Helical drive belt

Description

[0001] The invention relates to a helical drive belt according to the preamble of claim 1. Furthermore, the invention relates to a steering gear in an electromechanical steering system of a motor vehicle according to claim 12. State of the art

[0002] DE 10 2011 002 230 A1 discloses a drive belt with a base made of a polymeric material and a tensile cord made of aramid. The tensile cord has two or three strands, each formed from a group of yarns. The tensile cord has a ratio of the yarn twist factor to the strand twist factor between 0.7 and 2.5. Such drive belts are used in vehicles to drive lightly loaded auxiliary components.

[0003] US 2007137766 A1 describes a helical drive belt comprising equally twisted tension cords, wherein the direction of rotation of the tension cords and the helix angle of the teeth are opposite to each other in a specific angle-dependent relationship. Both aramid and glass are disclosed as tension cord materials. The tension cord is formed from several strands. A disadvantage is that the twist of the tension cord is only based on the helix angle of the teeth and not on other product requirements such as longitudinal stiffness or fatigue strength.

[0004] JPH09159000 A describes a timing belt with a tensile cord, which has a diameter of 0.1 to 0.2 mm and is made of single-twisted aramid filaments. Such thin cords can break when transmitting large torques and are therefore disadvantageously only suitable for use in low-load drives.

[0005] Timing belts with aramid tensile cords are also used in industrial applications. These timing belts are typically made of polychloroprene rubber or polyurethane and, thanks to the embedded aramid tensile cords, exhibit high longitudinal stiffness and thus corresponding positioning accuracy.

[0006] Power steering systems in motor vehicles are now an important application area for drive belts. Electromechanical power steering systems are increasingly replacing other steering assistance systems in today's motor vehicles, such as electrohydraulic steering systems and, of course, direct steering systems. One of the reasons for this is the relatively easy possibility of electronically controlling and regulating such systems, especially in connection with the increasing proliferation of driver assistance systems, such as lane keeping systems, parking systems, etc., and even systems for autonomous driving.

[0007] The electric motor assistance of such power steering systems is applied via a gear box either to the steering column or to a steering rod / rack. Until now, a mechanical transmission of the steering wheel's rotation to the steering rod was also typically provided, so that, as its name suggests, the power steering system merely transmits an assist torque via the various types of gear boxes. With the development of steer-by-wire systems, however, steering is then achieved solely through the electronic sensing of steering movements on the steering wheel and their transmission to appropriately controlled steering motors.

[0008] For servo steering systems, for the now emerging autonomous driving functions and especially for vehicles with steer-by-wire systems, the steering is of course an absolutely safety-relevant component.

[0009] This also applies to the gears used there. In addition to the worm gears often used, belt drives are also used in electromechanical servo steering systems. In these, the assistance torque generated by an electric motor is transmitted via a usually helical toothed belt and appropriately designed gears from the motor shaft, for example, to a ball screw acting on the rack. Such toothed belts have to transmit considerable steering forces and vibrations and must be designed with a high level of safety and redundancy so that no damage can be feared during maintenance intervals and maintenance measures. The toothed belts basically consist of a so-called substructure, which has the tooth profile, at least one layer of tension cords or strength members arranged in the longitudinal direction of the belt, and the belt back.Depending on the application, the tooth profile and belt back can be provided with additional coatings or fabrics.

[0010] Typically, timing belts made of elastomeric material are used here, which are equipped with tensile cords or reinforcements made of, for example, glass fibers. Such tensile cords made of glass fibers have proven their worth due to their low thermal expansion, their temperature resistance, and, in particular, their dynamic stability—that is, their tolerance to high-frequency, alternating vibration loads, such as those that can occur in a steering gear. Even under these conditions, timing belts with tensile cords made of glass fibers retain their tear strength for a long time.

[0011] DE102019212056A1 discloses a helical drive belt with an aramid tensile member consisting of two strands, suitable for use in steering gears of an electromechanical steering system in a motor vehicle. Unfortunately, the production of the aramid tensile member is also complex and costly in this case, since both the strands and the cord must be twisted.

[0012] Another significant problem with timing belts is their noise during operation. Especially in high-speed drives, the meshing noise can be very loud. This can be reduced by equipping the belt with helical teeth, as is well known from the cited prior art.

[0013] Furthermore, the smallest possible tooth pitch reduces noise levels. However, a narrow tooth pitch also requires smaller teeth. This, in turn, means that belts with a smaller tooth pitch or smaller tooth spacing are more likely to skip under higher loads, or the teeth may shear due to excessive load.

[0014] This results in a conflict of objectives between high power transmission and good acoustic properties at the same time. Task

[0015] Therefore, the object of the invention is to provide a timing belt that is well suited to the high-frequency pulsating and bending loads in a steering gear, that is hydrolysis-resistant, that remains stable under dynamic loads, that is insensitive to kinking and handling and transport stresses, and that can be manufactured simply and cost-effectively using conventional methods. Furthermore, the timing belt should generate low running noise while simultaneously enabling the greatest possible power transmission. Particularly for very small drive belts, cost reduction through simpler designs combined with more efficient manufacturing processes is particularly important. Therefore, manufacturing should also be more cost-effective than before.Compared to the drive belts known from the state of the art, a belt is to be provided that is flexible and can transmit high power without breaking. Solution to the task

[0016] The solution to this problem is provided by a helical drive belt having the features of independent claim 1.

[0017] Claim 12 discloses a steering gear in an electromechanical steering system of a motor vehicle with a helical drive belt according to the invention.

[0018] Further advantageous developments are disclosed in the dependent claims. Advantages of the invention

[0019] The helical-toothed drive belt disclosed in claim 1 according to the invention comprises a base body made of elastomeric material with teeth spaced apart by a tooth pitch, wherein a plurality of tensile strands are arranged in a longitudinal direction in the base body and enclosed by the elastomeric material of the base body. The tensile strands are composed of a single-twisted p-aramid yarn. The para-aramid, or p-aramid for short, can preferably be in the form of a monofilament, multifilament, or staple fiber, with the use of a multifilament being particularly preferred. According to the invention, the outer diameter of the tensile strand is between 0.25 mm and 0.5 mm.

[0020] In other words, the angle between the longitudinal direction of the belt and the longitudinal direction of the tooth, which is essentially oriented perpendicular to the longitudinal direction of the belt, is less than or greater than 90°. Helical toothing can improve the belt's noise emission.

[0021] Elastomers based on vulcanizable rubber mixtures, comprising at least one rubber component and mixture ingredients, are mostly used.

[0022] In principle, any elastomer known to a competent person can be used. The terms rubber and elastomer are used synonymously in this document.In a preferred embodiment, the elastomer is selected from the group consisting of ethylene-propylene copolymer (EPM) or ethylene-propylene-diene copolymer (EPDM) or nitrile rubber (NBR) or (partially) hydrogenated nitrile rubber (HNBR) or fluororubber (FKM) or chloroprene rubber (CR) or natural rubber (NR) or styrene-butadiene rubber (SBR) or isoprene rubber (IR) or butyl rubber (IIR) or bromobutyl rubber (BIIR) or chlorobutyl rubber (CIIR) or butadiene rubber (BR) or chlorinated polyethylene (CM) or chlorosulfonated polyethylene (CSM) or polyepichlorohydrin (ECO) or ethylene-vinyl acetate rubber (EVA) or acrylate rubber (ACM) or Ethylene acrylate rubber (AEM) or silicone rubber (MQ, VMQ, PVMQ, FVMQ) or fluorinated methyl silicone rubber (MFQ) or perfluorinated propylene rubber (FFPM) or perfluorocarbon rubber (FFKM) or polyurethane (PU).

[0023] The rubbers mentioned can be used alone or in a blend.

[0024] Advantageously, the elastomeric material comprises 70 to 100 phr of at least one polyalphaolefin rubber. The polyalphaolefin rubber can, in particular, be an ethylene-propylene copolymer (EPM) or an ethylene-propylene-diene rubber (EPDM), or a combination of EPM and EPDM.

[0025] Polyalphaolefin rubbers such as EPDM or EPM offer the decisive advantage over polychloroprene or polyurethane of improved low-temperature flexibility and significantly better aging resistance. Timing belts made of EPDM can be used at temperatures as low as -40°C. These materials are also characterized by high heat resistance, thus becoming less brittle and also achieving very good abrasion resistance.

[0026] The elastomeric material may also preferably be peroxide crosslinked.

[0027] To prevent the entire drive belt from twisting or unwinding, the yarns of adjacent tension cords in the drive belt can be twisted alternately or in groups in an S-lay and a Z-lay. For better adhesion, the tension cords or each individual filament of a yarn can have an internal finish with isocyanate or an isocyanate compound and an external adhesive layer, preferably made of resorcinol formaldehyde latex (RFL).

[0028] According to DIN 60 900 Part 1, a yarn is a linear structure made from textile fibers.

[0029] According to the invention, the outer diameter of the tensile strand is between 0.25 and 0.5 mm, preferably between 0.3 and 0.4 mm.

[0030] This is the best way to reconcile the mechanical and economic factors described above.

[0031] The advantage of such a construction of tensile cords and belts is essentially that the fineness and geometry of the yarns and tensile cords achieve a high tensile strength against stresses caused by high-frequency loads and alternating bending, which is not possible with other tensile cord designs of larger diameter.

[0032] Compared to tension cords with a larger outer diameter, economic advantages can also be generated due to the lower material usage.

[0033] Furthermore, tensile cords with a larger outer diameter lose their tensile strength with decreasing bending radii. The tensile cord according to the invention can therefore be particularly suitable for belt drives with small-diameter pulleys, since the reduction in tensile strength is less pronounced compared to tensile cords with a larger diameter.

[0034] In addition, the tensile strength of thinner tensile cords is lower, which is why a larger number of tensile cords would have to be arranged in the belt to compensate for the lower tensile strength. Therefore, compared to tensile cords with an outer diameter of less than 0.25 mm, the production of the drive belt is more economical, as the time required to wind the tensile cord around a cylindrical tool on which the belt blank is built is reduced.

[0035] As the tooth pitch decreases, i.e., the distance between two belt teeth decreases, noise performance can be optimized. This, in turn, reduces the power transmission capacity, which is why the belt must be made wider to transmit the same power as the tooth pitch decreases.

[0036] Simultaneous good noise performance and high power transmission performance can be achieved by manufacturing the timing belt with a pitch between 1.5 mm and 2.5 mm and a width between 10 mm and 35 mm. The positive impact on acoustics of the smaller pitch exceeds the negative impact of a larger belt width.

[0037] It is also particularly advantageous that para-aramid tensile cords have greater kink resistance compared to glass cords known from the state of the art, or in other words, a lower reduction in tear strength under kink stress.

[0038] According to a further aspect of the present invention, the weight of the tensile strand is between 200 and 2000 dtex.

[0039] According to DIN 60 900 Part 2, the tex system indicates the fineness, or length-related mass, of a yarn. In other words, the tex system represents the quotient of the yarn's mass and length. One decitex, or dtex for short, corresponds to 1 dg / km.

[0040] The best results can be achieved within the specified weight range while maintaining cost-effective production.

[0041] According to a further aspect of the present invention, the weight of the tensile strand is between 400 dtex and 1500 dtex.

[0042] This is the best way to reconcile the mechanical and economic factors described above.

[0043] According to a further advantageous aspect of the present invention, the twist of the yarn is between 250 to 450 tpm (turns per meter), preferably between 300 to 400 tpm.

[0044] The advantage of this design is that only as many twists are made as necessary for the intended application. The number of twists should not be chosen less, as otherwise the cutting ability of the cords may deteriorate and the fatigue resistance may decrease.

[0045] If the rotation is between 300 and 400 tpm, the tear strength and the bending fatigue resistance of the tensile cords and thus of the entire belt are at a particularly good level.

[0046] A further advantageous embodiment consists in the tensile strand having an internal treatment with isocyanate or an isocyanate compound and an external adhesive layer, wherein the adhesive layer preferably comprises or is formed from RFL. This allows for particularly good adhesion of the tensile strand to the surrounding elastomer material.

[0047] A further advantageous design is that the angle of the helical toothing relative to an axis transverse to the longitudinal direction of the drive belt is between 4° and 8°.

[0048] This allows for the advantageous achievement of good acoustic properties while simultaneously tolerating lateral forces.

[0049] According to a further aspect of the present invention, the tooth pitch is between 1.5 and 2.5 mm.

[0050] A pitch in this range offers the best compromise between good acoustic properties and sufficient power transmission capacity.

[0051] According to a further aspect of the present invention, the width of the drive belt is at least 6 times the tooth pitch.

[0052] Such a ratio between the pitch and the belt width offers particularly advantageous acoustic properties and a sufficiently high power transmission capacity.

[0053] According to a further aspect, the drive belt according to the invention is, in addition to other fields of application, particularly suitable for use in a steering gear in an electromechanical steering system of a motor vehicle due to the load situation.

[0054] The invention will now be explained in more detail using comparative and exemplary embodiments, which are summarized in Table 1.

[0055] For all examples, a specification of a helical toothed belt with a para-aramid yarn material was selected. The inventive specifications E1 to E3, with a single-twisted yarn, are compared in terms of running time with reference specifications V1 to V2. In the reference specifications, the number of first and second twists, as well as the cord thickness, differs significantly. In the inventive specifications E1 to E3, however, the yarn is twisted only once.

[0056] The belt's service life until failure was determined. The target for passing the test is a service life of more than 100 hours. Typical failure mechanisms include a belt tear, which often occurs as a result of individual belt teeth shearing off. The tension cord can also tear directly due to excessive loading.

[0057] The test is carried out at a constant temperature of 120 °C with a tolerance of + / - 3 °C. The belts are tested dynamically with a width of 12 mm and a length of 310 mm on a two-pulley drive with reversing rotation. The two-pulley drive has a gear ratio, with the first pulley having 41 teeth and the second pulley having 117 teeth. The tooth pitch, which can be understood as the distance between two teeth, is 2 mm. The belts are pretensioned with an axial force of 167 N with a tolerance of + / - 10 N. The cycle time in one direction of rotation is 3 seconds with a tolerance of + / - 0.1 seconds, with the first pulley making 5 revolutions to the left or right with a tolerance of + / - 0.3 revolutions. The maximum drive speed of the first pulley is 285 revolutions per minute with a tolerance of + / - 10 revolutions per minute.The maximum output torque on the second toothed pulley is 13 Nm with a tolerance of + / - 1 Nm.

[0058] Table 1 shows that sufficiently good running times can be achieved with only one twist of the p-aramid yarn and a cord thickness according to the invention, see E1 to E3. Table 1: specification V1 V2 E1 E2 E3 Yarn material p-aramid p-aramid p-aramid p-aramid p-aramid Yarn type Filament yarn Filament yarn Filament yarn Filament yarn Filament yarn construction 440x1x2 440x1x2 1100x1 1100x1 1100x1 1st rotation [tpm] 450 300 360 100 300 2nd rotation [tpm] 450 300 - - - Cord thickness [mm] 0,36 0,25 0,4 0,35 0,36 RFL Yes Yes Yes Yes Yes Running time in h, target > 100h 51,5 80,5 175 122 200,5

Claims

1. Helically toothed drive belt comprising a main body made of elastomeric material comprising teeth spaced apart from one another by a tooth pitch, wherein a plurality of tensile strands is arranged in the main body in a longitudinal direction and surrounded by the elastomeric material of the main body, characterized in that the tensile strands are constructed from a singly twisted p-aramid yarn, the external diameter of the tensile strand being between 0.25 mm and 0.5 mm.

2. Helically toothed drive belt according to Claim 1, characterized in that the external diameter of the tensile strand is between 0.3 mm and 0.4 mm.

3. Helically toothed drive belt according to either of the preceding claims, characterized in that the weight of the yarn is between 200 and 2000 dtex.

4. Helically toothed drive belt according to Claim 3, characterized in that the weight of the yarn is between 400 dtex and 1500 dtex.

5. Helically toothed drive belt according to any of the preceding claims, characterized in that the twist of the yarn is between 250 and 450 tpm.

6. Helically toothed drive belt according to Claim 5, characterized in that the twist of the yarn is between 300 and 400 tpm.

7. Helically toothed drive belt according to any of the preceding claims, characterized in that the yarn is constructed from multifilaments.

8. Helically toothed drive belt according to any of the preceding claims, characterized in that the tensile strand has an internal preparation with isocyanate or an isocyanate compound and an additional adhesive layer on the outside.

9. Helically toothed drive belt according to any of the preceding claims, characterized in that the angle of the helical teeth relative to an axis perpendicular to the longitudinal direction of the drive belt is between 4° and 8°.

10. Helically toothed drive belt according to any of the preceding claims, characterized in that the tooth pitch is between 1.5 and 2.5 mm.

11. Helically toothed drive belt according to any of the preceding claims, characterized in that the width of the drive belt is at least 6 times the tooth pitch.

12. Steering gear in an electromechanical steering system of a motor vehicle having a helically toothed drive belt according to any of Claims 1 to 11.