The present invention relates to a driveshaft design and method of making same, capable of utilizing the advantages of magnetic pulse forming technology to simplify a driveshaft sliding spline-type joint and to eliminate electric erosion of the active working mandrel surface.
Torque transmitting shafts (driveshafts) are widely used for transferring rotational power from a source of rotational power to a rotatably driven mechanism. For example, in most land vehicles in use today, a drive train system is provided for transmitting rotational power from an output shaft of an engine/transmission assembly to an output shaft of an axle assembly so as to rotatably drive the wheels of the vehicle. To accomplish this, a typical vehicular drive train system includes a hollow cylindrical driveshaft tube. A first universal joint is connected between the output shaft of the engine/transmission assembly and the first end of the driveshaft tube, while a second universal joint is connected between the second end of the driveshaft tube and the input shaft of the axle assembly. The universal joints provide a rotational driving connection from the output shaft of the engine/transmission assembly through the driveshaft tube to the input shaft of the axle assembly while accommodating a limited amount of misalignment between the rotational axes of the shafts.
Where the driveshaft tube has two or more sections, connection of the first driveshaft section and the second driveshaft section is typically accomplished by using a sliding spline-type slip joint having cooperating male and female members with respective pluralities of splines formed thereon. The male member is generally cylindrical in shape and has a plurality of outwardly extending splines formed on the outer surface thereof. The female member is generally hollow and cylindrical in shape and has a plurality of inwardly extending splines formed on the inner surface thereof. The male member is inserted within the female member such that the outwardly extending splines of the male member cooperate with the inwardly extending splines of the female member. As a result, the male and female members are connected together for concurrent rotation movement and for relative axial movement.
Theoretically, if the male and female members of the driveshaft are made from aluminum tubes, the splines may be formed using magnetic pulse forming techniques. Magnetic pulse forming was developed as a means of shaping and assembling metal parts. The technology is especially convenient for treatment of tubular parts using cylindrical inductors, but also is capable of forming sheet metal with the help of flat inductors. If necessary, both forming and assembly can be executed in a single step. The materials that are the most easily formed by pulse magnetic field are ones that have a high electrical conductivity such as copper, brass and aluminum. Material with low conductivity can be formed if very high frequency pulses or driving rings made from aluminum or copper are used.
In comparison with other metal forming methods, magnetic pulse forming has several advantages: the process is easy to control, the forming tool (inductor) is not mechanically connected with the to-be-formed work piece (following no tooling indentations); the same inductor can be used to form parts of different shapes; forming can be made through insulated walls and vacuum; only one piece of tooling (mandrel or die) needs to be used; there is a very high specific pressure of forming; very high velocity of forming increases the metal plasticity, and there is high productivity. The basic disadvantages of magnetic pulse forming are: it is hard to perform deep elongation; it is practically impossible to form most types of steel without a driver and impossible to form parts when the free passage of electric current is restricted.
Magnetic pulse forming technology uses a high voltage capacitor bank, a high current switch, a forming inductor, a high voltage power supply, and an electrically conductive work piece. The magnetic field is created by the discharge of the bank capacitor into the forming inductor. The work piece is typically placed very close to the inductor coil so that an eddy current is inducted into the work piece. From the moment the magnetic field is created, an eddy current is induced electromagnetically into the work piece in the opposite direction of the current of the coil. The interaction of the opposite flowing currents in the inductor coil and the work piece causes a mutual repulsion. The mutual repulsion causes a pressure impulse on the surfaces of the inductor coil and work piece. The inductor has to be mechanically very strong to withstand this pressure without deformation. So, the pulse of electromagnetic pressure causes high-velocity deformation of only the part of the work piece located under the inductor coil. That piece rapidly moves away from the coil until all the beginning kinetic energy is spent for deformation, or until the to-be-formed part collides with the shaping surfaces, for example the mandrel. The pulses of the magnet field are typically kept short to avoid reduction of magnetic pressure as a result of the magnetic field penetrating through the wall of the work piece. If the inductor is disposed about the exterior of the work piece, then the work piece can be deformed inwardly into engagement with the support surface of the mandrel to form the female splines. If, on the other hand, the inductor is disposed within the interior of the work piece, then the work piece can be deformed outwardly into engagement with the support surface to form the male splines.
However, it is hard to realize the described technique in practice, especially under the high volume production conditions typical for manufacturing driveshafts. Indeed, from the point of view of efficiency, reliability and safety, to form the splines of aluminum tubes, the most appropriate magnetic pulse forming machine is one with voltage of capacitor battery below 10 kV and frequency of the magnetic pulse about 10 kHz. But because the wall thickness of the typical aluminum driveshaft tube it 2.0-2.5 mm, it is inevitable that the magnetic field of this frequency will partly penetrate through the tube wall in the space between the internal tube surface and the supporting surfaces of the mandrel. In the process of high velocity tube deformation, the penetrated magnetic flux (the product of the average magnetic field times the perpendicular area that it penetrates) will be captured and compressed quickly in the gap. As a result, very high opposing currents will be induced along the perimeter of those surfaces and at the moment the tube collides about the mandrel, electric erosion of the contact surfaces occurs. The erosion is especially intense in the bottom of the mandrel grooves, where the tops of the splines are in contact first. This electric erosion destroys the active mandrel surface very fast. Electric erosion of these surfaces is the main restriction to wide proliferation of magnetic pulse forming technology in applications where mandrels/dies have to be used. Theoretically the mandrel surface could be protected from erosion by using dielectric coating or spacers, for example. Also a higher frequency and resultant higher voltage magnetic pulse machine could reduce the intensity of erosion. But it is technically difficult and expensive to realize those opportunities to cure the problem in practice.