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.
