Flexible drive shaft and laboratory centrifuge with flexible drive shaft

Abstract

The invention relates to a flexible drive shaft (10) for transmitting a torque from a motor (4) to a high-speed rotor (2), in particular for a laboratory centrifuge (1), wherein the material of the drive shaft (10) has a defined yield strength and a defined modulus of elasticity. In order to provide a novel drive unit, for example for a laboratory centrifuge, which at least maintains the necessary flexibility at critical rotational speeds but has a higher strength with regard to the bending stresses, according to the invention a material is used for the drive shaft (10) in which a quotient of the yield strength of the material in N / mm² divided by the modulus of elasticity of the material in N / mm² assumes a value > 0.0045.

Description

[0001] Flexible drive shaft and laboratory centrifuge with flexible drive shaft

[0002] The invention relates to a flexible drive shaft for transmitting torque from a motor to a high-speed rotor, in particular for a laboratory centrifuge, wherein the material of the drive shaft has a defined yield strength and a defined modulus of elasticity.

[0003] The invention further relates to a laboratory centrifuge equipped with such a drive shaft, the sample-receiving rotor of which is arranged on a drive shaft mounted vertically in the bearing shield, wherein the drive shaft is in turn connected to a drive motor and has a defined yield strength and a defined modulus of elasticity.

[0004] The invention is described below for the specific case of a drive shaft for a laboratory centrifuge. However, this does not preclude its use in comparable high-speed centrifuges.

[0005] To achieve the desired high rotational speeds (usually up to 6,000 rpm, but often exceeding 18,000 rpm), the laboratory centrifuge passes through at least a critical rotational frequency, at which the system is prone to high vibrations and oscillations, and the rotor exhibits oscillatory movements. The difference between the operating rotational frequency and the critical rotational frequency should be as large as possible to reduce or eliminate vibrations during operation.

[0006] Drive shafts in laboratory centrifuges are a well-known feature. The components used, including the drive shaft, should be made of a corrosion-resistant material, as the drive shaft often comes into contact with liquids inside the centrifuge or, if the laboratory centrifuges are cooled significantly during operation, must not be susceptible to corrosion from condensation. Therefore, considerable emphasis has been placed on using corrosion-resistant steels, which typically have a 0.2% proof strength (po,2) between 190 and 520 N / mm². 2 exhibit the yield strength R P 0.2 is used in this patent application because the yield strength of some materials is not pronounced but gradual, and one can use the proof strength R P 0.2 can determine a defined size. With a Young's modulus of around 210,000 N / mm². 2Very thin and relatively long drive shafts are necessary so that a flexible drive shaft supports the self-centering of high-speed rotors. This is described, for example, in German patent application DE 1 001 684. This patent application also discloses the positive effect of centrally symmetrically arranged buffers, i.e., spring-damper systems.

[0007] It is quite obvious that only steel or a steel alloy has been used as the material for the drive shaft so far, because this material is inexpensive, easy to obtain and machine, and also corrosion-resistant.

[0008] The particular importance of the thin (steel) drive shafts is clearly illustrated in German patent application DE 2 251 614 A. A length-to-diameter ratio of between 15:1 and 30:1 is assumed. Additionally, elastic bushings between the drive shaft and a surrounding pipe section are recommended.

[0009] It is therefore known to those skilled in the art that a soft design of the drive shaft as a bending spring is advantageous for rotors operating at supercritical speeds, since this lowers the system's resonant frequency and also reduces the restoring forces resulting from self-centering. However, a softer design leads to a reduction in strength. This is a conflict of objectives which the present invention aims to resolve.

[0010] Furthermore, it is known that the flexible drive shaft in a centrifuge can assume different natural shapes. DE 28 14 719 C2 describes how the drive shaft bends during the vibration phases. This invention addresses this by means of several tubular sleeves that surround the drive shaft with specific dimensions. This is intended to prevent permanent deformation when passing through critical natural frequencies. However, this document also describes the drive shaft as being very slender. It assumes a length of 14.6 cm and a diameter of only 6.4 mm. Even though it is coated, this disclosure also assumes that the drive shaft is made of stainless steel.

[0011] The object of the invention is therefore to create a new drive unit, for example for a laboratory centrifuge, which retains the necessary flexibility at critical speeds, but has a higher strength with regard to bending stresses.

[0012] The problem is solved with regard to the drive shaft by the features of claim 1 and in particular by using a material for the drive shaft in which a quotient yield strength of the material in N / mm 2 by elastic modulus of the material in N / mm 2 takes on a value > 0.0045.

[0013] The inventor recognized that the elastic modulus of the drive shaft material had been insufficiently considered. He realized that a lower elastic modulus allows for a larger shaft diameter with the same bending stiffness, thereby reducing bending stress.

[0014] Since the permissible bending stress and the yield strength must also be taken into account, the ratio of yield strength to modulus of elasticity was calculated. It was found that materials with a ratio above 0.0045 prove to be much more suitable than conventional steel drive shafts.

[0015] The following materials were tested as examples (the steels that may be state-of-the-art are listed at the top of the table):

[0016] This led to the discovery of new materials for the drive shaft that are preferable to steel alloys: beryllium copper, aluminum alloys, titanium alloys, magnesium alloys, or brass. A significant advantage of all the new materials mentioned is their corrosion resistance.

[0017] It should be expressly noted that the new material is intended to be the main component of the drive shaft. A coating or, for example, a chrome plating of the drive shaft is understandably also covered by the invention.

[0018] Preferably, a material is used whose ratio of yield strength to modulus of elasticity is even above 0.0072.

[0019] By using these materials, up to three times the safety is achieved compared to a steel alloy, while maintaining the same spring stiffness. Conversely, the spring stiffness can be reduced accordingly while maintaining comparable safety.

[0020] This means that with materials that satisfy the equation, drive shafts can indeed be made thicker to achieve higher strength values. However, elasticity is maintained, and often even significantly improved.

[0021] It is advantageous to achieve a length-to-thickness ratio of less than 12 for the drive shaft. In many cases, particularly suitable materials can even achieve a ratio below 10 or even below 8.

[0022] This has a significant impact on the strength of the drive shaft, which is subjected to bending loads, particularly during the critical start-up phases. The bending stresses acting on the drive shaft are considerably reduced because, according to the simplified formula Ob = Mb / W, where Mb is the bending moment and W is the section modulus (and the section modulus increases with the cube of the drive shaft diameter), the bending stress decreases as the drive shaft diameter increases. Furthermore, the deflections remain within the same limits thanks to the lower Young's modulus of the new material compared to steel.

[0023] Furthermore, as is already known from the prior art, it is also preferred with the new drive shaft material if the flexible drive shaft is at least partially surrounded by a hollow shaft.

[0024] This protects the drive shaft against contamination on the one hand, and on the other hand limits the deflection of the flexible drive shaft through the hollow shaft due to the ring gap width.

[0025] The new material of the drive shaft, from which the hollow shaft should also be made in order to have the same elongation properties, advantageously allows the annular gap formed between the hollow shaft and the drive shaft to have a radial thickness in cross-section of at least 1%, preferably at least 2% and most preferably at least 3% of the length (L) of the drive shaft.

[0026] While previously this thickness had to remain below 1% of the drive shaft length (L) for safety reasons, significantly larger gap widths are now possible with the same level of safety.

[0027] The annular gap in conventional laboratory presses had a maximum thickness of 1 mm, but can now exceed 3 mm without reducing strength or negatively impacting safety. The resulting lower profile and shorter drive shafts are a significant advantage for laboratory centrifuges. Due to the more elastic material, critical resonance frequencies are no longer transmitted to the rotor with the same degree of vibration. It's important to remember that vibrations, for example during blood centrifugation, are also a quality criterion.

[0028] The object of the invention with regard to a laboratory centrifuge, the sample-receiving rotor of which is arranged on a drive shaft mounted vertically in the housing, wherein the drive shaft is in turn connected to a drive motor and has a defined yield strength and a defined modulus of elasticity, is solved by the features of claim 4 and in particular by the fact that the drive shaft consists of a material in which a quotient yield strength of the material in N / mm 2 by elastic modulus of the material in N / mm 2 takes on a value > 0.0045.

[0029] Understandably, even more significant improvements can be achieved if the quotient exceeds the value 0.0072.

[0030] For the reasons already described, it is also advantageous to modify the slenderness of the drive shaft in the laboratory centrifuge so that the length-to-diameter ratio of the drive shaft is less than 12, preferably less than 10, and most preferably even less than 8. The use of a new and more suitable material for the drive shaft can also advantageously benefit from the advancements made in very slender steel drive shafts.

[0031] The positive effects of using and redesigning hollow shafts around the drive shaft have already been mentioned.

[0032] However, it is also advantageous that the drive motor is mounted on at least one spring-damper system, such as the buffer known from the prior art. In this case, oscillations of the rotor at the critical start-up speed are better absorbed.

[0033] For practical use, the drive shaft of the laboratory centrifuge preferably features a quick-change mount for the rotor at its upper end. This allows the rotor to be attached with a quick-release fastener or mounted on a high-friction cone. Test tubes (prouvettes) or other centrifugation chambers that rotate with the rotor and whose contents are separated into components of different densities by inertia can thus be changed more quickly. The laboratory centrifuge can then be used with a different batch while the analysis of the first batch is still underway.

[0034] The invention will now be explained in more detail with reference to Figures 1 and 2. Figure 1 shows a longitudinal section of a laboratory centrifuge according to the invention, and Figure 2 shows the same laboratory centrifuge in a three-dimensional view.

[0035] Both figures show a laboratory centrifuge 1, as it can be designed with the invention, for example.

[0036] In the upper part of Fig. 1 and Fig. 2, the high-speed rotor 2 can be seen, which has several recesses that represent the sample holders 3. Test tubes, for example, can be held here; their contents are separated into components of different densities due to inertia. The lower part of the figures shows the drive motor 4. An induction motor is shown as an example. Among other things, the stator winding 6, the stator lamination stack 7, the rotor lamination stack 8, and the rotor short-circuit ring 9 are indicated in the cross-section of Fig. 1.

[0037] However, the invention is not limited to this type of motor. All other motor types known to those skilled in the art can be used, in particular permanent magnet motors, reluctance motors, separately excited motors, stepper motors or commutator motors. The surrounding bearing shield 5 is essentially simultaneously the housing of the motor 4 and of the part of the laboratory centrifuge 1 up to the rotor 2.

[0038] To prevent vibrations and compensate for gyroscopic movements at critical frequencies, at least one spring-damper system 15 is provided. However, several of these (for example, three) can also act on the circumference on which the motor 4 is supported.

[0039] A flexible shaft 10 is mounted in the bearing shield 5, connecting the motor 4 to the rotor 2 and transmitting the torque. In the illustrated embodiment, a rotating hollow shaft 11 is interposed between the bearing 13 and the drive shaft 10, maintaining an annular gap 14 over a free length L of the drive shaft 10. This annular gap 14 has proven effective in the prior art for preventing excessive deflection of the drive shaft 10. While in the prior art the radial thickness of the annular gap was always chosen to be less than 1% of the length L of the drive shaft for safety reasons, in the invention it can be significantly greater than 2%. Thus, the entire laboratory centrifuge, due to the potentially thicker drive shaft 10 and the correspondingly thicker annular gap 14, features a drive component with higher strength than in the prior art.The hollow shaft 11 can be made of the same material as the drive shaft to ensure it experiences the same thermal expansion. And, as in the prior art, the drive shaft 10 shown in Fig. 1 also has a quick-change mount 12, in the simplest design in the form of a cone, to allow for faster rotor 2 changes. However, all other known quick-change mounts can be used here.

[0040] The fundamentally new aspect of the invention is that a different material than the usual stainless steel is used for the drive shaft 10. The hollow shaft 11 can also be made of the new material.

[0041] The new material is selected based on its yield strength (in N / mm²). 2 ) divided by its modulus of elasticity (in N / mm²) 2) exhibits a value above 0.0045. The vibration behavior and load reduction have proven particularly effective when the value is even above 0.0072. The high slenderness ratio (length L divided by diameter D) required for elasticity in steel shafts, which was previously at least above 15, often 30, can also be reduced with the new material. This has a very positive effect on lower bending stress at critical start-up frequencies. Self-regulation that is just as good as previously achieved with steel shafts is generally achieved with a slenderness ratio of 10. This increases the strength of the drive shaft 10 by up to three times.

[0042] Reference symbol list

Claims

Patent claims 1. Flexible drive shaft (10) for transmitting torque from a motor (4) to a high-speed rotor (2), in particular for a laboratory centrifuge (1), wherein the material of the drive shaft (10) has a defined yield strength and a defined modulus of elasticity, characterized in that a material is used for the drive shaft (10) in which a quotient yield strength of the material in N / mm² 2 divided by the material's modulus of elasticity in N / mm 2 takes on a value > 0.0045.

2. Drive shaft according to claim 1, characterized in that a material is used for the drive shaft (10) in which a quotient yield strength of the material in N / mm² 2 by elastic modulus of the material in N / mm 2 takes on a value > 0.0072.

3. Drive shaft according to claim 1 or 2, characterized in that the ratio of length (L) to diameter (D) of the drive shaft (10) is less than 12, preferably less than 10, most preferably less than 8.

4. Drive shaft according to one of claims 1 to 3, characterized in that it is surrounded by a hollow shaft (11 ) such that an annular gap (14) is formed between the hollow shaft (11 ) and the drive shaft (10) at least over most of the length (L) of the drive shaft (10), the thickness of which in cross-section is at least 1%, preferably at least 2% of the length (L) of the drive shaft.

5. Drive shaft according to claim 4, characterized in that the hollow shaft is made of the same material as the drive shaft.

6. Laboratory centrifuge, the sample-receiving rotor (2) of which is arranged on a drive shaft (10) mounted vertically in the bearing shield (5), wherein the drive shaft (10) is in turn connected to a drive motor (4) and has a defined yield strength and a defined modulus of elasticity, characterized in that the drive shaft (10) according to claim 1 consists of a material in which a quotient yield strength of the material in N / mm 2 by elastic modulus of the material in N / mm 2 takes on a value > 0.0045.

7. Laboratory centrifuge according to claim 6, characterized in that a material is used for the drive shaft (10) wherein a quotient yield strength of the material in N / mm² 2 by elastic modulus of the material in N / mm 2 takes on a value > 0.0072.

8. Laboratory centrifuge according to claim 6 or 7, characterized in that the ratio of length (L) to diameter (D) of the drive shaft is less than 12, preferably less than 10, most preferably less than 8.

9. Laboratory centrifuge according to one of claims 6 to 8, characterized in that the drive motor (4) is mounted on at least one spring-damper system (15).

10. Laboratory centrifuge according to one of claims 6 to 9, characterized in that the flexible drive shaft (10) is at least partially surrounded by a hollow shaft (11).

11. Laboratory centrifuge according to claim 10, characterized in that 12. An annular gap (14) between the flexible drive shaft (10) and the hollow shaft (11) has a cross-sectional thickness of at least 1%, preferably at least 2%, of the length (L) of the drive shaft (10).

13. Laboratory centrifuge according to claim 10 or 11, characterized in that the hollow shaft (11) is made of the same material as the drive shaft (10).

14. Laboratory centrifuge according to any one of claims 6 to 12, characterized in that the drive shaft (10) has a quick-change receptacle (12) for the rotor (2) at its upper end.

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