WRENCH WITH RECESSES ENCLOSING CORNERS OF A FASTENING ELEMENT WITHOUT TOUCHING THE CORNERS Background Wrenches, such as socket wrenches and box wrenches, have long been made with their inner corner made as rounded recesses to reduce the risk of cracks at the corner, where the thickness is small and the corner cause stress concentrations. Examples of this are described in patents US 3,125,910, US 3,273,430 and US 4,581, 957. Wrenches of these types are commonly used in mechanical power wrenches which often have an impact function when a preselected static torque value is exceeded. The stresses during the impacts can then reach high values and cause fatigue cracks in the wrenches if the corner are not enough rounded.

Another desire is that it should be possible to use the wrench for nuts with worn or otherwise slightly differing profile. In that case it is important that the corner of the nut do not become overstressed and deformed, which is achieved by leaving them free of contact in the recesses. It is also desirable to avoid sharp edges of the recesses cutting into the surface of the nut, damaging rust protection. This may be avoided by making the surface of the wrench slightly sloping as in US 3,273,430, fully rounded as in US 3, 125,910 or both as in US 4,581,957. One result of this is, however, that nuts with already deformed corner are contacte near the center of the hexagon side, causing a reduced leverage and very large radial forces which may damage the wrench. It will also make the rotational contact very elastic, which will partly cancel the effect of the impact function.

The present invention concerns a type of socket wrenches, box wrenches or other fixed wrenches which reduces the stress concentration at the corner, and which will make a well defined contact far enough from the corner and from the center, even at deformed nuts.

Description The wrench profile is described with reference to figure 1 showing a section through a socket wrench with rounded recesses according to the invention, figure 2 showing a detail of the section of a rounded recess at a corner according to the invention and figure 3 showing a rounded recess at a corner according to US 3,273,430.

The depth of the recess should be such that it coincedes with the nominal corner (11) of the hexagonal nut as shown in the figures, since damage to the corner of a nut will result in shortening or tangential deformation, not in radial expansion. For a given external diameter, this depth will allow the largest thickness at the corner.

Figure 3 shows a recess according to US 3,273,430 with a uniform radius of curvature (16) equal to the width (12) of one half of the recess. Since the stress concentration factor depends on the radius, it can be clearly reduced by giving the recess an oval or elliptical section according to figure 2, where the bottom radius (13) is 30-50 % larger than the width (12) of one half recess, preferrably 40-50 % larger, and by using a radius (14) 40- 70 % smaller than the width of one half recess closer to the edges of the recess where the thickness is larger. This allows a reduction of stresses of approximately 9 % compare to the section according to figure 3, in spite of equal thickness at the bottom.

The transition between larger (13) and smaller (14) radius of curvature may occur stepwise or gradually. The largest radius of curvature (13) should be at the bottom of the recess, and should be larger than the width (12) of one half recess, and less than the distance from the center line of the socket to the bottom of the recess. At increasing distance from the bottom line of the recess, the thickness will increase, and the tangential stress and the bending stress without correction for the stress concentration factor will decrease. This allows a limited increase of the concentration factor at the transition to smaller radius, while still keeping the stress approximately the same as at the bottom line.

Compare to a constant radius, the improvement is 8-10 %. If the stresses are detemilned by an external deformation rather than by external torque, the improvement will be even higher, since the deformation is distributed over a longer portion.

The transition to the active contact surface (10) is in prior art such as US 3,273,430 often made as a sharp corner, which may cause some damage to nuts, especially if they are made with rust protection coatings such as zinc or chrome. The contact surface is often made with an outward slope of 2-3 degrees so as not to damage nuts with smaller width, but will instead cause the forces to act too near the center of oversize nuts, causing very high forces and stresses in the wrench. Sharp corner are also difficult to forge and their forging tools are easily damage.

Rounded transitions on the contact surface (10) are also previously known, as in US 3,125,910, where the radius at the transition is as large as in the recess. This has two disadvantageous effects: the recess becomes unnecessary narrow unless the contact point is far away from the corner of the nut, and the friction against painted or greasy nuts is low. Both effects cause high stresses in the wrench.

According to the invention, the contact surface close to the recess is rounded with a radius (15) smaller than the width of one half recess, preferrably 30-60 %. This leads to a well defined point of attack of the force between the wrench and the nut, and a contact which is concentrated enough to get a sure grip on painted surfaces but not enough to damage metal coatings.

In the figures, the invention has been described as it is applied to wrenches with a closed hexagonal or dodecagonal profile, but it can also be applied to other wrench types such as open or semi-open wrenches, where the bending stresses at the profile corner determine the strength.