Comparison of clamping systems
in the processing of rivets & sealing plugs
The following elaboration provides an insight into the many different facets of the design of the clamping mechanisms when processing fasteners and sealing plugs. The use of manual tongs, hydro-pneumatic setting devices and fully automatic systems results in different requirements. The different design forms of the elements and the level of the setting forces have a great influence, not to forget the accessibility of the processing point and the materials.
Clamping systems in general
During the setting process of a blind rivet, plastic deformation forms a shooting head on the shank of the blind rivet by exerting very high forces on the mandrel head via the mandrel.
During the setting process of a sealing plug, a bore is closed by plastic deformation or by drawing in a taper by exerting very high forces on the mandrel.
The force is determined by the breaking load of the mandrel at the predetermined breaking point, which is approximately 80% of the wire breaking force.
Simple hand pliers have two-part versions of the jaws with a wrap-around tooth form, which are normally designed for smooth and thinner mandrels.
Many manufacturers of semi- or fully automatic setting tools have three-part jaws, with a wrap-around tooth form for larger mandrel diameters and a straight tooth form for thinner mandrels. With the straight tooth form, a triangular cross section results when gripping.
The wrap-around tooth shape is always oriented to the largest mandrel diameter to be gripped so that notches on the mandrel are avoided.
The clamping jaws perform a key task in the processing of blind rivets and tension sealing plugs. A variety of designs influence the function, service life and sensitivity to dirt of the clamping mechanisms.
Wedge angle
The wedge angles of the clamping jaws are, for example, 9°, 10°, 12° or 18°. The smaller the angle, the greater the forces so that the teeth of the jaws can dig into the mandrel by plastic deformation. This plastic deformation is necessary so that the forces to produce the mandrel break are transmitted by positive locking. In the case of grooved mandrels, the form fit is also given without plastic deformation and the wedge angle can be made correspondingly larger. With a pure frictional connection, the required forces cannot usually be transmitted.
A larger wedge angle of e.g. 12° puts less stress on the mandrel and requires less return stroke to release the mandrel for removal. The tooth shape is relatively pointed and wrapped around the tooth in order to keep the notches pressed into the mandrel as small as possible.
The path for releasing the mandrel reduces the stroke of the setting tool, which is available for forming the closing head or setting the sealing plug.
Tooth forms
Tooth form symmetrical, blunt or pointed to the direction of force
In the case of smooth mandrels, the teeth of the clamping jaws must be pressed into the mandrel so that the forces can be transmitted (clamping jaws on the right and left outer side). If the mandrels are prepared for positive locking by a grooving, the teeth can be blunt (clamping jaw in the middle).
The following pictures show two different 3-part 18° clamping jaws with a pitch of 0.794 mm or 1.2 mm and a trapezoidal tooth shape. These clamping jaws achieve a secure frictional connection and are designed for maximum forces. A disadvantage is the resulting relatively large diameter of the clamping sleeve, as the wedge shape extends over all teeth.
Clamping jaw 3-part, 18°, 0.794 mm
Clamping jaw 3-part, 18°, 1,2 mm
Division
The following pitches are known for the grooving of the mandrels or tooth spacing of the clamping jaws:
| 0,794 | mm | (corresponds to 32 teeth per 1 in = 25,4 mm) |
| 0,8 | mm | (see above, rounded off metrically) |
| 0,9 | mm | (old construction of a clamping jaw) |
| 1,0 | mm | (metric version of a clamping jaw) |
| 1,06 | mm | (Pitch on a battery unit) |
| 1,2 | mm | (see bottom, metrically rounded) |
| 1,25 | mm | (Gripping aid) |
| 1,27 | mm | (corresponds to 20 teeth per 1 in = 25,4 mm) |
The left mandrel in the illustration on the right has a trapezoidal groove with a clamping jaw with trapezoidal, snagged teeth with the appropriate pitch.
The middle mandrel in the figure has a trapezoidal groove with a clamping jaw with sharp teeth with the appropriate pitch.
The right mandrel in the figure has a trapezoidal groove with a jaw with sharp teeth with a mismatched pitch.
The pitch error adds up to the number of teeth in mesh. For example, if the error is 0.07 mm and 15 teeth are engaged, the incorrect position of the last tooth adds up to a striking 1.05 mm. This error leads to the fact that only very few teeth really transmit the force. The breakage of the teeth and increased wear are pre-programmed.
Grooved mandrels
The mandrels are grooved for higher quality blind rivets and sealing plugs. In terms of manufacturing technology, they are produced by a rolling process. Optimal are trapezoidal grooves, where the teeth of the clamping jaws have a negative form.
The grooves should have a depth of 0.1 to 0.2 mm, the greater value is preferable. It is also important that the depth of the grooves is constant over the length of the mandrel. If the grooves do not reach the end of the mandrel, it must be ensured that the mandrel is long enough for the clamping jaws to grip in the area of existing grooves.
On the mandrels shown above, one can see the break at the predetermined breaking point and the notches caused by the clamping jaws on the mandrels despite the presence of weakly pronounced grooves.
If the mandrel is grooved too short, or the clamping mechanism is incorrectly designed, the mandrel will end up with unacceptable plastic deformation. This can lead to malfunctions or at least to increased wear of the clamping mechanism.
Execution of the mandrel ends
The ends of the mandrels are divided into the truncated cone, the quadruple tip and the chisel tip. The chisel tip is widely used for cost reasons. The quadruple tip hits the mouthpiece well, but the tip can cause injury. The truncated cone is advantageous for automatic processing because it can be easily inserted into the mouthpiece. For manual processing, the lower risk of injury due to the truncated cone and the accuracy is an advantage.
Predetermined breaking point/squeeze-on mandrel
The predetermined breaking point of the mandrel ensures that it breaks off at the right place with the right force, so that the setting process for blind rivets or sealing plugs is completed in a process-safe manner. The upper two mandrels in the illustration have the pinch between the predetermined breaking point and the mandrel head so that this part of the mandrel remains in the rivet joint, e.g. to increase the shear strength. The lower two rivets have the predetermined breaking point close to the mandrel head. The mandrel head does not remain in the rivet joint and cannot rattle.
The mandrel breaking load amounts to approx. 80% of the wire breaking load, so that the mandrel leaves safely at the right place without protrusion:
The pinch and the predetermined breaking point have a similar shape with different characteristics.
The predetermined breaking point corresponds to a notch which is tempered with the same impact as the chisel tip.
The predetermined breaking groove is rolled up all around and ensures the cleanest break.
Clamping sleeve
The clamping sleeve accommodates the clamping jaws and is screwed onto the piston extension via a thread. The diameter of the clamping sleeve determines the diameter of the front sleeve which, together with the mouthpiece, forms a disturbing edge to the workpiece. A large wedge angle of the jaws often results in a large diameter of the clamping sleeve or the front sleeve.
Clamping jaw 9° with sharp teeth and a small clamping sleeve
The adapter sleeves are also called clamping housings in an industry called "sealing plugs".
Mouthpiece
The bore of the mouthpiece and the diameter of the end face is adapted to the blind rivet or sealing plug to be processed.
Special mouthpieces have a front surface which corresponds to the head shape of the setting head of the blind rivet.
The external hexagon or spanner flat of e.g. 13 mm or 15 mm is used for tightening via the threads M10x1 or M12x1 in the front sleeve.
The bore of the mouthpiece serves to centre the mandrel and is usually 0.1 mm larger than the crimping and 0.2 mm larger than the mandrel diameter.
At the end of the mouthpiece there is a pin which, in the initial position of the setting tool, opens the clamping jaws so that the clamping jaws are released from the mandrel. Only when the mouthpiece is unscrewed more than one turn should the teeth of the clamping jaws begin to grip the mandrel.
Extended mouthpieces are used for workpieces that are difficult to access, but they also require correspondingly extended mandrels so that all teeth of the clamping mechanism can grip the mandrel securely.
Guide sleeve
The guide sleeve presses the clamping jaws into the clamping sleeve via a spring so that the mandrel is securely gripped when the working stroke begins. During the return stroke the clamping jaws meet the pin on the mouthpiece and the clamping jaws open. The mandrel is centred through the hole in the guide sleeve so that it is released from the microserration and can be sucked off by the vacuum.
In the case of high mandrel breaking forces, wear on the collar of the guide sleeve and on the face of the piston extension can be cushioned by an O-ring.
Vacuum
The vacuum is used to transport the torn off mandrel into the mandrel container. With hand-held setting tools, the air consumption is limited by a so-called adjustable interval suction. With automatic riveting systems in vertical arrangement, the rivet or sealing plug is held in the mouthpiece by vacuum. The air must be able to escape at the mandrel container so that the vacuum suction is not hindered. If one wants to avoid the high air consumption in automatic systems, it is also possible to work with a small pre-pressure at the pressure intensifier.
Three possible solutions are known: The vacuum can be generated in the piston extension, in the piston or behind the setting tool.
Special clamping jaws
The clamping jaw in special design shown in the picture below is designed for gripping a thread M6 without the need to spin up or down the thread. When positioning the jaws in the clamping sleeve, the correct sequence of the jaws must be observed.
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