Patent Publication Number: US-2021180679-A1

Title: Case

Description:
This is a U.S. national phase application of PCT/JP2019/020211, filed on May 22, 2019, which claims priority to Japanese Patent Application No. 2018-110618, filed on Jun. 8, 2018. The entire disclosure of Japanese Patent Application No. 2018-110618 is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a case. 
     BACKGROUND ART 
     With an automatic transmission for a vehicle, using a magnesium alloy for the material of the components to reduce weight is known. In Japanese Laid-Open Patent Publication No. 2010-90405, it is disclosed that a transmission case (one of the cases) can be considered as one application of a magnesium alloy. 
     SUMMARY 
     When the inventors did trial production of a case made of a magnesium alloy, they discovered that the inner wall surface of the case turned black with a mottled appearance. The mottled black state had a poor appearance, and there was concern that it would give a disagreeable feeling to the delivery recipient. 
     In light of that, the purpose of the present invention is to provide a case that does not give a disagreeable feeling to the delivery recipient. 
     One aspect of the present invention is a case made of a magnesium alloy in which oil is housed in the interior, wherein the inner wall surface of the case is a coated region on which a black film is formed. 
     According to the present invention, it is possible to provide a case for which the appearance of the case is improved, and that does not give a disagreeable feeling to the delivery recipient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing for explaining an automatic transmission case. 
         FIG. 2  is a drawing for explaining the automatic transmission case. 
         FIG. 3  is a drawing for explaining a transmission case. 
         FIG. 4  is a drawing for explaining a coating of the transmission case. 
         FIG. 5  is a drawing for explaining the transmission case. 
         FIG. 6  is a drawing for explaining the transmission case of a modification example. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Following, a mode for carrying out the invention is explained using an automatic transmission case  4  for a vehicle, that includes a transmission case  1  (an example of a case including a main body), as an example. 
       FIG. 1  is a schematic diagram for explaining the automatic transmission case  4 . 
       FIG. 2  is a schematic diagram for explaining the automatic transmission case  4 . FIG. (a) is an exploded perspective view of the automatic transmission case  4 . FIG. (b) is a drawing showing a cross section of an oil pan  5  cut at surface A in FIG. (a). 
       FIG. 3  is a schematic diagram for explaining the transmission case  1 . FIG. (a) is a drawing of the transmission case  1  in  FIG. 2  seen from a converter housing  2  side. FIG. (b) is an A-A cross section view of the transmission case  1  in FIG. (a). FIG. (c) is a drawing of the transmission case  1  in  FIG. 2  seen from the oil pan  5  side. 
     For convenience of the explanation, for the connection of the transmission case  1  and the converter housing  2 , a case is explained of screwing in a bolt B 1  facing the converter housing  2  from the transmission case  1  side. 
     As shown in  FIGS. 1 and 2 , the automatic transmission case  4  is configured from the transmission case  1  that houses a transmission (not illustrated) and a control valve unit (not illustrated), converter housing  2  that houses a torque converter (not illustrated), and a reduction case  3  that houses an output shaft (not illustrated). 
     Also, using the installation state of the automatic transmission case  4  in  FIG. 1  as a reference, the oil pan  5  that stores a lubricating oil OL is fixed to the bottom part of the transmission case  1  in a vertical line VL direction (in the drawing, the vertical direction). 
     The transmission (transmission mechanism) housed inside the transmission case  1  has a plurality of rotating bodies, and a plurality of friction engagement elements (clutch, brake). 
     These plurality of rotating bodies and plurality of friction engagement elements are provided along a rotation axis X, and with the transmission, by changing the engage/release combination of the plurality of friction engagement elements, the transmission route of the rotation drive force is switched, realizing the desired gear ratio. 
     On one end of the transmission case  1  in the rotation axis X direction (left end in  FIG. 1 ), the converter housing  2  is joined from the rotation axis X direction, and is fixed by a bolt. On the other end of the transmission case  1  in the rotation axis X direction (right end in  FIG. 1 ), the reduction case  3  is joined from the rotation axis X direction, and is fixed by a bolt. 
     The oil pan  5  blocks a bottom opening of the transmission case  1 , and the lubricating oil OL used for friction engagement element operation and rotating body lubrication is made to return to inside the oil pan  5  through the bottom opening of the transmission case  1 . 
     Transmission Case  1   
     As shown in  FIGS. 1 and 2 , the transmission case  1  has a substantially cylindrical base  10 . With the transmission case  1 , the center line of the base  10  and the rotation axis X of the transmission are matched. A one end surface  10   a  (end surface  10   a  at the left side in  FIG. 1 ) of the base  10  in the rotation axis X direction is a flat surface orthogonal to the rotation axis X. 
     As shown in  FIGS. 3 ( a ) and ( b ) , a flange part  11  is formed on the one end surface  10   a  side of the base  10 . The flange part  11  extends from a cylindrical outer wall surface  102  of the base  10  in the direction separating from the base  10  in the radial direction of the rotation axis X. The flange part  11  is provided across the entire circumference of the base  10  in the circumferential direction around the rotation axis X. 
     A through hole  11   c  penetrates the flange part  11  in the rotation axis X direction. The through hole  11   c  penetrates between one side surface  11   a  (side surface  11   a  in the left side of  FIG. 3 ( b ) ) of the flange part  11  in the rotation axis X direction and the other side surface  11   b  (side surface  11   b  in the right side of  FIG. 3 ( b ) ). A plurality of the through holes  11   c  are provided in the circumferential direction around the rotation axis X (see  FIG. 3 ( a ) ). A center line Lx of the through hole  11   c  is parallel to the rotation axis X. 
     Here, when there are two components (or members), a mating surface is formed by adhering a first surface having a first component to a second surface having a second component. In this case, the first surface is the mating surface region in the first component, and the second surface is the mating surface region in the second component. 
     The one side surface  11   a  of the flange part  11  is provided to form the same plane as the one end surface  10   a  of the base  10 . The one side surface  11   a  of the flange part  11  and the one end surface  10   a  of the base  10  become a mating surface region S 1  with the converter housing  2  described later. 
     A bearing surface  11   d  in a prescribed range around the through hole  11   c  is formed on the other side surface  11   b  of the flange part  11 . 
     As shown in  FIGS. 3 ( a ) and ( b ) , the other end surface  10   b  (end surface  10   b  of the right side in  FIG. 1 ) of the base  10  in the rotation axis X direction is a flat surface orthogonal to the rotation axis X. A bolt hole  10   c  is formed on the other end surface  10   b  of the base  10 . The bolt hole  10   c  is provided facing along the rotation axis X, and a plurality are provided with a gap open in the circumferential direction around the rotation axis X (see  FIG. 2 ). The other end surface  10   b  becomes the mating surface region with the reduction case  3  described later (with the explanation hereafter, the other end surface  10   b  is also noted as the mating surface region  10   b ). 
     With the installation state of the automatic transmission case  4  in  FIG. 1  as a reference, a wall part  103  is provided on the bottom part of the base  10  in the vertical line VL direction. 
     As shown in  FIGS. 3 ( a ) and ( c ) , the wall  103  extends in the direction separating from the base  10  along the vertical line VL direction, and also surrounds the bottom opening of the base  10  around the entire circumference (see  FIG. 3 ( c ) ). 
     Also, an opening  105  that connects the interior and exterior of the base  10  is formed in the region surrounded by the wall  103  in the base  10 . 
     Viewing from below the base  10 , the opening  105  is formed inside the region surrounded by the wall  103 . 
     A bottom end surface  103   a  of the wall  103  is a flat surface orthogonal to the vertical line VL. A plurality of bolt holes  103   c  are formed on the bottom end surface  103   a . The bolt holes  103   c  are provided facing along the vertical line VL. The bottom end surface  103   a  becomes the mating surface region with the oil pan  5  described later (in the explanation hereafter, the bottom end surface  103   a  is also noted as the mating surface region  103   a ). 
     Details will be noted later, but with the transmission case  1 , except for prescribed regions such as mating surface regions S 1 ,  10   b ,  103   a , etc., a coating C of magnesium hydroxide (Mg(OH) 2 ) is formed on the entire surface of the base  10  (inner wall surface  101 , outer wall surface  102 ). 
     Converter Housing  2   
     As shown in  FIGS. 1 and 2 , the converter housing  2  has a substantially cylindrical base  20 . The converter housing  2  is placed in a state with the center line of the base  20  matched to the rotation axis X. One end surface  20   a  (end surface  20   a  of the left side in  FIG. 1 ) of the base  20  and another end surface  20   b  (end surface  20   b  of the right side in  FIG. 1 ) in the rotation axis X direction are a flat surface orthogonal to the rotation axis X. 
     The one end surface  20   a  of the base  20  is connected to an engine side fixing member E with the engine (not illustrated) housed inside (end surface  20   a  of the left side of the base  20  in  FIG. 1 ). In this case, the end surface  20   a  is the mating surface region that is adhered to the engine side fixing member E. 
     The other end surface  20   b  of the base  20  is connected to the transmission case  1  (end surface  20   b  of the right side of the base  20  in  FIG. 1 ). In this case, the other end surface  20   b  becomes the mating surface region that is adhered to the mating surface region S 1  of the transmission case  1  noted above. 
     A bolt hole  20   c  is formed in the rotation axis X direction on the other end surface  20   b . The bolt hole  20   c  is provided at a position corresponding one-to-one with the abovementioned through hole  11   c  formed on the flange part  11  in the circumferential direction around the rotation axis X. 
     The transmission case  1  and the converter housing  2  are fixed with a bolt B 1  that passes through the through hole  11   c  and the bolt hole  20   c  interposed (see  FIG. 5 ). By the fastening force of the bolt B 1 , the transmission case  1  is adhered with the converter housing  2  at the mating surface region S 1  along the entire circumference around the rotation axis X. 
     Reduction Case  3   
     As shown in  FIGS. 1 and 2 , the reduction case  3  has a substantially cylindrical base  30 . The reduction case  3  is placed in a state with the center line of the base  30  matched to the rotation axis X. One end surface  30   a  (end surface  30   a  of the left side in  FIG. 1 ) of the base  30  in the rotation axis X direction is a flat surface orthogonal to the rotation axis X. 
     At the one end surface  30   a  side of the base  30 , a flange part  31  is formed. The flange part  31  extends from a cylindrical outer wall surface  302  of the base  30  in the direction separating from the base  30  in the radial direction of the rotation axis X. The flange part  31  is provided across the entire circumference of the base  30  in the circumferential direction around the rotation axis X. 
     A through hole  31   c  penetrates the flange part  31  in the rotation axis X direction. The through hole  31   c  penetrates between one side surface  31   a  and the other side surface  31   b  of the flange part  31  in the rotation axis X direction. The through hole  31   c  is provided at a position corresponding one-to-one to the bolt hole  10   c  formed on the other end surface  10   b  of the abovementioned base  10 . 
     The one side surface  31   a  of the flange part  31  is provided to form the same plane as the one end surface  30   a  of the base  30 . The one end surface  30   a  of the base  30  and the other side surface  31   a  of the flange part  31  become a mating surface region that is adhered to the mating surface region  10   b  of the transmission case  1 . 
     The transmission case  1  and the reduction case  3  are fixed with interposition of a bolt B 2  that passes through the bolt hole  10   c  and the through hole  31   c . By the fastening force of the bolt B 2 , the transmission case  1  is adhered to the reduction case  3  along the entire circumference around the rotation axis X at the mating surface region  10   b.    
     Oil Pan  5   
     As shown in  FIGS. 1 and 2 , the oil pan  5  has a substantially rectangular bottom wall  50  in the plan view, and a peripheral wall  51  that surrounds the peripheral edge of the bottom wall  50  along the entire circumference. On an end surface  51   a  of the side opposite to the bottom wall  50  of the peripheral wall  51  in the vertical line VL direction, a flange part  52  is formed. The flange part  52  is formed across the entire circumference along the outer peripheral edge of the peripheral wall  51 . A through hole  52   c  penetrates the flange part  52  in the vertical line VL direction. The through hole  52   c  penetrates between one side surface  52   a  and another side surface  52   b  of the flange part  52  in the vertical line VL. The end surface  51   a  of the peripheral wall  51  is a flat surface orthogonal to the vertical line VL. The one side surface  52   a  of the flange part  52  is provided to form the same plane as the end surface  51   a  of the peripheral wall  51 . 
     The through hole  52   c  is provided at a position corresponding one-to-one with the bolt hole  103   c  formed on the wall  103  of the abovementioned base  10 . 
     The one side surface  52   a  of the flange part  52  forms the mating surface region that adheres to the mating surface region  103   a  of the transmission case  1 . 
     The transmission case  1  and the oil pan  5  are fixed with interposition of a bolt B 3  that passes through the bolt hole  103   c  and the through hole  52   c . By the fastening force of the bolt B 3 , the transmission case  1  is adhered to the oil pan  5  along the entire circumference around the vertical line VL at the mating surface region  103   a.    
     Here, when the constitutional elements of the automatic transmission case  4  (transmission case  1 , converter housing  2 , reduction case  3 ) and the oil pan  5  are produced using an alloy containing magnesium, the inner peripheral surface of each of these constitutional elements and the inner peripheral surface of the oil pan  5  are blackened with a mottled appearance. 
     The inventors considered as follows the cause of blackening with a mottled appearance. With the explanation hereafter, the transmission case  1  is explained as an example. 
     Magnesium alloys are highly reactive. A chemical reaction is caused when a magnesium alloy contacts water (H 2 O). On the surface of the magnesium alloy in the part that contacted water, a black coating of magnesium hydroxide (Mg(OH) 2 ) is formed. 
     On the inside of the transmission case  1 , the lubricating oil OL used for lubrication of the rotating body or operation of the friction engaging elements moves along the inner circumference of the transmission case  1  to the oil pan  5  side. 
     Here, the lubricating oil OL contains moisture, so when the lubricating oil OL moves along the inner circumference of the case, the moisture within the lubricating oil OL reacts with the magnesium alloy that configures the transmission case  1 , and a coating of magnesium hydroxide is formed on the inner wall surface  101  of the transmission case  1 . 
     Also, the lubricating oil OL is not in even contact with the entire region of the inner wall surface  101 . For example, the lubricating oil scraped up by the rotating body is in contact with substantially the same region of the inner wall surface  101 . Also, the lubricating oil OL that contacted the inner wall surface  101  moves to the oil pan  5  side through substantially the same path of the inner wall surface  101 . 
     For that reason, with the inner wall surface  101 , as a result of the occurrence of areas that frequently contact the lubricating oil OL and areas that do not frequently contact it, the coating on the inner wall surface  101  is generated with a mottled appearance. 
     Having done that, with areas at which the coating is generated to be thick and looks dark and areas where it is not generated to be thick, the shading differs, so when looking at the inner wall surface  101 , the presence and absence of the coating gives a mottled pattern, resulting in a poor appearance. 
     In light of that, the inventors decided to prevent having a poor appearance by providing the coating C of magnesium hydroxide in advance on the entire surface of the transmission case  1 . 
     Coating C 
     The coating C of the transmission case  1  is explained. 
       FIG. 4  is a drawing for explaining the coating C of the transmission case  1 . FIG. (a) is a drawing for explaining formation of the coating C, and shows a transmission case  1 ′ as a raw material. FIG. (b) is a drawing showing a cross section with the transmission case  1 ′ cut at surface B in FIG. (a). FIG. (c) is a drawing for explaining the mating surface region S 1  and the bearing surface  11   d . FIG. (d) is a drawing for explaining the transmission case  1  as a product. The thickness of the coating C is shown exaggerated. 
     The transmission case  1  of the embodiment is produced by implementing a prescribed machining after forming the coating C of magnesium hydroxide on the entire surface (including the inner wall surface  101  and the outer wall surface  102 ) of the transmission case  1 ′ as the raw material. The transmission case  1 ′ as the raw material is molded by casting. 
     As shown in  FIG. 4 ( a ) , formation of the coating C is performed by immersing the entire transmission case  1 ′ in a container filled with water, for example. 
     When the transmission case  1 ′ is immersed, the entire surface of the transmission case  1 ′ is in contact with water. Then, the magnesium alloy and water chemically react. By doing this, the coating C of magnesium hydroxide is formed on the entire surface of the transmission case  1 ′ (see  FIG. 4 ( b ) ). 
     Because magnesium hydroxide is black, the transmission case  1 ′ has the entire surface change color to black. 
     The entire transmission case  1 ′ does not have to be immersed. For example, it is also possible to form the coating C on the desired area of the transmission case  1 ′ by applying water using a hose, etc. As noted previously, by water that has mixed in the lubricating oil OL, of the surface of the transmission case  1 , the inner wall surface  101  side is the part that has a mottled appearance. Therefore, it is possible to apply water to the inner wall surface  101  and form the coating C only on the inner wall surface  101  to be black. 
     In this case, it is not absolutely necessary for the coating C to be formed on the entire surface (100%) of the inner wall surface  101 , and a slight peeling or scraping, etc., is allowed. In other words, the entire surface of the inner wall surface  101  is sufficient provided the area for which the entire inner wall surface  101  can be recognized as being black when seen by a human (90% or greater, preferably 95% or greater, more preferably 98% or more of the surface area) is black. 
     Also, before forming the coating C, it is preferable to perform the process of removing impurities generated on the surface of the transmission case  1 ′ using a shot blasting process, etc. 
     For example, at the casting stage, there are cases of impurities being generated on the surface of the transmission case  1 ′. Contact of the surface of the transmission case  1 ′ and water (chemical reaction) is blocked by impurities. This is because having done that, formation of the coating C is partially blocked, so partial changing of color to black does not occur. 
     Here, the coating C is not limited to being only a coating of magnesium hydroxide. For example, it is also possible to apply a black colored paint to the surface of the transmission case  1 ′. 
     In this case, it is possible to apply paint instead of the coating C of magnesium hydroxide, and also possible to further coat paint on top of the coating C of magnesium hydroxide. 
     It is preferable that the coating C be an item formed by chemical reaction such as magnesium hydroxide, or a paint that is coated as appropriate, etc., but the material is not limited as long as it can be made black. It can be said that an item formed by chemical reaction is preferable compared to a paint which is susceptible to heat (organic matter). 
     As shown in  FIG. 4 ( c ) , after the coating C is formed on the transmission case  1 ′, the mating surface region S 1  and the bearing surface  11   d  are formed. In specific terms, by partially removing the coating C by machining, the mating surface region S 1  and the bearing surface  11   d  are formed (in the drawing, see the dotted line). 
     Though omitted in the drawing, the mating surface region  10   b  (see  FIG. 3 ( b ) ) and the mating surface region  103   a  (see  FIG. 3 ( c ) ) are handled in the same manner as the mating surface region S 1 , and are formed by removing the coating C using machining. 
     Here, when the transmission case  1 ′ is immersed to form the coating C, though overall it becomes black, there is some variation in film thickness of the coating C. The same is true when applying water using a hose. 
     In the transmission case  1 , the coating C is formed on the joining surface with the other constitutional elements of the automatic transmission case  4  (converter housing  2 , reduction case  3 , oil pan  5 ), and when there is variation in the thickness of this coating C, the assembly precision with the other constitutional elements is poor. 
     In light of that, from the perspective of ensuring assembly precision, after the coating of magnesium hydroxide is formed, it is preferable to implement machining on the joining surface with the other constitutional elements. At the regions in which machining is implemented, the shape precision (flatness, positioning) is higher than regions in which machining is not implemented, making it possible to ensure assembly precision. 
     By doing this, the surface of the transmission case  1 ′ is divided into a coated region R 1  on which the coating C is formed, and an uncoated region R 2  on which the coating C is removed (see  FIG. 4 ( c ) ). Because the uncoated region R 2  is formed by machining, there is a higher flatness degree than the coated region R 1 . 
     Uncoated Region R 2   
     As shown in  FIG. 4 ( d ) , of the uncoated region R 2 , the through hole  11   c  is formed at a prescribed position in the region corresponding to the mating surface region S 1  and the bearing surface  11   d . The through hole  11   c  is formed by drilling, for example. 
     Also, of the uncoated region R 2 , at the mating surface region  10   b  (see  FIG. 3 ( b ) ), the bolt hole  10   c  is formed at a prescribed position. The bolt hole  10   c  is formed by drilling and tapping, for example. 
     Also, of the uncoated region R 2 , in the mating surface region  103   a  (see  FIG. 3  ( c )), the bolt hole  103   c  is formed at a prescribed position (see  FIGS. 1 and 2 ). The bolt hole  103   c  is formed by drilling and tapping, for example. By doing this, the transmission case  1  is completed as a product (see  FIG. 4 ( d ) ). 
     After the through hole  11   c , and the bolt holes  10   c  and  103   c  are formed, it is possible to also form the mating surface regions S 1 ,  10   b , and  103   a . However, forming the through hole  11   c  and the bolt holes  10   c  and  103   c  after the mating surface regions S 1 ,  10   b , and  103   a  are formed allows higher degree of positioning of the through hole  11   c  and the bolt holes  10   c  and  103   c.    
       FIG. 5  is a drawing for explaining the transmission case  1 , and is an enlarged view of region A in  FIG. 1 . 
     The mating surface region S 1  is a flat surface that is orthogonal to the rotation axis X. Also, the other end surface  20   b  of the converter housing  2  in the rotation axis X direction is also machined in the same manner as the mating surface region S 1 , and is a flat surface orthogonal to the rotation axis X. 
     When the other end surface  20   b  of the converter housing  2  and the mating surface region S 1  of the transmission case  1  are matched, these are in contact with no gap across the entire surface. By doing this, leaking of the lubricating oil OL from between the transmission case  1  and the converter housing  2 , etc., is prevented. 
     Here, the transmission case  1 , the converter housing  2 , the reduction case  3 , and the oil pan  5  expand and contract according to changes in the ambient temperature. 
     This expansion and contraction is easily affected by temperature changes in the lubricating oil OL. In this case, the transmission case  1  and the converter housing  2 , the reduction case  3 , and the oil pan  5  are not evenly affected by temperature changes. For example, during transmission driving, the high temperature lubricating oil OL scraped up by the rotating body is mostly on the transmission case  1 . Therefore, the transmission case  1  is the most greatly deformed. That being the case, there is concern for example that a gap will occur between the transmission case  1  and the converter housing  2 , and the lubricating oil OL will leak from this gap. The same is also true between the transmission case  1  and the reduction case  3  or the oil pan  5 . 
     With this embodiment, the mating surface region S 1  of the transmission case  1  and the other end surface  20   b  of the converter housing  2  are formed with a high degree of flatness. For that reason, compared to cases when the coating C is not removed, there is less susceptibility to a gap being formed even if there is thermal deformation of the transmission case  1  and the converter housing  2 , so it is possible to maintain an adhered state across the entire surface. 
     Thus, leaking of the lubricating oil OL from between the transmission case  1  and the converter housing  2  is more suitably prevented. The same is also true between the reduction case  3  and the oil pan  5 . 
     Also, the bearing surface  11   d  is a flat surface that is orthogonal to the rotation axis X (center line Lx of the through hole  11   c ). The bearing surface  11   d  is formed by removing the coating C in the range of the inner diameter D 2  that is slightly larger in diameter than the outer diameter D 1  of a bolt flange B 11  of the bolt B 1  (D 1 &lt;D 2 ). 
     When the bolt B 1  is tightened, the bolt flange B 11  contacts along the entire surface of the bearing surface  11   d . By doing this, the fastening holding force of the bolt B 1  is received at the entire surface of the bearing surface  11   d  and is evenly distributed and stabilized. 
     Here, the coating C (magnesium hydroxide) is softer than the magnesium alloy, and is plastically deformed more easily (slackens easily). When the bearing surface  11   d  is not formed (when not removing the coating C), as a result of the fastening holding force of the bolt B 1  being absorbed by plastic deformation (slackening) of the coating C, the fastening holding force of the bolt B 1  decreases. 
     By forming the bearing surface  11   d  (removing the coating C), absorption of the fastening holding force of the bolt B 1  by slackening of the coating C is prevented (improved resistance to slackening), and it is possible to prevent a decrease in the fastening holding force of the bolt B 1 . 
     When using only the bolt B 1  without the bolt flange B 11 , it is sufficient to remove the coating C in the range of the inner diameter of a slightly larger diameter than the outer diameter D 3  of a bolt head B 10  of the bolt B 1 . This makes it possible to obtain the same effect as when using the abovementioned bolt flange B 11 . 
     Here, the uncoated region R 2  of the transmission case  1  is the mating surface regions S 1 ,  10   b , and  103   a , and the bearing surface  11   d . At the uncoated region R 2 , this becomes the ground color of the magnesium alloy. Thus, though the uncoated region R 2  is not black, the overall transmission case  1  has a regular color scheme. 
     Therefore, the delivery recipient recognizes the transmission case  1  as having that kind of design (color scheme). The delivery recipient does not judge this to be a transmission case with a mottled appearance as in the past, so does not have a disagreeable feeling. 
     As described above, the transmission case  1  (case) of the embodiment has the following configuration. 
     (1) The transmission case  1  (case) is made of a magnesium alloy. 
     The transmission case  1  has the lubricating oil OL (oil) housed inside. 
     The inner wall surface  101  of the transmission case  1  is used as the coated region R 1  on which the coating C (black film) of magnesium hydroxide is formed. 
     When configured in this way, by having the delivery recipient recognize that this is that kind of design by intentionally forming the black film on the inner wall surface  101 , it is possible to provide the transmission case  1  (case) that does not give a disagreeable feeling to the delivery recipient. 
     (2) The mating surface regions S 1 ,  10   b , and  103   a  in the transmission case  1  are used as the uncoated region R 2  on which the coating C is not formed. 
     For example, to prevent leaking of lubricating oil, etc., the mating surface region S 1  with the converter housing  2  in the transmission case  1  requires a high degree of flatness. Thus, removal of the coating C is necessary. 
     Using the configuration noted above, the coating C (black region) is removed only at the end part of the transmission case  1 . Therefore, regularity is generated in the color scheme of the overall transmission case  1 . Thus, the delivery recipient recognizes it to be such a design (color scheme), and is not given a disagreeable feeling. 
     (3) The bearing surface  11   d  (contact surface with the bearing surface of the fastening member) of the bolt B 1  in the transmission case  1  is used as the uncoated region R 2  on which the coating C is not formed. 
     To increase the fastening holding force of the bolt B 1  and the resistance to slackening, etc., it is preferable that the bearing surface  11   d  be the uncoated region having a high degree of flatness. 
     Using the configuration noted above, the coating C (black portion) is removed in a regular pattern. Therefore, regularity is generated in the color scheme of the overall transmission case  1 . Thus, the delivery recipient recognizes the item as having that kind of design, and is not given a disagreeable feeling. 
     (4) The side surface of the transmission case  1  through hole  11   c  and the bolt holes  10   c  and  103   c  are used as the uncoated region R 2  on which the coating C is not formed. 
     For the assembly of the converter housing  2 , the reduction case  3 , and the oil pan  5  in the transmission case  1 , it is necessary that the parts being assembled are formed with high shape precision. Therefore, it is preferable that the parts for assembly be uncoated regions. 
     When the parts for assembly configured as described above are uncoated regions, the coating C (black portion) is removed in a regular pattern. Thus, the delivery recipient recognizes this as having that kind of design, and is not given a disagreeable feeling. 
     (5) The transmission case  1  is placed in the vehicle. 
     The outer wall surface  102  of the transmission case  1  is used as a coated region R 1  on which the coating C is formed. 
     During traveling of the vehicle, there are cases when water or contaminants, etc., from outside the vehicle infiltrate from outside into the engine room in which the transmission case  1  is housed. When that happens, due to the water, contaminants, etc., that infiltrated from outside, there is a risk that the outer wall surface  102  of the transmission case  1  will change color to black with a mottled appearance over time. 
     By using the configuration noted above, the outer wall surface  102  also becomes black by the coating C, so it is possible to prevent it being seen as having a mottled appearance. 
     Modification Example 
     With the abovementioned embodiment, an example was shown of a case when the inner diameter D 2  of the bearing surface  11   d  was formed to be slightly larger than the outer diameter D 1  of the bolt flange B 11  (D 1 &lt;D 2 ), but the invention is not limited to this mode. 
     For example, it is also possible to have a transmission case  1 A for which the inner diameter D 4  of the bearing surface  11   d  is smaller than the outer diameter D 1  of the bolt flange B 11  (see  FIG. 6 ). 
       FIG. 6  is a drawing for explaining the transmission case  1 A of the modification example, and shows a region correlating to region A in  FIG. 1 . For the modification example, only the parts that differ from the embodiment noted above are explained. The thickness of the coating C is shown exaggerated. 
     As shown in  FIG. 6 , the bearing surface  11   d  of the transmission case  1 A of modification example 1 is formed by removing the coating C in the range of the inner diameter D 4  that is a smaller diameter than the outer diameter D 1  of the bolt flange B 11  of the bolt B 1  (D 1 &gt;D 4 ). 
     The bolt flange B 11  is placed across the coated region R 1  and the uncoated region R 2  in the radial direction of the center line Lx of the through hole  11   c.    
     At the outer circumference side of the bolt flange B 11  in the radial direction of the center line Lx, the coating C is in a state sandwiched by the bolt flange B 11  and the flange part  11  in the center line Lx direction. 
     In  FIG. 6 , the thickness of the coating C is shown exaggerated, but the actual thickness of coating C is extremely thin. Thus, when the bolt B 1  is tightened, the inner circumference side of the bolt flange B 11  is in contact with the uncoated region R 2 , and the outer circumference side of the bolt flange B 11  is in contact with the coated region R 1 . 
     Here, as noted above, the coating C (magnesium hydroxide) is softer than the magnesium alloy. Thus, in the process of tightening the bolt B 1 , the coating C sandwiched by the bolt flange B 11  and the flange part  11  is pressed out facing the center line Lx side (see the arrow in the drawing). 
     The pressed out coating C exhibits an effect as a sealing member. This makes it possible to suppress moisture, etc., from outside the transmission case  1 A from infiltrating. 
     The inner circumference side of the bolt flange B 11  is in contact with the uncoated region R 2 . By doing this, there is no absorption of the fastening holding force of the bolt B 1  due to slackening of the coating C as described above and also no decrease in the fastening holding force of the bolt B 1 . 
     The transmission case  1 A (case) of the modification example has the configuration below. 
     (6) The contact surface with the inner circumference side of the bolt flange B 11  (inner periphery of the bearing surface) is used as the uncoated region R 2 , and the contact surface with the outer circumference side of the bolt flange B 11  (outer periphery of the bearing surface) is used as the coated region R 1 . 
     By configuring in this way, it is possible to suppress infiltration of moisture to the uncoated region R 2  that exists on the inner circumference side of the bolt flange B 11  using the coated region R 1  that exists on the outer circumference side of the bolt flange B 11 . 
     The fastening member is not limited to being a bolt. For example, it can also be the bearing surface of a screw, nut, etc., or of these combined with a washer (when using as standalone, the bearing surface of the standalone component, and when using a washer, the bearing surface of the washer). 
     Above, embodiments of the present invention were explained, but the present invention is not limited to only the modes shown in these embodiments. Changes can be made as appropriate within the scope of the technical ideas of the invention. 
     For example, as long as it is made of a magnesium alloy, the embodiments of the present invention can also be applied to the converter housing  2  and the reduction case  3 . Also, the engine case that houses the engine, a motor case that houses a motor, and a reducer case that houses a decelerator that decelerates the output of the motor, etc., may also be any kind of item as long as it is a case in which the lubricating oil OL is housed. The case may be constituted from only one case, but it is also possible to configure by connecting two or more cases with bolts. 
     Also, in the connection of the transmission case  1  and the converter housing  2 , when screwing the bolt B 1  from the converter housing  2  side, a part correlating to the bearing surface  11   d  may be formed on the converter housing  2  side. 
     Also, for example the lubricating oil OL is lubricating oil or hydraulic oil, etc., of the drive members placed within the case (drive source (engine, motor, etc.) or power transmission member (transmission, decelerator, accelerator, gear alone, engagement element (clutch, etc.) etc.)), but the invention is not limited to this. There are also cases when the hydraulic oil and the lubricating oil are used together.