Transmission unit

Abstract

A transmission housing 112 comprises a housing wall that defines an internal volume of the transmission housing; and a gear shroud 120 within the internal volume, the gear shroud comprising: a first shroud wall 122 sealingly engaged with the housing wall; and a second shroud wall 124 extending from the first wall such that the first shroud wall, the second shroud wall and the housing wall define a gear volume for receiving at least part of a gear 108.

Description

TECHNICAL FIELD The present disclosure relates to a transmission unit. Aspects of the invention relate to a transmission unit, an electric drive unit housing, an electric drive unit, a vehicle and a method of assembling an electric drive unit housing. BACKGROUND It is known to provide transmission units for housing rotating components and volumes of fluid such as coolant. Where the rotating component interacts with the fluid, aeration of the fluid can degrade the performance of the fluid, for example if the fluid is used as a coolant. Further, the efficiency of a rotating component moving through a fluid such as liquid coolant may be less than the efficiency of a rotating component moving through air alone. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a transmission unit, an electric drive unit housing, an electric drive unit, a vehicle, and a method of assembling an electric drive unit housing as claimed in the appended claims. According to an aspect of the invention, a transmission unit comprises a transmission housing having a housing wall that defines an internal volume of the transmission housing; a gear rotatable about a gear axis; and a shroud within the internal volume about at least a portion of the gear and defining a gear volume that is open to said internal volume. Liquid lubricant is in the internal volume having a fill level at least approximately parallel said gear axis when the transmission unit is in a normal operative orientation. The fill level, when the transmission unit is not operating, immerses at least a portion of the gear. During operation of the transmission unit and rotation of the gear, lubricant within the shroud is ejected by the gear from the a gear volume, whereby the fill level of lubricant within the gear volume falls below the gear. In this way, not only is the gear not permanently immersed in lubricant, adversely affecting the efficiency of the transmission by retarding rotation of the gear by virtue of the viscosity of the lubricant, but also the lubricant is not continuously stirred and agitated by the gear to entrain air into the lubricant and reduce the efficacy of the lubricant. The shroud may have an aperture below the fill level of lubricant in the internal volume whereby lubricant overspills into the gear volume. This replenishes lubricant in the gear volume so that lubricant is picked up by the gear as it rotates, but the aperture is sized so that the overspill is insufficient to fill the gear volume to the fill level of the internal volume during operation of the transmission unit. That is, lubricant is ejected faster than it overspills through the aperture whereby the level of lubricant in the gear volume falls to below the gear. Clearly, once the lubricant level is below the gear it stops being ejected to the same extent as at initial startup of the transmission and is replenished via the aperture until the gear picks it up again, whereby its level during normal operation of the transmission is at or around the bottom of the gear. According to another aspect of the present invention there is provided a transmission unit comprising: a transmission housing comprising a housing wall that defines an internal volume of the transmission housing; and a gear shroud within the internal volume, the gear shroud comprising: a first shroud wall sealingly engaged with the housing wall; and a second shroud wall extending from the first wall such that the first shroud wall, the second shroud wall and the housing wall define a gear volume for receiving at least part of a gear. The gear volume defined by the housing wall, the first shroud wall and the second shroud wall is separated from an adjacent portion of the transmission unit by the shroud and a seal engagement between the first wall and the housing wall. This may restrict fluid (e.g. oil) in the internal volume from entering the gear volume and by extension interacting with a gear housed therein. In particular, this may be advantageous where oil may pool adjacent to the gear. The shroud may prevent pooled oil from interacting with the gear. This in turn may reduce drag losses compared to a gear at least partially immersed in oil and may reduce aeration of the oil, in turn reducing oil degradation and / or improving heat transfer. The second shroud wall may extend from the first shroud wall in a substantially perpendicular direction. By providing a shroud having a second shroud wall that extends perpendicularto the first shroud wall, the gear volume may be increased, such that when the shroud is immersed in a larger pool of oil the gear may be isolated from the oil. A portion of the transmission unit internal volume may define an oil collection volume. The first shroud wall may be arranged between the oil collection volume and the gear volume. In this way, the transmission unit may collect oil, such as for transferring to an oil sump or for recirculation. Arranging the first shroud wall between the oil collection volume and the gear volume may restrict collected oil from entering the gear volume and may thereby reduce drag on a gear in the gear volume. The housing wall may comprise a groove. The first shroud wall may be received in the groove. The second shroud wall of the gear shroud may extend from a first side of the first shroud wall. The first shroud wall may comprise a first shroud wall edge along a second side of the first shroud wall, the second side being opposite the first side. The profile of the groove may match the profile of the first shroud wall edge such that the first shroud wall edge is insertable in the groove. A sealing element, optionally comprising a deformable rubber seal, may be fitted to the first shroud wall edge to be received in the groove. The groove may improve the seal between the first shroud wall and the housing wall by resiliently locating the first shroud wall relative to the housing wall. This may also provide mechanical support to the shroud, for instance when the transmission housing is subjected to acceleration due to movement of a vehicle the transmission housing is arranged in. The groove may comprise at least one groove wall aligned parallel to the first shroud wall. The first shroud wall may abut the at least one groove face such that movement of the shroud in a direction perpendicular to the groove wall is prevented. The transmission unit may further comprise a gear at least partially received in the gear volume, the gear being arranged to rotate about an axis. The first shroud wall may be curved such that it is configured to follow the profile of the gear. The first shroud wall may be arranged at a substantially constant distance from the axis and / or from the profile of the gear. Providing a gear shroud with a first wall curved to follow the profile of the gear housed therein may reduce the volume occupied by the gear and shroud together hence improving packaging of the transmission housing. The second shroud wall may extend from the first wall to cover at least a portion of a face of the gear. The gear is arranged to rotate about an axis. The gear may comprise an end face which is perpendicular to the axis. The second shroud wall may cover at least a portion of the end face of the gear. By providing a significant overlap between the second shroud wall and the gear, the gear may be separated from a significant volume of oil collected in the transmission housing, and the prospect of oil spilling over into the gear volume may be reduced. The first shroud wall may further comprise an aperture arranged to permit flow of fluid into the gear volume. Providing an aperture in the shroud may permit for example a fill to spill service / inspection, where the transmission unit is filled with oil to assess its condition. In this case, the shroud may influence the oil level within the transmission housing which could yield a false result of the inspection. Providing an aperture to allow oil into the gear shroud may reduce the likelihood of a false result. The gear shroud may be manufactured from plastics material. By selecting plastics, the shroud may be sufficiently stiff and thin to allow compact packaging and maintain a position resiliently. The shroud may comprise one or more connectors facing away from the gear volume, the one or more connectors being arranged to fix the gear shroud to an adjacent component. The gear shroud may be fixed to an adjacent component using one or more connectors facing away from the gear volume partially defined by the first shroud wall and the second shroud wall. The transmission unit may further comprise a gear housed at least partially in the gear volume. The gear shroud may comprise one or more connectors facing away from the gear, the one or more connectors being arranged to fix the gear shroud to an adjacent component. By providing connectors on the shroud facing away from the gear volume, the shroud may be fixed in place before the gear is assembled within the assembly. This may allow an easier assembly of the transmission housing and gear. According to another aspect of the invention, there is provided an electric drive unit housing comprising: the transmission unit of any preceding claim, and a main housing arranged to receive an electric machine, the main housing being fixed to the transmission unit such that the main housing and the transmission unit define a continuous internal space. According to a further aspect of the invention, there is provided an electric drive unit comprising: the electric drive unit housing of the another aspect of the invention; and an electric motor arranged within the main housing. Where the electric drive unit housing comprises a transmission unit which in turn comprises a gear at least partially received in a gear volume, the gear may be a differential gear arranged to receive a torque from the electric motor. The gear shroud may be fixed to the main housing. Fixing the gear shroud to the main housing may allow the gear shroud to be fixed to the electric drive unit before the transmission unit is assembled. This may improve ease of manufacture of the electric drive unit and ease of maintenance. According to a still further aspect of the invention, there is provided a vehicle comprising the electric drive unit of the further aspect of the invention. According to another further aspect of the invention, there is provided a method of assembling an electric drive unit housing, the method comprising: fastening a gear shroud to a main housing at a first side of the gear shroud, wherein the gear shroud comprises a first shroud wall and a second shroud wall extending from the first shroud wall, such that the gear shroud is open on a second side of the gear shroud, the second side being opposite to the first side. With such an assembly method, there is provided a gear shroud comprising a first wall and a second wall means that, prior to incorporation with the transmission housing, is open on at least one side. The gear may therefore be inserted into the gear shroud through the open side. This may improve ease of manufacture since each component may be fastened to the main housing sequentially. It is further preferable to fasten the gear to the transmission housing and provide the two as a single component to fasten to the main housing. According to another still further aspect of the invention, there is provided a method of assembling an electric drive unit comprising: the method of the another further aspect of the invention; coupling a gear to a transmission housing, the transmission housing comprising a housing wall that defines an internal volume; and fastening the transmission housing to the main housing such that the first shroud wall is sealingly engaged with the housing wall, and the gear is at least partially received in a gear volume defined by the first shroud wall, the second shroud wall and the housing wall. According to another still further aspect of the invention, there is provided a transmission unit comprising a transmission housing having a housing wall that defines an internal volume of the transmission housing, a gear rotatable about a gear axis, and a shroud within the internal volume about at least a portion of the gear and defining a gear volume that is open to said internal volume, wherein liquid lubricant is in the internal volume having a fill level at least approximately parallel said gear axis when the transmission unit is in a normal operative orientation, which fill level, when the transmission unit is not operating, immerses at least a portion of the gear, and wherein, during operation of the transmission unit and rotation of the gear, lubricant within the shroud is ejected by the gear from the a gear volume whereby the fill level of lubricant within the gear volume falls below the gear. Optionally, the shroud has an aperture below the fill level of lubricant in the internal volume whereby lubricant overspills into the gear volume. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle comprising an electric drive unit (EDU); Figure 2 shows a cross sectional view of an electric drive unit (EDU); Figure 3 shows a cross sectional view of a portion of the electric drive unit; Figure 4 shows a first gear shroud for a transmission unit according to the invention; Figure 5 shows a second gear shroud for a transmission unit according to the invention; Figure 6 shows a cross sectional view of a first transmission unit according to the invention; Figure 7 shows a cross sectional view of a second transmission unit according to the invention; Figure 8 shows a cross sectional view of a third transmission unit according to the invention; Figure 9 shows an end view of the third transmission unit; Figure 10 shows a cross sectional view of an EDU housing according to the invention; Figure 11 shows a flowchart illustrating a method of assembling an EDU housing according to the invention; Figure 12 shows a first partially assembled EDU; and Figure 13 shows a second partially assembled EDU. DETAILED DESCRIPTION Figure 1 shows a vehicle 10 to provide context for this invention. The vehicle 10 may be a fully electric vehicle having only an electric machine for propulsion and no internal combustion engine or may be a hybrid vehicle having both an electric machine and an internal combustion engine arranged to propel the vehicle. The vehicle 10 may be a plug-in hybrid electric vehicle (PHEV) or a mild hybrid electric vehicle (MHEV). The vehicle 10 has at least one vehicle drive unit 100 containing a power source such as an electric machine or an internal combustion engine and one or more drivetrain components such as a gear or a differential arranged to transfer torque from the power source to the wheels of the vehicle. The vehicle drive unit 100 may be an electric drive unit (EDU) that contains an electric machine and one or more drivetrain components. Figure 2 shows an electric drive unit (EDU) 100. The drive unit 100 contains an electric machine for propelling the vehicle. The electric machine has a stator 102 and a rotor 104, the stator 102 being arranged to receive electrical power from a battery or generator and to impart a magnetic field to the rotor 104 to power the vehicle. The rotor 104 is coupled to a rotor shaft 106 which is arranged to transfer torque to downstream components of the electric drive unit. The rotor shaft 106 has a geared end portion 107 that is arranged to engage with and to transfer torque to a differential gear 108. The differential gear 108 is arranged to rotate and to transfer torque to the wheels of the vehicle. The electric drive unit 100 also has a lubricant jet 110 that is arranged to project lubricant, such as oil, onto the geared portion 107 of the rotor shaft 106 and the differential gear 108 at the point at which the two gears mesh. The electric drive unit 100 may generally be considered as a two-part unit having a power source portion 100a and a transmission portion 100b. The power source portion 100a contains the electric machine and the rotor shaft, whereas the transmission portion 100b contains the differential gear, the geared portion of the rotor shaft 107 and other transmission components including stub axle shafts (not shown) for transferring drive to the wheels. The transmission portion 100b may also be referred to as a transmission unit 100b. The electric drive unit 100 is formed of a two-part housing. The two-part housing comprises a first part 111 and a second part 112. The first part 111 substantially contains the power source and the second part 112 substantially contains the transmission component. The second part 112 may be referred to hereinafter as the transmission housing. The two-part housings 111,112 are bolted together at an interface. Figure 3 shows a detailed view of a known vehicle drive unit 100 (not the same drive unit shown in Figure 2). It is known to provide a collection volume in the vehicle drive unit 100 for temporarily storing oil before the oil is drained, for example to an oil sump to be recirculated. Where the transmission unit 110b houses a gear 108 or other rotating components, the gear 108 may be in contact with a volume of oil O in the transmission unit 110b. Figure 3 shows an example of a vehicle drive unit 100 housing a gear 108 and a volume of oil O. As shown by the oil level line 3000, the bottom of the gear 108 sits in the volume of oil O. Significant efficiency losses may occur as a result of the gear 108 moving through a volume of oil O compared to a gear 108 moving through air. This is due to the increased viscosity and hence drag when moving through oil compared to air. As the volume of oil O is disturbed by the gear 108, the oil may also become aerated as air is circulated through the fluid. This may negatively affect the performance of the oil when used to lubricate and / or cool components since the pockets of air within the oil reduce the thermal conductivity of the oil. The present invention provides a gear shroud to separate the gear 108 from the volume of oil O in the transmission unit 100b. Separating the gear 108 from the volume of oil O may allow the gear 108 to move through air rather than through oil, thus reducing the drag acting on the gear 108. Further, since the volume of oil O is not disturbed by the gear 108 moving through the gear aeration of the oil may be reduced. It will be appreciated that the gear 108 may be lubricated using oil, preferably with oil applied to the meshing point between the gear 108 and a further component. Oil may be applied to the gear 108 using a lubricant jet 110 as shown in Figure 2. As a result, a further volume of oil may run off the gear 108 and be stored in the gear shroud. The flow of oil used to lubricate the gear 108 may be controlled such that the further volume of oil does not excessively interfere with the gear 108. The further volume of oil may be maintained at a relatively constant volume as any excess oil which comes into contact with the gear 108 is ejected from the gear shroud by the gear as the gear rotates. Figure 4 shows an example of a gear shroud 120. The gear shroud 120 comprises a first part-cylindrical, shroud wall 122 and a second planar shroud wall 124 extending from the first shroud wall 122. The first shroud wall 122 and the second shroud wall 124 partially define a gear volume for receiving at least part of a gear 108. The gear volume may be defined by the first shroud wall 122, the second shroud wall 124 and part of a transmission housing wall that will be described shortly. Each shroud wall 122, 124 may be a relatively thin wall, optionally between 1 millimetre and 10 millimetres thick, preferably between 2 millimetres and 5 millimetres thick. The gear shroud 120 may be manufactured as a single component and may be formed of plastic. Any suitable method of manufacture such as injection moulding may be used to manufacture the gear shroud 120. The second shroud wall 124 may extend from the first shroud wall 122 in a substantially perpendicular direction. Generally, the second shroud wall 124 may extend from the first shroud wall 122 at an angle of between 60 and 120 degrees, preferably at an angle of between 85 and 100 degrees. The first shroud wall 122 may have a constant radius of curvature. It may be beneficial to provide a gear shroud 120 having a curved first shroud wall 122 to reduce the distance between the gear shroud 120 and a gear 108 housed therein such that the overall EDU may be more compact. An example of a gear shroud 120 comprising a first shroud wall 122 having a constant radius of curvature is shown most clearly in Figure 9. In a further embodiment (not shown) the first shroud wall 122 may be planar. It may be beneficial to provide a planar first shroud wall 122 such that the gear volume is substantially larger than the gear 108 housed therein. This may reduce the likelihood of the further oil which has run off the gear 108 interfering with the gear 108 as there would be an increased volume compared to a curved shroud wall that the further oil may occupy and not be in contact with the gear 108. Where the first shroud wall 122 is curved as shown in Figure 4, the second shroud wall 124 may effectively close the gear shroud 120 at a first side 126 of the gear shroud by extending from the first shroud wall 122. That is to say that the second shroud wall 124 extends between opposite ends of one of the curved edges of the first shroud wall 122 such that the gear shroud 120 is closed on one side. The gear shroud 120 therefore further comprises a second side 128 opposite the first side 126 which second side 128 is open. The second side 128 may be bounded by a first shroud wall edge 129. In some embodiments, the gear shroud 120 has a sealing element 125 such as for example a deformable rubber sleeve fixed to the first shroud wall edge 129 as shown in Figure 4. Figure 5 shows a second embodiment of a gear shroud 130 further comprising a plurality of mounting bosses 131 for fixing the gear shroud 130 to an adjacent component. Although shown with two mounting bossesl 31, it will be appreciated that the gear shroud 130 may comprise any number of mounting bosses 131. The gear shroud 130 comprises a first shroud wall 132 and a second shroud wall 134 extending from a first side 136 of the gear shroud 130. The gear shroud 130 further comprises a second side 138 opposite the first side 136. The first and second shroud walls 132, 134 of the gear shroud 130 may be substantially identical to the first and second shroud walls 122, 124 of the gear shroud 120 and therefore will not be discussed again in detail. Each mounting boss 131 is fixed to the second shroud wall 134 and extends away from the first shroud wall 132, optionally away from the gear volume partially defined by the firstand second shroud walls 132,134. The mounting boss(es) 131 may be manufactured as an integral part of the second shroud wall 134. Each mounting boss 131 may comprise a through hole accessible from the gear volume, such that a fastener may be inserted through the mounting boss 131. Figure 6 shows a cross-section of part of a transmission unit 100b comprising a transmission housing 112 and the gear shroud 120. The transmission housing 112 comprises a housing wall 112a that defines an internal volume of the transmission housing 112. The first shroud wall 122, is sealingly engaged with the housing wall 112 such that, together, the first shroud wall 122, the second shroud wall 124 and the housing wall 112 define the gear volume for receiving at least part of a gear. It may be beneficial to provide a sealing element 125 coupled to the first shroud wall edge 129 and arranged to sealingly engage with the housing wall 112a. Byway of non-limiting example, the sealing element 125 may be a deformable rubber sealing element arranged to form an impermeable connection with the housing wall 112a. Providing a sealing element between the housing wall 112a and the gear shroud 120 may reduce the likelihood of oil entering the gear volume through the joint between the housing wall 112a and the gear shroud 120. A portion of the transmission unit 100b may define an oil collection volume for temporarily storing oil before the oil is drained to a further reservoir, such as an oil sump. The oil collection volume may be defined by the housing wall 112a, the first shroud wall 122 and the second shroud wall 124. The first shroud wall 122, optionally the first shroud wall 122 and the second shroud wall 124, may be arranged between the oil collection volume and the gear volume as shown in Figure 6. Since the first shroud wall 122 is sealingly engaged with the housing walH 12a, the oil collection volume and the gear volume are not fluidically connected. As such, oil in the oil collection volume cannot enter the gear volume through the gear shroud 120 or its seal with the housing wall 112a. Since gravity will cause oil to pool at the bottom of the transmission housing 112, it may be beneficial to provide the oil collection volume at the bottom of the transmission unit 100b. During operation, the transmission unit 100b may roll and / or pitch such that the transmission unit 100b is inclined at a transmission unit angle, for example as a vehicle in which the transmission unit 100b is provided travels over an uneven surface. This may cause oil in the transmission unit 100b to move due to gravity, which may increase the likelihood of oil entering the gear volume. As such, the gear shroud 120 may be designed to separate the gear volume and the oil collection volume at all transmission unit angles expected during normal operation. As seen most clearly in Figure 4, the gear shroud 120, preferably the first shroud wall 122, may comprise an aperture 123 arranged to permit flow of fluid into the gear volume. The aperture 123 may be a slot extending down from a top edge of the first shroud wall 122 as shown in Figure 4. The aperture 123 shown in Figure 4 has a tapered width which decreases as it extends from the top edge of the gear shroud 120. Providing a slot with a tapered width may improve ease of manufacture of the aperture 123, particularly where the aperture is manufactured as an integral part of the gear shroud 120. By way of non-limiting example, the angle of taper of the aperture width may be selected to correspond to the required drafting angle for an injection mould. Alternatively, the aperture 123 may be manufactured by a subtractive manufacturing process once the gear shroud 120 has been formed. In this case, the slot may have a constant width, for example where the slot is milled. The distance which the aperture 123 extends down may be measured based on the position of the lowest portion of the aperture 123 relative to an oil level expected during normal use and / or a service fill oil level. The aperture 123 may extend down the first shroud wall by at least 10 millimetres, preferably by at least 15 millimetres, further preferably by at least 17 millimetres. Alternatively, the aperture 123 may extend down from the top edge of the shroud 120 by between 1% and 60% of the height of the gear shroud 120. It is preferable to provide the aperture 123, specifically the lowest portion of the aperture 123, outside of the oil collection volume to reduce the likelihood of oil entering the gear volume through the aperture 123 during normal operating conditions, which would reduce the effectiveness of the gear shroud 120. In a further embodiment not shown, the aperture 123 may comprise a bore through the first shroud wall 122, i.e. the aperture 123 may be surrounded by material and does not extend from an edge of the first shroud wall 122. This arrangement may allow the rate of flow of fluid through the aperture to be more closely controlled by controlling a diameter of the bore. The aperture 123 may allow maintenance and / or inspection of the transmission unit 100b to be conducted more easily. For example, a “fill to spill” process may be conducted to maintain the transmission unit 100b. This process involves filling the transmission unit 100b with oil via a port until the oil level is at the port. At this stage, it can be determined that the amount of oil in the transmission unit 100b is correct. The gear shroud 120 may influence the oil level within the transmission housing 112 by causing an oil-free volume below the desired oil level. Providing an aperture 123 to permit flow of oil into the gear volume during a filling process may reduce the likelihood of an insufficient volume of oil being in the transmission unit 100b. Figure 7 shows a second embodiment of transmission unit 200b comprising a further transmission housing 212 having a housing wall 212a which comprises a groove 214. The transmission unit 200b comprises the gear shroud 120 described previously. Details of the gear shroud 120 will therefore not be repeated here. The groove 214 is arranged to receive the first shroud wall edge 129. As such, the groove 214 may follow the profile of the first shroud wall edge 129. Where the gear shroud 120 comprises a sealing element 125 fixed to the first shroud wall edge 129 as shown in Figure 7, the groove 214 is arranged to receive and / or engage with the sealing element 125. The sealing element 125 may be arranged to elastically deform once received in the groove 214. The deformed sealing element 125 may fill the groove 214, reducing the likelihood of oil entering the gear volume through the seal between the first shroud wall 122 and the transmission housing 212. The groove 214 may improve the seal between the gear shroud 120 and the transmission housing 212 by preventing deflection of the first shroud wall 122 in a direction along the housing wall 212a. Where the gear shroud 120 is coupled to a component adjacent the second shroud wall 124, the first shroud wall 122 is effectively an unsupported cantilever structure as shown in Figure 6. Since the second side 128 of the gear shroud 120 is unsupported, it may deflect and create a gap between the first shroud wall 122 and the transmission housing 212 which would allow oil to enter the gear volume, reducing the efficiency of the gear. The groove 214 comprises two groove walls 214a extending into the transmission housing 212 to define the groove 214. Each groove wall 214a is aligned substantially parallel with the second side 128 of the first shroud wall 120. The angle between each groove wall 214a and the first shroud wall 120 may be between 70° and 110°. It will be appreciated that the groove 214 may have any cross-sectional profile, for example a semi-circle. In the case where the groove 214 is defined by a single continuous groove wall, the two groove walls 214a may be two portions of the single continuous groove wall. Since the first shroud wall edge 129 is received in the groove 214, any deflection of the first shroud wall 122 in a direction along the housing wall 212a will cause the first shroud wall 122 to abut one of the groove walls 214a. The groove wall 214a then provides a reaction force to prevent further movement of the first shroud wall 122 perpendicular to the groove wall 214a. In some cases, the dimensions of the groove 214 may be specified such that, when in the nominal position of the gear shroud, both the upper and lower surfaces ofthe first shroud wall 122 abut a groove wall 214a. In this way movement ofthe first shroud wall 122 from its nominal position is prevented. In a further embodiment not shown, the transmission housing 212 may comprise a step (or lip) in place of a groove 214. The step is defined by a step wall extending into the housing wall 212a. The step wall may be arranged substantially parallel to the second side 128 ofthe first shroud wall 122. The first shroud wall 122 is arranged to abut the step wall and hence movement ofthe first shroud wall 122 is prevented by the step wall. Providing a step wall for the first shroud wall 122 to abut may be beneficial where it is only required to prevent movement ofthe first shroud wall 122 in a single direction along the housing wall 212a. It will be appreciated that all embodiments described hereinafter comprising a transmission unit housing 100b may equally be applied to the second embodiment of the transmission unit 200b comprising the further transmission housing 212 comprising a groove 214. Figure 8 shows a third embodiment of a transmission unit 300b comprising the transmission unit 100b and a gear 108 partially received in the gear volume. The gear 108 is a differential gear arranged to receive a torque from a rotor shaft (not shown) as previously described with reference to Figure 2. The gear 108 and rotor shaft are arranged to rotate about a gear axis A1. The rotor shaft is partially supported by the transmission housing 112. The gear 108 has an outer profile that defines a generally cylindrical gear shape. The first shroud wall 122 may be curved such that it is configured to follow the outer profile ofthe gear 108. As seen most clearly in Figure 9, the first shroud wall 122 may be arranged at a substantially constant distance from the gear axis A1 and / or from an outer surface of a portion ofthe gear 108 adjacent the first shroud wall 122. Where the gear profile has a constant radius of R about the gear axis A1, the first shroud wall 122 may be arranged at a distance of less than 1,5R; preferably less than 1,2R, further preferably less than 1.1R from the gear axis A1. The first shroud wall 122 may be arranged at a distance of less than 10 millimetres, preferably less than 7 millimetres, further preferably less than 5 millimetres from the outer surface ofthe portion ofthe gear 108 adjacent the first shroud wall 122. Providing a first shroud wall 122 relatively close to the gear 108 may reduce the overall size of the gear shroud 120 and gear 108 together. The gear 108 may comprise a first end face 108a which is perpendicular to the gear axis A1 and facing the second shroud wall 124. The second shroud wall 124 may extend to cover a portion ofthe first end face 108a as shown in Figure 8. The second shroud wall 124 may cover between 5% and 50%, preferably between 15% and 25% ofthe area ofthe first end face 108a. In some embodiments (not shown) the second shroud wall 124 may extend to cover the entirety ofthe first end face 108a, such that the gear 108 is housed entirely within the gear volume. Since oil will pool at the bottom ofthe transmission unit 300b, it is preferable to position the gear shroud 120, particularly the first shroud wall 122, below the gear 108 as shown in Figures 8 and 9. As such, the gear shroud 120 may be centred directly below the gear axis A1. Where the gear 108 has a circular profile, the first shroud wall 122 may extend around at least a portion of the circumference of the gear profile. The first shroud wall 122 may extend around between 15% of the circumference of the gear profile (to subtend an angle of 54°) and 50% of the circumference of the gear profile (to subtend an angle of 180°), preferably around between 25% of the circumference of the gear profile (to subtend an angle of 90°) and 35% of the circumference of the gear profile (to subtend an angle of 126°). It may be preferable to provide a first shroud wall 22 that extends around less than 50% of the circumference of the gear profile (i.e. that subtends less than 180°) such that the gear 108 may be inserted into the gear volume in a direction perpendicular to the first shroud wall 122, for example during assembly of the transmission unit 300b. Figure 10 shows a portion of an EDU housing 400. The EDU housing 400 comprises the transmission unit 300b and a main housing 410. The main housing 410 is equivalent to the first part 111 of the EDU two-part housing described previously with reference to Figure 2. The main housing 410 is fixed to the transmission unit 300b, optionally to the transmission housing 112 such that the main housing 410 and the transmission housing 112 define an internal space contiguous with said internal volume. As such, the transmission unit 300b and the main housing 410 may together define the oil collection volume. In some embodiments, the gear shroud 120 may be fixed to the main housing 410, optionally using at least one fastener coupled to a corresponding mounting boss 131 on a gear shroud 130. The main housing 410 may comprise an outlet 412 arranged to supply oil to the internal volume of the transmission. It may be beneficial to direct the outlet 412 towards the second shroud wall 124 such that fluid dispensed from the outlet 412 impacts upon the second shroud wall 124. In some embodiments (not shown) the second shroud wall 124 comprises features such as ridges and / or channels to control the flow of oil impacting upon the second shroud wall 124. It may be beneficial to dissipate the flow of oil to reduce aeration of the oil. The EDU housing 400 of Figure 10 may form part of the EDU 100 of Figure 2. The EDU 100 may be a power source for a vehicle, optionally wherein the vehicle comprises multiple EDUs each arranged to apply a torque to a separate differential gear. The EDU housing 400 may be assembled according to the method of assembly 500 illustrated by the flowchart of Figure 11. The method of assembly will now be described with reference to Figures 11,12 and 13. At step 510 a first subassembly 512 is assembled. Figure 12 shows the first subassembly 512 which comprises a gear shroud 130 fastened to the main housing 410 at the first side 136 of the gear shroud 130. The first side 136 of the gear shroud 130 is the side from which the second shroud wall 134 extends from the first shroud wall 132 as described previously. The second shroud wall 134 is fastened to the main housing 410 such that the gear shroud 130 is open on the second side 138 of the gear shroud 130, the second side 138 of the gear shroud 130 being opposite the first side 136. The gear shroud 130 is fastened to the main housing 410 using a plurality of fasteners, optionally where the gear shroud 130 comprises a plurality of mounting bosses 131 to engage with the main housing 410 as shown in Figure 12. At step 520 a second subassembly 522 is assembled. It will be appreciated that steps 510 and 520 may be conducted simultaneously. Figure 13 shows the second subassembly 522 which comprises a gear 108 coupled to the transmission housing 112 described previously. The gear 108 may for example be mounted on a rotor shaft (not shown) and the rotor shaft may be rotatably coupled to the transmission housing 112 to allow rotation of the gear 108. At step 530, the first subassembly 512 and the second subassembly 522 are coupled together to provide the EDU housing 400 as shown in Figure 10. The transmission housing 112 of the second subassembly 522 may be fastened to the main housing 410 of the first subassembly 512, optionally using a plurality of fasteners. Once assembled, the first shroud wall 122 is sealingly engaged with the housing wall 112a and the gear 108 is at least partially received in the gear volume defined by the first shroud wall 122, the second shroud wall 124 and the housing wall 112a. Assembling the EDU housing 400 as a first subassembly 512 and a second subassembly 522 may improve ease of assembly of the EDU housing 400. Providing a first subassembly 512 with the gear shroud 120 having an open side 138 facing away from the main housing 410 may allow any other components within the EDU housing 400 to be more easily assembled. Further, by coupling the gear 108 to the transmission housing 112 rather than the main housing 410, the gear 108 may be disassembled from the EDU housing 400 by removing the transmission housing 112. This may improve ease of maintenance for the gear 108 and / or the gear shroud 130 as the two components may be worked on as part of a subassembly 512, 522 rather than the full EDU housing 400. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A transmission unit comprising:a transmission housing comprising a housing wall that defines an internal volume of the transmission housing; anda gear shroud within the internal volume, the gear shroud comprising:a first shroud wall sealingly engaged with the housing wall; anda second shroud wall extending from the first wall such that the first shroud wall, the second shroud wall and the housing wall define the gear volume .

2. A transmission unit according to claim 3, wherein the second shroud wall extends from the first shroud wall in a substantially perpendicular direction.

3. A transmission unit according to any preceding claim, wherein a portion of the transmission unit internal volume defines an oil collection volume, and wherein the first shroud wall is arranged between the oil collection volume and the gear volume.

4. A transmission unit according to any preceding claim, wherein the housing wall comprises a groove, and wherein the first shroud wall is received in the groove.

5. A transmission unit according to any preceding claim, further comprising a gear at least partially received in the gear volume, the gear being arranged to rotate about an axis, and wherein the first shroud wall is curved such that it is configured to follow the profile of the gear.

6. A transmission unit according to claim 5, wherein the second shroud wall extends from the first wall to cover at least a portion of a face of the gear.

7. A transmission unit according to any preceding claim, wherein the first shroud wall further comprises an aperture arranged to permit flow of fluid into the gear volume.

8. A transmission unit according to any preceding claim, wherein the gear shroud is manufactured from plastic.

9. A transmission unit according to any preceding claim, wherein the shroud comprises one or more connectors facing away from the gear volume, the one or more connectors being arranged to fix the gear shroud to an adjacent component.

10. An electric drive unit housing comprising:the transmission unit of any preceding claim, anda mam housing arranged to receive an electric machine, the main housing being fixed to the transmission unit such that the main housing and the transmission housing define an internal space contiguous with said internal volume.

11. An electric drive unit comprising:the electric drive unit housing of claim 10; andan electric motor arranged within the main housing;12. An electric drive unit according to claim 11, wherein the gear shroud is fixed to the main housing.

13. A vehicle comprising the electric drive unit of claim 12.

14. A method of assembling an electric drive unit housing, the method comprising:fastening a gear shroud to a main housing at a first side of the gear shroud, wherein the gear shroud comprises a first shroud wall and a second shroud wall extending from the first shroud wall, such that the gear shroud is open on a second side of the gear shroud, the second side being opposite to the first side.

15. A method of assembling an electric drive unit housing comprising:the method of claim 14;coupling a gearto a transmission housing, the transmission housing comprising a housing wall that defines an internal volume; andfastening the transmission housing to the main housing such that the first shroud wall is sealingly engaged with the housing wall, and the gear is at least partially received in a gear volume defined by the first shroud wall, the second shroud wall and the housing wall.16

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