The invention relates to a toothed belt drive with a toothed belt and at least two toothed pulleys, wherein the toothed belt has at least one drive side provided with a toothed profile, the toothed belt wraps around the toothed pulleys over a partial area of their circumference and the teeth of the toothed belt meshingly engage in the tooth gaps of the toothed pulleys.
One of the challenges when designing such toothed belt drives is in providing toothed belt drives that run as silently as possible and in reducing as far as possible the generation of noise. Running noises result in particular from the constructive and geometric conditions in the engagement of the teeth in the toothed belt pulley and from the resulting vibrations and oscillations in the strand. Even though a reduction in noise is already possible by changing from spur-geared toothed belt drives to helically-geared toothed belt drives, the noise emissions still present in the latter are also significant and undesirable.
According to the current state of the art, toothed belts are manufactured on vulcanization molds that have a tooth pitch that is as precise and uniform as possible over their circumference and which then also transfer this to the belt blanks to be vulcanized. Due to this constant, regular pitch, there occurs significant noise development during operation when the belt meshes with the equally regularly manufactured toothed pulleys, triggered substantially by a vibration in the tooth meshing frequency, the sound power level of which is audible and measurable. The energy of this vibration is radiated directly as airborne noise or transmitted as structure-borne noise and radiated indirectly elsewhere.
In order to achieve a low level in the tooth meshing frequency, it is known in the prior art, for example, to form groups of differently designed pitches and to arrange these “pitch groups” over the length or circumference of the belt in such a way that a uniform tooth meshing frequency cannot be built up or that destructive interference results. With such a so-called “low-noise toothing”, the individual groups can differ in the pitch by up to 20%. Of course, the pitch of the toothed pulleys in the form of groups must also be adjusted accordingly. This also creates a major disadvantage of this type of toothing. This is due to the fact that there has to occur an absolutely precise and adapted mounting of the belt drive with pitch groups that correspond exactly to one another on the toothed pulley and on the toothed belt, which of course makes installation more difficult and very error-prone. There must always be a multiple of a group of teeth that match one another on the belt and toothed pulley and the belt must be fitted accordingly. A further disadvantage arises from the fact that if the belt skips, for example in the event of an overload, the belt is immediately destroyed if different pitch groups necessarily have to engage with one another.
On the other hand, there are proposals in the prior art to reduce the noise development by changing the backlash. EP 1 614 933 B1 discloses in this respect a helically-geared belt drive in which noise and vibration are to be reduced in that the play, that is to say the backlash (“D”), between the helically-geared belt and a toothed pulley is set in a specified ratio to the tooth pitch at a specific tooth pitch angle and predetermined width of the toothed belt. EP 2 803 879 A1 proposes similar measures, in particular for the reduction in noise in the case of toothed belt drives in electrical power steering assemblies. Here, too, the gap clearance, the backlash, is adjusted depending on the tooth width and tooth height.
The design of the profile of toothed pulleys as the counterpart of toothed belts and the profile of the latter in helically-geared toothed belt drives thus always envisages a certain degree of play, that is to say a certain degree of “backlash”, not least in order to guarantee a certain capability of the belt to be displaced in relation to the toothed pulley in the circumferential direction and thus in order to compensate for production tolerances of the belt and for variable tensile forces and for dissimilar pitch lengths resulting from the latter in the wrapped arc. It is the superordinate objective herein for the play to be configured such that an unimpeded run-in of the toothed belt into both toothed pulleys is achieved in order for the generation of airborne noise and structure-borne noise to be minimized.
However, the measures known in the prior art for reducing the airborne noise and structure-borne noise in the case of toothed belt drives are no longer adequate in the case of an interaction between comparatively large and small toothed pulleys, in particular thus in toothed belt drives for comparatively large gear ratios. In the case of toothed belt drives in particular, which have step-up ratios/step-down ratios and for instance reduce the speed from input to output, the measures mentioned are already not sufficient because the meshing conditions when running into a small toothed pulley/pinion differ significantly from those when running into a larger toothed pulley.
The object of the invention was therefore to provide a belt drive that runs as quietly and with as little impact as possible when engaged, generates few vibrations and oscillations in the strand even at high speeds, does not require any pitch groups and can be easily assembled and installed regardless of the running direction and circumferential position.
This object is achieved by the features of the main claim. Further advantageous embodiments are disclosed in the dependent claims.
At least one of the toothings on the belt or pulley is designed with a series of varying tooth pitches, wherein
The nominal pitch P of the toothed belt and toothed pulley is the pitch at the height of the effective cord line, that is to say on the radius or arc defined by the center of the cord (cord center) of the toothed belt wrapped around the toothed pulley. The nominal pitch line or the nominal pitch arc is therefore radially outside the teeth of the toothed pulley, that is to say outside the tooth tip circle of the toothed pulley.
The variation according to the invention of the tooth pitches considerably reduces structure-borne noise and strand vibrations and thus the sound power level, by spectrally precalculating the series of tooth meshing with different deviations from the nominal pitch on the superimposition of their impulses when the belt runs into a toothed pulley and selecting the (best) sequence for production on the belt or the toothed pulley in which the level of the tooth meshing order is minimized, with the constraint that the levels of the secondary orders do not exceed the tooth meshing order.
The permissible backlash limits the variation of the tooth pitch according to the invention in principle as an upper limit, the backlash being defined as the free space between the belt tooth and two adjacent toothed pulley teeth along the engagement arc at half the tooth height of the belt. The design of the toothed belt drive according to the invention means that a considerable reduction in the sound power level of the usually monotonous, strongly dominant tooth meshing frequency can be achieved.
An advantageous development of the invention is that the changing tooth pitch on the belt or pulley is formed periodically or according to a predetermined pattern. With such a design, one approaches the basic idea of the low-noise toothing known in the prior art, but without having to accept the existing disadvantage which arise from the formation of pitch groups. The periodic tooth pitch, or one that varies according to a predetermined pattern, leads to further suppression of noise generation or to destructive interference with nevertheless occurring vibrations.
A further advantageous embodiment of the invention is that the varying tooth pitch on the belt or pulley is formed in the positive or negative direction, that is to say leads to an increased and/or reduced pitch with regard to the mean pitch. Both types of variation can significantly lower the sound power level.
A further advantageous embodiment of the invention is that the toothed belt or the toothed pulley is designed with a varying tooth pitch and the respective other part is designed with a regular tooth pitch. This expands the possibilities for reducing noise generation and can be further increased by the fact that, in the context of a further advantageous embodiment, the toothed belt and the toothed pulley are designed with a varying tooth pitch. Depending on the application, the belt drive can thus be precisely adapted to the required noise reduction. This is particularly important when such a toothed belt drive is used in electric power steering gears in motor vehicles, in particular designed with helical toothing.
A further advantageous embodiment of the invention consists in the fact that the series of varying tooth pitches is selected in such a way that the lowest possible level in the tooth meshing order results.
A further advantageous embodiment of the invention consists in the fact that the levels of the orders adjacent to the tooth meshing order do not exceed this.
The invention will be explained in more detail on the basis of an exemplary embodiment. In the figures:
If the noise of a toothed drive is subjected to an order analysis, the energy content of the noise is considered via an “order”, in the case of toothed belt drives via an order that is formed from a frequency spectrum that is calculated using a Fast Fourier Transform (FFT). An order spectrum can be mapped with an FFT, from which the level or the amplitude of each order can be determined with certain concurrent calculations/filters. This occurs with the frequency of the meshing engagement, the tooth meshing order, as well as with a number of secondary orders. This gives the diagrams shown in
Number | Date | Country | Kind |
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10 2019 215 965.6 | Oct 2019 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2020/074569 | 9/3/2020 | WO |