Patent Description:
Electric propulsion systems often include an electric machine that functions as an electric motor when it converts electricity to mechanical power, and as a generator when converts mechanical power to electricity. When acting as an electric motor the electric machine introduces torque into a drive mechanism, and that torque is transferred to a propulsion means, for example one or more propellers of an aeroplane or one or more wheels of a wheeled vehicle, for example a car. When acting as a generator the electric machine receives torque from the drive mechanism, and that torque is converted into electricity. That electricity may be stored in an electricity storage device, for example a rechargeable battery, or used to power a an electrical system. Some electric machines include one or more permanent magnets.

It is known that to manage system safety requirements for electric propulsion systems, including those that include one or more permanent magnets, there is a need to be able to disconnect the electric machine from the drive mechanism to ensure that the electric machine can be stopped from rotating or being rotated. This is necessary to manage a number of fault conditions. It also allows damage to the electric machine to be minimised or avoided once a fault has developed.

In some electric propulsion system architectures, a plurality of electric machines may provide torque to or receive torque from a single drive mechanism. In such architectures there may be a need to be able to disconnect the electric machines from the drive mechanism as a whole, or to disconnect the electric machines singly to ensure that one or more of the electric machines can be stopped from rotating or being rotated as required.

<CIT> discloses a system for conversion of a vehicle between two-wheel drive and four-wheel drive. The conversion is effected by a camming assembly and a shift mechanism which temporarily locks the camming assembly against rotation when advancing a drive gear into driving engagement with a receiving gear when converting to four-wheel drive. The camming assembly is reengaged under the control of the shift mechanism as a preliminary to disengaging the drive gear from the receiving gear when converting to two-wheel drive.

According to a first aspect of the present disclosure there is provided a drive mechanism for an electric propulsion system that comprises an electric machine. The drive mechanism comprises a drive shaft with a longitudinal axis A, an engagement means, and a disconnect mechanism. The engagement means comprises a drive shaft engagement element and an electric machine engagement element. The drive engagement element comprises a first and second shaft gear ring, and each shaft gear ring is supported on the drive shaft. Each shaft gear ring comprises a plurality of gear teeth which are circumferentially disposed around the drive shaft, and the gear teeth extend radially outward from the drive shaft. The gear teeth of the first shaft gear ring have an axial length L1, and the first and second shaft gear rings are axially spaced from each other along the drive shaft by a distance L2. The electric machine engagement element comprises a first and second gear set; and the first and second gear sets each comprise a plurality of gear teeth which are configured to mesh with the gear teeth of the first and second shaft gear rings respectively. The gear teeth of the first gear set have an axial length L3, and the first and second gear sets are separated from each other by a length L4 in the axial direction. The length L1 is less than length L4, and length L3 is less than length L2. The disconnect mechanism is configured to reversibly move one of the drive shaft engagement element or a part thereof and the electric machine engagement element or a part thereof relative to the other of the engagement elements in an axial direction between a first position and a second position. The first position is one in which the gear teeth of the first shaft gear ring are meshed with the gear teeth of the first gear set and the gear teeth of the second shaft gear ring are meshed with the gear teeth of the second gear set. The second position is one in which the gear teeth of the first shaft gear ring are axially between the first and second gear sets and the gear teeth of the first shaft gear ring are not meshed with the gear teeth of the first or second gear sets.

For the purposes of the present invention an electric machine is to be understood to be an apparatus that is configured to function as an electric motor only, an apparatus that is configured to function as a generator only, or an apparatus configured to function as both an electric motor and a generator.

The meshing of the gear teeth for the first and second shaft gear rings with the gear teeth of the first and second gear sets respectively is such that rotation of one of the first and second shaft gear rings and the first and second gear sets transmits torque to the other of the first and second shaft gear rings and the first and second gear sets with the effect that the gear teeth of both of the first and second shaft gear rings follow a circumferential path around the axis A, and the gear teeth of the first and the second gear sets each follow a circumferential path around a first and second gear set axis respectively.

In some embodiments the axis A, the first gear set axis and the second gear set axis are the same axis or in common.

In some embodiments the axis A, the first gear set axis and the second gear set axis are parallel axes.

In some embodiments the axis A, the first gear set axis and the second gear set axis are not all parallel to each other or in common with each other.

An advantage of the drive mechanism of the present disclosure is that because L1 is smaller than L4, and L3 smaller than L2 the teeth of the first shaft gear ring can sit axially between the teeth of the first gear sets, and the teeth of the first gear sets can sit axially between the teeth of the shaft gear rings without any of the teeth engaging or meshing with each other.

In an embodiment of any of the above embodiments, the drive shaft engagement element of at least one engagement means comprises n(<NUM>) shaft gear rings, and n(<NUM>) gear sets, where n(<NUM>) is greater than two, and n(<NUM>) is greater than two.

In an embodiment of the above embodiment, n(<NUM>) equals n(<NUM>).

In an embodiment of any of the above embodiments when n(<NUM>) and n(<NUM>) are greater than two, the gear teeth of the each shaft gear ring have an axial length L1, and axially adjacent shaft gear rings are axially spaced from each other along the drive shaft by a distance L2, the gear teeth of each gear set have an axial length L3, and gear sets that are adjacent in the axial direction are each separated from each other by a length L4 in the axial direction.

In an embodiment of any of the above embodiments, length L1 is approximately equal to length L3.

In an embodiment of any of the above embodiments, length L2 is approximately equal to length L4.

The maximum torque transmission that can occur between the drive shaft engagement element and the electric machine engagement element is dependent on the area of contact between the teeth of the shaft gear rings and the teeth of the gear sets. In known drive mechanisms there is one shaft drive ring and one gear set and the way to increase the maximum torque transmission between the shaft gear ring and the gear set is to increase the axial length of both the teeth on the shaft gear ring and the teeth on the gear set. This increases the contact area between those teeth which has the effect of increasing the area of contact and as such the maximum torque that is transmissible. This also has the effect of increasing the axial movement between the shaft gear ring and the gear set that is required to disengage or de-mesh the shaft gear ring from the gear set.

It is an advantage of the drive mechanism of the present disclosure that the axial movement of the shaft gear rings relative to the gear sets that is required to disengage the shaft gear rings from the gear sets is the larger of distances L1 and L3 whilst the contact length, and thus contact area, between the teeth of the shaft gear rings and the teeth of the gear sets is the smaller of twice L1 and twice L3. In embodiments where L1 is equal to L3, the effect is that the axial movement to disengage the shaft gear ring and the gear set is half that of the known mechanism which has the same contact length / area of twice L1 between the teeth on the shaft gear ring and the teeth on the gear set (when all other dimensions of the compared drive mechanisms are equal).

A further advantage of the drive mechanism of the present disclosure is that when n(<NUM>) and n(<NUM>) are greater than two the required axial movement of the shaft gear rings relative to the gear sets to disengage the shaft gear rings from the gear sets remains the larger of distances L1 and L3 whilst the contact length between the teeth of the shaft gear rings and the teeth of the gear sets is the smaller of n(<NUM>) x L1 and n(<NUM>) x L3.

In an embodiment of any of the above embodiments, the first and second shaft gear rings are rigidly fixed to the drive shaft such that they cannot move along the drive shaft in an axle direction.

In an alternative embodiment of the above embodiment, the first and second shaft gear rings are attached to the drive shaft in such a fashion that they may move at least a predetermined distance along the shaft in an axle direction when impelled to do so by the disconnect mechanism.

In an embodiment of any of the above embodiments, the drive mechanism is for use with two or more electric machines. The drive mechanism comprises two or more engagement means, and each engagement means is associated with a single or common drive shaft. The engagement means are axially spaced along the drive shaft. This arrangement or architecture is advantageous because all of the electric machines can apply torque to the same, common, drive shaft. This minimises the number of components in the drive mechanism and increases the torque supplied to the propulsion system relative to a single electric machine.

In an embodiment of any of the above embodiments, each engagement means is substantially the same as or similar to the other engagement means.

In an embodiment of any of the above embodiments, at least one of the gear sets of at least one engagement means comprises a second gear ring.

In an embodiment of any of the above embodiments, each second gear ring is co-axial with the drive shaft, and the gear teeth of each second gear ring extend radially inwards. An advantage of such an arrangement is that the drive mechanism can be, as a result, compact in size.

In an embodiment of any of the above embodiments, the gear teeth of each shaft gear ring are male or external splines, and the gear teeth of each gear set are female or internal splines.

In an embodiment of any of the above embodiments, the electric machine engagement element of at least one engagement means further comprises a hollow shaft, and each gear set of the electric machine engagement element is supported on the hollow shaft. In such embodiments the hollow shaft has a central axis that is the same as Axis A of the drive shaft.

In an embodiment of any of the above embodiments, the drive shaft further comprises a first stop element, one of the electric machine engagement elements of one of the engagement means further comprises a second stop element, and the first and second stop elements are so configured and located that they abut when the drive shaft engagement element and the electric machine engagement element of that engagement means and or the shaft gear rings and the gear sets are in the second position relative to each other. This is advantageous because the first and second stop elements prevent the shaft gear rings moving relative to the gear sets (or the gear sets moving relative to the shaft gear rings) by a distance that would cause the teeth of the shaft gear rings to begin to mesh with the teeth of the gear sets adjacent to the gear set (or the teeth of the gear sets to begin to mesh with the teeth of the shaft gear rings) that they are intended to mesh with.

In an embodiment of any of the above embodiments, each of the first and second stop elements comprises a stop surface, the stop surfaces of the first and second stop elements abut, and one or both of the stop surfaces comprise or are coated with a low friction material. This is advantageous because it minimises the friction between the stop surfaces and, as a result, minimises the transmission of torque between those stop surfaces.

In an embodiment of any of the above embodiments, each engagement means comprises a disconnect mechanism, the disconnect mechanism of each engagement means causes at least part of the electric machine engagement element to move axially relative to the drive shaft engagement element of that engagement means. In some embodiments the disconnect mechanism of each engagement means operates independently of the disconnect mechanisms of each other engagement means. This has the advantage that individual electric machines may be disengaged from the drive shaft without having to disengage other electric machines from the drive shaft.

In an alternative embodiment of any of the above embodiments, the drive mechanism is so configured that operation of the disconnect mechanism causes the relative movement of one of the drive shaft engagement element and electric machine engagement element relative to the other of the drive shaft engagement element and electric machine engagement element for each engagement means.

In an embodiment of any of the above embodiments, at least one disconnect mechanism comprises an actuator.

In an embodiment of any of the above embodiments, at least one disconnect mechanism comprises a solenoid mechanism.

According to a second aspect of the present disclosure there is provided an aeroplane or wheeled vehicle comprising an electric propulsion system, in which the electric propulsion system comprises at least one electric machine and at least one drive mechanism according to the first aspect of the present disclosure.

The present invention will be further described and explained by way of example with reference to the accompanying drawings in which.

In the following discussions of the accompanying drawings, where the same element is present in a more than one embodiment the same reference numeral is used for that element throughout. Where there are similar elements, similar reference numerals (the same numeral plus a multiple of <NUM>) are used.

With reference to <FIG>, an aeroplane <NUM> includes a propulsion system <NUM>. The propulsion system <NUM> includes a propeller <NUM> which has a hub <NUM>. The hub <NUM> is connected to a first end <NUM> of a drive shaft <NUM>. Between the first end <NUM> and a second end <NUM> of the drive shaft <NUM> an electric machine <NUM> is engaged with the drive shaft <NUM> via a drive mechanism <NUM>.

With reference to <FIG>, an aeroplane <NUM> includes a propulsion system <NUM>. The propulsion system <NUM> includes a propeller <NUM> which has a hub <NUM>. The hub <NUM> is connected to a first end <NUM> of a drive shaft <NUM>. Between the first end <NUM> and a second end <NUM> of the drive shaft <NUM> three electric machines 118A, 118B, 118C are each engaged with the drive shaft <NUM> via a drive mechanism 116A, 116B, 116C respectively. In the illustrated embodiment in <FIG> each electric machine 118A, 118B, 118C has the same configuration as electric machine <NUM> shown in <FIG>.

With reference to <FIG> and <FIG>, and with continued reference to <FIG>, the drive shaft <NUM> has a longitudinal axis A and is comprised of a first portion <NUM> that extends from the first end <NUM> to a radially inwardly extending shoulder or stop element <NUM>, and a second portion <NUM> which extends from the shoulder <NUM> to the second end <NUM>. Adjacent the second end <NUM> a stop ring <NUM> extends circumferentially around and radially outward from the radially outer surface <NUM> of the drive shaft <NUM>.

Located on the radially outer surface <NUM> of the second portion <NUM> of the drive shaft <NUM> is a drive engagement element <NUM> which is comprised of four gear rings. Each of the gear rings has the form of a male or external set of splines <NUM>. The splines <NUM> are spaced circumferentially around the outer surface <NUM> and extend radially outwardly. Each of the splines <NUM> has an axial length L1 in the direction of axis A. The splines <NUM> / each gear ring are axially spaced from the axially adjacent splines <NUM> / gear ring(s) by a distance L2. In other non-illustrated examples of the drive mechanism of the present disclosure a different number of gear rings may be present. That different number may be two, three, or more than four. The number of gear rings / splines included in the drive engagement element may be determined by the desired maximum torque transmission between the drive shaft and the electric machine.

The splines <NUM> of the gear ring that is axially closest to the second end <NUM> of the drive shaft <NUM> are located on the gear shaft <NUM> so that the end of the splines <NUM> closest to end <NUM> abut the stop ring <NUM>.

The drive mechanism <NUM> includes a hollow shaft <NUM> which is co-axial with the drive shaft <NUM>. The drive shaft <NUM> extends through the hollow shaft <NUM>, and the second portion <NUM> of the drive shaft <NUM> is substantially surrounded by the hollow shaft <NUM> together with the shoulder <NUM> and a part of the first portion <NUM> of the drive shaft <NUM> which is adjacent to the shoulder <NUM>. The surrounded parts of the drive shaft <NUM> are to be considered to be part of the drive mechanism <NUM> for the purposes of the present disclosure.

The hollow shaft <NUM> has a radially outwardly facing surface <NUM> and a radially inwardly facing surface <NUM>. Located on the radially inwardly facing surface <NUM> of the hollow shaft <NUM> is an electric machine engagement element <NUM>. The electric machine engagement element <NUM> includes four gear sets. Each of the gear sets has the form of an female or internal set of splines <NUM>. The splines <NUM> are spaced circumferentially around the inner surface <NUM> and extend radially inwardly. Each of the splines <NUM> has an axial length L3 in the direction of axis A. The splines <NUM> of each gear set are axially spaced from the splines <NUM> of the adjacent gear set(s) by a distance L4. In other non-illustrated examples of the drive mechanism of the present disclosure a different number of gear sets may be present. That different number may be two, three, or more than four.

In the embodiment illustrated in <FIG> and <FIG>, lengths L1 and L3 are same as each other as are distance L2 and L4.

Adjacent to the of the splines <NUM> of the gear set furthest from the second end <NUM> of the drive shaft <NUM> end furthest from the second end <NUM> of the drive shaft <NUM> is a stop element or stop block <NUM>. The face of the stop element <NUM> furthest from the end <NUM> of the drive shaft <NUM> is coated with a low friction material.

The electric machine <NUM> includes a casing <NUM> within which a stator <NUM> is fixed and an armature <NUM> is supported on two or more bearings (not shown). The armature <NUM> is caused to rotate when electrical power is supplied to the electric machine <NUM>. The armature <NUM> supports or is coupled to a set of female or internal drive splines <NUM>. The electric machine <NUM> is so configured that the set of female drive splines <NUM> is adapted to follow a circumferential path around the axis A.

Located on the radially outward facing surface <NUM> of the hollow shaft <NUM> are a set of male or external drive splines <NUM>. The hollow shaft <NUM> is so configured that the male drive splines <NUM> mesh with the set of female drive splines <NUM> coupled to the armature <NUM>.

Supported on the case <NUM> of the electric machine <NUM> is a disconnect mechanism in the form of a solenoid <NUM>. The solenoid <NUM> is coupled to the hollow shaft <NUM> via a coupling <NUM>. The engagement of the coupling <NUM> to the hollow shaft <NUM> is such that the coupling remains engaged to the hollow shaft <NUM> when the hollow shaft <NUM> is rotating around the axis A.

Actuation of the solenoid <NUM> causes the hollow shaft <NUM> to be moved between a first position shown in <FIG> (in which the gear sets / splines <NUM> on the hollow cylinder <NUM> and the gear rings / splines <NUM> on the drive shaft <NUM> are fully engaged or meshed with each other), and a second position shown in <FIG> (in which the gear sets / splines <NUM> on the hollow cylinder <NUM> and the gear rings / splines <NUM> on the drive shaft <NUM> are not engaged or meshed with each other).

When the hollow cylinder <NUM> is in the first position the splines <NUM> closest to the end <NUM> of the drive shaft <NUM> abut the stop ring <NUM>, and the hollow cylinder <NUM> is thus prevented from traveling any further toward or past the end <NUM> of the drive shaft <NUM>.

When the hollow cylinder <NUM> is in the second position the stop element <NUM> abuts the shoulder or stop element <NUM> and prevents the hollow cylinder <NUM> moving too far away from the second end <NUM> of the drive shaft <NUM> and the splines <NUM> and <NUM> reengaging with each other.

The configuration of the set of female drive splines <NUM> and set of male drive splines <NUM> is such that those drive splines <NUM>, <NUM> remain meshed or engaged with other throughout the movement of the hollow shaft <NUM> between the first and second positions.

With further reference to <FIG>, in a first embodiment of the aeroplane <NUM>, each of the drive mechanisms 116A, 116B, 116C are configured in the same fashion as drive mechanism <NUM> described above in connection with <FIG> and <FIG>. In particular, the hollow shafts <NUM> of drive mechanism 116A, 116B, 116C are separate from each other and each of the actuators <NUM> of drive mechanisms 116A, 116B, 116C are independently controllable by suitable control means (not shown). As a result, each of electric machines 18A, 18B, 18C can be engaged with or disengaged from the drive shaft <NUM> independently of the other electric machines. Thus the propeller <NUM> can be driven by or drive one, two or three of the electric machines 18A, 18B, 18C.

With reference to <FIG> and with further reference to <FIG>, in a second embodiment of the aeroplane <NUM> the drive shaft <NUM> has a longitudinal axis A and is comprised of a first portion <NUM> that extends from the first end <NUM> to a radially inwardly extending shoulder or stop element <NUM>, and a second portion <NUM> which extends from the shoulder <NUM> to the second end <NUM>. Adjacent the second end <NUM> a stop ring <NUM> extends circumferentially around and radially outward from the radially outer surface <NUM> of the drive shaft <NUM>.

Located on the radially outer surface <NUM> of the second portion <NUM> of the drive shaft <NUM> are three drive engagement elements 156A, 156B, 156C which are axially spaced along the second portion <NUM> of the drive shaft <NUM>. Each of the drive engagement elements 156A, 156B, 156C is comprised of four gear rings. The gear rings each have the form of a set of male or external splines 136A, 136B, 136C respectively (for clarity not all of not all of splines 136A, 136B, 136C are labelled). The splines 136A, 136B, 136C are spaced circumferentially around the outer surface <NUM> and extend radially outwardly. Each of the splines 136A, 136B, 136C has an axial length L1 in the direction of axis A. Within each drive engagement element 156A, 156B, 156C the splines 136A, 136B, 136C / each gear ring are axially spaced from the axially adjacent splines 136A, 136B, 136C / gear ring(s) by a distance L2. In other non-illustrated examples of the drive mechanism of the present disclosure a different number of gear rings in each drive engagement element may be present. That different number may be two, three, or more than four. There may be different numbers of gear rings in one or more of the drive engagement elements. The number of gear rings / splines included in each drive engagement element may be determined by the desired maximum torque transmission between the drive shaft and the electric machine with which the drive engagement element is associated.

The splines <NUM> of the gear ring that is axially closest to the second end <NUM> of the drive shaft <NUM> are located on the gear shaft <NUM> so that the end of the splines 136C closest to end <NUM> abut the stop ring <NUM>.

The drive mechanisms 116A, 116B, 116C share a common hollow shaft <NUM> which is co-axial with the drive shaft <NUM>. The drive shaft <NUM> extends through the hollow shaft <NUM>, and the second portion <NUM> of the drive shaft <NUM> is substantially surrounded by the hollow shaft <NUM> together with the shoulder <NUM> and a part of the first portion <NUM> of the drive shaft <NUM> which is adjacent to the shoulder <NUM>. The surrounded parts of the drive shaft <NUM> are to be considered to be part of the drive mechanisms 116A, 116B, 116C for the purposes of the present disclosure.

The hollow shaft <NUM> has a radially outwardly facing surface <NUM> and a radially inwardly facing surface <NUM>. Located on the radially inwardly facing surface <NUM> of the hollow shaft <NUM> are three electric machine engagement elements 158A, 158B, 158C. The electric machine engagement elements 158A, 158B, 158C each include four gear sets. Each of the gear sets has the form of an female or internal set of splines 130A, 130B, 130C respectively (for clarity not all of not all of splines 130A, 130B, 130C are labelled). The splines 130A, 130B, 130C are spaced circumferentially around the inner surface <NUM> and extend radially inwardly. Each of the splines 130A, 130B, 130C has an axial length L3 in the direction of axis A. The splines 130A, 130B, 130C / each gear set are axially spaced from the axially adjacent splines <NUM> gear set(s) by a distance L4. In other non-illustrated examples of the drive mechanism of the present disclosure a different number of gear sets may be present. That different number may be two, three, or more than four. There may be different numbers of gear sets in one or more of the electric machine engagement elements. The number of gear sets / splines included in each electric machine engagement element may be determined by the desired maximum torque transmission between the drive shaft and the electric machine with which the electric machine engagement element is associated.

In the embodiment illustrated in <FIG>, lengths L1 and L3 are same as each other as are distance L2 and L4.

Adjacent to the of the splines 130A of the gear set furthest from the second end <NUM> of the drive shaft <NUM> end furthest from the second end <NUM> of the drive shaft <NUM> is a stop element or stop block <NUM>. The face of the stop element <NUM> furthest from the end <NUM> of the drive shaft <NUM> is coated with a low friction material.

Each of the electric machines 118A, 118B, 118C includes a casing 150A, 150B, 150C within which a stator 148A, 148B, 148C is fixed and an armature 146A, 146B, 146C is supported on two or more bearings (not shown). The armatures 146A, 146B, 146C are caused to rotate when electrical power is supplied to the electric machines 118A, 118B, 118C. Each of the armatures 146A, 146B, 146C support or are coupled to a set of female or internal drive splines 144A, 144B, 144C respectively. The electric machines 118A, 118B, 118C are so configured that the set of female drive splines 144A, 144B, 144C are each adapted to follow a circumferential path around the axis A.

Located on the radially outward facing surface <NUM> of the hollow shaft <NUM> are three sets of male or external drive splines 142A, 142B, 142C. The hollow shaft <NUM> is so configured that the male drive splines 142A, 142B, 142C mesh with the female drive splines 144A, 144B, 144C coupled to the armature 146A, 146B, 146C respectively.

Supported on the case 150B of the electric machine 118B is a disconnect mechanism in the form of a solenoid <NUM>. The solenoid <NUM> is coupled to the hollow shaft <NUM> via a coupling <NUM>. The engagement of the coupling <NUM> to the hollow shaft <NUM> is such that the coupling remains engaged to the hollow shaft <NUM> when the hollow shaft <NUM> is rotating around the axis A.

Actuation of the solenoid <NUM> causes the hollow shaft <NUM> to be moved between a first position in which the gear sets / splines 130A, 130B, 130C on the hollow cylinder <NUM> and the gear rings / splines 136A, 136B, 136C on the drive shaft <NUM> are fully engaged or meshed with each other, and a second position shown in <FIG> (in which the gear sets / splines 130A, 130B, 130C on the hollow cylinder <NUM> and the gear rings / splines 136A, 136B, 136C on the drive shaft <NUM> are not engaged or meshed with each other).

When the hollow cylinder <NUM> is in the first position the splines 130C closest to the end <NUM> of the drive shaft <NUM> abut the stop ring <NUM>, and the hollow cylinder <NUM> is thus prevented from traveling any further toward or past the end <NUM> of the drive shaft <NUM>.

When the hollow cylinder <NUM> is in the second position the stop element <NUM> abuts the shoulder or stop element <NUM> and prevents the hollow cylinder <NUM> moving too far away from the second end <NUM> of the drive shaft <NUM> and the splines 130A, 130B, 130Cand 136A, 136B, 136C reengaging with each other.

The configuration of the set of female drive splines 144A, 144B, 144C and set of male drive splines 142A, 142B, 142Cis such that those drive splines 142A, 142B, 142C, 144A, 144B, 144C remain meshed or engaged with other throughout the movement of the hollow shaft <NUM> between the first and second positions.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the disclosure. Still other modifications which fall within the scope of the present disclosure will be apparent to those skilled in the art, in light of a review of this disclosure.

Claim 1:
A drive mechanism (<NUM>, 116A, 116B, 116C) for an electric propulsion system (<NUM>, <NUM>) that comprises an electric machine (<NUM>, 118A, 118B, 118C), in which
the drive mechanism (<NUM>, 116A, 116B, 116C) comprises a drive shaft (<NUM>, <NUM>) with a longitudinal axis A, an engagement means, and a disconnect mechanism (<NUM>); the engagement means comprises a drive shaft engagement element (<NUM>) and an electric machine engagement element (<NUM>); wherein
the drive shaft engagement element (<NUM>) comprises a first and second shaft gear ring, and each shaft gear ring is supported on the drive shaft (<NUM>, <NUM>);
each shaft gear ring comprises a plurality of gear teeth (<NUM>) which are circumferentially disposed around the drive shaft(<NUM>, <NUM>), and the gear teeth (<NUM>) extend radially outward from the drive shaft (<NUM>, <NUM>);
the gear teeth (<NUM>) of the first shaft gear ring have an axial length L1;
the first and second shaft gear rings are axially spaced from each other along the drive shaft by a distance L2;
the electric machine engagement element (<NUM>) comprises a first and second gear set;
the first and second gear sets each comprise a plurality of gear teeth (<NUM>) which are configured to mesh with the gear teeth (<NUM>) of the first and second shaft gear rings respectively;
the gear teeth (<NUM>) of the first gear set have an axial length L3;
the first and second gear sets are separated from each other by a length L4 in the axial direction,
length L1 is less than length L4;
length L3 is less than length L2; characterised in that the disconnect mechanism (<NUM>) is configured to reversibly move one of at least part of the drive shaft engagement element (<NUM>) and at least part of the electric machine engagement element (<NUM>) relative to the other of the engagement elements in an axial direction between a first position and a second position;
in which in the first position the gear teeth (<NUM>) of the first shaft gear ring are meshed with the gear teeth (<NUM>) of the first gear set and the gear teeth (<NUM>) of the second shaft gear ring are meshed with the gear teeth (<NUM>) of the second gear set, and in the second position the gear teeth (<NUM>) of the first shaft gear ring are axially between the first and second gear sets and the gear teeth (<NUM>) of the first shaft gear ring are not meshed with the gear teeth (<NUM>) of the first or second gear sets.