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Transmissions with HSG gears

The codification of a new theory for the design of special gears that takes into account not only the design but also technological problems is necessary to achieve the project objective, relating to the design, construction and testing of an innovative GearBox characterized by involute toothing medium and high flow. In one of the bibliographical works produced by the founding members of the Startup, the general theory underlying the design of cylindrical gears with medium and high-sliding involute teeth, indicated with the acronym HSG (High Sliding Gears), was exposed, entitled: ” Theoretical foundations for the design of cylindrical gears with medium and high slip involute teeth” published by Liguori editore (2022).

Thanks to the experience and skills of the internal working groups, which are formed from time to time based on specific technical needs, GearUp is able to propose the most suitable solutions to achieve the company’s innovative technical objectives within the expected timescales. Furthermore, the company offers the customer the possibility of establishing technical collaborations with universities and public research bodies, aimed at integrating skills for study and experimentation phases not present in GearUp.

What is meant by a high slip gear (HSG)?

The theory behind high sliding gear (HSG) allows a general mathematical formulation of any gear and in particular of those to be used in pairs for the transmission of motion between parallel shafts. The HSG theory, while including traditional design methods, allows the transmission of motion between two cylindrical gear wheels to be customized, allowing the creation of high transmission ratios with limited wheelbases. Furthermore, with the same transmitted power it is possible to reduce the total number of teeth to the advantage of overall compactness, managing to reduce both the weight and the total size. Therefore, the theory behind HSG gears enables a new concept of parallel shaft gears with high gear ratios that are simultaneously light, strong, silent, efficient and safe.

Assieme

Why choose an HSG gear?

Among the main mechanical characteristics of HSG gears there is undoubtedly robustness, as the design of a tooth (obtained with the proposed theory) is similar to that of a shelf fitted to the base with a profile such as to provide uniform resistance Figure 1(c). To approach the characteristics of uniform resistance, the tooth of a conventional wheel would require larger dimensions, but this need is compromised by the minimum number of teeth which cannot decrease very much without encountering a notch at the base of the tooth Figure 1(a). In fact, in modular proportioning (example ISO 53) a method is proposed for calculating the minimum number of teeth such as to avoid the “under-cut” (notch at the base), which often causes breakages in Figure 1(b). Furthermore, the design of the profile of an HSG tooth allows, in addition to approaching uniform resistance with respect to bending stresses, and avoiding the concentration of stresses produced by the notch at the base (which the ordinary “modular” theory would produce), to increase the resistance against the “pitting” phenomenon (damage and surface wear) is significantly increased, as the increase in the radius of relative curvature between the profiles in contact leads to a notable reduction in the Hertzian stress.

Thanks to the experience and skills of the internal working groups, which are formed from time to time based on specific technical needs, GearUp is able to propose the most suitable solutions to achieve the company’s innovative technical objectives within the expected timescales. Furthermore, the company offers the customer the possibility of establishing technical collaborations with universities and public research bodies, aimed at integrating skills for study and experimentation phases not present in GearUp.

(a)

(b)

(c)

Picture 1: schematization of the profile of a tooth (with the same module) in the following configurations (a) with notch at the base, (b) without notch at the base, (c) HSG theory.

How are HSG gears lubricated?

An equally important feature of HSG gears that differentiate them from all traditional gears concerns the motion transmission. In fact, the conjugated surfaces have a unidirectional relative (or sliding) speed. This is achieved by avoiding the reversal of the motion at the primitive point (Figure 2(a)) by imposing the contact segment entirely in the recess arc. Depending on whether the contact segment starts from the primitive point or deviates from it to a greater or lesser extent, we have all-addendum or long-addendum type teeth Figure 2(b). The unidirectionality of the relative motion between the conjugated profiles entails certain benefits on the lubrication regime that can be established between them, which can become hydrodynamic, a regime that is certainly favored by the increase in the relative curvature as one moves away from the original point .

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(b)

Figure 2: Butterfly diagram of the sliding speeds between the contacting profiles, (a) conventional dentition, (b) HSG all-addendum dentition.

How loud are HSG gears?

Another feature that positively distinguishes HSG gears is their silence. In fact, the noise of a gear is mainly attributable to the fluctuation of the load between the teeth, depending on variations in load (torque), unbalance of the rotating masses, construction errors. It is demonstrated that the sharpness of the noise is proportional to the number of teeth and to the square of the peripheral speed of the base circumferences. In conventional gears the techniques used to reduce noise are: use of helical toothing, reduction of the unbalance of the rotating masses, accurate control of the pitch and surface finish of the toothing. In HSGs, silence improves with interventions not permitted by traditional theory, i.e.:

  • Reductions in the radial dimensions of the pairs making up the gear. These allow the peripheral speed of the base circumferences and the inertia of the rotating masses to be reduced;
  • Reduction in the number and height of teeth;
  • Use of strongly inclined helical or bi-helical teeth.

What is the maximum gear ratio of HSG gears?

In HSG gearboxes it is possible to make significant changes to the transmission ratio without changing the wheelbase. In Figure 3, the same driven wheel with 32 teeth can be coupled to a driving wheel with 1, 2 or 3 teeth without having to change the wheelbase and with transmission ratios of approximately 32, 16 and 10.66 respectively. The versatility of use of HSG gears therefore allows you to change the transmission ratio simply by replacing a pinion without making changes to the wheelbase; an impracticable condition for modular gears as a variation in the transmission ratio of a pair of traditional gears imposes stringent constraints essentially linked to modularity and therefore a necessary modification of the wheelbase.

Figure 3: CAD rendering of a long-addendum gearbox consisting of a central toothed wheel and three toothed pinions with the same center distance.

Among the main peculiarities of HSG gears is undoubtedly linked to the possibility of creating high transmission ratios with small wheelbases. This characteristic, if extended to gear trains, allows the creation of very compact and light groups with high overall efficiency. Figure 4 shows a conceptual comparison between a naval gear system made up of three speed jumps (on the left) with an equivalent HSG gear system made up of two speed jumps (on the right) and the same overall ratio (100:1), all with the same transmitted power and the same input and output shafts from the groups. The reduction in overall mass and size is evident.

Figure 4: Comparison between a naval gear system consisting of three speed steps (left) with an equivalent HSG gear system consisting of two speed steps (right) and the same overall ratio (100:1).

Can HSG gears be irreversible?

Irreversibility between parallel shafts is not contemplated in traditional theory and it is not possible to obtain a high efficiency for orthogonal shaft pairs (screw-companion wheel) characterized by irreversibility. In the HSG theory, the vectorial decomposition of the force transmitted in the conjugated pair is exploited and not the pure sliding contact (as happens instead in the case of the conventional gearbox Figure 5 (a)), therefore, when the function of each wheel is exchanged from driving to moving and vice versa, the active component of the transmitted force makes movement impossible. Picture 5 (b) shows an example of an irreversible bi-helical couple with parallel shafts, with a transmission ratio equal to 30, whose efficiency in direct motion exceeds 80% compared to a traditional screw-companion wheel reducer with orthogonal shafts, whose efficiency would not exceed 50%, with the same dimensions and transmission ratios.

(a)

(b)

Picture 5: (a) Representation of a conventional worm gear reducer with irreversible orthogonal axes companion wheel, 50% efficiency; (b) Representation of an HSG worm gearbox with irreversible companion wheel with parallel axes, 80% efficiency.

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