Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
1997-04-18
2001-04-10
Ryan, Patrick (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S099000
Reexamination Certificate
active
06215085
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a stud welding device with an axially displaceable adjusting element for displacement from a front position into a rear position and back, in which a magnetic field and a coil interact in such a way that, when the coil is connected to a controllable current source an axial force is produced in order to displace the adjusting element, the movement of which is transmitted to a stud holder.
DESCRIPTION OF THE PRIOR ART
A stud welding device of this type is known from U.S. Pat. No. 5,321,226. The operating principle on which this device is based is shown in
FIG. 5
thereof. With that device, a permanent magnet designed as a bar magnet is moved axially to and fro so as to form an adjusting element on which a stud holder is arranged so that a stud fixed in the stud holder is axially displaced during displacement of the adjusting element. This displacement is then used in the conventional manner for igniting an arc and welding the stud to the workpiece. The forces to be applied for displacing the bar magnet originate from two coils arranged axially one behind the other with opposing winding directions which form electromagnets of opposing polarity during the flow of current. The bar magnet can move axially along the coil axes, more specifically to a stop at the two respective outer ends of the coils.
When the two coils are excited by the flow of electric current, the bar magnet which is prevented from moving outwardly by the stops is to move in the direction of the other coil by which it is initially repelled owing to the direction of the magnetic field produced by it. This repelling force has to be overcome by the coil which initially comprises the bar magnets so the bar magnet moves out of this coil and enters the other empty coil until it reaches the respective stop at its other end. For the return movement of the bar magnet, the polarity of the flow of current then has to be reversed via the two coils connected in series. This magnetic system is obviously dependent on special balancing of the interacting magnetic fields, with the result that, during continued movement of the permanent magnet, the forces acting on it vary to a considerable extent. Once the repelling forces initially emanating from the empty coil have been overcome, the bar magnet, once it has entered the empty coil, is drawn into the empty coil with continuously increasing forces. This is a problem for controlled movement of the adjusting element at a specific speed at the respective individual positions of the stud, particularly if the stud is to be prevented from sinking into the melt produced by the arc at an excessively high speed.
The object of the invention is, while utilizing the magnetic principle mentioned at the outset, to design the stud welding device in such a way that at a given current strength which excites the coil, the axial force acting by the magnetic field on the adjusting element remains substantially equal independently of position.
SUMMARY OF THE INVENTION
According to the invention this is achieved in that the coil is rigidly arranged on a hollow body which is connected to the adjusting element and is arranged axially movably in an air gap between a magnetizable core and a magnetizable sheath, the magnetic field being conveyed via at least one yoke connecting the core and the sheath while passing radially through the coil, the coil exerting on the adjusting element an axially force acting only in one direction as a function of the direction of current during current control.
With this design, the hollow body with the coil carried by it forms a relatively lightweight component. During its axial displacement at least in the range of movements during the welding process, it remains substantially in the radially extending magnetic field so the coil is exposed to a substantially steady axial force during the axial displacement of the coil while the current flowing via the coil is constant. The axial forces acting on the coil and therefore the hollow body with the adjusting element can therefore be calculated accurately for each position within the path of adjustment of the coil. This results in the significant advantage over the bar magnets which are axially displaced in the prior art that the components which are decisive for producing the axial forces, namely the coil and the hollow body carrying it, can be relatively light weight in construction. For example, the hollow body can consist of a light plastics material. On the other hand, the bar magnet according to the prior art has a considerable mass if it is to produce a magnetic field of significant intensity at all. This mass counteracts the accelerations which are required for the adjusting movement of the adjusting element and then have to be produced by correspondingly high currents through the two coils connected in series. On the other hand, higher accelerations and decelerations can easily be achieved with the design according to the invention owing to its relatively low mass.
The accuracy of adjustment of the adjusting element can be increased by designing the device in such a way that a spring which presses the adjusting element into an end position defined by a stop acts upon the adjusting element. If the spring acts against the axial force applied by the coil, the axial force has to overcome the spring force so the adjusting element is lifted from its stop. The axial force required can be adjusted very accurately by adjustment of a corresponding current flowing via the coil, and the increasing spring force of the spring can also be taken into consideration during compression of the spring. Conversely, it is possible to ensure by means of an appropriate axial force that the adjusting element moves back to the stop if the spring force is greater than the axial force.
The magnetic field required for producing the axial force can be applied by a permanent magnet, but it is also possible to provide an electromagnet for this purpose.
If a permanent magnet is used, it is preferably integrated into the casing of the stud welding device. If an electromagnet is used, it can be placed in the connection between core and yoke.
If the adjusting element is to be moved in accordance with a predetermined pattern of movement, the stud welding device is preferably designed such that it is provided with a linear displacement measuring device from which there can be derived a displacement signal which corresponds to the respective relative position of casing and adjusting element and, as an actual value, is compared stepwise in a comparator with a desired value from a predetermined displacement-time graph for the movement of the stud holder read stepwise from a memory, the comparison signal of the comparator controlling the current intensity of a current source for supplying the coil.
A control process is carried out by comparing the actual signal with the stored desired signal using the displacement measuring device and the comparator, this control process ensuring that the displacement signal derived from the displacement measuring device is invariably opposed by the corresponding datum from the displacement-time graph so the sequence of movement corresponding to the displacement-time graph may be maintained with great accuracy during the movement of the adjusting element. In particular, this allows a rapid stud lifting movement and a correspondingly rapid return of the stud with an adequate interval for the melting of the material at the weld, it additionally being possible to design the return movement of the stud into the melt at the end such that the weld stud sinks into the melt at a desired speed, in particular preventing the molten material from being sprayed away during impingement of the weld stud, this frequently occurring when a spring is used alone to produce the return force of a weld stud.
The storage of the displacement-time graph in the memory also permits different displacement-time graphs to be provided as a function of the workpieces to be welded in each case, and these displ
Cummings Gerald
Mauer Dieter
Dey Anjan
Emhart Inc.
Murphy Edward D.
Ryan Patrick
LandOfFree
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