Patent Description:
Rescue hoists deploy and retrieve a cable from a cable drum to hoist persons or cargo, and the rescue hoist may be mounted to an aircraft, such as a helicopter. The rescue hoist includes a drum off of which the cable is deployed. The cable drum rotates to spool or unspool the cable from the cable drum, with one end of the cable attached to the cable drum and the other end, which can include a hook or other device, deployed during operation. The cable drum requires a gear reduction between the motor and the cable drum to provide a desired rotational speed of the cable drum. The gear reduction typically includes several shafts arranged parallel to each other. The parallel shafts induce large radial combined forces, thus necessitating robust bearings and other supporting components within the rescue hoist. Moreover, each shaft and gear set must be individually installed and aligned with the various other components of the overall transmission system.

Hoist systems are taught in <CIT> and <CIT> and in <CIT> which also discloses the preamble of claim <NUM>.

According to one aspect of the invention, there is provided a drive train as defined by claim <NUM>.

1A is a perspective view of aircraft <NUM> and rescue hoist <NUM>. <FIG> is a cross-sectional view of rescue hoist <NUM>. 1A and <FIG> will be discussed together. Rescue hoist <NUM> is mounted to aircraft <NUM> by support <NUM>, and cable <NUM> extends from rescue hoist <NUM>. Rescue hoist <NUM> includes frame <NUM>, motor <NUM>, drive train <NUM>, linear bearing <NUM>, cable drum <NUM>, and level wind mechanism <NUM>. Linear bearing <NUM> includes input ring <NUM>. Cable drum <NUM> includes first flange <NUM>, second flange <NUM>, and barrel <NUM>. Barrel <NUM> extends between and connects first flange <NUM> and second flange <NUM>. Level wind mechanism <NUM> includes level wind gear <NUM> and screw <NUM>. Drive train <NUM> includes housing <NUM>. Housing includes first end <NUM> and second end <NUM>, and second end <NUM> includes mounting flange <NUM>. Mounting flange <NUM> includes fastener openings <NUM> and alignment openings <NUM>.

Rescue hoist <NUM> is mounted to aircraft <NUM> by support <NUM>. Cable <NUM> extends from rescue hoist <NUM> and is configured to raise and lower objects to and from aircraft <NUM>. Linear bearing <NUM> is rotatably mounted to frame <NUM>. Motor <NUM> extends from frame <NUM> and is disposed within linear bearing <NUM>. Drive train <NUM> is connected to motor <NUM> and linear bearing <NUM>, and drive train <NUM> is configured to transmit rotational power from motor <NUM> to linear bearing <NUM>. Cable drum <NUM> is mounted to linear bearing <NUM>. Level wind mechanism <NUM> is also mounted to linear bearing <NUM> and extends through cable drum <NUM>. Cable <NUM> wraps around barrel <NUM> of cable drum <NUM> and is retained between first flange <NUM> and second flange <NUM>.

Mounting flange <NUM> extends radially outward from second end <NUM> of housing <NUM>. Fastener openings <NUM> and alignment openings <NUM> extend through mounting flange <NUM>. Drive train <NUM> is installed within linear bearing <NUM> and is directly mounted to frame <NUM>. Mounting flange <NUM> preferably includes two fastener openings <NUM> and two alignment openings <NUM>, but it is understood that mounting flange can include any suitable number of fastener openings <NUM> and alignment openings <NUM>. Fasteners (not shown) extend through fastener openings <NUM> and engage frame <NUM> to secure drive train <NUM> in place. Aligning pins (not shown), such as dowels, are inserted through alignment openings <NUM> and extend into frame <NUM>. The aligning pins extend into frame <NUM> to ensure that drive train <NUM> is properly aligned when installed on frame <NUM>.

During operation, motor <NUM> is activated and provides rotational power to drive train <NUM>. Drive train <NUM> is a gear reduction drive, and drive train <NUM> outputs rotational power to linear bearing <NUM>, thereby causing linear bearing <NUM> to rotate about cable drum axis A-A. An output gear of drive train <NUM> meshes with input ring <NUM> of linear bearing <NUM> to provide rotational power to linear bearing <NUM>. In one embodiment, linear bearing <NUM> is a ball spline bearing, and as such linear bearing <NUM> is capable of transmitting torque to cable drum <NUM> to thereby cause cable drum to rotate about cable drum axis A-A to spool cable <NUM> onto cable drum <NUM> or unspool cable <NUM> from cable drum <NUM>, while also allowing cable drum <NUM> to translate along cable drum axis A-A.

Level wind mechanism <NUM> is mounted to linear bearing <NUM> such that level wind mechanism <NUM> rotates about cable drum axis A-A with linear bearing <NUM>. Level wind gear <NUM> is attached to screw <NUM> and is meshed with teeth on a housing of motor <NUM>. Because the housing of motor <NUM> remains stationary as linear bearing <NUM> rotates, rotating linear bearing <NUM> causes level wind gear <NUM> to rotate due to level wind gear <NUM> meshing with the teeth on the housing of motor <NUM>. Level wind gear <NUM> transmits the resulting rotational power to screw <NUM>, thereby causing screw <NUM> to rotate. Screw <NUM> is connected to cable drum <NUM> through a follower that tracks along a thread of screw <NUM> as screw <NUM> rotates. As screw <NUM> rotates, the follower maintains a connection with the thread of screw <NUM> and tracks along the thread. Level wind mechanism <NUM> thus causes cable drum <NUM> to translate along cable drum axis A-A due to the fixed connection of the follower to cable drum <NUM>. Translating cable drum <NUM> along cable drum axis A-A as cable drum <NUM> rotates about cable drum axis A-A ensures that cable <NUM> is deployed through a single point instead of through a moving elements on rescue hoist <NUM>.

While rescue hoist <NUM> is described as including cable drum <NUM> that translates along cable drum axis A-A, it is understood that cable drum <NUM> can be fixed such that cable drum <NUM> does not translate along cable drum axis A-A. Where cable drum <NUM> does not translate, rescue hoist includes a translating payout point. Drive train <NUM> can be directly meshed with barrel <NUM> of cable drum <NUM> to cause cable drum <NUM> to rotate about cable drum axis A-A. Level wind mechanism <NUM> is meshed with a payout mechanism through which cable <NUM> extends. Level wind mechanism <NUM> rotates with cable drum <NUM> and causes a follower to translate relative to cable drum <NUM>. Cable <NUM> is paid out and retrieved through a follower. The follower translates relative to cable drum <NUM> to ensure that cable <NUM> is levelly wound onto and off of cable drum <NUM>. To ensure level winding of cable <NUM>, the follower is connected to screw <NUM> of level wind mechanism <NUM>, but level wind mechanism <NUM> will be mounted outside of cable drum <NUM>.

Drive train <NUM> provides significant advantages. The gears of drive train <NUM> are disposed within housing <NUM>, which allows drive train <NUM> to be installed as a single unit. Drive train <NUM> thus forms a line replaceable unit that can be quickly installed or uninstalled on rescue hoist <NUM>. Moreover, drive train <NUM> is mounted to rescue hoist <NUM> at mounting flange <NUM> by fasteners and aligning pins. As such, drive train <NUM> requires minimal parts to install or uninstall drive train <NUM>, thereby reducing the complexity of installation and reducing the number of small components that can be misplaced. Drive train <NUM> being a line replaceable unit also provides for easy installation because the various gears of drive train <NUM> are properly positioned and aligned within housing <NUM>. As such, drive train <NUM> is easily installed by inserting drive train <NUM> into linear bearing <NUM>, aligning drive train <NUM> with aligning pins, and securing drive train <NUM> to frame <NUM> with fasteners. Drive train <NUM> being a line replaceable unit also reduces the downtime of rescue hoist <NUM> when drive train <NUM> is replaced because drive train <NUM> can be tested apart from rescue hoist <NUM> to ensure that drive train <NUM> is properly functioning, thereby eliminating additional testing of rescue hoist <NUM>.

<FIG> is an isometric view of drive train <NUM>. <FIG> is a cross-sectional view of drive train <NUM> taken along line <NUM>-<NUM> in <FIG> and <FIG> will be discussed together. Drive train <NUM> includes housing <NUM>, input housing <NUM>, and oil pump <NUM>. Housing <NUM> includes first end <NUM>, second end <NUM>, and drive slots <NUM>. Second end <NUM> includes mounting flange <NUM>. Mounting flange <NUM> includes fastener opening <NUM> and alignment opening <NUM>.

Drive train <NUM> further includes first stage <NUM>, second stage <NUM>, third stage <NUM>, first transmission shaft <NUM>, and second transmission shaft <NUM>. First stage <NUM> includes first epicyclic gear system <NUM> and load brake <NUM>. First epicyclic gear system <NUM> includes first planetary gears <NUM> and first ring gear <NUM>. First ring gear <NUM> includes input end <NUM> and output end <NUM>. Second stage <NUM> includes second epicyclic gear system <NUM> and overload clutch <NUM>. Second epicyclic gear system <NUM> includes second planetary gears <NUM>, second carrier <NUM>, and second ring gear <NUM>. Second carrier <NUM> includes main output <NUM> and auxiliary output <NUM>. Second ring gear <NUM> includes radial flange <NUM>. Third stage <NUM> includes third planetary gears <NUM>. Load brake <NUM> includes brake disc pack <NUM> and output shaft <NUM>. Overload clutch <NUM> includes clutch input <NUM>. First transmission shaft <NUM> includes input end <NUM> and output end <NUM>. Second transmission shaft <NUM> includes input end <NUM> and output end <NUM>.

Mounting flange <NUM> extends radially outward from second end <NUM> of housing <NUM>. Fastener openings <NUM> and alignment openings <NUM> extend through mounting flange <NUM>. Input housing <NUM> is attached to first end <NUM> of housing <NUM> and extends partially into first end <NUM> of housing <NUM>. Drive train <NUM> is installed within linear bearing <NUM> (shown in <FIG>) and is directly mounted to frame <NUM> (shown in <FIG>). Mounting flange <NUM> preferably includes two fastener openings <NUM> and two alignment openings <NUM>, but it is understood that mounting flange <NUM> can include any suitable number of fastener openings <NUM> and alignment openings <NUM>. Fasteners extend through fastener openings <NUM> and engage frame <NUM> to secure drive train <NUM> in place. Aligning pins, such as dowels, are inserted through alignment openings <NUM> and extend into frame <NUM>. The aligning pins extend into frame <NUM> to ensure that drive train <NUM> is properly aligned when installed on frame <NUM>.

First stage <NUM>, second stage <NUM>, third stage <NUM>, first transmission shaft <NUM>, and second transmission shaft <NUM> are disposed within and contained by housing <NUM>. First stage <NUM> is disposed within housing <NUM> proximate first end <NUM>. Second stage <NUM> is disposed within housing <NUM> proximate second end <NUM>. Third stage <NUM> is disposed within housing <NUM> between first end <NUM> and second end <NUM>. First stage <NUM>, second stage <NUM>, third stage <NUM>, first transmission shaft <NUM>, and second transmission shaft <NUM> are all coaxial with drive train axis B-B. It is understood that drive train axis B-B is preferably coaxial with cable drum axis A-A (shown in <FIG>) when drive train <NUM> is installed on rescue hoist <NUM> (best seen in <FIG>).

First planetary gears <NUM> are supported by input housing <NUM>, such that input housing <NUM> is the carrier for first stage <NUM>. Input housing <NUM> holds first planetary gears <NUM> and prevents first planetary gears <NUM> from rotating about drive train axis B-B. Motor shaft <NUM> of motor <NUM> extends into input housing <NUM> and meshes with first planetary gears <NUM> to power drive train <NUM> from motor <NUM>. Input end <NUM> of first ring gear <NUM> extends about and meshes with first planetary gears <NUM>. First ring gear <NUM> is supported on housing <NUM> by a bearing disposed between first ring gear <NUM> and housing <NUM>. First ring gear <NUM> is powered by first planetary gears <NUM> and rotates about drive train axis B-B. Output end <NUM> of first ring gear <NUM> is connected to and powers load brake <NUM>.

Brake disc pack <NUM> and output shaft <NUM> of load brake <NUM> are driven to rotate about drive train axis B-B by first ring gear <NUM>. Load brake <NUM> creates a proportional clamping force across brake disc pack <NUM> in response to tension caused due to a load on cable <NUM> (shown in FIGS. Load brake <NUM> prevents slippage of cable <NUM> through the proportional clamping of brake disc pack <NUM> and thereby facilitates a smooth lowering motion for cable <NUM>. Load brake <NUM> thus assists in controlling the speed at which cable <NUM> is lowered from rescue hoist <NUM>. Load brake <NUM> also resists movement of cable <NUM> when motor <NUM> is not activated. First stage <NUM> provides a speed reduction between motor <NUM> and second stage <NUM>, and as such, load brake <NUM> rotates one stage slower than motor <NUM>. Output shaft <NUM> of load brake <NUM> is connected to input end <NUM> of first transmission shaft <NUM>. Output shaft <NUM> preferably includes a female spline configured to receive a male spline of input end <NUM> of first transmission shaft <NUM>. Output shaft <NUM> is supported by a bearing disposed between output shaft <NUM> and housing <NUM> and a bearing disposed between output shaft <NUM> and input housing <NUM>. Load brake <NUM> provides rotational power to first transmission shaft <NUM> through output shaft <NUM>.

First transmission shaft <NUM> extends between output shaft <NUM> and second stage <NUM> and provides power to second stage <NUM> from first stage <NUM>. First transmission shaft <NUM> extends along and is configured to rotate about drive train axis B-B. Output end <NUM> of first transmission shaft <NUM> extends into second stage <NUM> and meshes with second planetary gears <NUM>. As such, output end <NUM> of first transmission shaft <NUM> forms an input sun gear for second epicyclic gear system <NUM> of second stage <NUM>. Output end <NUM> of first transmission shaft <NUM> is also connected to and powers oil pump <NUM>. Output end <NUM> of first transmission shaft <NUM> can include both a male spline and a female spline to connect to and power both second stage <NUM> and oil pump <NUM>.

Second planetary gears <NUM> are supported by second carrier <NUM>. Second carrier <NUM> is supported for rotation about drive train axis B-B by a second bearing. Second ring gear <NUM> extends about second planetary gears <NUM>. Radial flange <NUM> extends from second ring gear <NUM> and is disposed adjacent mounting flange <NUM>. Radial flange <NUM> is attached to mounting flange <NUM> by fasteners extending through fastener openings <NUM>. Second ring gear <NUM> is held stationary by the connection of radial flange <NUM> and mounting flange <NUM> such that second ring gear <NUM> does not rotate about drive train axis B-B.

Auxiliary output <NUM> of second carrier <NUM> is disposed outside of housing <NUM>. Auxiliary output <NUM> is configured to provide power to auxiliary components of rescue hoist <NUM> (shown in FIGS. 1A-1B), such as a traction sheave. Main output <NUM> of second carrier <NUM> is connected to clutch input <NUM> of overload clutch <NUM>. In one embodiment, main output <NUM> is splined to input ring <NUM> of overload clutch <NUM>. It is understood that main output <NUM> can include either a male spline or a female spline and that clutch input <NUM> can correspondingly include either a female spine or a male spline. Clutch input <NUM> of overload clutch <NUM> is rotationally supported on housing <NUM> by a bearing disposed between clutch input <NUM> and housing <NUM>.

Input ring <NUM> is driven by second carrier <NUM> and input ring <NUM> provides rotational power to overload clutch <NUM> through a clutch disc pack. Input ring <NUM> and overload clutch <NUM> are configured to rotate about drive train axis B-B. Overload clutch <NUM> is connected to input end <NUM> of second transmission shaft <NUM>. Torque from second carrier <NUM> is transmitted to second transmission shaft <NUM> through overload clutch <NUM>. The clutch disk pack of overload clutch <NUM> is configured to slip when a load on cable <NUM> reaches a set point that is greater than the rated load of rescue hoist <NUM>, thereby decoupling second transmission shaft <NUM> from second stage <NUM>, first transmission shaft <NUM>, load brake <NUM>, first stage <NUM>, and motor <NUM>. As such, the excess load on cable <NUM> is prevented from being transmitted to motor <NUM> by overload clutch <NUM>, thereby providing overload protection.

Second transmission shaft <NUM> extends concentrically with first transmission shaft <NUM> about drive train axis B-B. Second transmission shaft <NUM> provides rotational power to third stage <NUM> from overload clutch <NUM> of second stage <NUM>. Output end <NUM> of second transmission shaft <NUM> extends into third stage <NUM> and meshes with third planetary gears <NUM> such that output end <NUM> of second transmission shaft <NUM> forms the input sun gear for third stage <NUM>. Third planetary gears <NUM> are disposed in drive slots <NUM> and supported on housing <NUM>. Housing <NUM> thus forms the carrier for third stage <NUM>. Third planetary gears <NUM> extend through drive slots <NUM> and are configured to mesh with input ring <NUM> (shown in <FIG>) and provide rotational power to input ring <NUM>, thereby causing linear bearing <NUM> (shown in <FIG>) and cable drum <NUM> (shown in <FIG>) to rotate. With third planetary gears <NUM> supported by housing <NUM>, third planetary gears <NUM> are prevented from rotating about drive train axis B-B.

During operation, motor <NUM> is activated to cause cable <NUM> to spool onto or unspool from cable drum <NUM>. Output shaft <NUM> of motor <NUM> meshes with and causes first planetary gears <NUM> to rotate. First planetary gears <NUM> are held stationary relative to drive train axis B-B by input housing <NUM>. First planetary gears <NUM> cause first ring gear <NUM> to rotate about drive train axis B-B and first ring gear <NUM> provides power to load brake <NUM> thereby causing load brake <NUM> to rotate about drive train axis B-B and provide power to first transmission shaft <NUM>. First stage <NUM> thus provides power to first transmission shaft <NUM> from motor <NUM>. First epicyclic gear system <NUM> provides a first speed reduction such that load brake <NUM> and first transmission shaft <NUM> rotate one stage slower than output shaft <NUM> of motor <NUM>.

First transmission shaft <NUM> transmits power to second stage <NUM>. Output end <NUM> of first transmission shaft <NUM> meshes with and causes second planetary gears <NUM> to rotate. Second planetary gears <NUM> and second carrier <NUM> rotate about drive train axis B-B, and second carrier <NUM> provides rotational power to overload clutch <NUM> causing overload clutch <NUM> to rotate about drive train axis B-B. Overload clutch <NUM> is connected to second transmission shaft <NUM> and transmits torque from second stage <NUM> to second transmission shaft <NUM>. Overload clutch <NUM> thus powers second transmission shaft <NUM> to rotate about drive train axis B-B. Second epicyclic gear system <NUM> provides a second speed reduction such that overload clutch <NUM>, and thus second transmission shaft <NUM>, rotate one stage slower than first transmission shaft <NUM> and load brake <NUM>. Second transmission shaft <NUM> thus rotates two stages slower than output shaft <NUM> of motor <NUM>.

Second transmission shaft <NUM> transmits rotational power to third stage <NUM>. Third planetary gears <NUM> are supported by housing <NUM> and provide power to linear bearing <NUM> from second transmission shaft <NUM>. Third stage <NUM> provides a speed reduction between second transmission shaft <NUM> and linear bearing <NUM> causing linear bearing <NUM> to rotate one stage slower than second transmission shaft <NUM>. Third stage <NUM> thus provides a third speed reduction between motor <NUM> and linear bearing <NUM>, such that linear bearing <NUM> rotates three stages slower than output shaft <NUM> of motor <NUM>. While third planetary gears <NUM> are described as meshing with input ring <NUM> of linear bearing <NUM>, it is understood that in other embodiments, third planetary gears <NUM> can mesh directly with an input ring disposed on an inner surface of barrel <NUM> (shown in <FIG>) of cable drum <NUM>. As such, third planetary gears <NUM> can be either directly or indirectly connected to cable drum <NUM> to provide rotational power to cable drum <NUM>.

Drive train <NUM> provides significant advantages. First stage <NUM>, second stage <NUM>, third stage <NUM>, first transmission shaft <NUM>, and second transmission shaft <NUM> all share drive train axis B-B, thereby reducing the forces on the supporting bearings of drive train <NUM>. Because the driving components are aligned on drive train axis B-B, the radial forces cancel out, so smaller bearings are utilized in drive train <NUM>, thereby reducing the material cost and the weight of drive train <NUM>. Moreover, drive train <NUM> is relatively compact, thereby reducing the footprint of rescue hoist <NUM>.

A drive train includes a housing having a first end and a second end, a first stage disposed within the housing proximate the first end, a second stage disposed within the housing proximate the second end, and a third stage disposed within the housing between the first stage and the second stage. The first stage, the second stage, and the third stage are disposed coaxially on a drive train axis.

The drive train of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
The first stage includes a first stage epicyclic gear system configured to receive an output shaft of a motor as a sun gear of the first stage epicyclic gear system, and a load brake connected to and driven by the first stage epicyclic gear system. The first stage epicyclic gear system and the load brake disposed coaxially on the drive train axis.

The second stage includes a second stage epicyclic gear system configured to receive an output of the first stage as an input sun gear of the second stage epicyclic gear system, and an overload clutch connected to and driven by the second stage epicyclic gear system. The second stage epicyclic gear system and the overload clutch are disposed coaxially on the drive train axis.

The second stage epicyclic gear system includes a plurality of second stage planetary gears supported by a second stage carrier, the second stage carrier configured to rotate about the drive train axis, and the second stage carrier including a main output connected to and driving the overload clutch.

The second carrier further comprises an auxiliary output extending outside of the housing.

A first transmission shaft extending between and connecting the first stage and the second stage, and a second transmission shaft extending between and connecting the second stage and the third stage.

The first transmission shaft extends through the second transmission shaft, and the first transmission shaft and the second transmission shaft are disposed coaxially on the drive train axis.

The first transmission shaft includes a first input end and a first output end, the first input end connected to a load brake of the first stage, and the first output end connected to a plurality of second stage planetary gears of the second stage, the first output end forming a second stage sun gear. The second transmission shaft includes a second input end and a second output end, the second input end connected to at least one friction disc of an overload clutch of the second stage, and the second output end connected to a plurality of third stage planetary gears of the third stage, the second output end forming a third stage sun gear.

An input housing extending into and connected to the first end of the housing, the input housing supporting a plurality of first stage planetary gears of the first stage.

The housing supports a plurality of third stage planetary gears of the third stage.

The third stage planetary gears are disposed in slots extending through the housing.

A rescue hoist includes a cable drum rotatable about a cable drum axis, a stationary frame supporting the cable drum, and a drive train disposed within the cable drum and configured to drive the cable drum about the cable drum axis. The drive train includes a housing having a first end and a second end disposed opposite the first end, a first stage disposed within the housing proximate the first end, a second stage disposed within the housing proximate the second end, and a third stage disposed within the housing between the first stage and the second stage. The first stage, the second stage, and the third stage are disposed coaxially on the cable drum axis.

The rescue hoist of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
The housing directly supports a plurality of third stage planetary gears of the third stage, the plurality of third stage planetary gears disposed in slots extending through the housing.

A linear bearing extending through the cable drum, the cable drum mounted on the linear bearing. The drive train is disposed within the linear bearing, and the plurality of third planetary gears directly engage an input ring of the linear bearing.

A mounting flange extending radially from the second end, the mounting flange attached to the stationary frame.

At least one fastener opening extending through the mounting flange, the at least fastener opening configured to receive a fastener to secure the housing to the stationary frame. At least one alignment opening extending through the mounting flange, the at least one alignment opening configured to receive an aligning pin to position the drive train on the stationary frame.

The first stage includes a first stage epicyclic gear system, the first stage epicyclic gear system configured to receive an output shaft of a motor as a first stage sun gear of the first stage epicyclic gear system, and a load brake connected to and driven by the first stage epicyclic gear system. The first stage epicyclic gear system and the load brake are disposed coaxially on the cable drum axis.

The second stage includes a second stage epicyclic gear system configured to receive an output of the first stage as a second stage sun gear of the second stage epicyclic gear system, and an overload clutch connected to and driven by the second stage epicyclic gear system. The second stage epicyclic gear system and the overload clutch are disposed coaxially on the drive train axis.

The drive train further includes a first transmission shaft extending between and connecting the first stage and the second stage, the first transmission shaft coaxial with the cable drum axis. An output end of the first transmission shaft being the second stage sun gear.

A second transmission shaft extending between and connecting the second stage and the third stage, the second transmission shaft coaxial with the cable drum axis. The first transmission shaft extends through the second transmission shaft. An output end of the second transmission shaft is a third stage sun gear for a plurality of third stage planetary gears of the third stage.

Claim 1:
A drive train comprising:
a housing (<NUM>) having a first end (<NUM>) and a second end (<NUM>);
a first stage (<NUM>) disposed within the housing proximate the first end;
a second stage (<NUM>) disposed within the housing proximate the second end; and
a third stage (<NUM>) disposed within the housing between the first stage and the second stage;
a first transmission shaft (<NUM>) extending between and connecting the first stage and the second stage; and
a second transmission shaft (<NUM>) extending between and connecting the second stage and the third stage;
wherein the first stage, the second stage, and the third stage are disposed coaxially on a drive train (<NUM>) axis; and characterized in that
the first transmission shaft extends through the second transmission shaft, and the first transmission shaft and the second transmission shaft are disposed coaxially on the drive train axis.