Coupling Technology Blog

Mechanically Generated Friction Fits in Power Transmission Couplings 

In the world of electric motors and power transmission coupling products, torque is increasingly being transferred between directly coupled shafts by means of purely mechanically generated friction rather than by positive drive connections like keys and keyways or splines. Mechanical friction fits evolved into modern standardized dimensions during the twentieth century as machine tools and cutting tooling evolved into their present states of precision and accuracy. For many decades power transmission component design lagged behind the machining world when it came to the use of mechanically generated frictional clamping, with a large portion of connecting elements still relying on keyways and interference fits to guarantee transmission.

Interference fits, wherein the diameter of the male component is actually slightly larger than the diameter of the mating female component, do have their merits when it comes to ensuring reliable backlash free transmission.  But they can be difficult to assemble, as the shaft must be cooled with a cryogenic such as liquid nitrogen, or the bore needs to be expanded with heat. If either high heat or extreme cold are used, safety is a key concern during the assembly process. Heating a metal component will also often change its mechanical properties such as temper, so cooling the male fitting part is preferred when maintaining the material properties is a concern. Due to the inconveniences associated with interference fits, most newly designed frictional clamping coupling hubs are made using compression by means of screw thread fasteners instead. 

This type of mechanical connection is very predictable and can be made by applying a set amount of torque to radial or axial screws. For easy assembly the fit between the male and female part typically should not have overlapping diametrical tolerances, but should also not allow for excessive clearance. When the fits can be made by hand, proper tightening of the fasteners is all that is necessary to create a friction fit which will reliably and predictably transmit torque for the life of the machine.  A shrink disk style fit on a female member of a coupling will maintain concentricity and will compensate for tolerances on a male shaft while maintaining the ability to transmit torque across a range of diameter dimensions and torque values. This type of conical clamping hub can be manufactured in different styles to promote modularity, ease of assembly and concentricity between shafts, often allowing for easier balancing for high speed applications. 

The fastener assisted friction fit has streamlined design of couplings as it allows for the use of smooth shafts which are not subject the imbalance and complications shaft key or spline tolerances can cause. The next time you are sizing and selecting rotating components, consider going with keyless frictional fits and reap the benefits of modern machine construction.

Vibration damping couplings:  sometimes bigger is better

A customer sizing an elastomer insert coupling recently asked us if using a larger body size would provide better vibration damping.  In a word, the answer was, yes.  Vibrations in machine shafts come from a wide variety of sources. Machines that crush and shred materials can impart quite a bit of vibration into the drive line and prime mover. These types of machines can cause motors and mechanical driveline components to fail prematurely if proper damping measures are not in place.  Conversely, driving components, especially internal combustion engines, can impart severe vibration down the driveline.  While R+W specializes in vibration damping couplings for electric motor drives, the concept of increasing driving inertia for smooth continuous rotation is most easily explained and widely understood in terms of internal combustion engines.

One of the oldest methods mechanical designers have used to damp vibrations is employing a flywheel. This device is still widely in use, in many parts of industry. Using laws of basic classical mechanics, “a body in motion tends to stay in motion, while a body at rest tends to stay at rest,” designers know that it is difficult to change the rotating velocity of a massive, high inertia flywheel easily. This is why a flywheel is used on the output shaft of many combustion engines, since they tend to continue rotating smoothly rather than jerking severely every time a cylinder fires. A large drive coupling or belt pulley would have the same effect. Piston air compressors often have massive cast iron sheaves driving them for this reason. 

In situations where inertia, space and power consumption are not of concern, it can be beneficial to use shaft couplings with larger and heavier hubs than are necessary to simply transmit the torque.  At R+W we have machined flywheel features directly into steel coupling hubs on many occasions in order to assist customers with torsional vibration damping, especially in pump coupling applications. Another reason to consider upsizing for vibration damping is that in the case of elastomer insert couplings, the vibration is spread over a larger area which can also provide damping. In general, using the “bigger is better” concept is often a good policy when the priority is to make a robust and smooth running machine more so than a quick moving, light or compact machine. For many folks this concept is basic common sense, however in this day and age of making everything more compact, this principle can be overlooked when it might make a lot of sense.