Technology for the Production of Stator and Rotor Packets for... Adhesive Bonding

Technology for the Production of Stator and Rotor Packets for Electric Motors by
Adhesive Bonding
Holger Thiede, Andreas Winkel, Sascha Mechtold, Stefan Böhm
Institute of Cutting and Joining Manufacturing Processes (tff), University of Kassel
Kurt-Wolters-Straße 3, 34125 Kassel, Germany,
h.thiede@uni-kassel.de
Introduction
The battery is not only of vital importance for the efficiency of future electric vehicles, but also the electric motor.
Essential components of an electric motor are the stator
and the rotor, which are composed of stacked magnetic
steel sheets and fixed to so-called packets. In this paper a
new adhesive-based production technology is presented
which enables a greater degree of freedom when designing
the geometry of the plates. Simultaneously, the efficiency
of electric motors is optimized. Moreover, a more efficient
manufacturing process lowers the manufacturing costs.
This innovative approach involves the integration of the
gluing process in the cutting process, so that full surface
bonding is enabled in the punching cycle. Especially the
short time available for curing the precoated adhesive is
taken into account. Therefore, suitable adhesive coatings
are developed and tested. In this paper, first results of the
investigations are presented.
State of the Art
In literature, numerous methods are known for the manufacture of stator and rotor packets for electric motors. The
processing step which involves joining singular lamellae
together to form a packet of metal sheets is generally referred to as packetizing; the according methods are called
packetizing methods. The aim of the packetizing is to support the ensuing assembly processes. Depending on the
area of application, quantity, or requirements regarding the
efficiency, various packetizing methods are employed. At
present, interlocking [1], the baked paint system method
[2], the Glulock method [3, 4] and packetizing by welding
[2] have the highest significance in industry.
Conventionally, the separation of the sheets is carried out
in progressive cutting tools, in which the geometry of the
metal sheet lamellae is cut in a series of steps. In order to
save material, the rotor and stator are punched out of the
same band of magnetic steel sheets; the rotor and stator are
simply pressed out in different places. During interlocking,
so-called packetizing studs are embossed in the material by
means of additional punching sequences. They are pressed
into one another so that a mechanical grouting of the metal
sheet lamellae is achieved [1].
Disadvantages include, in particular, eddy current losses,
which occur due to damage to the insulating layer located
on the metal sheet and as a result of electric contact of the
metal sheet lamellae in the vicinity of the packetizing
studs. The positions of the packetizing studs has to be specifically selected to ensure that the lowest possible losses
arise. The positioning also restricts the constructive freedom of design. Furthermore, the joints by interlocking are
characterized by a low strength, which in turn leads to high
scraps. In addition, high investments are necessary for the
required tools.
In the baked paint system method, the metal sheet is entirely coated with an adhesive that functions as an insulator, but also bonds the metal sheet lamellae together. In
contrast to interlocking, no mechanical studs are embossed
into the material during the baked paint system method.
Instead, the metal sheet lamellae are packetized without a
joint after separation, and positioned in a clamping device.
In a subsequent processing step, the adhesive is hardened
in an oven at temperatures ranging from about 150 to 200
°C for 30 to 240 minutes [2, 5].
The benefits of the baked paint system method are a high
strength and temperature stability of the joint. Additionally, this process makes it possible to obtain a high degree
of freedom regarding construction, as well as good insulation of the singular lamellae against one another and thus,
good electric properties. The high costs and expenditure of
energy that arise when hardening the adhesive in the oven
are a definite disadvantage. Consequently, the utilization
of this method is economically only useful for small series.
An adhesive bonding of metal sheet lamellae is also produced when using the Glulock method. A low-viscosity
adhesive is selectively applied onto the metal sheet; either
before it is or whilst it is in the punching tool. Chemically
cross-linking adhesives with a short hardening time, i.e.
cyanoacrylate, are employed and require constant and
clean production and environmental conditions to guarantee a safe process. The complex dosing units are also disadvantageous because they demand high precision concerning the location and amount of the dose. Moreover,
this method only enables selective adhesion, not complete
adhesion. An additional varnish is required to ensure electric insulation. Here, compatibility of the varnish with the
employed adhesives is only partially on hand. The strength
and temperature stability are limited. The speed of the
method and the simultaneously low expenditure of the
packetizing tool are advantageous [3, 4].
During packetizing by welding, the punched metal sheet
lamellae are positioned in a unit, and connected on the
outer radius using several welding seams that are orthogo-
nal to the metal sheet lamellae level. Via these welded
seams, an electric contact is created between the metal
sheet lamellae. Moreover, the insulating layer between the
singular lamellae is damaged, leading to an increased
amount of eddy current losses. The advantages include a
high strength and high temperature stability in regards of
the joint. [2]
1465 was carried out. The tensile shear samples are produced using a hot press. The pressing time equaled < 0.5 s.
Moreover, the reaction of the adhesives was characterized
using Differential Scanning Calorimetry (DSC). In particular, isothermic DSC measurements are completed. By
means of the assessment of the time span till the end of the
reaction, a measure for the reaction speed is generated.
Solution Statement
Results and Discussion
In order to eradicate the disadvantages of thus far known
packetizing methods, so-called thermic packetizing by
gluing of magnetic metal sheets is being examined at the
tff. As is the case for interlocking, the metal sheet lamellae
should be combined together in the punching step. However, the joining process is not accomplished via a mechanical joint, but by means of adhesive bonding. The adhesive bond is characterized by the fact that it covers the
complete area of the material and an initial strength can be
achieved during punching.
An adhesive is applied onto the magnetic metal sheet before punching and represents a tack-free surface before the
material is inserted into the punching tool. The adhesive is
applied to the underside of the material, in order to enable
contact with the source of heat on the upper side. The
metal band is punched throughout a series of steps in a
modified packetizing tool using a classic method. In doing
so, the coated adhesive is activated and a joint is generated.
Essential adhesive technical challenges which result from
observations of the new technology include the extremely
short time (<1 s). Simultaneously, the requirements for the
temperature stability are just as high, because temperatures
higher than 180 °C can occur both in the rotor and stator
when utilizing these components in electric motors. In
addition, the provision of an adhesive as a dry, tack-free
and punchable coating represents a special challenge.
Adhesives
The identification of an adhesive suitable for this technology is of decisive significance. The adhesive is either
commercially available, or is a commercial adhesive that is
modified. Besides the classic magnetic metal sheet varnishes, which are employed for reference purposes, epoxy
dispersions are employed which are made into adhesive
coatings; they fulfil the requirements by means of the addition of different hardeners and catalysts. The adhesives are
then converted into adhesive varnishes, which are applied
in thin coats in order to create sufficiently thin layers.
Experimental
The adhesives are characterized according to their achievable temperature and ageing resistances in correlation with
the hardening time. The results are then compared to existing packetizing methods. In order to determine the
strengths, a tensile shear experiment according to DIN EN
Numerous examinations were carried out which aimed to
obtain a suitable formulation. Here, we chose to leave out
an illustration of the previous investigations. Instead, only
the best result is listed, and compared to a classic magnetic
metal sheet.
The goal of the isothermic DSC examinations is, above all,
to gain an impression of the possibilities concerning the
reaction rate of the adhesive.
dispersion with accelerator
dispersion without accelerator
baked coating
Figure 1. Isothermic DSC measurements at 200 °C
Figure 1 depicts the result of the isothermic DSC measurements for a baked coating and a newly formulated adhesive system (dispersions). One dispersion is formulated
with an accelerator and one without an accelerator. While
the reaction of the baked coating completes after approximately 7.5 minutes, the newly formulated system achieved
the hardening maximum after only 1.5 minutes (with accelerator) respectively 4.0 minutes (without accelerator).
When regarding the fact that heat for hardening is not only
available during punching when using packetizing by gluing, but also when the metal stamp consisting of several
hundred sheets heats with every punching action, the time
for hardening should be sufficient for a temperature and
ageing resistant adhesive bond.
In order to examine this, tensile shear examinations in accordance with DIN EN 1465 were carried out. The sample
sheets were coated with both adhesives. Coating thicknesses of 3 to 6 µm were used. The samples were then
bonded in a hot press for 0.5 s using a heating plate tempered to 200 °C. Testing was performed at room tempera-
ture, 150 °c and 190 °c, as well as after completing the
ageing test VDA 621.415. The results of the examinations
are shown in Figure 2.
References
1.
2.
3.
4.
Figure 2. Results of tensile shear tests
In Figure 2, the results for the baked coating are shown in
red. Here, it is already evident that only very low strengths
of 0.5 MPa are achieved when testing at room temperature.
After testing at higher temperatures and after ageing,
strength is no longer discernible. This result was to be expected, for the DSC tests already proved that a significantly longer hardening time period is necessary to achieve
complete cross-linking.
The newly formulated adhesive coating displays strengths
between 2 and 8 MPa at room temperature. At 150 °C, the
material reaches a strength of 1 MPa, and, at 190 °C, the
strength equals approximately 0.2 MPa. After ageing,
strengths of up to 2 MPa (testing at room temperature) or
0.7 MPa (testing at 150 °C) can be achieved.
The enhanced cross-linking of the adhesive identified in
the DSC tests is confirmed by the achieved strengths.
Conclusion
A new technology for packetizing magnetic metal sheets
was introduced, which requires new adhesive coatings
which cure in extremely short amounts of time. A possible
adhesive formulation was introduced with which the requirements could be fulfilled. However, further examinations are necessary, especially those which concentrate on
enhancing hardening, examining precoating, and characterizing the storage suitability for the formulated adhesives.
Acknowledgements
The studies presented here are part of the joint project
„ProStaR- Produktionstechnologie für die serienflexible
Herstellung von Stator- und Rotorpaketen von EAntrieben“.
The
project
is
funded
by
the
“Bundesministerium für Bildung und Forschung” (BMBF)
in Germany in the context of „Forschung für die
Produktion von morgen“, and is supervised by the
“Projektträger Karlruhe” (PTKA).
5.
Hellwig, Waldemar: Spanlose Fertigung: Stanzen,
Grundlagen für die Produktion einfacher und
komplexer Präzisionsstanzteile, Springer-Verlag,
2006.
Brachthäuser, N.: Elektromobilität – Neue Herausforderungen an den Werkstoff Elektroband. 11. Umformtechnischen Kolloquium in Darmstadt, 2012.
Meissner: "Patentschrift DE 3829068: Verfahren und
Vorrichtung zum Verkleben von gestanzten
Elektroblechen", 1988.
Blocher, D, Bauer, S.: Patentanmeldung EP1 833145
A2: „Verfahren, Werkzeug und Vorrichtung zur
Produktion von Lamellenpaketen sowie
Lamellenpaket“.
Vendler, Ulreich: Dupont, "Selfbonding Varnish",
2010, http://www.dupont.de