Patent Application
20060135305
Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to a belt drive mechanism that uniquely combines the desirable characteristics of a traditional harmonic drive mechanism with the desirable characteristics of a direct belt drive. In particular, the invention is a harmonic belt drive that directly advances an elongated belt useful for rotating or translating a mechanical load.
2. Description of Related Art
Harmonic drives and belt drives are well known in the art. Both have desirable properties that would be of great advantage to industry if properly combined. Harmonic drives have favorable properties such as nearly zero backlash and very high gear ratios as well as compact and lightweight construction.
Belt drives have the useful property of transferring mechanical power to remote offset loads. For example, the rotary motion of a rotating pulley mounted to the output shaft of a drive mechanism can be communicated to a remote torque load by way of a belt engaging both a pulley mounted to the input shaft of the torque load and the pulley connected to the output shaft of the drive mechanism. The following list of U.S. patents disclose various harmonic drive mechanisms or belt drive mechanisms:
Of particular interest is U.S. Pat. No. 2,906,143 to C. W. Musser, which discloses the original harmonic drive. Harmonic drives have three main elements, a multi-lobe cam commonly referred to as a strain wave generator, an elastic radially deflectable gear known as a flexspline and a usually stationary gear known as a rigid circular spline or ring gear.
The modern configuration for a flexspline is that of a thin cup-shaped element having an outside, an inside, a closed end, an open end and an adjacent toothed portion arranged around the outside of the open end of the cup. The ring gear has internal radially extending teeth that have a slightly different pitch diameter from the teeth of the flexspline. The multi-lobe cam or wave generator is concentrically rotatable within the flexspline and the flexspline is concentrically rotatable within the ring gear. The lobes of the multi-lobe cam deflect the flexspline from the inside, meshing a number of flexspline teeth with a number of ring gear teeth. For each revolution of the strain wave generator the flexspline advances by a small number of teeth. Exceedingly high gear ratios can be achieved by proper selection of pitch diameter differential and flexspline deflection distance.
The output of a harmonic drive can be taken from either the flexspline or the ring gear. With the ring gear held stationary the flexspline is the driven element and a torque load is usually axially coupled to the closed end of the flexspline. Conversely, if the flexspline is held fast, the ring gear becomes the driven element and a torque load is coupled to it. One disadvantage, in either case, is that extra members such as belts, pulleys and couplings are necessary to transmit power to remote loads.
One attempt to combine the useful properties of the harmonic drive with that of a belt drive is presented in U.S. Pat. No. 5,749,800 to Nagel et al. The disclosure describes a synchronous belt drive having a toothed belt having two rows of teeth with different pitches. The toothed belt engages a driven wheel, a return wheel and two interconnected wheels secured against independent rotation. In contrast to the harmonic drive, the “gear ratio” of the synchronous belt is a function of belt length as well as tooth pitch. Also, the synchronous belt drive does not rely on a multi-lobe cam to mesh the teeth of the belt with any other element. A disadvantage of the synchronous belt drive is its larger number of key elements compared to the harmonic belt drive. What is needed is a belt drive mechanism that combines the desirable properties of both a harmonic drive and a belt drive while at the same time reducing the number of key elements needed with either technology.
The present invention addresses these needs by providing a unique belt drive that combines useful properties of a traditional harmonic drive mechanism with the useful remote load motion transfer capability of a traditional belt drive. For instance, the present invention retains the high gear ratio of a traditional harmonic drive mechanism while directly driving a belt coupled to a remote load. For the purposes of this disclosure the term teeth used herein means a series of equally spaced equally sized projections or indentions.
In general, the present invention is a harmonic belt drive that comprises a rigid arcuate toothed member having an internal surface, an external surface and a row of teeth radially extending from the internal surface. A belt portal extends through the arcuate toothed member connecting the internal and external surfaces of the arcuate toothed member.
A multi-lobe cam for driving a toothed belt is rotatably mounted concentric within the arcuate toothed member and an elongated toothed belt is disposable along a belt pathway between the multi-lobe cam and the arcuate toothed member. The toothed belt has an inside surface and an outside surface. Teeth on the belt's outside surface are meshable with the teeth on the inside surface of the arcuate toothed member. The inside surface of the belt may also include teeth useful for driving a toothed motion transfer device such as a toothed pulley coupled to a rotatable load.
Furthermore, the toothed belt is radially deflectable from its inside surface by the multi-lobe cam such that a contact patch or area containing a number of belt teeth is forced to mesh with a number of arcuate toothed member teeth. A slight difference in the number of teeth between the arcuate toothed member teeth meshing with belt teeth must exist in order to advance the position of the toothed belt relative to the arcuate toothed member. This difference exists in the number of teeth per area of contact or mesh created by the shorter distance between teeth contact points on the toothed belt relative to the distance between teeth contact points on the rigid arcuate toothed member, or vice versa.
The present invention has some similarities with a traditional harmonic drive mechanism. For example, the arcuate toothed member of the present invention functions much like the circular spline of a traditional harmonic drive. However, a major structural difference exists in that the arcuate toothed member of the present invention has a belt portal for the toothed belt to enter and exit the confines of the arcuate toothed member.
The portal is an opening that radially extends from the inside surface of the arcuate toothed member to the outside surface of the arcuate toothed member. The circular spline of a traditional harmonic drive has no equivalent portal.
Furthermore, the toothed belt of the present invention functions similar to the flexspline of a traditional harmonic drive. Here too, a major structural difference exists. The difference is that the total length of the toothed belt of the present invention can be many times longer than the length of the belt pathway situated adjacent the teeth of the arcuate toothed member, whereas the circumference of the flexspline is necessarily shorter than the inside circumference of the circular spline.
Another distinguishing feature of the present invention is its ability to transfer power to a load coupled directly to the enlongated belt. An example of such an application in which this feature can be utilized is with a garage door opener. Moreover, the toothed belt of the present invention does not need to be endless. In fact, the toothed belt of the present invention can have two ends, each adapted to attach to separate loads. For example, each end of the belt could be attached to separate loads pulled down by the force of gravity. This way, as the toothed belt is forced to move in one direction, one load ascends while the other load descends. In contrast, a traditional harmonic drive would be inoperable if its flexspline had ends.
In even greater contrast, the toothed belt of the present invention carries its teeth outside the confines of the arcuate toothed member in order to directly transfer motion to a remote mechanical load. The teeth of a flexspline in a traditional harmonic drive are always confined within the perimeter of the circular spline. Therefore, additional elements such as couplings and pulleys must be used in order to drive a load.
The multi-lobe cam of the present invention functions much the same as the multi-lobe cam or strain wave generator found in a traditional harmonic drive, both are designed to induce a strain wave in their respective toothed belt or flexspline. However, the multi-lobe cam of the present invention cannot have all the lobes of the multi-lobe cam deflecting the toothed belt at all times. The reason for this is that each lobe of the multi-lobe cam of the present invention must pass once per rotation within a space opened by the belt portal extending between the inside surface and outside surface of the arcuate toothed member. On the other hand, all the lobes belonging to a multi-lobe cam of a traditional harmonic drive mechanism must always deflect some portion of the drive's flexspline because the flexspline and multi-lobe cam are completely encompassed by a circular spline.
In operation, a motor or other prime mover rotates the multi-lobe cam. As the multi-lobe cam rotates, each lobe at some point in time enters the confines of the arcuate toothed member. While within the confines of the arcuate toothed member each lobe radially deflects a portion of the toothed belt against a number of teeth belonging to the arcuate toothed member. This action forces a patch or contact area of teeth belonging to the toothed belt to mesh with a patch of teeth belonging to the arcuate toothed member. As a result, the toothed belt forcefully advances through the belt pathway around the inside perimeter of the arcuate toothed member by a small number of teeth with each rotation of the multi-lobe cam. The toothed belt's direction of travel opposes the direction of rotation of the multi-lobe cam. The toothed belt enters and exits through the belt portal extending from the inside surface to the outside surface of the arcuate toothed member as the toothed belt advances around the inside perimeter of the arcuate toothed member. It has been determined that at least one idler pulley, preferably two, mounted on a faceplate above the belt portal can provide sufficient belt tension to prevent undesirable belt vibrations.
The toothed belt of the present invention is not limited to a toothed belt as defined in the normal sense. Instead, the toothed belt of the present invention can be a link chain like those used to anchor a ship or the toothed belt can be a small ball chain similar to those used to raise and lower household blinds. Moreover, it is possible that the toothed belt could have relatively smooth surfaces without discrete teeth. An example of such a belt would be grocery checkout conveyor belt like used with supermarket checkout counters. Such a belt would be forced by frictional forces between the arcuate toothed member and multi-lobe cam to advance through the belt pathway.
An important aspect of the invention is that sections of the toothed belt external the confines of the arcuate toothed member are used to directly put in motion remote mechanical loads. Such loads can be, but are not limited to printer print heads, scanners, household blinds, garage doors and electric windows in cars.
Another aspect of the invention is the ability to disengage the toothed belt from the arcuate toothed member for the purpose of freely moving the toothed belt through the belt pathway by a means other than rotation of the multi-lobe cam. One way to disengage the toothed belt from the arcuate toothed member is to retract one or more spring-loaded lobes of the multi-lobe cam. Another way is to separate the arcuate toothed member away from the belt and multi-lobe cam, thereby releasing the toothed belt allowing it to be pulled freely through the belt pathway.
Other aspects of the invention is its smaller size, fewer components, lower cost and simpler installation when compared to comparable prior art belt drives. Furthermore, in certain ultra-clean applications such as those found in the medical and semi-conductor industries, the present invention can be operated dry, i.e. without grease or oil lubricants. Dry operation of the invention also solves low viscosity lubrication problems that plague many prior art belt drives that must be able to operate in cold weather environments. Yet still other aspects of the present invention will become apparent to one skilled in the art upon a reading of the following detailed description of the invention, taken with the drawings.
In the following description, terms such as left, right, top, bottom and the like are used solely for the purpose of clarity in illustrating the invention, and should not be taken as words of limitation. The drawings are for the purpose of illustrating the invention and are not intended to be to scale.
The present invention is a harmonic belt drive generally 10, shown in
In the preferred embodiment, multi-lobe cam 14 is rotated by an electric motor 26. However, multi-lobe cam 14 can be driven by other prime movers such as an internal combustion engine, stepper motor or hand crank. Furthermore, in the preferred embodiment, multi-lobe cam 14 has a unique construction. Unlike the rigid elliptoidal multi-lobe cams used with traditional harmonic drive mechanisms, multi-lobe cam 14 of the present invention is constructed from an assembly that includes planetary rollers 28 and 30, rotatably mounted to axles 32 and 34 respectively. Rollers 28 and 30 have an engaged position and a disengaged position selectable by a selector switch 36 having an “on” position corresponding to the rollers engaged position and an “off” position corresponding to the rollers disengaged position. In the engaged position, rollers 28 and 30 are positioned to engage and deflect toothed belt 16.
In operation, multi-lobe cam 14 is rotated by motor 26. As multi-lobe cam 14 is rotated, the perimeter sidewalls 38 and 40 of rollers 28 and 30 radially deflect toothed belt 16. This action presses a patch of teeth 15 of toothed belt 16 against a patch of teeth 22 belonging to arcuate toothed member 12. As teeth 15 mesh with teeth 22, toothed belt 16 advances position by a small number of teeth dependent upon the gear ratio. Every rotation of multi-lobe cam 14 advances toothed belt 16 by a small amount resulting in a relatively slow but forceful progression of toothed belt 16 into and out of portal 24.
