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Increasing Static Friction with Laser

There is need for flat and curved surfaces with the functionality of high static friction for force-fitted nonslip power transmissions. This need is especially true for con rod and cam structuring for high torque resistance or front face connections of sprockets, gears or cam shaft adjustments.

Production of surfaces that generate static frictional connections with the counterpart are increasingly realized with a modified laser beam. The contact surfaces actuated by adherence are laid out with topographies that assure the functioning of workpieces, and can be produced in a cost-efficient way. The requirements of the connections include loads by torques and shearing forces. The system is kept in a state of adhesive friction during the different operational conditions and does not allow any relative movement of the contact areas.

The principal feature of the adhesive system was revealed in a stress test. In it, hardened and structured contact surfaces are loaded against non-structured and unhardened ones with an axial force of 80 kN over 60 seconds for a twist angle of 4°. The course of torque was indicated as target value. The contact areas (di= 15 m, da= 30 mm) were dry and free of grease. At the junction of adhesive and sliding friction, there is a maximum of adhesive value.

Using the adhesive friction bench or the functional aggregate, the effects of structure can be evaluated. Every structure that increases adhesive friction is characterized by a raised profile contour that has influence on the counter body. Next, manifold micro form profiles are produced to absorb shearing forces. Furthermore, the micro form profiles result in a frictional connection with high adhesive friction value. Due to the thermal effect of the laser beam, hardness is generated due to the short heating time.

There are significant differences in the laser structured adhesive friction surfaces of cams, connecting rods, front face connections and pre-treated substrate surfaces. Cam topographies can be raised with low profile elevations and without deepening. Material melts evenly. The structure lines of connecting rods are equidistant superposed. Both cam shafts and connecting rods can absorb torque of shaft hub joints. Between the structures of front face connections, there are often deepenings caused by material transfer. Lastly, pre-treated substrate surfaces display a squamous structure with micro recesses and are suitable to join layer materials to substrates.

Laser structuring of adhesive friction surfaces in rigid friction type connections can substitute positive locking design connections or other expensive friction type connections like diamond interlayers. The achievable adhesive value depends on the depth of the height, the profile shape of the laser structured roughness profile, the arrangement of the structure density, the martensitic properties of lasered materials, the normal force and whether the lubricant is embedded in the contact surfaces or not.

Different connections with torsional strength have different cylindrical contact surfaces. A shrinking connection with the laser structured inner surface of the cam and the shaft, enables the cost-effective built cam shaft in serial production. In the case of the clamping connection shaft and the hub, the laser structured contact surfaces are engaged by clamping. This replaces the costly alternative of positive connections. In this way, pin or fitting connections can be replaced by friction type contact surfaces with high adhesive friction generated by the laser structured process.

Another category is the frictional connections of plain functional surfaces for the transmission of shearing forces of torques. The laser structured front surface, often designed with an alignment element, replaces cost-intensive solutions like form-locked joint coupling with frontal tooling or inserted diamond or SiC interlayers as slices for the increase of the friction value. Applications related to the combustion engine like pinion, cam shaft adjuster, ball bearing or related to the chassis frame are becoming frequently demanded alternatives to the expensive solutions with slices and interlayers.

Laser structuring of the connecting rods is realized by means of the crank pin, which joins the connecting rod and the crankshaft. The design of the bearing consists of the rod eye, the bearing shell and the crank pin of the crankshaft. There is a rotational relative movement between the crank pin and the inner side of the bearing shell. The joint is laid out as a sliding bearing and works with low friction. However, there is no movement allowed between the outer side of the bearing shell and the connecting rod bore. Here, a sufficient high adhesion is required to absorb the frictions within the sliding bearing. Next, a frictional connection is necessary to deliver the suitable torque strength by means of the elevated topography of the connecting rod and the resulting adhesion.

For the process of laser structuring, a solid body laser with scanning optics is used. The two beam sources are arranged above the connecting rod eye laterally, the connecting rod bore. The beam sources work simultaneously and structure each of the two areas with the connecting rod bore. In each unit, the pulsed beam is guided by the galvanometer scanners with two galvanometer mirrors each. There is no mechanically actuated displacement or positioning of the beam and the focal movement is carried out quickly, line by line, with the highest precision and simultaneously by the movable optics of the integrated scanning heads.

The laser structuring of cams takes place after the shaft and cams are machined as single parts and are assembled by thermal friction type connected processes. In order to achieve sufficient torque strength of the cams, their bores are structured for enhanced adhesion.

Friction type connections are applied in several sectors of mechanical engineering and vehicle design for the transmission of shearing forces and torque. Frontal face connections with plain contact surfaces are laser structured in serial production. Cam shaft adjuster, chain pinions or gears are the focus of the central valve of Schaeffler with the cam shaft in serial production. This valuable technology reduces carbon dioxide emissions. By laser structuring of the main bearing, a high torsion strength of the bearing shell can be realized by the roughly structured areas of a cylindrical type connection as well as heat transfer by the smooth unstructured areas.

In conclusion, the advantage of laser structuring with integrated scanning is that the structuring segments on curved and plain surfaces can be freely dimensioned and positioned. In addition, the structure data and the tolerances of the different surface values can be adjusted by the beam parameter. The structuring process is completely automatable and can be integrated in a production line. Laser structuring allows an increase of static friction up to five times. Compared with diamond interlayers, the costs and number of parts is significantly lower.

Authored by Gerhard Flores, Manager of Technology Development and Intellectual Property, Gehring

For more information contact:

Gehring L.P.

24800 Drake Road

Farmington Hills, MI 48335

248-427-3901 / 888-923-9760

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