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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.

Elastomer Jaw Couplings:  not all are created equal

Today’s mechanical drive components market is flooded with a wide variety of direct drive couplings. Many folks working with or designing drives are not familiar with the different technologies or differences in quality and precision between manufacturers. One prime example is with elastomeric jaw couplings. Elastomer jaw couplings have been around for 100 years or more. Most of these couplings use two hubs and an elastomeric spider between the jaws to transmit torque. Although they look essentially the same, the applications can vary greatly.

Traditional Jaw Couplings

Much of the market by volume of units sold consists of low manufacturing cost cast hub couplings with a rubber spider element.  These hubs are used in drives such as conveyors and centrifugal pumps turning in one direction where precise shaft position is not important.  Torque is usually transmitted accross the hubs by means of a keyway locked down with a set screw.  Many of the machine frames are formed and welded to loose tolerances.  The coupling design has plenty of play between the spider insert and jaws to accomodate shaft misalignment.  This design is optimal for producing machinery where labor time to align shafts is cost prohibitive, and they are ideal for less demanding applications. 

Precision Jaw Couplings

A somewhat smaller part of the market volume is precision backlash free or low backlash elastomeric couplings. The hubs of these couplings are generally machined from solid stock with clamping features incorporated into them for wear free and play free frictional connections to the shafting.  This can eliminate the need for keys and keyways and also enhance performance while improving longevity.  

 

The machined hubs typically also come with curved jaws held to a very tight level of concentricity in reference to the bore. Measuring concentricity from the jaw to the bore is what ensures smooth transfer of rotation from one jaw set to the next. 

       

 

 

 

 

 

Wear resistant polyurethane inserts are then put through a secondary “match molding” process to smooth out any inconsistencies on the driving lugs, before being press fit into the hubs for a preloaded zero backlash assembly. 

 

 

 

 

  

Being based on the same principle of mounting a resilient element between the jaws of two metallic hubs, they look similar, but are quite different in some key characteristics.  Because the hubs are precision machined from solid bar stock, they are naturally more expensive than a cast or sintered hub.  But considering that both coupling styles are relatively inexpensive, the cost differences are often far outweighed by the benefits in situations where high torques or shock loads must be transmitted through a small space envelope, as can happen in many pumping, conveying, crushing and grinding applications to name a few, or in cases where dynamic motion needs to be transmitted smoothly, like in servo and stepper motor driven machines.  In the latter situations the precision concentricity and backlash free characteristics, and not only the stronger material condition of the hub benefit the user and the process.  

    

Shaft alignment

One of the assumptions being made with this design concept is that that a higher value will also be placed on the overall quality of the drive line assembly, which always includes precision alignment.  It is not uncommon for someone to switch to a precision elastomer jaw coupling and hear an audible clicking as the equipment runs. Because the elastomeric spider element is preloaded in the hubs and is a slightly harder material, it will rub on the metal surfaces audibly if there is significant shaft misalignment. Although lubricant can be used to address the clicking, eventually the elastomer segment may need replacement.  But when alignment is addressed they can be wear-free for a theoretically infinite service life.  

 

 

 

Many motors, gearboxes, linear actuators and pumps have precision centering features already incorporated into their mounting frames by the manufacturer for this very reason. When the coupling is installed inside a housing or bracket which takes advantage of a centering feature on both pieces of coupled equipment, then alignment is normally within the allowable limits for the precision variety of elastomer coupling.  

 

 

 

 

When no such feature exists, then alignment must be checked manually.  There are several approaches which can be taken to address this potential issue. For a single elastomer element coupling the shafts need to be precisely aligned with dial indicators or laser alignment systems in case the shaft alignment is not inherent to the mechanical assembly.  

 

 

 

    

For situations where space is tight, split hub couplings are available to be installed laterally after the manual alignment check has been performed and any necessary adjustments have been made. 

 

 

     

Then, in cases where precision manual alignment is just not practical, there are other options.  Many manufacturers make the urethane elastomer segment available with a convex tooth geometry, which allows for a rolling action as the coupling hubs rotate under angular misalignment. 

 

 

 

 

 

When two are used in series with a coupling spacer, the ability to compensate for parallel shaft misalignment is magnified to many times greater than with the single element version, allowing for simple visual shaft alignment, while at the same time maintaining the benefits of relatively low inertia, zero backlash and high torque density.  

 

 

 

 

 

While these two different shaft coupling styles may look more or less the same, small differences in materials and manufacturing processes can mean big differences in terms of the end result.  When considering making the switch from traditional to precision varieties of elastomer insert jaw couplings, consider discussing your requirements with the manufacturer, and make sure to check all of the specifications to get the best coupling for your unique situation.

  

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.