Integrated electric motor and traction drive

An integrated electric motor and traction drive is disclosed. The device comprises an electric motor and a traction drive. The electric motor provides power at a high angular velocity to a sun roller. The sun roller transfers the power to the traction drive which reduces the power to a lower angular velocity and delivers it via an output shaft.

BACKGROUND OF THE INVENTION

Electric motors are one of the most widely used machines. The applications range from automobiles to earth movers, from aerospace to marine machinery, from home appliances to medical equipment. Recently, there have been increasing demands for motors with greater power density. For the purposes of this application, power density can be defined as the amount of power delivered either per unit weight or per unit volume, expressed respectively as W/kg or W/liter. One way to increase power density is to elevate the speed of a motor. As the speed of a motor increases, so does its overall power. As a result, there is a trend in machine designs toward using smaller electric motors operating at higher speeds. In some applications, this results in the speed of the motor being higher than the required speed of the driven member. Therefore, it is often deemed necessary to include a speed reduction unit between the motor and the driven member to reduce the speed of the motor to the required speed of the driven member. Although this results in an overall higher power density, the speed reduction unit still limits power density because of the additional weight and volume.

One solution to this is a so-called gear-head motor where a gear reduction unit is integrated with an electric motor. There are many types of gear-head motors including “precision” gear-head motors, which are capable of running at higher speeds and generally are much more expensive than “regular” gear head motors. However, even with precision-made gear heads, gear-head motors are often limited to operating speeds of 5,000 to 6,000 rpm. This has, to a large degree, prevented the gear-head motors from achieving their ultimate power-density potentials.

Recent developments in traction drives have demonstrated that a well-built traction drive can operate at higher speeds up to and exceeding 10,000 rpm and cost much less than gear-head drives. Thus, integrating a traction drive with an electric motor can increase the system power-density potential and thus extend the scope of application of electric motors.

SUMMARY OF THE INVENTION

Briefly stated, the invention is a motor supplying power at a high angular velocity integrated with a traction drive for receiving the power at a high angular velocity and delivering the power at a lesser angular velocity. The motor comprises a stator, a rotor that revolves in the stator at a high angular velocity, and a sun roller with a first raceway affixed to the rotor. The traction drive comprises a carrier, an outer ring member with an output shaft and a fourth raceway eccentric to the first raceway of the sun roller, and a loading planetary roller supported by the carrier with a third raceway. The third raceway engages with the first raceway of the sun roller and the fourth raceway of the outer ring in a convergent wedge formed by the first and fourth raceways for transferring power between the sun roller and the outer ring. The output shaft of the outer ring delivers power at a lesser angular velocity.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description illustrates the invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the invention, describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believed to be the best mode of carrying out the invention.

As shown inFIGS. 1 and 2, an embodiment of an integrated electric motor and traction drive1comprises an electric motor100and a traction drive200. As used in this specification, “integrated” is defined as “combining multiple parts to form a single unit.”

As shown inFIGS. 1–3, the electric motor100comprises a motor housing110, a motor cover120, a bearing cup plate130, a stator140, a rotor150, and a sun roller153. The motor housing110is a hollow cylinder defining a front face111with holes112for attaching to the motor cover120, a back face113with holes114for attaching to the speed reducer200, an inner surface115for affixing to the stator140, and cooling fins116extending from an outer surface117to aid with the dispersal of heat produced during operation. The motor cover120is a disk with a raised concentric annulus defining a front face121with holes122for attaching to the bearing cup plate130, a back face123with holes124for attaching to the front face111and respective holes112of the motor housing110, and a first bore125for supporting the sun roller153via a bearing155. The bearing cup plate130is a disk defining a back face131with holes132for attaching to the front face121and respective holes122of the motor cover120. The stator140is a hollow cylinder with an outer surface141for affixing to the inner surface115of the motor housing110and internal poles142that are energized by electrical power to form magnetic poles for engaging the rotor150. The rotor150is a hollow cylinder defining a second bore for affixing to the sun roller153and external male poles152for engaging the internal female poles142of the stator140. The sun roller153is a shaft defining a first raceway154for engaging with the traction drive200.

As well known to those skilled in the art, electric connections are provided to supply electric power to and from the windings of the internal poles142of the stator140. For easier viewing, the windings are not shown in the drawings. While the embodiment inFIGS. 1–3discloses a typical switched reluctance motor, other types of motors may be used such as, brushless motors, DC motors, and AC induction motors. While the embodiment inFIGS. 1–3discloses a rotor150that is formed by laminated plates, other types of rotors may be used.

As shown inFIGS. 1–3, the traction drive200comprises a carrier210, a loading planetary roller230, supporting planetary rollers245, an outer ring member250, a double row bearing260, and a traction drive housing270. The carrier210comprises a base plate211and a cover plate220. The base plate211is a disk with a raised concentric annulus defining a front face212, a back face213, holes214for attaching to the motor housing110and the traction drive housing270, an obround pinhole215for supporting the loading planetary roller230, pinholes216for supporting the supporting planetary rollers245, a third bore217for supporting the sun roller153, and arcuately shaped wedges on the back face213, hereby referred to as islands218. The islands218act as spacers between the base plate211and cover plate220defining cavities for receiving supporting planetary rollers245and the loading planetary roller230. The cover plate220is a disk defining an obround pinhole221for supporting the loading planetary roller230, pin holes222for supporting the supporting planetary rollers245, and a through hole223for hosting the sun roller153.

The supporting planetary rollers245comprise pin shafts246and bearings247. The bearings247define second raceways248for engaging with the first raceway154of the sun roller153. The bearings247affix to the pin shafts246so that the second raceways248rotates freely. The pin shafts246insert into the pinholes216of the base plate211and the pinholes222of the cover plate220so that the supporting planetary rollers245reside within the cavities defined by the islands218.

The loading planetary roller230comprises a pin shaft231, an elastic insert232, a support bearing235and a roller238. The elastic insert232is circularly shaped with an outer surface233and a center hole234. The support bearing235is a circular anti-friction bearing, such as a ball bearing, with an inner race236and an outer race237. The roller238is also circularly shaped with an inner surface239and a third raceway240. When assembled, the support bearing235affixes to the elastic insert232with its inner race236fitted tightly over the outer surface233of the insert232. The roller238is fitted to the support bearing235with an interference fit between its inner surface239and the outer race237of the support bearing235so that the roller238can rotate freely. Next, the elastic insert232is affixed to the pin shaft231by inserting the pin shaft231through the center hole234of the elastic insert232. The pin shaft231is inserted into the pinhole215of the base plate211and the pinhole221of the cover plate220so that the third raceway240engages the first raceway154of the sun roller153and a fourth raceway251of the outer ring250. The obround shape of the pinholes215and221allow the pin shaft231to slide back and forth slightly. During operation, this allows the loading roller238to automatically shift to an effective position for the third raceway240of the loading roller238to engage in a convergent wedge between the first raceway154of the sun roller153and a fourth raceway251of the outer ring250allowing power to be transferred between the sun roller153and the outer ring250. While the pinshaft231shown inFIGS. 1–3is shown to be supported by pinholes215and221, it is also possible to have the pinshaft231supported by the carrier210through springs or elastomers.

The outer ring member250is an annular ring defining the fourth raceway251eccentric to the first raceway154of the sun roller153for engaging the third raceway240of the loading planetary roller230and the second raceways248of the supporting planetary rollers245, an output shaft252for transferring power, and spokes253connecting the fourth raceway251and output shaft252to accommodate any possible misalignment between the fourth raceway251and output shaft252. The output shaft252is supported by a back-to-back arranged double-row bearing260.

The traction drive housing270is a hollow cylinder with a raised concentric annulus defining a front face271with holes272for attaching to the base plate211and respective holes214, a fourth bore273for receiving the double row bearing260, and a back face274for external mounting.

To assemble the embodiment, the back face131of the bearing cup plate130is attached to the front face121of the motor cover120by aligning holes132with respective holes122and using appropriate mechanical means, such as bolts or rivets. Similarly, the back face123of the motor cover120is attached to the front face111of the motor housing110by aligning holes124with respective holes112and using appropriate mechanical means, such as bolts or rivets. The stator140is inserted in to the motor housing110so that the outer surface141of the stator140is affixed to the inner surface115of the motor housing110.

Bearing156is affixed to the sun roller153adjacent to the first raceway154for rotational support. A sleeve spacer159is affixed adjacent to the bearing156. The rotor150is affixed to the sun roller153adjacent to the sleeve spacer159. A nut157is affixed to the sun roller153to secure the rotor150. Bearing155is affixed to the end of the sun roller153opposite the first raceway154for rotational support. As shown inFIGS. 3 and 4, the sun roller153and rotor150insert into the stator140so that the bearing155affixes to the first bore125of the motor cover120and the bearing156affixes to the third bore217of the base plate211. In this position, the sun roller153and the rotor150rotate freely within the stator140. In addition, the first raceway154extends past the base plate211so that the first raceway154rotates within the carrier210.

To assemble the carrier210, the cover plate220attaches to the islands218of the base plate211. The pinshafts246of the supporting planetary rollers245are inserted into the pinholes216of the base plate211and the pinholes222of the cover plate220so that the second raceways248frictionally engage the first raceway154of the sun roller153. The pinshaft231of the loading planetary roller230is inserted into the pinhole215of the base plate211and the pinhole221of the cover plate220so that the third raceway240frictionally engages the first raceway154of the sun roller153.

The double row bearing260affixes to the fourth bore273of the traction drive housing270. To further secure the double row bearing260, a snap ring275may be used. The double row bearing260affixes to the output shaft252of the outer ring member250so that the outer ring member250can rotate freely. The front face271of the traction drive housing270attaches to the back face213of the base plate211by aligning holes272with respective holes214and using appropriate mechanical means, such as bolts or rivets. In this position, the fourth raceway251of the outer ring member250frictionally engages the third raceway240of the loading planetary roller230and the second raceways248of the supporting planetary rollers245.

In operation, electric power is supplied to the windings of the internal female poles142causing the rotor150and sun roller153to rotate and transfer power at a high angular velocity. Power is transferred from the first raceway154of the sun roller153to the second raceways248of the supporting rollers245and the third raceway240of the loading planetary roller230. Then, power is transferred from the second raceways248and the third raceway240to the fourth raceway251of the outer ring250. Finally, power is transferred via the spokes253of the outer ring250to the output shaft252where it is output at a lesser angular velocity.

As the sun roller153rotates inFIG. 5, the friction force FR(traction) generated at the contact between the first raceway154and third raceway240of the loading planetary roller230tends to rotate the loading roller230and generate a reaction friction force FRat the contact between the third raceway240and the fourth raceway251of the outer ring250. These friction forces pull the loading roller230into a converged wedge gap between the sun roller153and the outer ring250in either direction depending upon the rotation direction of the sun roller153. The friction forces FRare balanced by normal contact forces N at the contacts between the first raceway154and third raceway240and between the fourth raceway251and third raceway240, and by a supporting force FSprovided from carrier210via pin shaft231, elastic insert232, and support bearing235to the loading roller238.

The amount of normal force N generated in response to friction force FRis controlled by the supporting stiffness KSof the loading roller230assembly in relationship with the contact stiffness at the contacts along with the structural flexibility of outer ring250and the flexibility of other relevant components. Assume the lumped effective contact stiffness, representing Hertzian contact stiffness, structural flexibility of outer ring250and all other relevant components, be denoted as KR. The following relationship generally holds true.KSKR=μo⁢sin⁢⁢δ-2⁢sin2⁡(δ2)(1)
whereKS=effective support stiffness of loading rollerKR=effective contact stiffness between the loading roller and the sun roller and between the loading roller and the outer ringμo=operating traction coefficientδ=operating wedge angle (different from initial wedge angle)

To prevent the traction drive200from excessive slip at the contacts, the following inequality must be held.KSKR=μo⁢sin⁢⁢δ-2⁢sin2⁡(δ2)≤μm⁢sin⁢⁢δ-2⁢sin2⁡(δ2)(2)
whereμm=maximum available traction coefficient.

The second raceways248of the supporting rollers245are placed between and in contact with the first raceway154of the sun roller153and fourth raceways251of the outer ring250. The supporting rollers245provide appropriate forces at the contacts between the outer ring250and the respective supporting rollers245to balance out the contact forces at the contact between the outer ring250and the loading roller230. Likewise, the supporting rollers245provide appropriate forces at contacts between the sun roller153and the supporting rollers245to balance out the contact forces at the contact between the sun roller153and the loading roller230. Thus, forces acting on the outer ring250and the sun roller153are internally self-balanced.

As can be appreciated, the frictional forces may also be generated at the contacts between the supporting rollers245and the outer ring250and between the supporting rollers245and the sun roller153. These friction forces can also help to transmit torque and power between the sun roller153and outer ring250.

For efficiency considerations, conventional traction drives have to operate with a wedge angle smaller but close to the so-called friction angle δfdefined as:
δf=2 Arc tan μ  (3)
where μ is the friction coefficient at the contact.

This imposes an undesirable design constraint on the azimuth position of the loading roller230since the wedge angle δ is directly related to the azimuth position α of the loading roller230in relation to the eccentricity e of the sun roller's first raceway153with respect to the outer ring's fourth raceway251.

As indicated by equation (2), by choosing appropriate ratio of effective supporting stiffness KSto effective contact stiffness KRit is possible to operate the traction drive200at a wide range of given operating wedge angle regardless of the friction coefficient, without sacrificing the drive's efficiency. That is to say, the traction drive200is capable of operating with operating traction coefficient close to the maximum available value even at a small wedge angle. This allows the loading roller230to be placed at or in vicinity to the azimuth position corresponding to the widest wedge gap. Consequently, the same loading roller230can be used as a bi-directional loading mechanism, substantially simplifying the design and construction of the traction drive200.