WO1989009115A1

WO1989009115A1 - Tongs

Info

Publication number: WO1989009115A1
Application number: PCT/DK1988/000057
Authority: WO
Inventors: Axel Grønlund NIELSEN
Original assignee: Nielsen Axel Groenlund
Priority date: 1986-10-03
Filing date: 1988-03-30
Publication date: 1989-10-05
Also published as: DK473586D0; DK473586A

Abstract

A self-locking tong to transmit a moment of rotation round about the centre lines of round and hexagonal bodies, the jaws of the tong interacting with the body at three places, essentially separated 120° from each other. The tong comprises a revolving jaw section (1) having two jaw surfaces (3) forming an angle with each other of mainly 60° and interacting with the body (6) at two places. The jaw section is pivotally fixed to a handle (2) carrying a third jaw surface (4) constituting the third point of interacting with the body. The third jaw surface (4) follows such a curve shape that the angle (alpha) between the pressure spot of the axis of rotation and of the jaw surface against the body and the normal through said pressure spot is mainly constant in the whole span of the tong.

Description

TONGS

The invention relates to tongs of the type described in the preamble of claim 1. Such tongs can be used for gripping with a view to transmitting torques to circular and hexagonal objects such as rods and tubes of round material, bolts, nuts and hexagonal rod material of any kind.

Self-locking tongs of this type have been known for a long time. For instance, in GB patent specification no. 2524 from 1863 tongs are described in which the jaw face of the handle part is formed as a part of a spiral of Archimedes, and from European patent no. 40.231 are known other self-locking tongs in which the jaw face is formed by a part of a circle evolvent, the pivot of the movable jaw part being the centre of the involute circle. With either type of curvature the perpendicular to the tangent at the points of contact is itself always a tangent to a circle, such that all pressure points are located on said perpendicular, which also coincides with the angle bisector of the 60° angle formed by the locking faces of the loose jaw part. In the patent specifications it is explained that with tongs of this type direct proportionality is achieved between the force applied to the tongs by the user and the resulting force acting on the object gripped, e.g. a tube, the explanation given for the desired effect being that the moment arm of the forces acting within the entire gripping range of the tongues is the same.

This explanation does not correspond to the real conditions, for on the one hand it presupposes that the loose jaw part of the tongs is clamped during use, for instance in a vice, while on the other hand only moments about the pivot are considered. Another point is that the frictional conditions have not been taken into account at all, which quite certainly are of less importance in the large diameter range but very predominant in the low diameter range. In fact, the entirely decisive moment and the force system resulting therefrom occurs about the centre line of the tube as the tongs grip, a torque being exerted to the latter consisting in the sum of the moments of three tangential forces of equal size acting at the pressure points about the centre line of the tube, constituting the product of the three corresponding perpendicular forces multiplied with the given coefficient of friction. The reaction force due to the driving tangential force acting at the pressure point of the curve is accordingly of very considerable size and, in addition, acts in the same direction as the moment applied by the user, as a result of which it is self-boosting. The reaction force is resolved into two components, i.e. a perpendicular force, which must be added to the perpendicular force applied by the user, and a component acting within the polar vector itself in the direction towards the pivot, also this component being added to the force already present. The degree to which the tong forces are boosted, which is defined as the ratio between the components and the size of the reaction, and hence its capacity of giving rise to a frictional moment, which is always sufficiently larger than the moment applied by the user, both at the start and at the end, depends on the angle between these two components and also the angle between the polar vector and the corresponding radius in the tube. This angle thus exerts a decisive influence on the possibilities of limiting the forces tending, on the one hand, to deform the tube and, on the other hand, to load the tongs, as a result of which this angle is of extreme importance when dimensioning the tongs as regards both size and strength, especially if the tongs are subject, in the lowest part of the diameter range to very large moments in conjunction with massive and possibly hardened material.

With certain spans of jaws or jaw openings said boosting offorces becomes uncontrollably large if the jaw face is of evolvent form, since the angle between the pressure point and centre of rotation and the perpendicular through the pressure point approaches 180°, in which case the force component in the direction towards the point of rotation increases towards infinite. In practice it has also proved that tongs with evolvent curvature frequently break.

It is the object of the invention to provide tongs of the kind mentioned initially, but shaped in such a manner that the tongs can transmit the torque to the object within the entire jaw opening range without damaging either the tongs or the object. Accordingly, the invention is based on the knowledge that the jaw part of the tongs must have a curvature as a result of which the jaw face of the rotatable jaw part is designed in such a way that angle α between the pivot and the point at which the jaw face presses against the object, and the perpendicular through said pressure point is substantially constant throughout the entire jaw opening range.

By making certain that said angle α is substantially constant, substantially constant boosting of forces is achieved at all points of the jaw opening range, inasmuch as said boosting of forces depends, as described above, on the angle α. The mathematical representation of the curve is a logarithmic spiral, the centre of which is on the pivot of the jaw part, the logarithmic spiral being, as is known, characterised in that the angle between the polar vector and the tangent to the curve is constant.

With an embodiment preferred in practice the angle o. is selected so that the force component at right angles to the perpendicular through the pressure point is smaller than the frictional force between the rotatable jaw face and the object.

The frictional force between the rotatable jaw face and the object depends on the materials coefficient of friction and the angle of friction. On the basis of common material values it is possible to select α so that said force component will be somewhat smaller than the frictional force throughout the available jaw opening range. By this means sliding of the jaw part on the object is prevented.

Calculations have shown that a value of α substantially between 150 and 155° will meet the above conditions with conventional tool steel and common bolt and tube materials.

Calculations have also shown that in the case of tongs with a surface hardness of 62 Rc, which in practice is regarded as the best hardness achievable with the steel alloys available at the present time 153-49° is an advantageous value of α.

With the embodiment in which the tongs are used for hexagonal objects it is advantageous to design the curvature of the movable jaw face in accordance with the further criterion that the pressure point shall be located between the centre line of the hexagonal flat and the first corner in the locking direction of the tongs.

This makes it possible to apply to the object an additional torque over and above the torque produced by the frictional forces at the three points of contact. This accordingly gives rise to an additional torque resulting from the pressure force at right angles to the hexagonal flat multiplied by the perpendicular distance from the centre of rotation of the object. This effect corresponds to the conditions obtaining with the well known ringspanners, where the pressure point is shifted to the vicinity of the hexagon's corners.

With a possible embodiment of tongs according to the invention, the pressure point coincides with the centre line of the hexagonal flat at the largest possible jaw opening.

In this way one achieves the least possible deviation from the pure mathematical logarithmic spiral form defined in claim 1, angle α and, as a result, the degree to which the forces are boosted being kept, throughout the entire jaw opening range, within acceptable limits. With large hexagon dimensions, the torque brought about by the frictional forces is, as a rule, amply adequate.

By allowing, as proposed according to the invention, the pressure point to be located, at the lowest possible jaw opening, near the hexagon corner in the direction of rotation of the object, one obtains, on the other hand, an additional torque, due to the pressure force exerted upon the hexagon flat in the region in which the relatively small dimensions of the object do not permit the large moment transmissions owing to frictional forces along the hexagon flats. According to the invention the curved part and/or jaw part can be provided with teeth, whereby the front edge of the teeth of the curved part are, viewed in the locking direction, shaped approximately as circular arcs the centre of which is at the pivot of the jaw part.

As a result, the teeth of the jaw part readily slide off the object once the locking operation is concluded, inasmuch as the tooth form ensures that the teeth can be swung back and away from the small indentations or burrs in the surface of the object, which are produced as a result of hard locking. With many tooth shapes, on the other hand, a self-locking effect occurs, since the material displaced in the course of pressing in prevents the return of the tooth surface.

The invention is explained in detail below with reference to the drawing in which fig. 1 shows tongs designed in accordance with the invention, the handle part being marked by dot-and-dash lines in several positions about round objects, fig. 2 shows at a larger scale the movable jaw part with hexagon flats, one position being marked by dot-and-dash lines, figs. 3 and 4 illustrate a method for calculating the curvature of the jaw part, and fig. 5 illustrates the boosting of forces with two round objects with largest and smallest diameters in accordance with the possible jaw opening range.

Fig. 1 shows self-locking tongs comprising a handle part 2 with a jaw part 2 capable of being rotated about a pivot 5, said jaw part having two jaw faces 3 forming in respect of one another an angle of 60°. The tongs grip a round object 6 and are self-locking and self-adjusting with a view to adjusting the grip, when a force indicated by arrow P acts upon the handle, since jaw face 4 of the handle part is designed with a curvature forming part of a corrected logarithmic spiral, on which the tangential angle can be progressively corrected within a variation range of about 8% comprising the value zero. Fig. 1 also shows in graphic form, how the tongs act in self-locking manner as they grip an object with a smallest possible diameter and as they grip an object with the largest possible diameter, and the tongs' special design according to the invention is explained with reference to fig. 2.

The curve according to fig. 1 is substantially designed as a logarithmic spiral which in its standard form has the characteristic that the angle between a polar vector and the corresponding tangent is constant throughout the length of the curve.

Its parametric representation is as follows:

r = length of polar vector a = basic polar vector length with a preselected starting position e = logarithmic base φ = angle of rotation of polar vector m = cot α = angle between a polar vector and the corresponding tangent to the curve n = number of angle degrees spanned by curve

Looking in fig. 2 at the basic curve which is based on an α value of 65.5° and considering the case in which the tangent to the polar vector coincides with the uppermost flat of the largest possible hexagon while the pressure point is located on the angle bisector, it will be seen that the corresponding pressure points on the hexagons move, as the jaw opening becomes smaller, further and further towards the corner of the hexagon until they finally end relatively far outside. This characteristic exerts a particularly large influence when the tongs are used on hexagons. If the tongs are provided with teeth, they can for -example loosen any rusted-in bolt or nut, but the tongs are rough on the edges. On the other hand, if the tongs are not provided with teeth, they slide off at the corners of the hexagon. The first part of the curve is designed in accordance with an exponential function corresponding to a logarithmic spiral in a polar system of coordinates. With a pre-selected polar angle a negative correction is applied, which must also follow an exponential function with the same base.

The resultant curve can be regarded as a corrected logarithmic spiral and will be made up of polar coordinates in respect of which it can be proved that

if the differential quotient relating to r is designated r' . The exponential correction term developed has come to correspond to the following parametric representation

with k. and k₂ to be regarded as constants if their numerical values are in agreement with the required characteristics. If the correction starts with an n value corresponding to a pre-selected angle cf rotation φ_k, the reεμltant curve corresponds to the following representation

If m is selected as 0.5. becomes 0.5 or tgcc = 2 and α = 63.43°,

and if the value "a" amounts to 15.5, the function is reduced to

In the case shown in fig. 2 the tangential angles have been modified to 69.15°, but the correction, which, incidentally, is entirely omitted or generally reduced in special cases, starts only if the pressure point of the curve has been displaced so far out of the flat of the smaller hexagons that it has just reached the corner of the hexagon, corresponding to a quite definite jaw opening. This point has a corresponding φ_k value which may be 90°. If the largest value of n is 181.5°, the correction is therefore effected over 181.5 minus 90 = 91.5°.

At the point with the largest correction (largest n-value) corresponding to the smallest hexagon, the angle α_k is so selected that the difference between α_k and the angle between the polar vector and the hexagon flat amounts to about 4-5° but can in special cases be further reduced, however never with circular cross-sections.

Once the values of the corrected tangential angle α_k, (fig. 5 α_n = 69°) and the longest polar vector r have been decided upon, one determines the size of the correction. Thereupon k₁ is selected and k₂ calculated.

The differential equation is solved. By inserting k₁ and k₂ in the expression for ,

is calculated provisionally.

Once the correct value for k₂ has been determined (in this case 2.5856) the expression is recalculated, arriving at α_k = 69-15° which meets

the above requirement. By inserting k₁ and k₂ in the derived formula one can calculate, with the angle of rotation as the only variable, all points on the curve, possibly direct in the computer of a machine.

The advantageous characteristic achieved in accordance with the invention can be brought about in different ways. Instead of a certain deduction, which increases as a function of the angle of rotation and is therefore applied together with a spiral having an α of relatively low value, it is possible to select a logarithmic spiral with a larger value of α and to calculate a positive correction corresponding to a declining value of φ. One can also calculate an evolvent, the curvature of which is identical with the invention but can, of course, not make use of the evolvent's own centre as the tong pivot. Such a design will therefore be an imitation.

It is also possible to achieve the design of the logarithmic spiral in as such known manners. Designing a curve which is to embody a certain correction, one may regard figs. 3 and 4. Angle α and the sides b and d are related as follows:

By means of pre-selected values of the angles α and φ one can calculate, for the angle (φ + 1)°, the increase of the polar vector and sides b and d, by taking account of the fact that the small triangle (with additions) is equiangular and similar to the large triangle. The required correction can be included in the increase of d, whereupon the curve can be drawn degree by degree.

Using the stated designations we obtain:

If c. = 65.49° , tgα = tg 63.48 = 2, and we have:

and hence d = 2 . b, and one sees that the ratio between "d" and "b" is always the same, as determined by the angle α.

The self-locking tongs are based on friction, which presupposes that it is capable of utilising a coefficient of friction of a certain suitable size. In the case of hardened materials with smooth surfaces, coefficients of friction occur, which are 70 to 80^ lower. If, on the other hand, these hardened surfaces are rough or provided with grooves, hooks or edged projections conventional tongs are almost immediately destroyed.

This can be counteracted by hardening both the curved part and the jaw part to a surface hardness of 60 to 62 Rc, which is achieved by cooling steel alloyed with boron, the characteristics of which are considerably superior to those of plain or low-alloyed steel, the hardness of which must, for many reasons, be kept at values as low as 47 to 52 Rc, which is usual with conventional tongs. The self-locking tongs can also be provided with loose jaw parts made of compound steel.

Existing tongs are not capable of gripping very smooth objects such as chromium-plated metal, since they either slide on the surface or damage the surface or destroy the entire object.

If excessively large moments are applied to self-locking tongs, the tongs exert a correspondingly hard grip on the surface of the object thus giving rise to a corresponding opposite moment. With soft materials this can result in the tong teeth penetrating into the surface of the object, and this entails significant disadvantages .when they are to be released from such an object.

In fig. 5 the constant boosting of forces Is illustrated by two extreme positions of the movable jaw part in relation to circular objects.

Claims

C L A I M S

1. Tongs for gripping preferably round and hexagonal objects, said tongs being of the type comprising a fixed jaw part integral with a handle part and a jaw part rotatable in relation thereto, whereby the rotatable jaw part is located about a pivot on the fixed handle part, characterised in that the jaw face of the rotatable jaw part is designed in such a way that angle (α) between the pivot and the pressure point of the jaw face in relation of the object and the perpendicular through said pressure point is in the main constant over the entire jaw opening range of the tongs.

2. Tongs according to claim 1, characterised in that angle (α) is so selected that the pressure component at right angles to the perpendicular through the pressure point is smaller than the frictional force between the rotatable jaw face and the object.

5. Tongs according to claim 2, characterised in that angle (α) amounts substantially to 150-155°.

4. Tongs according to claim 3, characterised in that angle (α) is selected as 153.49°.

5. Tongs according to claims 1 and 2, whereby the tongs are used for hexagonal objects, characterised in that the curvature of the movable jaw face is fixed on the basis of the additional criterion that the pressure point shall be located between the centre line of the hexagon flat and the first corner occurring in the locking direction of the tongs.

6. Tongs according to claim 5, characterised in that the pressure point coincides with the centre line of the hexagonal flat, at the largest possible jaw opening.

7. Tongs according to claim 6, characterised in that the pressure point is located near the corner of the hexagon in the direction of rotation of the object, at the smallest possible jaw opening.

8. Tongs according to one of the preceding claims, characterised in that the curved part and/or the jaw part is provided with teeth, whereby the edge of the teeth of the curved part, which, viewed in the clamping direction, is in front as approximately a circular-arc shape, the centre of which is on the pivot of the jaw part.