DE102022134047A1 - Stator and electric motor and method for producing a stator - Google Patents

Abstract

The invention relates to a stator, comprising a plurality of hollow cylindrical sheets (101) which have recesses (133) on a respective inner radius and which are stacked horizontally between a first end face (105) of the stator and a second end face of the stator arranged opposite the first end face, such that vertically oriented grooves (103) are present which are formed by the recesses, in which copper wires are arranged, wherein a first ring (131) is arranged on a first hollow cylindrical sheet on the first end face and wherein a second ring is arranged on a second hollow cylindrical sheet on the second end face, and an insulating material (151) which is present in the grooves (103) and encases the first ring (131) and the second ring, wherein a first outer contour (155) is impressed on the first ring (131) and a second outer contour is impressed on the second ring in such a way that a thermal voltage, in particular a notch voltage, is formed between the insulating material (151) and the first hollow cylindrical sheet and/or the insulation material (151) and the second hollow cylindrical sheet.

Description

The invention relates to a stator having a plurality of hollow cylindrical sheets which have recesses on a respective inner radius and which are stacked horizontally between a first end face of the stator and a second end face of the stator arranged opposite the first end face in such a way that vertically oriented grooves are present which are formed by the recesses and in which copper wires are arranged, wherein a first ring is arranged on a first hollow cylindrical sheet on the first end face and wherein a second ring is arranged on a second hollow cylindrical sheet on the second end face, and an insulating material which is present in the grooves and encases the first ring and the second ring and an electric motor and a method for producing a stator.

The main components of an electric motor are, for example, a rotor, a stator, a commutator, brushes, a coil and a shaft. Rotational energy on the shaft of the electric motor is generated by the rotor, which moves between the two poles of the stator. Depending on the type of electric motor, two main components in particular influence the performance class of the electric motor: a laminated core, which serves to guide the windings in its slots and to locally concentrate magnetic field lines, and the electrical winding itself that generates the magnetic field.

The laminated core generally consists of a large number of thin sheets with thicknesses of approximately 0.1 mm to 0.6 mm. The mostly punched sheets are stacked in an axial direction and can be electrically insulated from one another. The stacking and the shape of the sheets create grooves for guiding the winding inside the laminated core.

The winding of an electric motor, which can be distributed or concentrated and is inserted into the slots of the laminated core, generally consists of copper wires. The wires are usually electrically insulated from each other and from the laminated core.

The achievable performance of the electric motor depends in particular on the thermal resilience of the insulation of the stator wires. The insulation also has to meet a number of other requirements, such as high tracking resistance, good thermal conductivity or high temperature resistance.

Paper insulators are known in the state of the art; these are inserted into the grooves of the laminated core for high-voltage insulation of the copper wires. The process of installing the insulating paper is time-consuming and therefore costly.

Furthermore, plastic-coated rotors and stators or plastic-coated segments of a stator are known in the state of the art ( www.muehlbeyer.de/media/ injection_molding_tools_for_insulating_stators_and_rotors.pdf ). The plastic sheath, usually duroplast or thermoplastic, envelops the sheet metal packages and acts as insulation for the wires inserted in the grooves. In the state of the art, the sheet metal package is usually placed in a mold for overmolding, positioned in the middle and then overmolded with plastic.

Since the laminated core material has a different coefficient of thermal expansion than the overmolding material, thermal stresses, particularly notch stresses, occur when the electric motor is in operation. The notch stresses can cause material failure and/or material decoupling. If the material fails, for example, the insulation material of the stator can break. If decoupling occurs, a connection between the stator and the insulation material can break. If this type of failure occurs, an undesirable electrical connection can arise between components, particularly windings and laminated core and/or housing. This occurs particularly in high power classes (high-voltage applications) or at low temperatures, because the overmolding material shrinks faster than the laminated core material. The overmolding can also break due to such different expansions.

The WO 002022 207763 A1 discloses an insulation device for electrically insulating a winding head of a stator winding from connection elements, wherein the insulation device has a plate-shaped body which has a first side, a second side which is arranged opposite the first side, and at least one recess which extends from the first side to the second side and through which at least one of the connections can be passed from the first side, wherein an insulation arrangement is provided which has a first section which extends from the first side of the body in a direction away from the second side.

The EN 10 2021 117 768 B3 discloses a manufacturing method for introducing electrical insulation into a stator slot of a stator, wherein the stator comprises a laminated core with a plurality of of stator slots for receiving a stator winding. The manufacturing method comprises the following steps: providing the laminated core; introducing at least one tool slot core into a respective stator slot so that a gap is formed in the stator slot between its inner wall and the tool slot core; injecting a plastic as an electrical insulation layer into the gap between the tool slot core and the inner wall of the stator slot; and removing the tool slot core, wherein the injection pressure is adjusted depending on a vibration property of the tool slot core in question.

The WO 00 2022 054381 A1 discloses an insulating paper, a stator for a rotating electric machine, and a rotating electric machine having improved adhesive strength between a cover and the insulating paper compared to the prior art. An insulating paper according to the present invention is provided between a coil and a stator core in a rotating electric machine, comprising: the stator core having a slot; the coil installed in the slot; and a cover fixing the coil to the slot. The insulating paper is characterized by comprising: a substrate; and an adhesive strength improving layer provided on the surface on the side of the substrate facing the stator core and containing a substance that interacts with the cover and the substrate.

The US 2004 0046477 A1 discloses a stator core used as an outer die, a cavity between the stator core and a die formed by a protruding portion inserted into a slot. The cavity has both side portions gradually widening in depth from a slot opening portion. The cavity thus widens at a depth-side corner portion. Liquefied resin generally evenly flows through this wide portion into each portion around the stator core thus provided with an insulating member. As a result, filling with the resin as an insulating member can be properly performed while ensuring a slot space factor and winding insensitivity in the core.

The EN 10 2020 111 542 A1 discloses a method for casting an FSM rotor for an electrical machine, which has a rotor body and a rotor winding applied thereto, with the steps: - applying a radial seal to the FSM rotor to be cast; - arranging the radially sealed FSM rotor in a transfer molding tool, which has a first tool plate and a second tool plate, such that the FSM rotor is aligned substantially axially perpendicular to these tool plates; - carrying out a transfer molding process in which a heated casting compound is pressed into the rotor winding via one tool plate or both tool plates; - removing the cast FSM rotor from the transfer molding tool and an FSM rotor with transfer-molded rotor casting.

The EN 10 2012 212 772 A1 discloses a rotor for an electrical machine, in particular for an electric motor, which is arranged concentrically around a rotor shaft and rotationally fixed thereto, wherein the rotor has a casing, wherein the casing hermetically surrounds the rotor and an electrical machine with such a rotor as well as a method for producing a rotor protected against corrosion.

Consequently, there is a need for a stator with an insulating material in which no or only minimal thermal stresses occur between the insulating material and the laminated core of the stator even at different temperatures, e.g. when the electric motor is in use and when it is switched off.

The object of the invention is to improve the state of the art.

The object is achieved by a stator having a plurality of hollow cylindrical sheets which have recesses on a respective inner radius and which are stacked horizontally between a first end face of the stator and a second end face of the stator arranged opposite the first end face in such a way that vertically oriented grooves are present which are formed by the recesses. Copper wires are preferably arranged in the grooves. A first ring is preferably arranged on a first hollow cylindrical sheet on the first end face and a second ring is arranged on a second hollow cylindrical sheet on the second end face. An insulating material is preferably present in the grooves and encases the first ring and the second ring, wherein a first outer contour is impressed on the first ring and a second outer contour is impressed on the second ring in such a way that a thermal stress, in particular a notch stress, between the insulating material and the first hollow cylindrical sheet and/or the insulating material and the second hollow cylindrical sheet is reduced.

This provides a stator which, even at different temperatures, has little or no thermal stress between the insulation material and the laminated core of the stator.

The reduced thermal stress is achieved in particular by the first outer contour impressed on the first ring or by the second outer contour impressed on the second ring.

The outer contour is designed in particular to reduce a notch stress between the insulation material and the first wooden cylindrical sheet or a notch stress between the insulation material and the second wooden cylindrical sheet.

An essential idea of the invention is based on the fact that when the insulation material is introduced, a geometry of the insulation material is created which, due to its shape, leads to a reduction of thermal stresses at material transitions.

• Unter einem „Stator“ wird insbesondere ein Element eines Elektromotors verstanden. Ein Elektromotor ist insbesondere ein Energiewandler, welcher elektrische Energie in mechanische Energie wandelt und zumindest einen Stator und einen Rotor aufweist. Ein Elektromotor mit einem erfindungsgemäßen Stator ist vorzugsweise ein Innenläufermotor, bei dem der Rotor insbesondere in einem Statorauge läuft. Der Stator weist vorzugsweise ein Blechpaket aus einer Vielzahl Bleche, eine Wicklung aus Drähten und eine Isolierung der Wicklung gegenüber dem Blechpaket auf. Das Blechpaket besteht vorzugsweise aus einer Vielzahl axial gestapelter, hohlzylindrischer Bleche. Die Wicklung wird vorzugsweise in dafür vorgesehene Nuten des Blechpakets eingelegt. Die Isolierung kann insbesondere ein Kunststoff, insbesondere ein Duroplast oder Thermoplast sein.

The following terminology is explained:

• A “stator” is understood to mean, in particular, an element of an electric motor. An electric motor is, in particular, an energy converter which converts electrical energy into mechanical energy and has at least one stator and one rotor. An electric motor with a stator according to the invention is preferably an internal rotor motor in which the rotor runs in particular in a stator eye. The stator preferably has a laminated core made up of a large number of sheets, a winding made up of wires and insulation of the winding relative to the laminated core. The laminated core preferably consists of a large number of axially stacked, hollow cylindrical sheets. The winding is preferably inserted into grooves provided for this purpose in the laminated core. The insulation can in particular be a plastic, in particular a duroplastic or thermoplastic.

A “hollow cylindrical sheet” is understood to mean, in particular, a punched sheet. Alternatively, a hollow cylindrical sheet can be produced by means of laser welding. A hollow cylindrical sheet preferably has an outer contour and/or an inner radius/inner contour. The inner radius is preferably rotationally symmetrical. The hollow cylindrical sheet can have receptacles for connecting elements, such as through holes and/or threaded holes, in particular on an end face. Additionally or alternatively, the hollow cylindrical sheet can have centering elements, such as elevations, mandrels and/or holes. The hollow cylindrical sheet can, for example, have a soft magnetic material/ferromagnetic material. The soft magnetic material can preferably be metallic. The soft magnetic material can, in particular, have iron, steel with a low carbon content, steel with added silicon (FeSi), a nickel-iron alloy (FeNi), a cobalt-iron alloy (FeCo), other alloys (e.g. FeAl, FeAlSi) and/or ferrites. The thickness of a hollow cylindrical sheet is preferably 0.05 mm to 0.6 mm, particularly preferably 0.1 mm to 0.35 mm. The hollow cylindrical sheet can be covered by an electrical insulating material. According to the invention, the inner radius of a hollow cylindrical sheet preferably has recesses. The recesses can be designed as a tooth contour, for example. In the case of a tooth contour, the material can be designed to be recessed, in particular at a large number of positions on the inner radius, over a predefined length along the circumference of the inner radius. Additionally or alternatively, the recesses can be designed in a star shape, in particular to a central axis of the hollow cylindrical sheet. The recesses can in particular be oriented radially to a central axis of the stator. The recesses of a hollow cylindrical sheet preferably correspond to the recesses of the large number of hollow cylindrical sheets of the stator.

The stacked hollow cylindrical sheets are preferably joined to form a mechanically stable composite. The mechanically stable composite of the stacked hollow cylindrical sheets can be produced by means of a packaging process. The stacked hollow cylindrical sheets can in particular be glued to one another and/or connected by means of baking varnish and/or punched into packages and/or connected by means of laser welding. The sheet package preferably has 50 to 500 hollow cylindrical sheets, particularly preferably 100 to 300 hollow cylindrical sheets.

A "vertically oriented groove" is understood to mean in particular a vertical depression created by the corresponding stacking of the large number of hollow cylindrical sheets, which have recesses on the inner radius. The design of the recesses preferably leads to a large number of radially distributed, in particular vertically oriented grooves. In other words, the grooves are created by the recesses in the hollow cylindrical sheets lying next to one another. In particular, copper wires are arranged in the grooves as the winding of the stator. The copper wires can be insulated.

A "front side" of the stator is understood to mean in particular an upper or lower end of the stator in the axial direction. In other words, a front side lines an opening in the laminated core. The front side is formed in particular by a first or last hollow cylindrical sheet of the plurality of stacked hollow cylindrical sheets. With In other words, according to the invention, the plurality of hollow cylindrical laminations are axially stacked as a laminated core between a first end face of the stator and a second end face of the stator. The last hollow cylindrical lamination can also be a second hollow cylindrical lamination.

A “first ring” is understood to mean in particular a sheet that corresponds to the shape of the hollow cylindrical sheets, in particular the shape of the inner radius of the hollow cylindrical sheets. The first ring can preferably have an inner diameter of 30 mm to 300 mm, in particular 80 mm to 200 mm. The first ring preferably has an outer contour that is preferably rotationally symmetrical. The outer contour of the first ring can particularly preferably be designed parallel to the inner contour of the first ring. The first ring can preferably have an outer diameter of 50 mm to 400 mm, in particular 100 mm to 250 mm. Furthermore, the first ring can preferably have a thickness of 0.3 mm to 3 mm, in particular 0.8 mm to 2 mm. The first ring can be a stamped part. The material of the first ring can be metallic, for example. The first ring is preferably arranged on a first end face of the sheet stack after the hollow cylindrical sheets have been stacked to form a sheet stack and is aligned to correspond to the orientation of the hollow cylindrical sheets. For this purpose, the first ring can have centering elements, such as elevations or holes. The first ring can preferably have three centering elements. The material of the first ring preferably has a similar thermal expansion coefficient to the insulation material. The same considerations apply to the second ring as to the first ring, which is why they will not be repeated here.

The first or second ring is preferably stamped with an “outer contour”, which is understood to mean a rotationally symmetrical profiling of the ring. The rotationally symmetrical profiling creates a predefined height profile in the cross section of the ring. Due to the height profile, the ring preferably does not lie flat on an interface surface. The interface surface is in particular a segment of the hollow cylindrical sheet on one of the front sides of the stator. The outer contour can be stamped, for example. Additionally or alternatively, the outer contour can be lasered. Finally, the outer contour can be etched. The contour is preferably oriented to the geometry of the ring. The outer contour is preferably designed in such a way that a cavity is created on segments of the hollow cylindrical sheet that are oriented at right angles to one another on one of the front sides of the stator. On the hollow cylindrical sheets, the tooth flank segments in particular are arranged at right angles to the tooth surface segments. The outer contour of the second ring can correspond to the outer contour of the first ring and/or be mirror-symmetrical to it.

An “insulating material” is understood to mean in particular a material which is applied to a surface of a component for electrical insulation and/or is introduced between two elements/segments/components for electrical insulation. The insulating material therefore preferably leads to an interruption of electrical conductivity between a first element and a second element. The insulating material can be a plastic, for example. The insulating material can preferably be a thermosetting plastic. The insulating material can particularly preferably comprise PPS-GF40. Furthermore, the insulating material can preferably be applied in a liquid state and, after introduction, can be thermally cured, for example. The insulating material is preferably introduced into the grooves of the laminated core. In addition, the insulating material preferably encloses the first ring and/or the second ring on the end faces of the stator. The insulating material preferably has a similar thermal expansion coefficient to the material of the hollow cylindrical sheets. The insulating material also preferably has a high temperature resistance and/or a high chemical resistance.

A "notch stress" is understood to mean in particular a concentration of stresses at a notch or cross-sectional transition. The stresses can be induced in particular by mechanical loads, e.g. tension, pressure, bending and/or torsion and/or a different thermal expansion of interconnected elements, in particular the hollow cylindrical sheets and the insulation material.

A “reduced thermoelectric voltage” is understood to mean in particular a thermoelectric voltage which is so low that no temperature-related material failure or temperature-related decoupling occurs.

A "relief cut" is understood to mean a material removal with a specific shape and fixed dimensions. A relief cut is defined in particular according to DIN 509. The relief cut according to the invention is preferably arranged at the transition from the axial flat surface of the ring to the radial cylinder surface on the outer radius of the ring. The relief cut according to the invention preferably has a width of 0.05 mm to 1.5 mm, particularly preferably 0.3 mm to 1.0 mm.

According to a second aspect, the object is achieved by an electric motor comprising a stator according to claims 1-6 and a rotor as well as a power storage device, wherein the power storage device is in particular a high-voltage storage device.

The term “power storage device” is understood to mean, in particular, a device which is designed to store electrical power. The power storage device according to the invention is preferably a high-voltage storage device.

In order to avoid repetition, reference is made here to the comments on the first aspect, which apply analogously.

According to a third aspect, the object is achieved by a method for producing a stator according to the first aspect. In a first step, a plurality of hollow cylindrical sheets, which have recesses on a respective inner radius, are stacked horizontally between a first end face of the stator and a second end face of the stator arranged opposite the first end face, so that a plurality of vertical grooves are created which are formed by the recesses. In a second step of the method according to the invention, a first ring is arranged on a first hollow cylindrical sheet on the first end face and, in a third step, a second ring is arranged on a second hollow cylindrical sheet on the second end face, wherein a first outer contour is embossed on the first ring and a second outer contour is embossed on the second ring. In a fourth step, a hardening insulating material is introduced into the grooves and the first ring as well as the second ring, so that through the interaction of the outer contour of the first ring and the outer contour of the second ring, an insulated stator with low thermal stress, in particular notch stress, is present between the insulating material and the plurality of hollow cylindrical sheets.

In order to avoid repetition, reference is made here to the comments on the first aspect, which apply analogously.
The invention is explained below using exemplary embodiments.

• 1 eine schematische Darstellung eines erfindungsgemäßen Blechpakets und
• 2 eine schematische Darstellung eines erfindungsgemäßen ersten Ringes,
• 3 einen Ausschnitt einer schematischen Darstellung einer Scheibe des, unter Verwendung eines erfindungsgemäßen Ringes hergestellten, Isolationsmaterials,
• 4 einen Ausschnitt einer schematischen Darstellung eines Schnitts durch das, unter Verwendung eines erfindungsgemäßen Ringes hergestellten, Isolationsmaterial sowie durch den Ring und das Blechpaket und
• 5 Verfahrensschritte eines erfindungsgemäßen Verfahrens zur Herstellung eines erfindungsgemäßen Stators.

Show it

• 1 a schematic representation of a laminated core according to the invention and
• 2 a schematic representation of a first ring according to the invention,
• 3 a section of a schematic representation of a disc of the insulating material produced using a ring according to the invention,
• 4 a detail of a schematic representation of a section through the insulation material produced using a ring according to the invention as well as through the ring and the laminated core and
• 5 Method steps of a method according to the invention for producing a stator according to the invention.

An electric motor (not shown) has a rotor and a stator. The stator comprises a laminated core 101. The laminated core 101 is made from a large number of axially stacked sheets which are glued together. A large number of grooves 103 are arranged on the inner circumference of the laminated core 101 and are designed to receive a copper wire. On the first end face 105 of the laminated core 101, three 2 mm wide and 1 mm deep holes are arranged as a first centering element 107 as well as a second centering element 109 and a third centering element 111, which correspond to centering elements (137, 139, 141) of a ring 131. The laminated core has four through holes (113, 115, 117, 119) for fastening.
The ring 131 has a tooth contour 133 on the inner radius that corresponds to the grooves 103 of the laminated core 101. The thickness of the ring 131 is 1 mm. Furthermore, a fourth centering element 137 and a fifth centering element 139 and a sixth centering element 141 are arranged on the second end face 135 of the ring 131, wherein the centering elements (137, 139, 141) of the ring 131 correspond to the centering elements (107, 109, 111) of the laminated core 101. The ring 131 also has a recess 155 with a depth of 1 mm and a width of 2 mm running all around and corresponding to the tooth contour 133.
A groove 157 is present on the outer circumference of the insulating material 151, running all the way around and following the tooth contour 133. The groove 157 is created during the overmolding of the ring 131 according to the invention and the grooves 103 by the undercut 155 arranged on the ring 131. The groove 157 is thus present in the insulating material 151 at the transition between the insulating material 151 and the laminated core 101, running all the way around and following the tooth contour 133.
In a first step 100 of a method for producing a stator, a plurality of hollow cylindrical sheets, which have recesses 133 on a respective inner radius, are stacked. The laminated core 101 stacked in this way has a plurality of vertical grooves 103 on the inner surface, which are formed by the recesses 133. In a second step 200, a first ring 131 is arranged on a first hollow cylindrical sheet of the laminated core 101 and in a third step 300, a second ring is arranged on a second hollow cylindrical sheet of the laminated core 101, wherein the first ring 131 is stamped with a first outer contour 155 and the second ring is stamped with a second outer contour, and in a fourth step 400, a hardening insulating material 151 is introduced into the grooves 103 and enveloping the first ring 131 and the second ring, so that the interaction of the outer contour 155 of the first ring 131 and the outer contour of the second ring results in an insulated stator with low thermal stress, in particular notch stress, between the insulating material 151 and the laminated core 101.

List of reference symbols

101

Sheet metal package

103

Grooves

105

first frontal surface

107

first centering element

109

second centering element

111

third centering element

113

first through hole

115

second through hole

117

third through hole

119

fourth through hole

131

ring

133

Tooth contour

135

second front surface

137

fourth centering element

139

fifth centering element

141

sixth centering element

151

Insulation material

155

Freeway

157

groove

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• WO 002022207763 A1 [0009]
• DE 102021117768 B3 [0010]
• WO 002022054381 A1 [0011]
• US 20040046477 A1 [0012]
• DE 102020111542 A1 [0013]
• DE 102012212772 A1 [0014]

Cited non-patent literature

• www.muehlbeyer.de/media/ injection_molding_tools_for_insulating_stators_and_rotors.pdf [0007]

Claims (9)

Stator, comprising a plurality of hollow cylindrical sheets (101) which have recesses (133) on a respective inner radius and which are stacked horizontally between a first end face (105) of the stator and a second end face of the stator arranged opposite the first end face, such that vertically oriented grooves (103) are present which are formed by the recesses, in which copper wires are arranged, wherein a first ring (131) is arranged on a first hollow cylindrical sheet on the first end face and wherein a second ring is arranged on a second hollow cylindrical sheet on the second end face, and an insulating material (151) which is present in the grooves (103) and encases the first ring (131) and the second ring, characterized in that a first outer contour (155) is impressed on the first ring (131) and a second outer contour is impressed on the second ring in such a way that a thermal voltage, in particular a notch voltage, is formed between the insulating material (151) and the first hollow cylindrical sheet and/or the insulation material (151) and the second hollow cylindrical sheet. Stator according to one of the preceding claims, wherein the outer contour of the first ring (131) and/or the outer contour of the second ring has a relief groove (155). Stator after Claim 2 , wherein the undercut (155) is arranged parallel to the recesses on the first hollow cylindrical sheet and/or parallel to the recesses on the second hollow cylindrical sheet. Stator according to one of the preceding claims, wherein the insulating material (151) is a thermosetting plastic. Stator according to one of the preceding claims, wherein the insulating material (151) is inserted or applied in particular by means of injection molding. Stator according to one of the preceding claims, wherein the first ring (131) and/or the second ring has or have a centering element (137, 139, 141). Stator according to one of the preceding claims, wherein the first ring (131) and/or the second ring has an outer diameter of 50 mm to 400 mm, in particular 100 mm to 250 mm and a thickness of 0.3 mm to 3 mm, in particular 0.8 mm to 1.2 mm. Electric motor comprising a stator according to Claim 1 - 6 and a rotor and a power storage device, wherein the power storage device is in particular a high-voltage storage device. Method for producing a stator according to one of the preceding claims, comprising the steps: • horizontally stacking (100) a plurality of hollow cylindrical sheets, which have recesses (133) on a respective inner radius, between a first end face (105) of the stator and a second end face of the stator arranged opposite the first end face, so that a plurality of vertical grooves (103) are created, which are formed by the recesses, • arranging (200) a first ring (131) on a first hollow cylindrical sheet on the first end face (105), • arranging (300) a second ring on a second hollow cylindrical sheet on the second end face, wherein the first ring (131) is stamped with a first outer contour (155) and the second ring is stamped with a second outer contour and • introducing (400) a hardening insulation material (151) into the grooves (103) and enclosing the first ring (131) and the second ring, so that through the interaction of the outer contour (155) of the first ring (131) and the outer contour of the second ring, an insulated stator with low thermal stress, in particular notch stress, is present between the insulation material (151) and the plurality of hollow cylindrical sheets (101).

Images