Productivity is a shared goal in industrial applications involving designers, component makers, and end users. Everyone plays a role. In this report, Motion System Design editors polled coupling and shaft experts for their advice on optimizing productivity. Here are their responses, which we believe you'll find most helpful.
What design features in couplings and shafts contribute to higher productivity, and why?
Galen/Rockwell: Couplings that warn of failure — noise and minor vibration — allow scheduling ahead for downtime and part replacement. Flexible, elastomeric couplings provide misalignment capability, which creates less cumbersome installations and shorter replacement times. Those with tapered bushings install quickly and easily as they slip over and collapse down on the shaft concentrically. During removal, they don't mar the shaft or cause fretting corrosion.
Bill/Rexnord: Since bearings, seals, and alignment work with couplings, they frequently require maintenance. High durability, fatigue life, low weight, high balance, and low-reaction forces contribute to coupling design features. Repeated equipment changes demand assembly and disassembly ease.
Edward/Ringfeder: A torsionally rigid, backlash-free coupling — such as the double-disc pack — suits most production machines, as it synchronizes timing on packaging equipment and prevents ink blurring on printing equipment. It's maintenance-free, functions in high-temperatures, and comes in various lengths.
Parthiv/Emerson: In selection software, four features speed coupling choice:
Pull-down menus requesting coupling design criteria
Built-in industry specs
Mass elastic data (weight, inertia, stiffness) that enables modifiable parameters and quick, iterative runs
Downloadable CAD files that integrate couplings into the overall design
Andrew/R+W America: Introducing cavities removes excess material from coupling hubs. As a result, lighter-weight couplings optimize inertia between the motor and load, enabling acceleration. Also: Self-supporting shafts made from hollow tubing (instead of solid steel) can eliminate using pillow-block bearings at long lengths, simplifying machine assembly and reducing component cost and count.
Frank/Stafford: Coupling selection should be based on torque, shaft geometry, and reducing/increasing shaft size. In addition, design and application engineers must communicate about proper product use.
Andrew/KTR: Considering overall system dynamics translates to higher productivity.
Tom/Helical: Attachment methods affect assembly ease and performance. Integral clamps simplify removal and replacement without marring the shafts. Other options include combining components on either end of the coupling and incorporating multiple pieces into one. Material choice also contributes to higher productivity and can lead to greater durability. For example, changing from aluminum to stainless steel can double torque capacity and triple torsional rates.
What can designers do to ensure higher productivity from the couplings and/or shafts they place in machines?
Galen/Rockwell: Flexible couplings with elastomeric inserts require little maintenance and aid productivity. Because couplings are inexpensive and easily replaced, they're often a system's weakest link. This is why during overload, the coupling fails first: to protect gearboxes, shafts, and motors.
Bill/Rexnord: On all applications, proper assembly of bushed arrangements assures squareness and shaft capture. Also, correct shaft and key fits prevent unbalance and excessive stress.
Edward/Ringfeder: Maintenance-free, easily removable couplings allow inspection of adjacent components during normal downtime; some connections provide simple shaft removal from hubs.
Parthiv/Emerson: Program input parameters minimize weight, increase load carrying capacity, and can fit compact designs into the envelope.
Andrew/R+W America: Flexible couplings eventually break or overload the shaft with restoring forces — especially common in zero-backlash designs. To find concentricity tolerances on the motor and mounting flange, designers should measure shaft-to-shaft runout with a dial indicator prior to installation. This saves time and money when replacing shafts and couplings in the field.
Frank/Stafford: Designers must consider machine design and intent. For instance, set couplings loosen from vibration or misalignment, but rigid couplings have a “set it and forget it” design. In high-torque applications, keys can reinforce clamp-style couplings — the frictional hold of which may slip. That's because keys and keyways allow additional torque and fixed rotational shaft alignment.
Andrew/KTR: Sizing involves estimating load profiles that the coupling might experience during normal operation, start-up, shutdown, and upset conditions. Most important are maximum peak torque loads, long-term nominal torque and speed, and torsional vibration. For common applications, most manufacturers provide safety factors to estimate loads.
Tom/Helical: Designers should know the operating and peak torque being transferred and understand the type and amount of misalignments between shafts.
What can end users do to ensure higher productivity from the couplings and/or shafts on their machines?
Galen/Rockwell: Special alignment requires extra installation time so couplings don't fail prematurely or cause bearing and shaft failure. Also, couplings requiring lubrication must be maintained with fresh grease to avoid internal metallic wear, heat, and failure.
Bill/Rexnord: Use the right coupling for the right application and follow its recommended installation and maintenance practices.
Edward/Ringfeder: Torsionally rigid, metal-disc pack couplings withstand harsh environments, lengthen time between maintenance cycles, and easily mount and dismount when combined with shaft-hub locking devices.
Parthiv/Emerson: Optimized couplings are designed for indefinite life due to customized parameters such as speed, power, misalignment, and thrust, which translates to lower maintenance and replacement costs for end users.
Andrew/R+W America: Many mounting systems are available to facilitate fast and easy installation. While some options appear to be overkill in the design phase, saving service techs' and machine assembly time is valuable.
Frank/Stafford: As downtime is the biggest concern, quick part replacement is critical.
Andrew/KTR: Training operators and understanding the machine's capacity ensure trouble-free operation and increase productivity. Periodic machine maintenance extends component life and identifies high-wear items. Proper coupling alignment, spacing, and installation boost the machine's lifespan.
Tom/Helical: End users should carefully align shafts during installation, center the coupling between shafts, tighten fasteners with torque wrenches and follow OEM maintenance recommendations.
What are the failure modes and weak links in couplings and/or shafts? What design/construction features are most susceptible to early failure, and why?
Darrin/Gerwah: Most couplings fail due to shaft misalignment. Since zero misalignment is nearly impossible, it must be accounted for somewhere in the system to extend motor and actuator bearing lives. Most couplings absorb misalignment through a soft flexible element, or by using relief in the coupling metal. Consequently, these couplings wear gradually, while transmitting rotational torque.
Chris/Diequa: Couplings primarily fail due to overload or misalignment. Overloads occur when system torque is inaccurate or startup torque is overlooked. Exceeding max alignment parameters causes the flexible element to quickly fatigue or transmit vibrations, frequencies, and radial loads to connecting drive components. In lineshafts, torque tubes allow higher speeds over longer spans than solid shafts.
Paul/Servometer: Collision, abrasion, chemical exposure, and extreme temperatures can damage couplings. Weak links in flexible shaft couplings include the flexing element and interconnection between it and the solid end pieces. If all forces acting on the coupling are not considered, the flexing element overstresses, fatigues, and fails prematurely. The interconnection is weak due to its location between flexible and rigid components.
August/Mayr: Most backlash-free couplings can tolerate limited shaft misalignment. When these limits are exceeded, fatigue life becomes finite, and with today's high-speed drives, is reached in little time.
Kevin/Lovejoy: Coupling failures are due to misapplication, incorrect selection, and poor installation. End users must understand that couplings have limitations, and if exceeded, will fail and cause downtime.
What are some of the common mistakes (leading to less productivity) designers make when selecting couplings and/or shafts?
Darrin/Gerwah: One potential problem is applying the wrong service factor. For example, couplings in reversing or high-shock load applications have a service factor greater than one. Not applying the higher service factor results in an undersized coupling that performs poorly and fails prematurely.
Chris/Diequa: Since the most common mistake is choosing the wrong coupling, designers must identify motion or power transmission requirements upfront. With lineshafts, speed, torsion, span length, and materials are considerations during selection.
Paul/Servometer: Sometimes, designers overlook forces — starting and stopping torque, shaft misalignments, lateral and axial loads, vibrations, and environment — working on couplings. For example, extreme temperatures or hot-to-cold cycles diminish coupling response to applied forces. Further, corrosive or caustic environments can corrode and weaken couplings.
August/Mayr: Often, designers do not select the right coupling for a drive. Those with excessive backlash and/or minimal torsional rigidity produce lags between the drive and driven components, causing unexpected and unwanted errors. Operating below max productivity levels corrects this.
Kevin/Lovejoy: Incorrect service factors can decrease productivity and lead to coupling undersizing during system torque calculations. Thus the coupling won't perform as required by the system/application.
What are some mistakes end users make regarding couplings and/or shafts, and how do they affect productivity?
Darrin/Gerwah: The most common mistake is not aligning couplings to manufacturer specs. Clamps and set screws should be tightened with a properly calibrated torque wrench. Excessive torque overstresses parts of the hub, which can break during installation or at peak moments in operation. A lack of torque causes couplings to slip around the shaft, leading to hub scarring, control issues, and damaged machine components.
Chris/Diequa: End users commonly disregard realignment when system components are replaced or serviced. As such, installing a new motor or gearbox requires attention to mounting tolerance differences compared to the original. When ignored, failed drive components may transmit shock loads or damaging frequencies to the coupling, which later fatigue flexible elements or connecting hubs.
Paul/Servometer: A common mistake is operating the coupling outside design parameters, particularly torque range. Another is overlooking forces (load) that the coupling generates through shaft misalignment, causing premature bearing failure.
August/Mayr: End users sometimes view couplings as sacrificial weak links, meant to fail if problems occur. This approach causes extensive repairs, downtime, and lost productivity. When jams or crashes are possible, combining a torque-limiting clutch with the coupling provides a positive disconnect between the drive and driven components.
Kevin/Lovejoy: Proper coupling installation and maintenance prolong life and reduce operating costs. Misaligned couplings introduce reactionary loads in the system, which can cause bearing, seal, and shaft failures.
Meet the experts
Emerson Power Transmission
Custom Order Engineering Manager
Dodge Bearings & PT Components
Gerwah Drive Components
Forest Park, Ga.
Director of Engineering
Cedar Grove, N.J.
Product Specialist, Coupling Group
Rexnord Industries Inc. - Falk & Rexnord Couplings
R+W America L.P.
Stafford Mfg. Corp.
North Reading, Mass.
Helical Products Company Inc.
Santa Maria, Calif.
Director of Product Management
Downers Grove, III.
Michigan City, Ind.