Couplings provide the vital link between the elements within a drive train - motor to gearbox or output shaft to driven machinery for instance - and few drive systems would survive without them. Like drive systems, they come in all sizes from enormous to minute and with various performance profiles from precision to power. “Drive Lines’ principal coupling manufacturer is R+W in Germany, which splits its products into two ranges, ‘industrial’ (Black) and ‘precision’ (White),” Ian Carr, of Drive Lines, said.
“Included in the R+W Black range are a number of different types of couplings, each of which has performance characteristics that make it suitable for particular heavy-duty applications,” he continued. “Selecting the best coupling for a given job and ensuring the optimum size is specified requires some skill and experience; this is what Drive Lines provides to its customers. The R+W range of industrial couplings includes safety, disc pack, flexible gear, metal bellows and elastomer types.”
Safety couplings are often referred to as torque limiters. They are designed to disengage the drive in the event the load exceeds a safe amount, thus protecting the drive system and possibly the motor, driven load and/or nearby personnel. The simplest form of safety coupling is based on shear pins, sized to break at the torque limit; but such designs are not particularly accurate or reliable and they need to be rebuilt if they are tripped.
The concept can be taken a considerable step forward by replacing the shear pins with a sprung ball and detent or plunger mechanism that activates at the set torque to disengage the coupling. With such a mechanism, the coupling is easily reset. It is also worth noting that some mechanisms can be adjusted to the application’s specific requirements.
“R+W offers safety coupling options that automatically reset after one full revolution of the drive shaft, after rotating a set number of degrees or which require manual resetting, in sizes from 0.2kNm to 250kNm, depending on the application,” Carr said. “R+W disc pack couplings transmit torque through thin spring steel discs assembled into a pack and held tightly by pins. Misalignment is accommodated by deformation of the discs. The design is flexible, while remaining torsionally strong under high torque loads and finds many uses in industrial drives as well as being popular for some precision servo applications.
“A one-disc coupling can be used where there is no need to allow for parallel misalignment: in a double disc coupling the discs can bend in different directions to better manage any compound offset. Torsionally stiff yet flexible, disc couplings are often used in high speed applications. R+W offers a range of standard disc couplings from 350Nm to 20kNm, plus bespoke designs, couplings with integral torque sensors and special couplings that comply with API610, the standard for centrifugal pumps used in the petroleum industry.”
Flexible gear couplings consist of driving and driven hubs connected by a surrounding sleeve. The hubs have external gear teeth which mesh with internal teeth in the sleeve. The design is torsionally rigid yet it can accommodate both angular and offset misalignment as well as axial forces, so finds a wide variety of applications in many different industries. “The R+W flexible gear couplings use a special crown gear tooth profile which minimises wear, allowing for a long, low maintenance life in almost all applications,” Carr added. “They are available in sizes from 1,300Nm to 348kNm and are often chosen when a compact coupling is required.”
Metal bellows couplings are widely used and often seen as a default choice by drive design engineers. Their basic principle is that the bellows style sleeve between the two hubs is flexible in several directions so can accommodate compound misalignments, yet provides an accurate and reliable drive. “Our bellows couplings are distinguished by the highly flexible, high grade stainless steel of the bellows and the top quality steel of the hubs,” Carr explained. “They are ideal for high torque applications and are based on a compact, simple design that allows easy mounting and dismounting. Bellows couplings offer high misalignment compensation, yet are backlash free and torsionally rigid. The bellows element is available in various lengths and custom designs can be produced to exactly match the requirements of particular applications. Sizes are from 10kNm to 100kNm.”
By incorporating an element made of an elastomeric material within the design of a coupling, it is possible to provide vibration damping and shock absorbing properties. “ We have developed a number of elastomer based couplings, including one design with a split clamping hub, another with a keyway connection and one with a conical clamping ring, in sizes from 1.95kNm to 25kNm,” Carr said. “Typical characteristics of elastomeric couplings include high clamping forces, vibration damping, electrical isolation, backlash free rotation, and a press fit design.”