Extreme temperature ranges, high pollution levels, high vibration levels, and 24/7/365 operation call for engineered drive components which can take the abuse. Extending component maintenance intervals is a priority, since industrial gearboxes, electric motors, bearings, and drive shafts typically require at least partial disassembly and replacement of wear parts. A new solution involves zero maintenance, stainless steel bellows couplings for up to 100,000 Nm, engineered to run smoothly for decades.
One challenge in the development of the BX series was the design of the metal bellows itself. The bellows serves to alleviate misalignment loading between adjacent bearing sets while transmitting rotary power. Traditional sizing calculations were used at first, having proved successful for high speed servo drive systems, which tend to be some of the most demanding of the customary applications for bellows couplings. After a short time it became clear that a widely marketable high torque bellows coupling could not be designed without FEA, as the first installed models were two to three times larger than the final product design. As life testing later confirmed, the software revealed some errors in the initial interpretation of the sizing, demonstrating that the traditional methods did not scale properly. In collaboration with a major bellows hydroformer, R+W reviewed and updated these traditional bellows sizing methods to better reflect the realities of what the formed bellows can tolerate in this physical size range.
A special welded joint between the bellows, intermediate tubing, and mounting flanges was also selected, involving a combination of TIG (perfect root) and MAG (low heat generation). This welding method is a recent development of RWTH Aachen University IME, a world renowned leader in mechanical and electrical engineering research, and has demonstrated superior fatigue characteristics in vibration and life testing. With these two important features in place, hubs, flanges, and intermediate tubing, fabricated from high strength steel, were configured to round off the complete product line. R+W has been able to launch the all new “BX” series, with a variety of standard and custom mounting options available, and presently broken down into three specific series.
BX1 with Flange Mounting
The BX1 is the basic model from which the complete BX series is configured. Mounting flanges are welded directly to the bellows, which are then used for adaptation to shaft mounting systems in the cases of the BX4 and BX6. Power is therefore transmitted through the BX1 by means of a frictional flange connection. A common misconception is that the torque is transmitted through the bolts themselves. This can have two negative consequences in that the coupling would exhibit backlash, and that ultimately the bolts would fatigue and fracture. In case of reversing applications or those with frequent load variation this failure would occur within a very short amount of time. Basically torque ratings of flanges are calculated based on the pressure generated by the bolt circle arrangement, the diameter into which they have been arranged, and the coefficient of friction of the flange surfaces. A number of safety and application factors are normally added to ensure a pure high-pressure frictional connection results.
BX4 with Keyway Mounting
The BX4 is described as a low backlash coupling, which transmits torque purely though the form fit of a shaft key or spline. Some potential for backlash always exists due to the clearance required to fit the hub over the shaft. However reduced, this is even in the case of an interference fit, and especially in cases where only set screws are used for the axial retention of the hub. The main advantage of a keyway connection is the quick and easy installation and removal of the coupling hubs. In lieu of keyway connections, spline connections provide additional security and surface area contact with superior stress distribution characteristics. In consideration of some extra cost, this can be a good solution for a more compact design, as the necessary shaft engagement length can then be reduced. Generally speaking, the BX4 is well suited to unidirectional drive applications, which must accelerate, even to high speeds, and maintain those speeds for prolonged periods. Drive systems with shock loading, reversals, and high duty cycles are better served by the BX1 and BX6 designs.
BX6 with Conical Clamping Ring
The BX6 makes use of a high pressure, keyless, frictional clamping connection. The conical clamping ring system consists of two components: the clamping ring and the hub. The clamping ring is drawn over a slotted tapered section of the hub by means of a set of fastening bolts. Sliding the two tapered surfaces against one another creates a high level of surface pressure in the vertical direction, compressing onto the cylindrical shaft. The resulting contact force eliminates the need for keyways and is suitable for transmitting extremely high torque levels depending on the diameter of the shaft. Since the BX6 has many planes of symmetry across its length, it is also well suited to very high rotational speeds, as a result of the low residual imbalance.
Variable Length with Internal Support
The intermediate section of the BX series consists of high strength steel tubing with adapter flanges common to the full product line. The length of the intermediate tubing section can be specified according to customer requirements and interchanged for machinery modifications as needed. The adapter flanges are made with a gimbal system which extends through the inside of the bellows and engages a concave receptacle on the opposite side. This support system causes the entire mass of the intermediate section to be carried by the shafting rather than the flexible bellows. It also serves as a safety feature in the event of a machine crash. Should the bellows or its connection be caused to fail by extreme overload, the tubing would still not be allowed to escape from the hubs, eliminating the need for coupling guards in some cases. This safety feature does not exist in flex disc or universal drive shaft products.
Summary and Outlook
The all new “BX” series offers standard torque ratings of up to 100,000 Nm and bore diameters of up to 280 mm. This technology provides a viable alternative to steel disc couplings, with improved torsional stiffness, balance, and safety features. Flexible design configurations allow for the BX to be made to suit applications with specialized flanges, hubs, materials and surface treatments for even the harshest of environments. The internal support system provides for a high degree of security unavailable in other similar types of couplings. The symmetrical design of the series BX1 and BX6 allows for an optimal balance quality in highly dynamic drive systems and test stands, with ISO Grade 2.5 certification available upon request.
The principle of the formed metal bellows as an equalizing element offers many unique performance advantages for today’s advanced drive technology. Designs capable of transmitting up to 2,000,000 Nm are currently in development.