Two types of shape deviation are especially important to consider: corner damage and differences in the across-flat dimension. It is also important that the wrenches can be used for nuts which are coated with paint or rust-protective layers.

In order not to cause or worsen damage on the nuts, the force acting from the wrench on the nut should be spread over a sufficient surface so as not to damage surface coatings, and should not act too near the corner. The force should be relatively close to normal to the surface, since frictional shear forces can easily damage coatings. The force value for a given torque depends on the leverage, measured as the distance from tille line of the force to the center line of the nut. If the force becomes excessive there is also an elastic defonnatioll of the wrench which makes the angular motion un-distinct and may cause large stresses ? in severe cases the wrench may break.

In the simples types of wrenches the jaws or contact surfaces are flat. This means that undersize nuts are contacte only at the corners, which are easily damage. On nuts where the convers are already damage, the forces will then act closer to the center of the sides and become so large that the wrench may be damaged and the angular motion un- distinct.

In patents US 3,242,775 and US 4,930,378 it has been suggested that the jaws of the wrench be made convex with large radius to spread the force over a larger area, but for such wrenches the contact point will vary very much with the across-flat dimension, and oversize nuts will overstress the wrench. If the wrench is made as in US 5,481,948 with jaws which are flat with convexly rounded ends, the forces will be acceptable and the contact points relatively independent of the across-flat dimension, but for nuts with damaged corner, the forces will be excessive and likely to worsen the damage. If thc jaws are made with fully serrate surfaces as in US 4., 126, 063 the forces on nuts with

corner damage will be limited since the frictional shear forces are utilized, but this may hurt painted or galvanized nuts. It is also known to make jaws which are serrate on the inner half and flat on the outer half, with the purpose of pulling the nut into the gap between the jaws.

According to the invention, the jaws or contact surfaces are made with a shape that gives a well defined contact point for the forces, little dependent on shape deviations of the nut, limited force values and little risk of surface damage.

Description The invention is described with reference to the figures, where figure l shows a socket or box wrench with hexagonal profile, figure 2 a socket or box wrench with dodecagonal profile and figure 3 an open wrench.

A hexagonal wrench according to figur 1 is used to rotate. a hexagonal nut or bolt with six flat sides (10) and six corners (11) which may theoretically be sharp, but in pactise always somewhat rounded from the manufacture or from later wear or corrosion. Forces near the corners (11) should be avoided since the may easily damage the nut, and the wrench is thus made with rounded recesses (12) with circular or oval section. The wrench is provided with contact surfaces shaped as two ridges on each side of the recess (12), one outer ridge (13) near the recess and one inner ridge (14) farther from the recess.

Between the pairs of ridges the profile has a substantially straight portion (15) which is recessed relative to the ridges so it will nut touch the flat side (10) of a nut. This makes it easier to use the wrench for nuts with severe previous corner damage. If the side had contacte the wrench between the pairs of ridges, the leverage would have been too small, and the force so large that the wrench might break. The ridge tops, the concave valley between them and the transition to the recess and to the center portion should preferrably be rounded to reduce the risk of surface damage on the nut.

Figure 2 shows a dodecagonal wrench, which has the avantage compare to a hexagonal of admitting more angular positions and to be slightly lighter, but it may be more

sensitive to certain kinds of shape deviations. The wrench profile is similar to the hexagonal wrench, but instead of a recessed center portion (15) there is a recess (22) with the same section as the recess (12) where the corner of the nut is. The protrusion between the recesses (12,22) will then have three convex ridges (13,20,21).

Figure 3 shows an end of an open fixed wrench with contact surfaces according to the invention. Each jaw has four ridges (32,33,34,35) as contact surfaces and one recessed center portion (36). The ridges are located so that they are all between the corner (37,38) of the hexagon side they are to contact when the nut is inserted into the wrench as far as possible, which may be when one corner (31) touches the wrench bottom between the jaws. The center portion (36) is so deeply recessed that it can not touch a fully inserted nut, and its transition (39) to the inner bridge (33) has such a sxmooth slope, preferrably less than 30 degrees, that the corner (37) of the nut will slide over the ridge (33) without stopping when the nut is being inserted.

The ends (40) of the jaws outside of the outer ridge (34) should be made divergent to facilitate insertion of the nut, and may be either flat or convex.

To distribute the forces between the ridges when working with standard size nuts, the outermost (34) and innermost (32) ridges should preferrably be slightly lower, so that the flat side of a nominal size nut will touch two adjacent ridges (34,35 or 32,33). Oversize or corner-damaged nuts will only touch the ridge (33 or 35) closer to the center portion, and undersize nuts will only touch the ridge (32 or 34) farther from the center portion.

The height difference between the rides should be such that the common tangent line to two adjacent ridges forms an angle (41) not over 3 degrees with the side of a maximum size nut.

The tops of two adjacent ridges should fit within a distance interval, measured from tile corner of a maximum size nut, of 10 % to 35 %, preferrably 12 % to 30 % of the length of the side of the nut, and the recessed center portion should comprise at least 30 preferrably at least 40 % of the length of the side.

Providing the jaw with pairs of adjacent ridges will, in comparison with flat or fully convex jaws, make the contact points well defined with a distinct grip and reduced risk of damaging the wrench or the nut.