SPRING BLADE AND METHOD FOR MAKING A SPRING BLADE

Description

[0001] The invention relates to a method with the features of claim 1 for manufacturing a leaf spring for a leaf spring, in particular a parabolic spring or a steering spring, wherein the leaf spring has two end regions, a middle region, a top surface which is subjected to tensile stress in the operating state, and a bottom surface which is subjected to compressive stress in the operating state, wherein at least one hole is provided in the bottom surface, and wherein the leaf spring is hot-formed, in particular rolled, and the top surface of the leaf spring is blasted.

[0002] Furthermore, the invention relates to a leaf spring with the features of claim 10 for a leaf spring, in particular a parabolic spring or a linkage spring, wherein the leaf spring has two end regions, a middle region, a top surface which is subjected to tensile stress in the operating state, and a bottom surface which is subjected to compressive stress in the operating state, wherein the bottom surface has at least one hole and wherein the leaf spring is hot-formed, in particular rolled, and the top surface of the leaf spring is blasted.

[0003] Leaf springs are used in the wheel suspensions of motor vehicles, particularly in commercial vehicles or trailers (towed vehicles). The requirements of motor vehicles are characterized, on the one hand, by the reduction of the weight of individual components. On the other hand, they are characterized by the fact that individual components are subject to increasingly higher mechanical stresses, for example, due to increasing engine power.

[0004] Many different types of leaf springs are known from the prior art, for example, parabolic springs, link springs, or trapezoidal springs. Such leaf springs consist of a single leaf or several leaves, each leaf having two end sections and an intermediate middle section. The middle section includes a clamping area in which the leaf spring is connected to a wheel axle or axle bolt of a vehicle via a connecting device, in particular via one or more U-shaped bolts or fittings that encompass the leaf spring in the clamping area.

[0005] If a leaf spring has several spring leaves arranged one above the other, the spring leaves are often connected to each other in the clamping area, in particular by a bolt guided through a central through-hole (hereinafter: center hole) in the clamping area of ​​each spring leaf, whereby the spring leaves either lie directly against each other or intermediate elements, such as intermediate plates, are arranged or clamped between the spring leaves.

[0006] Multi-leaf springs typically consist of a main leaf, also known as a guide leaf, and further leaflets arranged on it. Through-holes may be provided in at least one end section of the main leaf, through which connecting elements for attaching the leaf spring to vehicle components, particularly an axle, can be guided.

[0007] Leaflets of multi-leaf springs may have additional through holes and / or blind holes on their underside. Single-leaf springs may also have through holes, for example, through holes provided in at least one end section, through which connecting means for attaching the leaf spring to vehicle parts, in particular to an axle, can be guided, and / or blind holes on their underside.

[0008] Multi-leaf springs are known, for example, from DE 20 2011 001 703 U1 and EP 3 222 872 A1.

[0009] A known problem with leaf springs is that, under increasing mechanical stress, the spring leaves can break in areas where they have a hole. In this respect, the problem of center hole fractures is particularly noteworthy.

[0010] Center hole fractures are a well-known and undesirable fracture pattern in leaf springs. They usually originate on the underside of the spring leaf, which is subjected to compression during operation. Center hole fractures can be caused by static stress on the leaf spring (i.e., when the vehicle is at rest) or by dynamic stress (i.e., while driving).

[0011] The cause of center-hole fractures has not been clearly identifiable until now. In practice, hydrogen-assisted cracking (HAC) has generally been assumed to be the cause of center-hole fractures.

[0012] To prevent center hole fractures, it is known from CA 2 865 630 A1 to avoid hydrogen-induced crack growth by locally heating the spring leaf and then rapidly cooling it. This method is disadvantageous because it is technically complex and the heating and cooling process requires a high and expensive amount of energy. Furthermore, heating can only take place before shot peening the upper surface of the spring leaf, as heating the leaf should not occur after shot peening because heating itself has negative effects on the surface finish of the spring leaf.

[0013] To prevent center hole fractures, it is also known to chamfer the edges of the center hole on the underside of the leaf spring and to place a relatively soft steel spacer (compared to the leaf spring material) between the leaf springs and on the upper side. A relatively soft washer is placed on the underside of the leaf spring. These process steps are also technically complex and expensive, with the additional material costs incurring further expenses and generally increasing the weight of the finished leaf spring. Furthermore, center hole fractures also occur in the chamfered area.

[0014] The invention is based on the objective of providing a leaf spring for a leaf spring which reduces the risk of hole breakage, in particular the risk of center hole breakage, while avoiding the disadvantages of the prior art.

[0015] In the manufacture of spring leaves, forming processes, particularly rolling operations, take place to bring the spring leaf into the desired shape. It is known that the upper surface of the spring leaves, which is subjected to tensile stress during operation, is tempered and further processed after the rolling processes to optimize stress distribution, in particular shot peening or stress peening. The lower surface, which is subjected to compressive stress during operation, is generally not peened but remains in the material state it has after the rolling and tempering processes.

[0016] It has now been surprisingly discovered that the cause of center hole fractures can lie in the fact that stress-peening of the side of the spring leaf subjected to tensile stress during operation creates tensile stresses on the side of the spring leaf subjected to compression during operation. These tensile stresses are present on the side of the spring leaf subjected to compression during operation after stress-peening, i.e., after the spring leaf has been relieved of the load. This cause was previously unknown.

[0017] Because the upper surface of the spring leaf is further processed after the rolling processes to optimize the stress distribution, in particular stress blasting, but the underside is not, tensile stresses can remain on the underside in this material state.

[0018] The realization that these tensile stresses exist and can be responsible for the occurrence of center hole fractures is new.

[0019] In this material state, stress concentrations can occur at the edges of the holes of the leaf spring during operation. For example, when a multi-leaf spring is bolted together, force is transmitted, resulting in stress transfer from the bolt head to the underside of a leaf. Furthermore, after a leaf spring is installed in a vehicle's wheel suspension, surface stress can be superimposed by external stresses due to a convex or concave leaf surface. Additionally, process instabilities during leaf spring manufacturing can have further negative effects. These negative effects increase the risk of center hole failure due to the presence of tensile stresses on the side of the leaf spring that is subjected to compression during operation.

[0020] Furthermore, tensile stresses in near-surface areas can promote hydrogen-induced crack growth (HAC = Hydrogen Assisted Cracking), which in turn increases the risk of center hole fracture.

[0021] The invention makes use of this new knowledge by providing that the upper surface of the spring leaf is blasted under preload of the spring leaf, thereby creating tensile stresses on the underside of the spring leaf which is subjected to compression in the operating state, and that the underside of the spring leaf in the area around the hole is or has been locally blasted after the blasting of the upper surface of the spring leaf, thereby generating compressive stresses which superimpose the tensile stresses.

[0022] This allows for the generation of slight compressive stresses, which superimpose the tensile stresses surprisingly identified as the cause of center hole fractures and increase the local service life of the leaf spring. As a result, the risk of hole fracture is significantly reduced, while the manufacturing process, energy, and production costs of the leaf spring are not noticeably increased, and no additional components, which would incur further costs and add weight, are required.

[0023] Particularly advantageous is the avoidance of hydrogen-induced crack growth, whereby, in contrast to the method according to CA 2 865 630 A1, no tensile stresses are present on the underside of the spring leaf.

[0024] A spring leaf like this is manufactured from a profile bar. The profile bar is cut to length, heated, and subjected to forming processes, particularly rolling and stamping, so that it is brought into the desired shape of a spring leaf. After forming, heat treatment takes place. While the profile bar is being formed, it is essentially a semi-finished product. Only when all processing steps, including the heat treatments performed after the forming processes, are completed, is a finished spring leaf available as the final product.

[0025] However, the invention refers to a spring leaf even during the forming processes. This clarifies that the introduction of the hole(s) into the underside and the blasting of the underside in the area of ​​the hole(s) are process steps for forming features relating to the spring leaf, which is present as the finished end product after completion of all processing steps.

[0026] Within the scope of the invention, it is possible to blast the entire underside of the leaf spring, or at least a large part of it. However, since this incurs certain process costs and negatively impacts the capacity of the blasting system, a particularly preferred embodiment provides that the underside is blasted locally in the area around the hole. That is, the underside is blasted only in the area around the corresponding hole, whereas areas where no hole is provided remain unblasted. In a further preferred embodiment of the invention, the underside in the area around the hole can be shot-peened.

[0027] For example, blasting media with a grain diameter of 0.4 to 1.2 mm can be used. It is particularly economical to reuse previously used blasting media, such as those used when blasting the top of the spring leaf.

[0028] According to the invention, an abrasive can be applied via a blast nozzle, wherein the blast nozzle and the area of ​​the hole are aligned with each other. For this purpose, it can be provided that the nozzle is moved towards the spring leaf and / or the spring leaf towards the nozzle.

[0029] For example, in an automated blasting system, a robot can turn the spring blade over and hold it with the corresponding hole on the underside facing the blasting nozzle for a few seconds. In a manual blasting system, the spring blade can be placed on a conveyor belt after the top side has been blasted. The conveyor belt then moves the spring blade over a blasting nozzle.

[0030] Within the scope of the invention, it is preferred if an area around the hole is blasted, such that the distance from the hole to the transition between the blasted and unblasted surface of the underside is in the range of approximately 5 cm or 4 cm or 3 cm or 2 cm or 1 cm.

[0031] It is understood that during radiation beaming, no smooth boundary is created between the irradiated and unirradiated areas. Rather, the transition between the unirradiated and irradiated areas occurs gradually within a certain scattering range.

[0032] The invention allows for the provision of either a through hole or a blind hole. Multiple holes can be provided, and any combination of through holes or blind holes is conceivable.

[0033] If a blind hole is provided on the underside of a leaf spring, it is clear that this blind hole is machined into the leaf spring from the underside. A through hole, according to the invention, can be machined into the leaf spring from either the underside or the top side. The essential point is that a hole is machined into the underside of the leaf spring. In a particularly preferred embodiment of the invention, a clamping area with a through-hole is formed in the central region, and the area around the central hole is shot-blasted.

[0034] The invention provides that the spring leaf is made of flat bar steel, in particular spring steel.

[0035] A particularly preferred embodiment of the invention can be described as follows: The spring leaf is hot-formed, in particular rolled, and then tempered by blasting the upper surface of the spring leaf under preload. The underside of the spring leaf is blasted under preload in the area around the hole after blasting the upper surface. Blasting the underside of the spring leaf in the area around the hole can take place directly after blasting the upper surface, or further processes can take place between blasting the upper surface and blasting the underside.

[0036] It shows: Fig. 1 shows an area of ​​the underside of a spring leaf in the area of ​​a hole and Fig. 2 shows a block diagram with an embodiment of a process sequence according to the invention.

[0037] In Fig. 1 Figure 1 shows a region of the underside 1 of a leaf spring 2, in which a hole 3 is provided. The leaf spring can be, for example, a parabolic spring or a linkage spring. The hole 3 can be a through hole or a blind hole. The hole 3 can be a central hole provided in a clamping area or provided in another area of ​​the leaf spring 2, for example, in an end area. The underside 1 of the leaf spring 2 is subjected to compressive stress when the leaf spring is in operation.

[0038] The spring leaf 2 is preferably made of flat bar steel, in particular spring steel. The underside 1 of the spring leaf 2 is locally blasted, in particular shot-peened, in the area around the hole 3. The distance from the hole 3 to the transition 4 between the blasted and unblasted surfaces of the underside 1 is on the order of a few centimeters. During blasting, a first area 5 is created around the hole, which is uniformly blasted, as well as an adjacent second area 6, in which the transition between the unblasted and blasted areas occurs gradually within a certain dispersion range.

[0039] Blasting can be carried out via a blasting nozzle 7, which applies a blasting medium 8 to the underside 1, whereby the blasting nozzle 7 and the area of ​​the hole 3 are aligned with each other.

[0040] In Fig. 2An exemplary process flow for manufacturing a spring leaf 2 according to the invention is shown in a block diagram, wherein individual blocks provided with reference numerals can consist of single or several process steps.

[0041] In block 9, for example, a profile bar can be cut to a desired length and heated.

[0042] Heat processing and heat forming processes can be carried out in block 10. In particular, the heated profile bar can be rolled and, if necessary, further cut, whereby at least one hole 3 is introduced into the underside 1 of the spring leaf 2 to be produced. In block 10, a central hole can be introduced, in particular, into the clamping area. Alternatively or additionally, at least one longitudinal end of the spring leaf 2 can be processed by forming at least one end section into a means for connecting the spring leaf to a vehicle chassis or to connecting parts of a wheel suspension, in particular by rolling it into a spring eye. Likewise, a configuration is conceivable in which at least one end section is formed into a securing element, such as a safety roll.

[0043] Further mechanical processing can take place in block 11. Processes for bending, tempering, hardening, and annealing of spring leaf 2 can be carried out. The upper surface of spring leaf 2 is also shot-blasted under preload.

[0044] Blocks 9 to 11 comprise process steps that are known from the prior art.

[0045] In block 12, the process step proposed according to the invention takes place, in which the underside 1 in the area around the hole 3 is locally blasted, preferably shot peened.

[0046] After block 12, further processes can take place until a leaf spring is completed, whereby the leaf spring can have one leaf 2 or two or more leaf 2s.

[0047] If a spring eye has been rolled, an eye bearing can be pressed in, for example. The second leaf spring can then be painted. The leaf spring can be installed, inspected, and packaged.

[0048] The blasting of the underside of the leaf 2 described in Block 12 can take place immediately after the stress blasting of the top side of the leaf 2, or further processes can take place between the blasting of the top side and the blasting of the underside, for example, setting.

Claims

1. A method for manufacturing a spring leaf (2) for a leaf spring, in particular a parabolic spring or a trailing arm spring, wherein the spring leaf (2) has two end regions, a center region, an top side, which is subjected to tension in the operating state, and a bottom side (1), which is subjected to compression in the operating state, the spring leaf (2) being hot-formed, in particular rolled, at least one hole (3) being made in the bottom side (1) of the spring leaf (2) and the top side of the spring leaf (2) being blasted, characterized in that the top side of the spring leaf (2) is blasted with prestressing of the spring leaf (2), causing tensile stresses to arise on the bottom side of the spring leaf, which is subjected to compression in the operating state, and in that the bottom side (1) of the spring leaf (2) is locally blasted in the region around the hole (3) after the blasting of the top side of the spring leaf (2), causing compressive stresses to be generated which are superimposed on the tensile stresses.

2. A method according to claim 1, characterized in that the bottom side (1) is only blasted in the region around the corresponding hole (3), whereas no blasting is carried out in regions in which no hole (3) is provided.

3. A method according to claim 1, characterized in that the bottom side (1) is shot-blasted in the region around the hole (3).

4. A method according to one of the claims 1 to 3, characterized in that a blasting agent (8) is applied via a blasting nozzle (7), the blasting nozzle (7) and the region of the hole (3) being aligned with one another.

5. A method according to claim 4, characterized in that the blasting nozzle (7) is moved towards the spring leaf (2) and / or the spring leaf (2) is moved towards the blasting nozzle (7).

6. A method according to one of the claims 1 to 5, characterized in that a region around the hole (3) is blasted so that the distance from the hole (3) to the transition (4) between the blasted and unblasted surface of the bottom side (1) is in the range of approximately 5 cm or 4 cm or 3 cm or 2 cm or 1 cm.

7. A method according to one of the claims 1 to 6, characterized in that a through hole or a blind hole is introduced as the hole (3), a clamping region having a continuous center hole being formed in particular in the center region, and the region around the center hole being blasted.

8. A method according to one of the claims 1 to 7, characterized in that the spring leaf (2) is produced from flat steel bar, in particular spring steel.

9. A method according to one of the claims 1 to 8, characterized in that the spring leaf (2) is annealed after rolling but before blasting of the bottom side (1).

10. A spring leaf for a leaf spring, in particular a parabolic spring or a trailing arm spring, wherein the spring leaf (2) has two end regions, a center region, an top side, which is subjected to tension in the operating state, and an bottom side (1), which is subjected to compression in the operating state, the bottom side (1) having at least one hole (3) and the spring leaf (2) being hot-formed, in particular rolled, and the top side of the spring leaf (2) being blasted, characterized in that the top side of the spring leaf (2) is blasted while prestressing the spring leaf (2), causing tensile stresses to arise on the bottom side of the spring leaf, which is subjected to compression in the operating state, and that the bottom side (1) is locally blasted in the region around the hole (3), causing compressive stresses to be generated which are superimposed on the tensile stresses.

11. A spring leaf according to claim 10, characterized in that the bottom side (1) is blasted only in the region around the corresponding hole (3), whereas it is not blasted in regions in which no hole (3) is provided.

12. A spring leaf according to claim 10 or 11, characterized in that the bottom side (1) is shot-blasted in the region around the hole (3).

13. A spring leaf according to one of the claims 10 to 12, characterized in that the distance from the hole (3) to the transition (4) between the blasted and unblasted surface of the bottom side (1) is in the range of approximately 5 cm or 4 cm or 3 cm or 2 cm or 1 cm.

14. A spring leaf according to one of the claims 10 to 13, characterized in that the hole (3) is a through hole or a blind hole, a clamping region having a continuous center hole being provided in particular in the center region and the region around the center hole being blasted.

15. A Spring leaf according to one of the claims 10 to 14, characterized in that it is produced from flat steel bar, in particular spring steel.