After evaluating statistics from its fleet network, a well-known truck manufacturer has ascertained that a high percentage of reported crankshaft damage is due to particularly large loads on the bearings radii. Working closely with the German high-precision machine tool manufacturer Thielenhaus Microfinish, the manufacturer therefore decided to perform finishing work on the bearings as well as on the transition radii. In order to achieve the same surface quality on the crankshaft journals and radii, Thielenhaus developed a special tape contact shoe. This technology presses a finishing tape with a special profile on both sides into the radius, allowing radii to be machined up to 90 degrees. This represents a significant break-through from the previous 60 degrees. The manufacturer’s goal was thus successfully achieved.
Diameter Flexibility
The leading machine tool manufacturer for microfinishing is developing prototypes and filing patent applications for a variety of other innovative tape contact shoe systems. In addition to the conventional hard or soft shoes, Thielenhaus is developing a shoe that uses a kind of belt system to adapt to the component’s size. With the DiaFlex technology, crankshafts with the same bearing widths but different diameters can be machined without swapping the shoes. In practice, it often happens that, due to higher torques in three- or four-cylinder motors, a few millimetres more of bearing diameter are needed with the same motor concept and gauge. The shoe adapts to the respective diameter and can compensate for differences of up to 15 millimetres without changing shoes.
Correcting The Crown Shape
Also currently under development are tape contact shoes for generating or correcting profile and crowning on the bearings. With the ShapeFlex shoe, the tape contact shoe pressure can be varied over the diameters width. This makes it possible to work on the edges of the bearing points with a slightly elevated specific pressure to support or even generate a slight crown shape, or to achieve the desired crowned shape with minimal variation in crowning and symmetry.
Multiple Systems Integration
The different shoe systems can all be integrated in the CrankStar machine platform, which features horizontal and vertical versions for machining passenger car or truck crankshafts and can be adjusted to the specific needs of each manufacturer. Based on the manufacturer’s requirements regarding part handling, cycle times, quality tolerances, part variations and machining tasks, Thielenhaus can develop a machine concept that is precisely tailored to the needs of the customer.
Consistent Roundness Improvements
In order to achieve the same roundness improvements on main- and pin bearings, pneumatic technology is used in the machine to counterbalance pin bearing forces of +/- 5 newton. This ensures that the machining forces on the rotating crank pin are kept constant and that the mass forces that occur between the shoe and bearing are compensated. The roundness is thus improved to exactly the same extent on all bearings, whether pin or main bearings.
Negating The Risk Of Deformation
When microfinishing weight-optimised, small dimensioned crankshafts for compact engines, the machining forces used to clamp and oscillate the workpiece can produce minor deformations. This then has a negative impact on the crankshaft bearing diameters, including profile and cylindricity. To counteract this, the machine regulates axial tailstock forces utilising a hydraulic servo control mechanism. The technology is able to compensate variating oscillation strokes and frequencies as well as different part lengths.
Ergonomics & Flexible Automation
In the standard version, the CrankStar is capable of machining crankshafts for engines with three to six cylinders, in some cases even without major retooling. In addition to centre- and pin bearings and thrust faces, the machine can finish oil seal diameters and deburr bores in the flange. Thrust face machining occurs via a special unit while the workpiece is oscillated. Even with vertical machine concepts, the machine comes with up to 33 percent longer intervals between tool changes than the industry standard, featuring a tape capacity of 300+ metres. The vertical solution allows a quick belt change from the front of the machine, without requiring the withdrawal of an entire tool carrier package.
What the machine can do is that it can be adapted to a variety of automation systems. Vertical machines are designed to be loaded either from the front or the back. Thielenhaus is currently examining possibilities to minimise idle time during the loading/unloading operation.
Innovative machine solutions and tape contact shoe systems open up new possibilities for engine manufacturers and their suppliers, enabling the microfinishing of crankshafts of various designs simpler, faster and more cost-effective.
