Patent ID: 12196243

DETAILED DESCRIPTION

FIG.1is an exploded view of a torque-limiting axle nut100that includes a retaining ring110, a wave spring130, an inner nut300, and an outer nut500.FIG.2is a side cross-sectional view of the torque-limiting axle nut100ofFIG.1when secured to a threaded axle200.FIGS.3and4are perspective views of the inner nut300from the top and bottom, respectively.FIGS.5and6are perspective views of the outer nut500from the top and bottom, respectively.FIGS.1-6are best viewed together with the following description.

The inner nut300is ring-shaped, forming a threaded center hole310with internal threads320that match external threads220of the axle200. The center hole310and axle200are coaxial to a rotation axis190about which the inner nut300rotates and along which the inner nut300linearly translates to engage with the external threads220. As shown inFIGS.1,3, and4, the inner nut300may azimuthally extend entirely around the rotation axis190, in which case the inner nut300fully encircles the rotation axis190.

For clarity herein, it is assumed that the rotation axis190is parallel to the z axis of a right-handed Cartesian coordinate system120, where the inner nut300translates in the −z direction when engaging with the axle200(i.e., tightening) and in the +z direction when disengaging from the axle200(i.e., loosening). Also for clarity, several features of the present embodiments are described in terms of a cylindrical coordinate system that is equivalent to the Cartesian coordinate system120. Thus, the terms “axial” and “axially” refer to directions parallel to the z axis, the terms “radial” and “radially” refer to scalar distances from the rotation axis190in any plane that is parallel to the x-y plane, and the terms “azimuthal” and “azimuthally” refer to angular coordinates around the rotation axis190.

The inner nut300has an inner-nut body302with an outer radius r1that is greater than a radius R of the axle200. The inner nut300also has a flange304that is located azimuthally around the inner-nut body302and extends radially away from the inner-nut body302. Thus, the flange304has an outer radius r2that is greater than the outer radius r1. The flange304has a top face308that faces axially upward (i.e., in the +z direction) and a bottom face312that faces axially downward (i.e., in the −z direction). As shown inFIG.4, the bottom of the inner-nut body302may lie flush with the bottom face312, in which case the inner-nut body302and the flange304share the bottom face312. However, the bottom of the inner-nut body302need not lie flush with the bottom face312.

As shown inFIG.3, the flange304forms a plurality of recesses306at various azimuthal locations. The recesses306are centered identically at the same radial distance from the rotation axis190. Each recess306extends axially downward from the top face308. For clarity, only two recesses306are labeled inFIG.3. Each recess306is sized to accept a ball bearing222, and therefore may be shaped as a circle, a stadium, or another shape within which the ball bearing222can sit. As shown inFIG.2, only a portion of the ball bearing222sits within the recess306such that some of the ball bearing222lies above the top face308(also seeFIG.7). In embodiments, the ball bearings222all have the same diameter. Translationally, each ball bearing222is fixed in its corresponding recess306. However, each ball bearing222may rotate within its corresponding recess306when the outer nut500is rotated relative to the inner nut300(e.g., when the torque-limiting axle nut100is threaded onto the axle200).

FIG.3shows the inner nut300with fourteen recesses306. In this case, the torque-limiting axle nut100can accommodate up to fourteen ball bearings222. However, the inner nut300may have a different number of recesses306and ball bearings222without departing from the scope hereof. As shown inFIG.3, the recesses306need not all be identically shaped. Furthermore, it is not necessary that the recesses306be uniformly spaced around the rotation axis190(i.e., azimuthally). WhileFIGS.3and4show the recesses306are blind holes that do not pass, along z, entirely through the flange304, one or more of the recesses306may alternatively be a through-hole that extends entirely through the flange304.

The inner nut300forms a notch314that extends azimuthally around the inner nut300and radially inward from an outward-facing surface316of the inner-nut body302. The notch314is sized to accept a radially-inward portion of the retaining ring110. To facilitate insertion of the retaining ring110into the notch314, the retaining ring110may be split (see the gap112inFIG.1). In this case, the retaining ring110subtends an azimuthal range less than 360°. Alternatively, the retaining ring110may subtend an azimuthal range equal to 360°, in which case the retaining ring110fully encircles the inner nut300. The retaining ring110has an inner diameter less than the outer radius r1. Thus, when the retaining ring110is inserted into the notch314(seeFIG.2), the inner nut300axially constrains the retaining ring110. Examples of the retaining ring110include, but are not limited to, spiral retaining rings, a constant-section retaining rings, circlips, and snap rings. The retaining ring110may be axially assembled or radially assembled.

The outer nut500is ring shaped, forming an unthreaded center hole516whose radius is slightly larger than the outer radius r1of the inner nut300. This allows the outer nut500to encircle the inner-nut body302. Since the center hole516is unthreaded, the outer nut500can rotate relative to the inner nut300when the torque-limiting axle nut100is not fully torqued around the axle200. The outer nut500forms a ledge506that extends azimuthally around the unthreaded center hole516, as shown inFIG.5. The outer nut500also has a radial wall504that extends azimuthally around the ledge506. As shown inFIGS.1,5, and6, the ledge506and radial wall504may fully encircle the rotation axis190. However, the ledge506may form one or more azimuthal gaps. Similarly, the radial wall504may form one or more azimuthal gaps.

Transverse to the rotation axis190, the external shape of the radial wall504may be a regular octagon, as shown inFIGS.5and6. Alternatively, the external shape may be a regular hexagon or another type of regular or irregular polygon. Due to its external shape, the radial wall504does not exhibit fully continuous rotational symmetry (i.e., circular symmetry) about the rotation axis190. Instead, the radial wall504may exhibit discrete rotational symmetry about the rotation axis190when the external shape is a regular polygon. For example, the regular octagon shown inFIGS.5and6has eight-fold rotational symmetry about the rotation axis190.

The outer nut500forms a plurality of grooves510that extend axially upward from a bottom face508, as shown inFIG.6. For clarity, only one of the grooves510is labeled inFIG.6. Two azimuthally neighboring grooves510are separated by a ridge512. As shown inFIG.6, the bottom of each ridge512is flush with the bottom face508. Each ridge512subtends an azimuthal angle that is less than that subtended by each groove510. For example, inFIG.6, where there are eight grooves510and eight ridges512, each ridge512may subtend an azimuthal angle of 5°, in which case each groove510subtends an azimuthal angle of 40°. Each ridge512may subtend a different azimuthal angle without departing from the scope hereof. Similarly, the outer nut500may form a different number of grooves510without departing from the scope hereof.

As shown inFIG.2, each groove510acts like a track through which a ball bearing222is guided when the outer nut500is rotated relative to the inner nut300. Radially, each recess306of the inner nut300is centered at a radius rg. The groove510is also radially centered near rg. Therefore, the top of the ball bearing222is located near the radial center of the groove510. Furthermore, the groove has a radial width that is large enough such that the ball bearing222can partially fit inside the groove510.

InFIG.1, the wave spring130extends azimuthally around the rotation axis190. The wave spring130may fully encircle the rotation axis190, in which case the wave spring130is single-turn. Alternatively, the wave spring130may have an azimuthal gap similar to the gap112, in which case the wave spring130is split. Alternatively, the wave spring130may be a disc spring, Belleville washer, or other type of axially compressible ring-shaped component known in the art. As shown inFIG.2, the wave spring130is axially confined between the ledge506and the retaining ring110. When the torque-limiting axle nut100is unitized, as shown inFIG.2, the wave spring130is partially compressed such that each ball bearing222is seated in one of the recesses306and one of the grooves510. This compression ensures that the inner nut300and outer nut500do not become axially separated enough for the ball bearing222to fall out.

As shown inFIG.2, the outer nut500lies entirely between a top face318of the inner nut300and the bottom face312. Alternatively, part of the outer nut500(e.g., the radial wall504) can extend upward past the top face318or downward below the bottom face312.

FIGS.7-11illustrate how one ball bearing222cooperates with one ridge512to limit torque. For clarity in these figures, the movement of the ball bearing222around the rotational axis190has been “unwound” to appear linear. Thus, the horizontal dimension in these figures is the azimuthal angle ϕ. The views in these figures are taken at the radius rg(seeFIG.2).FIGS.7-11are best viewed together with the following description.

FIG.7shows how one ball bearing222moves within one groove510. The outer nut500forms a recessed surface708that is perpendicular to the rotational axis190(i.e., lies parallel to the x-y plane) and has an arc length L, as measured between a pair of azimuthally neighboring ridges512. Each ridge512has a front face704forming an obtuse front angle θ relative to the recessed surface708. The groove510has an axial depth d such that the top of the ball bearing222touches the recessed surface708. As shown inFIG.8, the bottom face508of the ridge512is separated from the top face308of the inner nut300by a minimum axial gap gmin.

FIG.8shows how a torque applied to the outer nut500rotates the outer nut500(i.e., right-to-left in the figure) relative to the inner nut300. This torque advances the groove510relative to the ball bearing222. InFIG.8, the outer nut500has advanced, relative to the inner nut300, such that the front face704of the ridge512contacts the ball bearing222. The applied torque creates to a tangential force Fτ. Due to the obtuse front angle θ, the ridge512acts like a wedge that converts the tangential force Fτinto an angled force Fαhaving a tangential component Fϕand an axial component Fz. The ball bearing222transmits the axial component Fzto the bottom of the recess306and the tangential component Fϕto the side of the recess306. The tangential component Fϕis part of a transferred torque that causes the inner nut300to rotate around the rotational axis190. In this case, the outer nut500and inner nut300will rotate together, or co-rotate, until the inner nut300bottoms out (i.e., the torque-limiting axle nut100is fully threaded onto the axle200).

FIG.9shows the ridge512riding up the ball bearing222when an excessive torque is applied to the outer nut500. In this case, the axial component Fzis exerted downward on the inner nut300, causing the internal threads320to push downward against the external threads220of the axle200(seeFIG.2). When the inner nut300cannot move downward in response to the axial component Fz, it will instead produce an upward normal force Fnthat the ball bearing222transmits back to the ridge512. This normal force Fncauses the outer nut500to move upward, thereby compressing the wave spring130(seeFIGS.12and13). As the tangential force Fτincreases, so does the axial component Fzand normal force Fn. As the normal force Fnincreases, the outer nut500moves increasingly upward, thereby compressing the wave spring130more. As the outer nut500moves upward relative to the inner nut300, the bottom face508of the ridge512is separated from the top face308of the inner nut300by an axial gap g that is greater than the minimum axial gap gmin.

FIG.10shows the ridge512on top of the ball bearing222. Here, the ridge512no longer acts like a wedge and the tangential force Fτcauses the ridge512to slide over the top of the ball bearing222. When the ridge512is directly on top of the ball bearing222, the bottom face508of the ridge512is separated from the top face308of the inner nut300by a maximum axial gap gmax. As discussed below with respect toFIGS.12and13, this position results in the maximum compression of the wave spring130.

FIG.11shows the ridge512after it has passed over the ball bearing222. As the outer nut500is torqued starting from the orientation shown inFIG.10, the ball bearing222will “snap back” into the next groove510′. Specifically, the wave spring130will push the outer nut500downward until the ball bearing222contacts the recessed surface708of the next groove510′. After this “snap back”, the ball bearing222again behaves as shown inFIG.7and the wave spring130is no longer maximally compressed.

The smallest torque applied to the outer nut500that results in the situation shown inFIG.10is referred to herein as a limiting torque. Any torque applied to the outer nut500that exceeds the limiting torque is not transferred to the inner nut300. Specifically, when the applied torque exceeds the limiting torque, the inner nut300is still torqued at the limiting torque. Thus, it is only the excess of the applied torque over the limiting torque that is not transferred to the inner nut300.

Advantageously, the “snapping back” shown inFIG.11may produce an audible click that indicates to the user that an excessive torque exceeding the limiting torque was applied. A click may be produced each time any ridge512passes over any ball bearing222. Thus, if the excessive torque is applied for an extended period of time, a series of rapid clicks will be generated. This audible effect is similar to that produced by torque limiters in power drills.

FIGS.12and13are side views illustrating axial movement of the outer nut500relative to the inner nut300when a ridge512passes over a ball bearing222. For clarity, the wave spring130is not shown inFIGS.12and13. InFIG.12, the ball bearing222is inside the groove510, corresponding to the situation shown inFIGS.7,8, and11. In this case, the bottom of the retaining ring110is located a height h above the ledge506. The bottom face508of the outer nut500is separated from the top face308of the inner nut300by the minimum axial gap gmin.

InFIG.13, the outer nut500has been rotated with enough torque that the ridge512is on top of the ball bearing222, corresponding to the situation shown inFIG.10. In this case, the bottom of the retaining ring110is located a height h′<h above the ledge506. The bottom face508of the outer nut500is separated from the top face308of the inner nut300by the maximum axial gap gmax. Compared toFIG.12, the wave spring130will be additionally compressed inFIG.13by h−h′=gmax−gmin=d.

The limiting torque is determined in part by the front angle θ. For θ=90°, there is no axial component Fzand all torque applied to the outer nut500is transferred to the inner nut300. In this case, the limiting torque is essentially infinite and the torque-limiting axle nut100will not protect against excessive torque. As θ increases from 90° to 180°, the limiting torque is lowered.

The limiting torque is also determined in part by the axial depth d. As described above, the wave spring130is compressed by the axial depth d every time a ridge512passes over a ball bearing222. Due to Hooke's law, compression by the axial depth d requires an increase in the axial component Fz, and therefore the normal force Fn. Therefore, an increase in the axial depth d causes the limiting torque to increase.

WhileFIGS.8-11illustrate torque limiting when tightening the torque-limiting axle nut100, the same concept also applies when loosening the axle nut100. In this case, the outer nut500moves left-to-right in these figures relative to the inner nut300. As shown inFIG.7, each ridge512also has a rear face706that forms a rear angle φ relative to the recessed surface708. The torque applied to the outer nut500is transferred to the inner nut300via the ball bearing222when the rear face706contacts the ball bearing. Similar to the situation shown inFIG.8, this causes the outer nut500and inner nut300to co-rotate, thereby loosening the axle nut100.

InFIG.7, the rear angle φ is 90°. In this case, there is no torque-limiting when loosening the torque-limiting axle nut100. Alternatively, the rear angle φ may be obtuse, similar to θ. In this case, an excessive torque applied to the outer nut500when loosening the torque-limiting axle nut100will cause the ridge512to pass up and over the ball bearing222(i.e., “snapping back”), but in the opposite direction to that shown inFIGS.8-11. To ensure that the axle nut100can be loosed, the limiting torque for loosening the axle nut100should be greater than that for tightening.

When the torque-limiting axle nut100is fully threaded on the axle200, the wave spring130is not compressed enough to provide an axial load that is sufficient to keep the torque-limiting axle nut100secured to the axle200. As known by those trained in the art, an axial load ensures that the threads of a nut (e.g., the torque-limiting axle nut100) push against the threads of an axle (e.g., the axle200) with enough force to resist rotation of the nut. This resistance to rotation is particularly important for vehicles like trucks and trains where motion and vibration can generates torques that cause the nut to loosen over time.

To understand this need to resist rotation in more detail, consider when the torque-limiting axle nut100is fully torqued around the axle200. In this case, the bottom face312of the inner nut300would directly contact an end surface204of the axle200. As the axle nut100is tightened and the bottom face312is increasingly pushed downward against the end surface204, the axle200responds by applying an upward normal force to the inner nut300. This normal force gives rise to a friction force between the bottom face312and the end surface204. However, the friction force will decrease over time as motion and vibration causes the bottom face312and end surface204to rub against each other and erode, thereby reducing the coefficient of friction. In addition, the wheel end will likely have oil, which can also reduce the coefficient of friction. As the friction force is reduced, it is only a matter of time before a vibration creates a torque large enough to overcome the reduced friction force, thereby completely loosening the axle nut100.

In some embodiments, the torque-limiting axle nut100includes a washer that is placed between the axle200and the bottom face312of the inner nut300. As the axle nut100is tightened, the inner nut300increasingly presses against the washer to compress it, thereby providing an axial load that keeps the axle nut100secured to the axle200. The washer may be a Belleville washer, split washer, spring lock washer, wave washer, disc spring, or another type of axially compressible ring-shaped component know in the art. The washer may also have a key that engages with a slot of the axle200to prevent the washer from rotating relative to the axle200. When using the axle nut100, the maximal axial load that can be applied will be determined by the torque threshold. Thus, if a certain axial load is targeted (e.g., 1000 lb), then the axle nut100should be designed such that it can produce the targeted axial load at, or near, the torque threshold.

While the washer can help resist rotation of the torque-limiting axle nut100, it has the same problem, as described above, when the bottom face312and end surface204directly contact each other. Specifically, the washer is held in place against the bottom face312and end surface204via friction forces. Therefore, wear-and-tear will eventually cause these friction forces to reduce over time, thereby increasing the possibility that a vibration will create a torque large enough to overcome these reduced friction force, thereby completely loosening the axle nut100.

In other embodiments, the torque-limiting axle nut100includes a locator plate1400and a plurality of spring plungers1402that cooperatively provide part of an axial load that secures the axle nut100to the axle200. As shown inFIGS.1and2, the locator plate1400is located between the bottom face312of the inner nut300and the end surface204of the axle200. For clarity, the spring plungers1402are not shown inFIGS.1and2. Advantageously, the locator plate1400establishes a minimum non-zero torque needed to loosen the inner nut300. As described in more detail below, this minimum non-zero torque is not established by a friction force that can decrease over time. Accordingly, these embodiments provide better long-term resistance to rotation, as compared to embodiments without the locator plate1400and spring plungers1402.

FIG.14shows the locator plate1400in more detail. The locator plate1400is ring shaped, forming an unthreaded center hole1410whose diameter is slightly larger than that of the axle200. The locator plate1400forms a plurality of detent holes1406at various azimuthal locations. The detent holes1406are centered identically at the same radial distance from the rotation axis190. Each detent hole1406extends axially downward from a top face1408of the locator plate1400. The edges of each detent hole1406are filleted or chamfered. The angle formed by the fillets/chamfers relative to the top face1408establish, in part, how much torque can be applied before the axle nut100becomes loose (see the minimum loosening torque discussed below). In one embodiment, the locator plate1400forms a key1404that is shaped to engage with an axial slot of the axle200(not shown in the figures) to prevent rotation of the locator plate1400relative to the axle200.

InFIG.14, the detent holes1406are all shaped as countersunk stadiums. However, one or more of the detent holes1406may alternatively have a different shape (e.g., a countersunk circular hole). Similarly, the detent holes1406are shown inFIG.14as being uniformly spaced around the rotation axis190(i.e., azimuthally). However, the detent holes1406may alternatively be non-uniformly spaced around the rotation axis190. WhileFIG.14shows the locator plate1400with40detent holes1406, the locator plate1400may have a different number of detent holes1406without departing from the scope hereof.

FIG.15is a side cross-sectional view of the torque-limiting axle nut100ofFIG.1that illustrates operation of the locator plate1400ofFIG.14. InFIG.15, a spring plunger1402is located inside one of a plurality of plunger mounting holes322. As shown inFIG.4, the plunger mounting holes322extend upward from the bottom face312of the inner nut300. Each plunger mounting hole322is sized to accept one spring plunger1402. In the example ofFIG.15, the spring plunger1402includes a spring1506that pushes downward against a contact1504such that the contact1504engages with one detent hole1406when the contact1504is positioned over the detent hole1406. In the example ofFIG.15, the contact1504is a ball bearing. However, the contact1504may be a different component that can engage with the detent holes1406, such as a pin. Also inFIG.15, the spring1506is shown as a disc spring. However, the spring1506may alternatively be a Belleville washer, wave spring, helical spring, or other type of spring.

The spring plungers1402cooperate with the detent holes1406to create a detent mechanism that arrests motion of the inner nut300while it is being tightened onto the axle200. As the inner nut300rotates with respect to the locator plate1400, the contact1504engages with one or more of the detent holes1406. Specifically, consider a contact1504initially engaged with a detent hole1406when the torque-limiting axle nut100is near the bottom of the axle200. Due to the chamfering of the detent hole1406, rotation of the inner nut300pushes the contact1504upward to compress the spring1506. As the rotation continues and the contact1504enters the next detent hole1406, the spring1506decompresses, resulting in a “snapping back” effect similar to that described above forFIG.11. Thus, as the inner nut300rotates, the contact1504will produce an audible click for each of the detent holes1406it engages with.

Assuming that the detent holes1406are uniformly spaced around the rotation axis190, the azimuthal angle ϕnbetween a neighboring pair of the detent holes1406is ϕn=360°/N, wherein N is the number of the detent holes1406. In the example ofFIG.14, where N is 40, the azimuthal angle ϕnis 9°. In the exampleFIG.4, the inner nut300has two plunger mounting holes322, and therefore two spring plungers1402, that are azimuthally separated from each other by a neighboring-plunger angle of ϕnp=175.5° (and the corresponding conjugate angle of 184.5°), which is divisible by ϕn/2. Thus, at most only one of the two spring plungers1402engages with a detent hole1406. As the inner nut300rotates, the one spring plunger1402engaged with a detent hole1406alternates back-and-forth between the two spring plungers1402. Thus, by azimuthally offsetting the two plungers1402from each other in this manner, the minimum rotation angle ϕmof the inner nut300that creates one audible click is reduced by half to ϕm=ϕn/2=4.5°.

This concept of reducing the minimum rotation angle ϕmcan be extended to more than two spring plungers1402to reduce the minimum rotation angle ϕm. For example, consider when the inner nut300has four spring plungers1402, each being azimuthally separated from its nearest neighbors by a neighboring-plunger angle ϕnpthat is divisible by four (e.g., the plunger mounting holes322are located at azimuthal angles of 0°, 87.75°, 175.5°, and 263.25°). Again, at most only one of the four spring plungers1402engages with a detent hole1406. As the inner nut300rotates, the one spring plunger1402that is engaged with a detent hole1406changes cyclically between the four spring plungers1402. In this example, the minimum rotation angle ϕmof the inner nut300that creates one audible click is reduced by a factor of four to ϕm=ϕn/4=2.25°.

In general, the inner nut300may form any integer number M of two or more plunger mounting holes322where each is azimuthally separated from its two nearest neighbors by a neighboring-plunger angle ϕnpthat is divisible by ϕn/M. In this case, the inner nut300can accommodate up to M spring plungers1402and the minimum rotation angle ϕmof the inner nut300is reduced by a factor of M to ϕm=ϕn/M. Note that the neighboring-plunger angles ϕnpdo not have to be identical, provided that each is divisible by ϕn/M.

In one embodiment, the inner nut300forms only one plunger mounting hole322, in which case the torque-limiting axle nut100has only one spring plunger1402and the minimum rotation angle Øm equals Øn. In other embodiments, multiple spring plungers1402are azimuthally positioned such that their contacts1504engage with multiple detent holes1406simultaneously. In this case, the spring plungers1402will click simultaneously as the inner nut300rotates.

Before the torque-limiting axle nut100is fully torqued, the contact1504will extend past the bottom of the locator plate1400. In this position, the spring1506is relaxed. As the axle nut100approaches the end of its travel, the contact1504will be pushed upward by the end surface204, thereby compressing the spring1506. At this point, the inner nut300will only be rotatable by a small additional angle (e.g., 10° or less) before the torque limit is reached. For this reason, it may be beneficial for the locator plate1400to form as many detent holes1406as possible. This reduces the angle ϕn, ensuring that after the inner nut300is rotated by the small additional angle, there is at least one spring plunger1402aligned with a detent hole1406. By contrast, when the angle ϕnis relatively large, the torque limit may be reached when all of the spring plungers1402are located between detent holes1406. In this latter case, none of the spring plungers1402will engage with the detent holes1406.

When the torque-limiting axle nut100is fully torqued, most of the axial force may be thought of as having a friction component and a spring component. Most of the axial force is the friction component, which is established by friction between the locator plate1400and the bottom face312of the inner nut300. The remaining spring component, however, is established by the compression of the spring1506and therefore is not a friction-based force. A vibration can only loosen the axle nut100if it generates a torque large enough to compress the spring1506by a thickness t of the locator plate1400. The smallest such torque is referred to herein as a “minimum loosening torque” and its magnitude depends on the angle formed by the chamfered edges of the detent holes1406. When the vibration gives rise to a torque that is greater than the minimum loosening torque, rotation of the inner nut300relative to the locator plate1400compresses the spring1506such that the inner nut300can rotate. In this case the contact1504will “snap back” into the next detent hole1406after the inner nut300has been rotated by ϕm.

Advantageously, the spring component of the axial force does not rely on friction and therefore will not degrade over time in response to normal wear-and-tear. Note that the friction component of the axial force will be reduced due to abrasion between the locator plate1400and bottom312. However, even if the friction component is reduced to a small fraction of its initial value, the minimum loosening torque will not be reduced over time. The value of the minimum loosening torque may be selected to ensure that the torque-limiting axle nut100will not become completely detached from the axle200, even if the friction component is zero.

As a quantitative example, the torque-limiting axle nut100may be designed such that at a limiting torque of 100 ft·lb, the axial force is 1000 lb. The frictional component may constitute 950 lb of axial force. Compression of the spring1506may provide another 20 lb of axial force. When there are two spring plungers1402(e.g., seeFIG.4) and only the first is engaged with a detent hole1406, the second will be additionally compressed by the thickness t of the locator plate1400. In this case, the second spring plunger1402may provide the remaining 30 lb of axial force. Since this second spring plunger1402is not engaged with a detent hole1406, its axial force does not contribute to the minimum loosening torque.

In some embodiments, the locator plate1400and spring plungers1402are unitized with the rest of the torque-limiting axle nut100to ensure that the locator plate1400and inner nut300do not become axially separated to the point where the contact1504can fall out. The locator plate1400may contain features to simplify assembly of the axle nut100into a unitized device. For example, inFIG.14the locator plate1400has a wall1412that encircles the top face1408and extends axially upwards. At the top of the wall1412are a plurality of tabs1414. After the axle nut100is assembled, each tab1414may be bent radially inward to overlap a lip324of the inner nut300(seeFIG.15). As another example, the upper portion of the wall1412is bent radially inward via roll-forming. This creates an upper lip at the top of the wall1412that extends continuously around the entire locator plate1400. The upper lip may be “crimped” radially inward enough to overlap the lip324, thereby achieving unitization.

FIG.16is a side cross-sectional view of a torque-limiting axle nut1600that is similar to the torque-limiting axle nut100ofFIG.1. For clarity inFIG.16, the cross-section does not pass through the rotational axis190. The axle nut1600includes an inner nut1630that is similar to the inner nut300ofFIGS.1-4except it forms a plurality of upward nubs1606that extend axially upward from the top face308of the flange304. The upward nubs1606replace the recesses306. The axle nut1600also includes an outer nut1660that is similar to the outer nut500ofFIGS.1-2and5-6except that it forms a plurality of downward nubs1604that extend axially downward from the bottom face508.

The downward nubs1604are functionally similar to the ridges512except that they are not defined by grooves510(seeFIG.6). The upward nubs1606are functionally similar to the ball bearings222(seeFIG.2) except that they cannot rotate. Accordingly, contact between the upward nubs1606and downward nubs1604will generate abrasion, as compared to the contact between the ball bearings222and ridges512(seeFIGS.8-10). Although this increased abrasion will cause the nubs1604and1606to wear out faster, the upward nubs1606are integral to the inner nut300, which simplifies assembly and unitization of the torque-limiting axle nut1600.

FIG.17shows one downward nub1604and one upward nub1606in more detail. The downward nub1604has a front face1704that is similar to the front face704of the ridge512(seeFIG.7) in that it forms an oblique front angle θ with respect to the bottom face508. Similarly, the downward nub1604has a rear face1706that is similar to the rear face706of the ridge512in that it forms a rear angle φ with respect to the bottom face508. The rear angle φ may be right (as shown) or oblique. Similarly, the upward nub1606has a front face1714that forms the oblique front angle θ with respect to the top face308of the inner nut300. The upward nub1606also has a rear face1716that forms the rear angle φ with respect to the top face308.

When tightening the torque-limiting axle nut1600, the front faces1704and1714will eventually contact each other and slide against each other as the downward nub1604moves up and over the upward nub1606. The limiting torque is determined, in part, by the front angle θ and a height l of the nubs1604and1606. When loosening the axle nut1600, the rear faces1706and1716will contact each other and slide against each other. In this case, a limiting torque can be set by making the rear angle φ oblique. In one embodiment, the rear angle φ is 90°. In this embodiment, there is no torque limiting when loosening the axle nut1600.

As shown inFIG.16, the torque-limiting axle nut1600may further include the locator plate1400to establish an axial force for resisting rotation once the axle nut1600is fully tightened. In one of these embodiments, the inner nut1630includes the spring plungers1402for engaging with the detent holes1406(seeFIG.15). In another of these embodiments, and shown inFIG.16, the inner nut1630forms locator nubs1608that extend axially downward from the bottom face312. For clarity, only one locator nub1608is shown inFIG.16. Each locator nub1608is sized and radially located to engage with the detent holes1406as the inner nut1630rotates with respect to the locator plate1400. Because the locator nubs1608are integral to the inner nut1630, they cannot be axially compressed. As a result, once the axle nut1600(with locator nubs1608) is tightened, it cannot be subsequently loosened. Therefore, this embodiment may be useful for applications where the axle nut1600, once tightened, never needs to be loosened.

FIG.18is a side cross-sectional view of a torque-limiting axle nut1800that is similar to the torque-limiting axle nut100ofFIG.1except that the inner nut300has an extended portion1802that extends axially upward from the inner-nut body302.FIG.19is a perspective view of the torque-limiting axle nut1800ofFIG.18.FIGS.18and19are best viewed together with the following description.

Unlike the inner-nut body302, the extended portion1802of the inner nut300is not internally threaded. As shown inFIG.18, the extended portion1802may be externally shaped as a polygon (e.g., hexagon, octagon, etc.). The extended portion1802has an height b that is large enough for a wrench to engage with the extended portion1802. In an alternative embodiment, the inner nut1630(seeFIG.16) extends axially upward to form the extended portion1802.

The torque-limiting axle nut1800is convenient for applications where an initial torque used to seat the bearing is greater than the final installation torque. For example, a bearing-seating torque of 200 lb·ft may be used to seat the bearing, but the final installation torque only needs to be 100 lb·ft. In this case, a torque wrench can be used with the extended portion1802to apply the bearing-seating torque. After the bearing is seated, the axle nut1800can be loosened, again using a wrench that only engages with the extended portion1802. Finally, the outer nut500can be torqued to the final installation value without a torque wrench, as described above (i.e., using a conventional wrench that engages with the outer nut500).

FIGS.20and21are side cross-sectional views of a torque-limiting axle nut2000that is similar to the torque-limiting axle nut100ofFIG.1except that it uses an inner nut2300instead of the inner nut300. The inner nut2300is similar to the inner nut300except that it has a flange2304that is thinner (along z) than the flange304(seeFIGS.3and4). Furthermore, the inner nut2300does not form any of the mounting holes322. Instead, the flange2304forms recesses2406that are similar to the recesses306ofFIG.3except that they are through holes passing entirely through the flange2304. The thickness of the flange2304is less than the diameter of the ball bearing222. As a result, when the ball bearing222is in a recess2406, it can extend past both a top face2312of the flange2304and a bottom face2314of the flange2304.

FIG.20shows the torque-limiting axle nut2000when the ball bearing222is between a pair of neighboring detent holes1406of the locator plate1400(seeFIG.14). In this case, the bottom of the ball bearing222contacts the top face1408of the locator plate1400. The top of the ball bearing222sits inside one groove510of the outer nut500, pushing the outer nut500upward to compress the wave spring130against the retaining ring110. In this position, the bottom of the outer nut500(see bottom face508inFIG.5) and the top face2312of the flange2304are separated by a maximum gap hmax.

FIG.21shows the torque-limiting axle nut2000when the ball bearing222is fully seated inside a detent hole1406. In this case, the ball bearing222has moved axially downward. The wave spring130pushes the outer nut500downward, thereby decreasing the force in the wave spring130. Due to the chamfered edges of the detent hole1406, the bottom of the ball bearing222does not extend past the bottom face of the locator plate1400. Accordingly, the ball bearing22does not directly contact the end surface204of the axle. Due to the downward movement of the ball bearing222into the detent hole1406, the bottom of the outer nut500and the top face2312of the flange2304are separated by a minimum gap hminthat is less than the maximum gap hmax. The thickness of the flange2304, diameter of the ball bearing222, and depth of the groove510may be selected such that hminis greater than zero, which ensures that the outer nut500and inner nut300do not directly contact each other.

FIG.21also shows the recesses2406, detent holes1406, and grooves510are all centered at the same radial distance r from the rotation axis190. Accordingly, the ball bearing222, when located in its corresponding recess2406, will also be centered at approximately the same distance r from the rotation axis. This configuration ensures that the top portion of the ball bearing222can engage with all of the grooves510as the outer nut500rotates relative to the inner nut300, and that the bottom of ball bearing222can engage with detent holes1406as the inner nut300rotates relative to the locator plate1400.

The torque-limiting axle nut2000provides the same advantages as the torque-limiting axle nut100ofFIG.1, including the establishment of a limiting torque and minimum loosening torque. However, the axle nut2000advantageously excludes the spring plungers1402(e.g., the contact1504and spring1506inFIG.15). As a result, the axle nut2000is easier to assemble than the axle nut100. Furthermore, the inner nut2300excludes the plunger mounting holes322, making it simpler to fabricate (e.g., less machining) than the inner nut300.

Combination of Features

Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following examples illustrate possible, non-limiting combinations of features and embodiments described above. It should be clear that other changes and modifications may be made to the present embodiments without departing from the spirit and scope of this invention:

(A1) A torque-limiting axle nut includes an inner nut having an inner-nut body forming a threaded center hole that defines a rotation axis. The inner nut also has a flange located azimuthally around the inner-nut body and extending radially away from the inner-nut body. The flange forms a plurality of recesses. The torque-limiting axle nut also includes a retaining ring located azimuthally around the inner-nut body and an outer nut located azimuthally around the inner-nut body. The outer nut forms a plurality of grooves that (i) extend axially upward from a bottom face of the outer nut and (ii) extend azimuthally around the rotation axis. For each pair of neighboring grooves of the plurality of grooves, the outer nut forms a ridge that separates the neighboring grooves of said each pair. The torque-limiting axle nut also includes a wave spring located azimuthally around the inner-nut body. The wave spring is located axially between the retaining ring and a top face of the outer nut. The torque-limiting axle nut also includes a plurality of ball bearings seated in the plurality of recesses and extending into the plurality of grooves.

(A2) In the torque-limiting axle nut denoted (A1), the ridge has a front face that forms an obtuse angle with respect to a recessed surface of a first groove of said each pair of neighboring grooves.

(A3) In either of the torque-limiting axle nuts denoted (A1) and (A2), the ridge has a rear face that forms a right angle or an obtuse angle with respect to a recessed surface of a second groove of said each pair of neighboring grooves.

(A4) In any of the torque-limiting axle nuts denoted (A1) to (A3), the ridge has a bottom face that lies flush with the bottom face of the outer nut.

(A5) In any of the torque-limiting axle nuts denoted (A1) to (A4), the plurality of ball bearings identically have a ball-bearing diameter. The plurality of grooves identically have a radial width that is larger than the ball-bearing diameter.

(A6) In any of the torque-limiting axle nuts denoted (A1) to (A5), the inner-nut body has a top face and a bottom face, the bottom face of the inner-nut body lying parallel to the top face of the inner-nut body and axially beneath the top face of the inner-nut body. The outer nut is fully located between the top and bottom faces of the inner-nut body.

(A7) In any of the torque-limiting axle nuts denoted (A1) to (A6), a bottom face of the flange lies flush with a bottom face of the inner-nut body.

(A8) In any of the torque-limiting axle nuts denoted (A1) to (A7), the outer nut is shaped as a polygon.

(A9) In the torque-limiting axle nut denoted (A8), the polygon is a regular octagon or a regular hexagon.

(A10) In any of the torque-limiting axle nuts denoted (A1) to (A9), the plurality of upper ball-bearing holes are centered identically at a radial distance from the rotation axis.

(A11) In the torque-limiting axle nut denoted (A10), the plurality of grooves are centered identically at the radial distance.

(A12) In any of the torque-limiting axle nuts denoted (A1) to (A11), the plurality of grooves extend identically around the rotation axis by an azimuthal range.

(A13) In any of the torque-limiting axle nuts denoted (A1) to (A12), the plurality of upper ball-bearing holes are uniformly spaced around the rotation axis.

(A14) In any of the torque-limiting axle nuts denoted (A1) to (A13), the torque-limiting axle nut is unitized.

(A15) In any of the torque-limiting axle nuts denoted (A1) to (A14), the retaining ring is engaged with a retaining-ring notch that extends radially inward from a cylindrical outer face of the inner-nut body.

(A16) In any of the torque-limiting axle nuts denoted (A1) to (A15), the inner-nut body extends axially upward to form an extended portion that is shaped as a polygon.

(A17) In the torque-limiting axle nut denoted (A16), the polygon is a regular octagon or a regular hexagon.

(A18) In either of the torque-limiting axle nuts denoted (A16) and (A17), the extended portion forms an unthreaded center hole.

(A19) In any of the torque-limiting axle nuts denoted (A1) to (A18), the inner nut forms a plurality of plunger mounting holes extending axially upward from a bottom face of the inner nut. The torque-limiting axle nut further includes a plurality of spring plungers affixed in the plurality of plunger mounting holes and a locator plate forming a center unthreaded hole that is centered on the rotation axis. The locator plate forms a plurality of detent holes located azimuthally around the rotation axis. The locator plate faces the bottom faces of the inner nut such that the plurality of spring plungers can engage with the plurality of detent holes as the inner nut rotates around the rotation axis.

(A20) In the torque-limiting axle nut denoted (A19), the plurality of spring plungers are centered identically at a radial distance from the rotation axis.

(A21) In the torque-limiting axle nuts denoted (A20), the plurality of detent holes are centered identically at the radial distance.

(A22) In any of the torque-limiting axle nuts denoted (A19) to (A21), the plurality of spring plungers are uniformly spaced around the rotation axis.

(A23) In any of the torque-limiting axle nuts denoted (A19) to (A22), the locator plate forms a key shaped to engage with a slot of a threaded axle to rotationally constrain the locator plate relative to the threaded axle.

(A24) In any of the torque-limiting axle nuts denoted (A19) to (A23), each of the plurality of detent holes is shaped as a countersunk hole or a countersunk stadium.

(A25) In any of the torque-limiting axle nuts denoted (A1) to (A13), the plurality of ball bearings identically have a ball-bearing diameter, the flange has an axial thickness less than the ball-bearing diameter, and each of the plurality of recesses is a through hole.

(A26) In the torque-limiting axle nut denoted (A25), the torque-limiting axle nut of further includes a locator plate forming a center unthreaded hole that is centered on the rotation axis. The locator plate forms a plurality of detent holes located azimuthally around the rotation axis. The locator plate faces a bottom face of the inner nut such that the plurality of ball bearings can engage with the plurality of detent holes as the inner nut rotates around the rotation axis.

(A27) In the torque-limiting axle nut denoted (A26), the plurality of detent holes are centered identically at a radial distance from the rotation axis, the plurality of recesses are centered identically at the radial distance, and the plurality of grooves are centered identically at the radial distance.

(A28) In either of the torque-limiting axle nuts denoted (A26) and (A27), the locator plate forms a key shaped to engage with a slot of a threaded axle to rotationally constrain the locator plate relative to the threaded axle.

(A29) In any of the torque-limiting axle nuts denoted (A26) to (A28), each of the plurality of detent holes is shaped as a countersunk hole or a countersunk stadium.

(A30) In any of the torque-limiting axle nuts denoted (A26) to (A29), the torque-limiting axle nut is unitized.

(B1) A torque-limiting axle nut includes an inner nut having an inner-nut body forming a threaded center hole that defines a rotation axis of the torque-limiting axle nut. The inner nut also has a flange located azimuthally around the inner-nut body and radially away from the inner-nut body. The inner nut also has a plurality of upward nubs extending axially upward from the flange. The torque-limiting axle nut also includes a retaining ring located azimuthally around the inner-nut body and an outer nut located azimuthally around the inner-nut body. The outer nut has a plurality of downward nubs that extend axially downward from a bottom face of the outer nut. The plurality of downward nubs are radially located to engage with the plurality of upward nubs as the outer nut rotates with respect to the inner nut. The torque-limiting axle nut also includes a wave spring located azimuthally around the inner-nut body. The wave spring is located axially between the retaining ring and a top face of the outer nut.

(B2) In the torque-limiting axle nut denoted (B1), each of the plurality of downward nubs has a front face that forms an obtuse angle with respect to the bottom face of the outer nut.

(B3) In either of the torque-limiting axle nuts denoted (B1) and (B2), each of the plurality of downward nubs has a rear face that forms a right angle or an obtuse angle with respect to the bottom face of the outer nut.

(B4) In any of the torque-limiting axle nuts denoted (B1) to (B3), each of the plurality of upward nubs has a front face that forms an obtuse angle with respect to a top face of the flange.

(B5) In any of the torque-limiting axle nuts denoted (B1) to (B4), each of the plurality of upward nubs has a rear face that forms a right angle or an obtuse angle with respect to the top face of the flange.

(B6) In any of the torque-limiting axle nuts denoted (B1) to (B5), the plurality of upward nubs are located identically at a radial distance from the rotation axis.

(B7) In the torque-limiting axle nut denoted (B6), the plurality of downward nubs are located identically at the radial distance.

(B8) In any of the torque-limiting axle nuts denoted (B1) to (B7), the plurality of upward nubs are spaced uniformly around the rotation axis.

(B9) In any of the torque-limiting axle nuts denoted (B1) to (B8), the plurality of downward nubs are spaced uniformly around the rotation axis.

(B10) In any of the torque-limiting axle nuts denoted (B1) to (B9), the inner-nut body has a top face and a bottom face. The bottom face of the inner-nut body lies parallel to the top face of the inner-nut body and axially beneath the top face of the inner-nut body. The outer nut is fully located between the top and bottom faces of the inner-nut body.

(B11) In any of the torque-limiting axle nuts denoted (B1) to (B10), a bottom face of the flange lies flush with a bottom face of the inner-nut body.

(B12) In any of the torque-limiting axle nuts denoted (B1) to (B11), the outer nut is shaped as a polygon.

(B13) In the torque-limiting axle nut denoted (B12), the polygon is a regular hexagon or a regular octagon.

(B14) In any of the torque-limiting axle nuts denoted (B1) to (B13), the torque-limiting axle nut is unitized.

(B15) In any of the torque-limiting axle nuts denoted (B1) to (B14), the retaining ring is engaged with a retaining-ring notch that extends radially inward from a cylindrical outer face of the inner-nut body.

(B16) In any of the torque-limiting axle nuts denoted (B1) to (B15), the inner-nut body extends axially upward to form an extended portion that is shaped as a polygon.

(B17) In the torque-limiting axle nut denoted (B16), the polygon is a regular octagon or a regular hexagon.

(B18) In either of the torque-limiting axle nuts denoted (B16) and (B17), the extended portion forms an unthreaded center hole.

(B19) In any of the torque-limiting axle nuts denoted (B1) to (B18), the inner nut has a plurality of locator nubs that extend axially downward from a bottom face of the inner nut. The torque-limiting axle nut further includes a locator plate forming a center unthreaded hole that is centered on the rotation axis. The locator plate forms a plurality of detent holes located azimuthally around the rotation axis. The locator plate faces the bottom face of the inner nut such that the plurality of bottom nubs can engage with the plurality of detent holes as the inner nut rotates around the rotation axis.

(B20) In the torque-limiting axle nut denoted (B19), the plurality of bottom nubs are centered identically at a radial distance from the rotation axis.

(B21) In the torque-limiting axle nut denoted (B20), the plurality of detent holes are centered identically at the radial distance.

(B22) In any of the torque-limiting axle nuts denoted (B19) to (B21), the plurality of bottom nubs of the inner nut are uniformly spaced around the rotation axis.

(B23) In any of the torque-limiting axle nuts denoted (B19) to (B22), the locator plate forms a key shaped to engage with a slot of a threaded axle to rotationally constrain the locator plate relative to the threaded axle.

(B24) In any of the torque-limiting axle nuts denoted (B19) to (B23), each of the plurality of detent holes is shaped as a countersunk hole or a countersunk stadium.

Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.