevated portions <NUM> and the centerline <NUM>, as well as the crest portion <NUM> and the flanks <NUM>, extend along the entire circumferential length of the turn <NUM>. The circumferential length is meant to be measured on the helical path of the turn <NUM>, along the crest portion <NUM> of the turn <NUM>, between the two opposite ends of the turn <NUM>.

During the rolling process, the two elevated portions <NUM> are deformed so as to be elevated with respect to the center line <NUM>. According to the invention, this deformation continues until the two elevated portions <NUM> close up to mutual contact above the center line <NUM>.

It should be noted that the steps of elevation and closing up of the elevated portions <NUM> are carried out by the same rolling tool in a single rolling step. Between the steps of elevation and closing up the rolling element is not removed, for example for application of a separate rolling element.

The rolling tool is distinct from the torque transmission element complementary to the element <NUM> as described above. In particular, in the case of a nut, the rolling tool is distinct from the screw with which the nut will then be coupled.

The deformation of the two elevated portions <NUM>, which will cause them to close up to mutual contact is a plastic deformation, and not an elastic deformation. In other words, this deformation causes the elevated portions <NUM> to remain in contact with each other even after removal of the rolling instrument, without creating forces that might spread the elevated portions <NUM> apart.

After contact, the crest portion <NUM> of the turn <NUM> has a contact surface <NUM> between the two elevated portions <NUM>, i.e. a physical interface whereat the elevated portions <NUM> contact each other.

The contact surface <NUM> can be identified in axial section as a discontinuity in the thread material <NUM> which delimits the two elevated portions <NUM>. Therefore, the contact surface <NUM> is not a mere ideal surface, but a material, physical surface.

More in detail, each elevated portion <NUM> is delimited toward the center axis X-X by a respective material surface <NUM>, which extends between the flank <NUM>, adjacent to the elevated portion <NUM>, and the center line <NUM>.

During a first step of the rolling process, as shown in <FIG> (which are also representative of a rolled thread according to the prior art), before the elevated portions <NUM> contact each other, the material surfaces <NUM> of the two elevated portions <NUM> only contact each other at the center line <NUM>.

At the end of the rolling process, as shown in <FIG>, in the finished product, the two material surfaces <NUM> of the elevated portions <NUM> have respective contact portions 64a, which provide contact between the two separate elevated portions <NUM>, at the contact surface <NUM>.

The details of the material surfaces <NUM> in the finished product may be particularly recognized in <FIG>, where a channel <NUM> (as discussed in greater detail below) is shown with a given depth specifically selected for clearer illustration of these details.

The material surface <NUM> of each elevated portion <NUM> also has a free portion 64b, which does not contact the other elevated portion <NUM>. For each elevated portion <NUM>, the contact portion 64a and the free portion 64b are adjacent to each other, with the contact portion 64a proximal to the centerline <NUM> and the free portion 64b proximal to the flank <NUM> of the turn <NUM>.

In the finished product, the free portions 64b of the two elevated portions <NUM> are contiguous to each other, due to the contact between the contact portions 64a. The free portions 64b define together a single ideal crest surface essentially perpendicular to the radial direction Y-Y. This means that the resulting thread <NUM> has an essentially trapezoidal axial cross-section.

The ideal crest surface may not be perfectly planar, but may have, for example, a small hump at each elevated portion <NUM>. However, this does not cause weaknesses, unlike the double crests of the prior art. A small sag may be present in the ideal crest surface between the two humps, whose depth is much less than the depth of the center line <NUM>. For example, the ratio between the two depths may be less than <NUM>%, preferably less than <NUM>%.

Thus, the contact between the two elevated portions <NUM> takes place above the center line <NUM>. Accordingly, the contact surface <NUM> is above the center line <NUM>, i.e. closer to the center axis X-X.

In the illustrated embodiments, the crest portion <NUM> of the turn <NUM> has a channel <NUM> at the center line <NUM>. Therefore, the channel <NUM>, which is hollow, is below the contact surface <NUM>. While in the embodiment of <FIG> and <FIG> the channel <NUM> is shown to be almost tangent to the crest surface that defines the top of the profile of the thread <NUM>, in <FIG>, as mentioned above, the channel <NUM> is shown at a greater depth.

The channel <NUM> represents a trace, as shown in axial section, confirming that the thread <NUM> has been obtained by rolling. It is a trace of the sag between the two elevated portions <NUM>, which is only temporarily formed during rolling, and then substantially closes up by contact between the elevated portions <NUM>.

This does not exclude embodiments in which no channel <NUM> is present at the end of rolling. Here, the contact surface extends deep from the ideal crest surface to the centerline <NUM>.

Advantageous features of one or both of the end turns 5a, 5b will be now discussed, only one of which will be described for brevity.

The end turn 5a has a substantially constant thickness along its entire circumferential length, and has an essentially trapezoidal shape. Conversely, the threads <NUM> of the prior art generally have end turns 5a with a thickness increasing from the end of the thread <NUM>, along the circumferential extent of the end turn 5a, toward the subsequent turn <NUM>. The thickness is measured along the direction of the center axis X-X.

This is achieved by initially forming a thread <NUM> having a sacrificial end portion <NUM>, and by removing the sacrificial end portion <NUM>. <FIG> and <FIG> show a condition in which the sacrificial end portion <NUM> has not been removed yet, whereas after removal the end turn 5a is as shown in <FIG> and <FIG>.

The sacrificial end portion <NUM> is that portion of thread <NUM> whose thickness increases from the end of the thread <NUM>, along the circumferential extent of the sacrificial end portion.

The sacrificial end portion <NUM> is removed by machining, preferably by turning, hence by material removal. The removal causes the height of the thread <NUM> to be leveled, i.e. brought to the same height as the grooves <NUM>, or even below, along the entire circumferential extent of the sacrificial end portion <NUM>.

It should be noted that the sacrificial end portion <NUM>, before removal, is in a position of the thread <NUM> that is more external than the position of the end turn 5av in the finished product. In other words, before removal, the thread <NUM> comprises an lead-in turn subsequent to the sacrificial end portion <NUM>. Upon removal, the lead-in turn becomes the end turn 5a.

Preferably, in addition to the sacrificial end portion <NUM>, the crest portion <NUM> of the lead-in turn is also at least partially removed. In other words, the height of the lead-in turn is at least partially reduced to obtain the end turn 5a. This could remove the elevated portions <NUM> of the crest portion <NUM> on at least part of the end turn 5a, 5b.

As a result, in the preferred embodiments, at least one of the end turns 5a in the finished product has a height that increases from a minimum height point <NUM>, defined by one end of the thread <NUM>, to a maximum height point <NUM>, where the end turn 5a connects to a subsequent turn. The thickness of the end turn 5a is constant in particular from the minimum height point <NUM> to the maximum height point <NUM>.

The described configuration of the end turn 5a is free from the weakness features of the end turns 5a, 5b according to the prior art.

Embodiments of torque transmission elements <NUM> incorporating the characteristics and advantages as described for the end turn 5a are also to be understood as falling within the scope of the invention as described.

However, preferred embodiments include both these advantageous aspects, with the thread <NUM> being also thermally treated in an oxide-free environment. This prevents the build-up of oxides and of other powders in the grooves <NUM> of the thread <NUM>, which are difficult to eliminate, such build-up leading to premature wear during the use of the torque transmission element <NUM>.

Claim 1:
A mechanical torque transmission element (<NUM>), comprising a body extending along an axis of extension (X-X), said body having:
- a through or blind hole (<NUM>) extending about said axis of extension (X-X),
- an inner surface (<NUM>) of the hole (<NUM>) facing said axis of extension (X-X);
- a trapezoidal thread (<NUM>) complying with ISO <NUM>:<NUM>, formed on said surface (<NUM>), said thread (<NUM>) comprising a plurality of turns (<NUM>) which extend in a helical pattern between two end turns (5a, 5b), spaced apart from each other in the direction of the axis of extension (X-X),
- each turn (<NUM>) defines a crest portion (<NUM>) projecting toward the axis of extension (X-X), and
- said thread (<NUM>) defines respective groove portions (<NUM>) between the crest portions (<NUM>) of each pair of successive turns (<NUM>), wherein
- at the crest portion (<NUM>) of at least one turn (<NUM>), two elevated portions (<NUM>) of the crest portion (<NUM>), separated by a centerline (<NUM>) of the crest portion (<NUM>) which extends all along the turn (<NUM>), are deformed so as to be elevated with respect to the centerline (<NUM>) and to close up by plastic deformation in mutual contact above the centerline (<NUM>).