Edited by Jessica Shapiro
“Coupling Motion Systems for Zero Backlash,” Machine Design, Feb. 2, 2009, tinyurl.com/MDzerobacklash
“What to Look for in a Servocoupling,” Machine Design, Aug. 5, 2004, tinyurl.com/MDservocoupling
When engineers select couplings with the capabilities, environmental resistance, and service parameters their applications demand and install and use them as designed, they can be confident the couplings will work reliably over their designed lifetimes. Unfortunately, ignoring one or more of these factors can cause premature coupling failure with results ranging from small inconveniences to serious financial loss and personal injury.
Mechanical couplings connect and transfer rotary motion and torque between rotating shafts. Although the concept is elementary, the details of selecting and using couplings may not be. Here are some common mistakes and how to avoid them.
Far too often, engineers select motion-control couplings too late in the design process. When this happens, the couplings often do not meet the system’s complex requirements.
Designers who have made this error and had to make changes to the system to get the couplings to function properly can tell you: Couplings are a critical component for overall system performance. Early selection reduces errors in estimating performance and makes premature coupling failure less likely.
Coupling selection involves a number of design criteria including torque, misalignment, stiffness, inertia, rpm, shaft mounting, environment, space limitations, service factors, and cost. Engineers selecting couplings must address all these criteria in the selection process.
In addition to thinking about design factors up front, engineers should be aware that subsequent changes to the application may make their coupling choice less suitable over time.
Shaft misalignment is one of the most common coupling conditions. Such misalignment may be angular, parallel, axial, or any combination of these — a case called complex misalignment.
Misalignment creates loads that can exceed coupling specifications. All flexible-shaft couplings are designed to allow some misalignment and varying degrees of flex. Understanding the allowable flex for the coupling under consideration is paramount.
For instance, Oldham couplings are designed to handle relatively large parallel misalignments, but don’t do much to compensate for angular misalignment or axial motion. A single-beam coupling, in contrast, easily accommodates angular misalignment and axial motion with less success in compensating for parallel misalignment.
Even with the right couplings, excessive misalignment between joined shafts is one of the most common reasons for coupling failure. Misalignment that produces loads exceeding coupling specifications can accelerate wear.
Also keep in mind that any coupling that bends during misalignment generates bearing loads. Therefore, excessive misalignment can cause premature failure in other system components, such as bearings.
Engineers should rectify misalignment beyond coupling specifications by first attempting to realign the shafts. The right coupling is secondary to a well aligned system.
Engineers who underspecify couplings often have not considered torque. Design selection must take into account not only steady-state torque but also maximum instantaneous torque. This is particularly important when torque varies, for instance with start-and-stop motion. In some cases, designers may wish to build in a degree of torsional compliance to dampen torque shock loads and peaks.
Flexible couplings’ static torque ratings depend on the coupling design. For example, a double-disc coupling and an Oldham coupling with an acetal disc may both be rated for a particular application, but the double-disc coupling has 15 to 20% greater static-torque capability.
Torsional compliance, torsional rigidity, or windup all refer to the rotational deflection between the driver (e.g., motor) and load. Think of it as winding up the coupling like a spring.
Windup can mean a difference in angular displacement from one end of the coupling to the other. In servo applications, this makes it difficult to maintain accuracy. Windup may also introduce resonance into the system that can cause instability in an improperly tuned servo.
Shock and backlash
Backlash refers to play in couplings and is essentially motion that is lost. Backlash interrupts or uncouples the transfer of power between the motor or other driver and the load. Backlash is not acceptable in motion-control applications where it diminishes positioning accuracy and makes tuning the system difficult.
In a motion-centric application such as a servo, backlash introduces timing problems that make the coupling move forward and backward more than necessary. It also introduces stresses that contribute to premature failure. For these reasons, zero-backlash couplings are ideally suited to servo applications.
Engineers my also want to minimize the transfer of shock and vibration across a coupling by damping. Damping is particularly important in motion-control and power-transmission applications where undesirable vibration wastes energy and stresses system components.
Shock damping helps reduce the effects of impulse loads and minimizes shock to the motor and other sensitive equipment. Engineers should select couplings that don’t contribute to system vibrations and have the desired dampening effects.
One type of coupling that dampens well is a zero-backlash jaw coupling composed of an elastomeric “spider” and two hubs. Spiders come in various durometers so engineers can choose the right level of damping for the magnitude of impulse load in their applications. The wrong coupling type or the wrong spider material can accelerate coupling failure.
Inertia and shaft speed
Inertia is a body’s resistance to change its angular velocity. It governs the tendency of the coupling to remain at a constant speed in response to applied external forces like torque. In a power-transmission system, inertia determined by mass and its distribution about the axis is an important part of drive-torque specifications.
Engineers selecting couplings for servodrive systems with intermittent starts and stops must consider inertia in addition to backlash and torsional stiffness. They must also understand the driven-system inertia and its effect on the coupling.
High coupling inertia can degrade system performance by introducing resonance and adding to the natural frequency of the system, possibly with unintended consequences. Low-inertia couplings let engineers tune systems for higher performance and are good choices for precision applications.
Shaft speed is another important factor. Leaving couplings’ safe operating speeds out of design criteria can quickly result in failure, sometimes with tragic consequences.
Engineers must pay attention to manufacturers’ speed ratings, but should also remember that any shaft misalignment detracts from a coupling’s safe operating speed. Coupling stiffness is another consideration because speed also causes deflection that lowers the maximum safe speed.
Especially in high-speed applications, coupling balance is essential to prevent excessive system vibrations. Engineers should be careful not to alter the dynamic balance of a coupling before or after installation.
Electrical isolation prevents the movement of electrical currents between functional components of mechanical systems while maintaining mechanical-energy transfer. Extraneous electrical currents can cause serious control problems when they pass between servodrives and driven components.
Oldham and jaw couplings with nonmetallic or polymer inserts are electrically isolating. Other coupling types can also be manufactured with electrically isolating materials.
Engineers may also want the option to mechanically isolate parts of a system. A fuse coupling keeps energy from transferring between parts when a failure occurs. In contrast, a fail-safe coupling is designed to stay engaged.
Some applications require a fail-safe coupling to protect personnel or equipment. For example, a fail-safe coupling in a material-handling application could prevent safety or process issues if the coupling failed when material flow was interrupted.
Jaw couplings are considered fail-safe because, even if the spider fails, the jaws of the two hubs interlock, allowing continued power transmission. In contrast, an Oldham coupling with a center disc that fails in a similar way will disengage and interrupt power transmission.
The best design effort and attention when selecting a shaft coupling is wasted if the coupling is installed improperly or if the actual application parameters are outside of original design criteria. Far too often a coupling is installed hastily or without regard for the manufacturer specifications, leading to premature failure.
Installation steps might include preparing the coupling and shafts prior to installation by cleaning mating parts and oiling shafts lightly; checking that any misalignment between shafts is within the coupling’s ratings; and tightening fasteners in the right order and to the specified torque.
In addition, installers should be sure to center any misalignment along the length of the coupling and avoid installing the coupling too far left or right of the center line. Couplings should be installed in a stress-free state, not compressed or stretched.
Finally, installers should position shafts at the right depth within the coupling hubs. Some couplings require a minimum gap between shafts. In most cases, the shaft depth within the hub is specified by the manufacturer based on the coupling design. Installing shafts too deeply or too shallowly in the hubs can lead to premature failure.
Motion-control couplings are, with specific exceptions, essentially maintenance-free. Regular and diligent system maintenance is important, however, for the entire system in which the coupling is an integral component. System-maintenance requirements and schedules are generally a function of the application, duty cycles, operating parameters, environment, and other factors.
Any maintenance or service plan for the system as a whole is intended to avoid component failure anywhere within the system, including shafts, couplings, motors, and bearings. The coupling may be adversely affected if other component operating characteristics force operation outside of design specifications.
Basic system-maintenance requirements might include checking for abnormal operating characteristics such as unusual noise or excessive component temperatures. Maintenance personnel may also look for excessive vibration or other indicators of a change in alignment within the system. Any signs of fastener wear should be noted and loose fasteners should be retorqued.
When using Oldham or jaw-type couplings, engineers should consider the duty cycle of the center discs or spiders. Wear on these components may result in backlash and performance issues.
A maintenance plan should include replacing center discs and spiders with vendor-specified parts when the duty cycle has been exceeded or when excessive wear is noted. The discs are low-cost, easily replaced items that will restore the coupling’s original capabilities.
If a coupling fails, engineers should document the system conditions at failure. This allows for appropriate corrective action, up to and including specifying a different coupling that better meets the needs of the application.
Consider this: Application concerns and options
Resolving motion-control or power-transmission problems requires a thorough understanding of the application, system, and the problem itself. Below are examples of problems engineers have reported and the alternatives coupling experts asked them to consider.
Concern: A bellows coupling was incorrectly chosen for an application requiring dampening and shock-absorbing capabilities. The bellows coupling failed soon after installation.
Concern: A small stepper motor was installed using a large-diameter, rigid-steel coupling. Performance is sluggish and positioning accuracy is difficult to control because the heavy coupling adds too much inertia to the system.
Concern: A packaging machine cannot be kept in close parallel alignment and the two shafts are transferring excessive forces to other system components. A rigid coupling connects the shafts, and the motor and gearbox bearings are damaged before the coupling breaks. Replacing the motor and gearbox is costly, not to mention lost production time.
Concern: The encoder-feedback system on a machine with a beam coupling receives electrical impulses from other machine components. These impulses affect encoder accuracy and cause process errors.
Concern: A curved-jaw coupling was selected for an application requiring high torsional stiffness for accuracy. The system is not performing as accurately as it should.
Concern: A high-speed application has the correct coupling yet has had two successive coupling failures. Analysis shows fatigue to be a factor in each instance of failure.