What are the differences between  disc couplings and diaphragm couplings?

         TRUMY: both  disc and diaphragm types are all-metal flexible couplings, they have many similarities, such as relying on metal bending to adapt to all kinds of misalignment, and feature free maintenance and long life, however, they are based on different working principles, and have respective advantages. 

For turbine equipment, in most cases both types of products can be selected and run reliably for long periods of time, however, for a specific unit, sometimes one may be more advantageous than the other. The two types of all-metal flexible element couplings both realize the functions of torque transmission and compensation, but the coupling is realized in different ways: 

      Disc coupling: function is realized by bending flexible elements (between adjacent bolts) on the coupling disc set, which are fixed to opposite flanges (Figs. 1 and 2). Entirely different torque capacity and bending characteristics are produced with specific diameter discs by optimizing the design of flexible elements. 

In disc couplings, the thinner discs (the basic elements of the laminated disc), the more flexible they are, and the greater compensation capacity can be achieved with the same diameter; the thicker discs, the stiffer they are, and the greater torque can be transmitted with the same diameter. With the same diameter, more bolts can transmit larger torque, but the chord length of curved part is shoten which decrease the ability of cpmpensation. How to optimize the design of these structures and shapes to achieve better characteristics is one of the differences between manufacturers. Because the discs are mounted together, laminated couplings are not prone to sudden failure. It usually starts from the outside and gradually transmit to the inside, so that the coupling donot fail in a short period of time, allowing the equipment to stop in time or fault to be detected. 

       Diaphragm couplings: although there are many different configurations and styles of diaphragms, the principle of torque transmission is the same: it is transmitted from the outer diameter to the inner diameter by integral designed flexible diaphragm elements. With the same diameter, manufacturers can continuously optimize the thickness and shape design to achieve a better match of torque and compensation capacity. 

Due to the very simple structure of diaphragm couplings, the wind resistance is very low at high speed (generally only the  design with coupling shieldfor the high-speed coupling needs to consider these details). The air inside produces friction at low surface speed, and usually the air in the cutting hood will lead to temperature rise. In addition, in large-scale gas turbines and steam turbines, because the flexible elements of the diaphragm coupling can be directly fixed on the turbine flanges with bolts, the flexible elements are very close to the turbine bearing, and a better center of gravity position can be obtained relative to the bearing. Diaphragm elements carefully designed by engineers can achieve very high axial compensation capacity, and some manufacturers can achieve +/-15 mm deformation capacity with a single diaphragm element; when necessary, multiple special shaped diaphragms can be connected in series to achieve large deformation capacity. Therefore, the diaphragms are more suitable for turbine units with relatively large thermal expansion or extremely misaligned applications. 

       Common features: in both types of couplings, all-metal flexible elements (discs and diaphragms) are the core components of couplings. These elements are made of high-strength stainless steel or high-strength alloy steel and are specially treated against corrosion or coated when necessary to reduce or eliminate fretting wear due to the relative movement of individual diaphragms during operation.  

What are the appropriate applications for the diaphragm coupling or  disc coupling?  

TRUMY: In the low speed applications of general equipment, disc coupling is usually used because economic demand is the first factor. High performance laminated disc/diaphragm couplings are usually suitable in special purpose applications. 

High-speed centrifugal machines usually require use of low bending moment couplings (disc couplings can be designed in the inverted form, so that the position of disc group or the center of gravity of the component is closer to bearing), therefore compressor manufacturers generally tend to use laminated disc coupling between high-speed gear and compressor; Laminated disc couplings also allow the use of a thicker shaft and are superior to diaphragm couplings in terms of low bending moment design. In addition, for specific torque values, laminated disc couplings can be designed in a variety of ways to achieve lighter weight and exert less force on the connected equipment than diaphragm couplings.  

Manufacturers of drive turbines (steam and gas) generally prefer diaphragm couplings. Turbine rotors are usually designed to have a flanged interface so that they can be bolted directly to diaphragm couplings or rigid couplings. In addition, diaphragm coupling is suitable for stable operation when the axial or angular offset is large, therefore more suitable for the applications where the axial position of the turbine unit may change significantly. Furthermore, in the presence of periodic torsional vibration when a synchronous motor is started, the fatigue resistance of diaphragm couplings is sometimes better than that of laminated disc couplings.