Flexible Membrane Couplings
As a key component in modern industrial transmission systems, flexible membrane couplings have become an ideal choice to replace traditional toothed couplings due to their unique metal elastic element design and excellent performance.
Flexible membrane coupling is a high-performance coupling device that uses metal elastic elements to compensate for the relative displacement between two shafts through the elastic deformation of stainless steel thin plate diaphragm groups. The flexible membrane coupling belongs to the category of seamless metal flexible couplings, and its core design concept is to use the elastic properties of metal materials to achieve a perfect combination of power transmission and deviation compensation.
The flexible membrane coupling mainly consists of three parts: two half couplings with wheel hubs, one or more sets of stainless steel diaphragms, and connecting bolts. The membrane is usually made of high-strength stainless steel, and each set of membranes is composed of several (usually 4-12) ultra-thin stainless steel sheets stacked together, which are fixed to the two halves of the coupling by precision bolts in a staggered manner. According to the compensation capability requirements, the membrane can be designed in different shapes such as linkage or integral, with linkage providing better flexibility and integral providing higher torsional stiffness.
The working principle of flexible membrane coupling: When the driving shaft rotates, torque is transmitted to the diaphragm through the bolt group, and then transmitted to the driven shaft through the diaphragm. When there is relative displacement between the two axes, the diaphragm absorbs these deviations through its own elastic deformation - radial displacement causes the diaphragm to undergo tensile or compressive deformation, angular displacement causes the diaphragm to undergo bending deformation, and axial displacement causes the diaphragm to undergo planar deformation. This elastic deformation mechanism enables the coupling to achieve deviation compensation without any sliding parts, fundamentally avoiding wear problems.
Classification: According to the number of membrane groups, it can be divided into two types: single membrane and double membrane. The single diaphragm coupling has a simple and compact structure, suitable for situations with small deviations, and can usually withstand angular deviations of about 0.5-1 degrees; The double diaphragm coupling adds a rigid element in the middle, and the two sets of diaphragms can bend in different directions, significantly improving the compensation ability. It can handle larger radial and angular deviations (radial deviation can reach 1.5mm, angular deviation can reach 1.5 degrees), especially suitable for installation environments with poor centering conditions.
Compared with traditional couplings, flexible membrane couplings have revolutionary advantages such as no lubrication, no wear, and zero backlash. Metal diaphragms do not age and fail like rubber or plastic elastomers, nor do they require regular lubrication like gear couplings, which makes them outstanding in terms of maintenance free performance. Meanwhile, the all metal structure enables it to operate stably within the extreme temperature range of -80 ° C to+300 ° C and withstand corrosive media environments, greatly expanding its application scenarios.
The reason why flexible membrane couplings can quickly replace traditional couplings in many industrial fields is due to their excellent mechanical properties and unique technological advantages. These characteristics make it an ideal choice for high-precision, high-speed, and heavy-duty working conditions.
Powerful deviation compensation capability: The diaphragm coupling can simultaneously compensate for three types of deviations: axial, radial, and angular. Its angular compensation capability can reach twice that of traditional gear couplings. When radial deviation occurs, the reaction force generated by the coupling is small and the system flexibility is large. Typical parameters are: axial displacement ± 0.5-5mm, radial displacement 0.2-1.5mm, angular displacement 0.5 ° -1.5 °. This multi-directional compensation capability is particularly suitable for situations where shaft alignment is difficult due to foundation settlement, thermal expansion, or manufacturing and installation errors.
Excellent vibration reduction and transmission performance: The metal diaphragm can effectively absorb vibration energy while transmitting torque, ensuring smooth and noise free system operation. Tests have shown that diaphragm couplings can reduce system vibration amplitude by up to 30% -50%. Its transmission efficiency is as high as 99.86%, with almost no power loss, which is significantly higher than most flexible couplings. More importantly, it can accurately transmit rotational speed without any slip during operation, and can even be used for precision mechanical transmissions with strict requirements, such as CNC machine tool spindles and measuring equipment.
Environmental adaptability and durability: The membrane made of stainless steel material has excellent corrosion resistance and can resist corrosion from acidic and alkaline media. The all metal structure does not require lubrication, avoiding oil pollution problems, and is particularly suitable for industries with high cleanliness requirements such as food and medicine. The product has a wide working temperature range, with conventional models ranging from -80 ° C to+300 ° C. Specially designed couplings can even adapt to deep cold environments of -196 ° C or high temperature conditions of+350 ° C. It can still operate safely under impact vibration conditions, and its service life can usually reach 10-20 years, far exceeding that of general elastic couplings.
Structural and maintenance advantages: The diaphragm coupling has a compact structure, light weight, and small installation space. The design with an intermediate shaft allows the equipment to be assembled and disassembled without moving, greatly simplifying maintenance work. Due to the absence of friction components that move relative to each other, the coupling is essentially maintenance free and only requires regular inspections of bolt tightening status and membrane for cracks, significantly reducing the overall cost of ownership.
From a technological development perspective, modern diaphragm couplings have continuously improved their performance through finite element optimization design and advanced material applications. If high-strength nickel alloy membrane is used, the fatigue life can be extended by 30%; Special surface treatment techniques such as nitriding and shot peening can significantly improve the wear resistance and corrosion resistance of the membrane; By using dynamic balance correction (with an accuracy of G2.5 level), the coupling can operate more smoothly at high speeds, and these technological advancements continue to expand the application boundaries of diaphragm couplings.
After years of development, flexible membrane couplings have formed a rich product line, with significant differences in structural design, size specifications, and performance characteristics among various models to meet the application needs of different industrial scenarios. Understanding the classification and characteristics of these models is crucial for proper selection.
DJM single key slot coupling film: adopting a single membrane design and key slot connection method, the structure is the simplest and most compact, suitable for medium torque transmission occasions with limited space. The nominal torque range is usually 10-5000Nm, and the shaft hole diameter ranges from 6mm to 120mm. This model has strong axial displacement compensation capability, but relatively limited angular compensation capability (about 0.5 degrees), making it suitable for installing equipment with high centering accuracy.
DJM-T expansion sleeve coupling: Innovatively using expansion sleeve connection technology instead of traditional key connection, the expansion sleeve forms an interference fit with the shaft through high-strength bolts, achieving keyless connection. This design eliminates stress concentration caused by keyways, making installation and positioning more precise. It is particularly suitable for situations that require frequent disassembly or high-precision positioning, such as the connection between servo motors and ball screws. The typical application torque range is 50-8000Nm.
ZJM cone sleeve coupling with double diaphragm coupling: Combining the advantages of cone shaft sleeve and double diaphragm structure, the cone sleeve design provides better installation centering, while the double diaphragm group provides greater deviation compensation capability. This model is particularly suitable for large power units such as ship propulsion systems, heavy-duty compressors, etc. Its nominal torque can reach over 50000Nm, and the maximum shaft hole diameter can reach 300mm. The cone sleeve connection can also effectively prevent micro motion wear between the shaft and the hub.
TJM external clamping diaphragm coupling: adopting a unique clamping wheel hub design, the shaft diameter is directly clamped from the outside through high-strength bolts, eliminating the keyway machining process and making installation more convenient. This structure transmits torque evenly, does not damage the shaft surface, and has automatic centering characteristics. There are two variants: single diaphragm (TJM) and double diaphragm (TJMJ). The former is suitable for small servo systems, while the latter is used for medium-sized transmission systems that require higher deviation compensation capabilities.
JM Micro Diaphragm Coupling: Designed specifically for precision small equipment, with a minimum outer diameter of up to 12mm and a torque range of 0.1-100Nm, it is widely used in micro transmission fields such as stepper motors, encoders, laboratory instruments, etc. Although these types of couplings are small in size, they still maintain the characteristics of zero backlash and high rigidity of diaphragm couplings.
In recent years, modular design has become a trend, such as the F-type diaphragm coupling, which combines the compactness of the R-type and the ability to accommodate large shaft diameters of the G-type. The same specification can be adapted to multiple shaft diameters, significantly improving product versatility. Specially designed for ultra high speed applications, such as the 8-cornea structure (replacing the conventional 6-cornea structure), enables more uniform torque transmission, with a maximum speed of up to 30000 rpm, meeting the demanding requirements of high-end fields such as turbomachinery and aviation engines.
The flexible membrane coupling is subjected to complex load combinations during operation, and accurate analysis of these stress conditions is crucial for ensuring reliable operation and extending the service life of the coupling. As the core elastic element of the coupling, the stress state of the diaphragm group directly determines the performance of the entire coupling.
Thin film stress generated by torque: When the coupling transmits torque T (N · m), for a typical structure with m groups of diaphragms and 8-hole bolts, the torque T1 borne by each group of diaphragms is T/m. Through mechanical analysis, it can be concluded that the force acting on each main bolt is F=T/(4mR), where R is the radius of the bolt distribution circle. This force causes shear stress and in-plane tensile stress on the membrane, which are proportional to the transmitted torque and inversely proportional to the square of the membrane thickness and quantity. Modern finite element analysis shows that the maximum stress usually occurs near the hinge point between the diaphragm and the hub, which is prone to stress concentration and is a common starting point for fatigue cracks.
Centrifugal stress: The centrifugal inertia force generated by various components of the coupling during high-speed rotation cannot be ignored. When calculating, the mass and position of the bolt and diaphragm need to be considered. The centrifugal force F=(2 π n/60) ² rp, where n is the rotational speed (rpm), r is the mass radius, and p is the mass. These centrifugal forces acting radially outward will cause additional tensile stress in the membrane. Especially when the speed exceeds 10000rpm, centrifugal stress may become the dominant factor, which is why high-speed couplings typically use lightweight aluminum alloy wheels and high-strength thin film sheets. By fixing the radial, circumferential, and axial displacement boundary conditions of the middle bolt hole, the stress distribution under this working condition can be accurately simulated.
Bending stress caused by axial deviation: Axial deviation during installation can cause bending deformation of the diaphragm along the axis direction. Applying this displacement load in the axial direction of the middle bolt hole, while fixing the radial and axial displacements, can establish a static simply supported mechanism model for analysis. Research has shown that the stress on the diaphragm caused by axial deviation is linearly related to the amount of deviation and inversely proportional to the square of the diaphragm length. Therefore, for applications that may generate significant axial displacement, such as steam turbines with significant thermal expansion, diaphragm couplings with longer intermediate sections should be selected to reduce bending stress.
Angular deviation stress: Angular installation errors cause periodic bending deformation of the diaphragm, which is the main cause of fatigue failure. The magnitude of the restoring moment H can be calculated by tilting the angle. Since the angular displacement of the diaphragm is usually very small (<1.5 °), the thin plate small deflection bending theory can be used for analysis. In practical engineering, the amplitude of alternating stress generated by angular deviation is often much greater than the average stress, which makes the fatigue life of the diaphragm extremely sensitive to the installation accuracy. Experimental data shows that when the angular deviation increases from 0.5 ° to 1 °, the lifespan of the membrane may be shortened by 60% -70%.
The stress concentration factor (Kt) is crucial in the design of diaphragm couplings. There are usually high Kt values (2.0-3.5) at the edges of bolt holes and transition corners on the membrane. By optimizing the geometric shape, such as using variable thickness design or stress relief grooves, Kt can be reduced to below 1.5. The latest design trend is to add reinforcement patches in high stress areas. Finite element analysis shows that this structure can reduce peak stress by 30% -40% and significantly extend service life.
Other stress factors need to be considered for special application environments. For example, in the propulsion system of a ship, the coupling may bear impact loads, and the instantaneous peak stress needs to be checked; In the petrochemical industry, the combined effect of corrosive media and stress may cause stress corrosion cracking, requiring the selection of corrosion-resistant materials and control of surface stress levels. Through comprehensive force analysis and precise calculation, the diaphragm coupling can ensure safe and reliable operation under various working conditions. The typical safety factor is generally taken as 2-3 (based on yield strength) or 4-6 (based on fatigue limit).
Proper installation and standardized maintenance are key factors in ensuring optimal performance and extending the service life of flexible membrane couplings. Unlike traditional couplings, diaphragm couplings have higher requirements for installation accuracy and operating procedures, and any improper operation may lead to premature failure or performance degradation.
Installation Guide
Pre installation inspection: Before installation, thoroughly clean the shaft end and coupling inner hole, check whether the shaft diameter size and keyway fit meet the requirements, and remove all burrs and sharp corners. Measure and record the actual dimensions of the shaft diameter and coupling hole to ensure that the interference fit is within a reasonable range (usually H7/k6 or H7/m6 mating). For precision equipment, it is recommended to use a dial gauge to check the radial runout (generally ≤ 0.02mm) and end face runout (generally ≤ 0.01mm/m) of the shaft end.
Axis deviation control: The axial, radial, and angular deviations that the coupling can compensate for are strictly limited. During installation, it is necessary to use a laser centering instrument or dual meter method for precise adjustment to ensure that the deviation is within the allowable range (usually radial ≤ 0.1mm, angular ≤ 0.5 °). When multiple deviations coexist, the total compensation capability will decrease, and each deviation should be controlled within 1/3 of the standard value. It should be noted that although diaphragm couplings can compensate for certain deviations, the ideal centering state can maximize their lifespan.
Bolt tightening process: The bolts of the diaphragm coupling must be tightened in stages in diagonal order. Firstly, pre tighten all bolts by hand, then use a torque wrench to evenly tighten them in three stages of 1/4, 1/2, and 3/4 of the rated torque, and finally fully tighten them to the rated torque (usually 70% -80% of the material yield strength). After tightening, it is recommended to apply anti loosening glue to the threaded part of the bolt. It is absolutely forbidden to use pneumatic tools for direct fastening to avoid overloading.
Maintain standards
Regular inspection: After 8 hours of operation, all bolts of the newly installed coupling should be tightened again, and then checked every 500 hours of operation thereafter. Focus on checking whether there are cracks or deformations in the membrane, whether the bolts are loose, and whether there are signs of relative sliding in the wheel hub. For high-speed equipment (>3000rpm), it is recommended to conduct vibration testing once a month to determine the coupling status through spectrum analysis.
Lubrication and anti-corrosion: Although the diaphragm coupling itself does not require lubrication, the supporting shaft extension, keyway and other parts should be regularly coated with an appropriate amount of lubricating grease. In corrosive environments, the surface of the membrane can be coated with molybdenum disulfide or treated with Teflon, which can reduce micro motion wear and enhance corrosion resistance. Couplings used in coastal areas require special attention to prevent stress corrosion caused by chloride ions.
