Servo Couplings

May 1, 2005
Transmitting rotary power from one shaft to another is a servo coupling's primary function. Doing this in the most productive manner is a shared goal

Transmitting rotary power from one shaft to another is a servo coupling's primary function. Doing this in the most productive manner is a shared goal in industrial applications, which involves designers, component makers, and end users. Everyone plays a role. In this report, the editors of Motion System Design polled servo coupling experts for their advice on optimizing productivity in power transmission applications. Here are their responses on both sides of the issue, which we believe you'll find most helpful.

What particular design/construction features in servo couplings contribute to higher productivity, and why?

Randy/Gam: From a performance standpoint, two important features in servo couplings are torsional rigidity (stiffness) and misalignment capability. Torsional stiffness is critical to accuracy and positioning. The higher the stiffness, the more accurate the positioning from one side of the coupling to the other. Misalignment capabilities are important because of servomotor characteristics. As a servomotor heats up, the motor's shaft tends to grow axially called thermal shaft expansion. If the coupling connected to the servomotor does not absorb this axial misalignment, bearings typically fail. Bellows in servo couplings buffer systems from these problems.

Vic/Rimtec: Servo couplings differ from general couplings as they have little misalignment and cause minimal wear when rotating. More specifically, bellow couplings can compensate for allowable misalignment, do not require spare parts, and have an extended life.

Coupling stiffness also contributes to higher productivity. Stiff couplings let machines accelerate and decelerate quickly without resonance or overshooting, translating to shorter cycle time. Certain bellow couplings offer very high stiffness, with a natural frequency two to three times higher than that of the machine slide. For systems that operate with almost no friction (linear bearings and high quality ball screws, for example) or servomotors with gearboxes, additional dampening may be needed for stable operation.

Charlie/Lovejoy: When accuracy and repeatability are foremost, a coupling with a high level of torsional stiffness/rigidity — like a bellow or mini disc — is most suitable. In applications where vibration damping and backlash elimination are necessary, curved-jaw couplings are fitting. Rather than use one coupling as a universal design for several applications, it is better to let the application dictate necessary features.

What can designers do to ensure higher productivity from the servo couplings they place in machines?

Vic/Rimtec: Designers should be sure that load inertia is equal to or less than motor inertia. This guarantees shorter acceleration and deceleration time without instability in closed-loop operation. When load inertia is higher than motor inertia, designers should use backlash-free planetary gearboxes to improve productivity.

Charlie/Lovejoy: Designers should review and prioritize coupling features most critical to an application, such as

  • Nominal and peak torques
  • Shaft size
  • Accuracy and repeatability
  • Vibration issues
  • Environmental factors (temperature, chemical)
  • Space
  • Weight and inertia constraints

Randy/Gam: If a production machine sees a crash condition, such as an E-stop, a safety coupling is recommended versus a standard servo coupling, since it is designed to disengage and avoid machine downtime. Although safety couplings initially cost more, users can save thousands of dollars in downtime and repair expenses.

What can end users do to ensure higher productivity from the servo couplings on their machines?

Charlie/Lovejoy: End users should ask the same questions as designers when reviewing the type of servo coupling for an application. They should also be careful to avoid misalignment during installation. Proper selection and installation will result in higher productivity.

Randy/Gam: Production machines are designed to produce a certain amount of parts or run up to a certain speed. Often, operators “speed up” the machines to produce more parts. Sometimes, in these situations, the coupling is the weak link and fails because it is not sized to handle higher production rates.

Vic/Rimtec: End users typically use servo couplings as spare parts and should follow misalignment instructions when changing, as improper installation results in shorter coupling life. Servo couplings are designed to run backlash free with limited axial, radial, and angular misalignment.

What are some common shortcomings in the way servo couplings are designed or constructed, and how do they affect productivity?

Robert/Zero-Max: The widely-used beam style coupling is machined out of one piece of material. Limitations include a lack of torsional stiffness and the amount of torque they can transmit through the flex element in shear.

Also common are bellow couplings with stainless tubing molded into the body and attached to aluminum hubs on either end. While torsionally stiff, they limit durability in high-speed reversing applications. Like the beam style, torque transmits through the flex element in shear, resulting in a high rate of fatigue. The best construction uses flex discs attached by high quality fasteners, not rivets. This design transmits torque through the flex discs in compression and tension and is generally more reliable.

Darrin/Gerwah: Servo couplings are designed to deliver exceptionally high torsional stiffness. Achieving this, however, sacrifices some misalignment liberties. For instance, servo couplings do not have the same lenient alignment characteristics as couplings with backlash, like U-Joints or straight-jaw couplings. As a result, machine builders should closely follow recommended alignment parameters, preferably by dial-indicating or laser-aligning the shafts during installation.

Chris/Diequa: Different servo coupling designs have advantages and disadvantages. Bellow couplings have the lowest inertia and accommodate thermal expansion well, but must be handled more delicately. Disc styles are very robust, but have higher inertia and increasing axial reaction loads when heated. Elastomer-insert designs sometimes lack the necessary rigidity, and beam styles can end up in reversing applications. For maximum productivity, the correct design must be applied.

Paul/Servometer: Ideally, a servo coupling moves axially (in compression and extension) with low torsional windup and zero-backlash. It requires little or no maintenance and should be free of speed restrictions. To choose the most appropriate servo coupling and avoid premature failure, the application's requirements must be fully understood.

Blair/Ruland: A common misconception about servo couplings is that they are designed to have infinite life. However, cyclical bending is required to accommodate inherent misalignment within the system, thus contributing to a finite life. Oftentimes, manufacturers view their product as a replaceable component, rather than an indestructible one.

What are some of the common mistakes designers make when selecting and applying servo couplings, and how do these mistakes affect productivity?

Blair/Ruland: Often, a system designer believes that one type of coupling will work in all applications, versus recognizing that many couplings are designed for specific applications. For example, a spiral beam coupling is ideally suited for low-torque, follower applications, as with encoders or tachometers. Sometimes, a designer may use a beam coupling for higher torque, like servo/lead screw assemblies. While the torque capacity is good, beam couplings have very low torsional stiffness and can compromise accuracy. The result is a system prone to errors.

Robert/Zero-Max: A common mistake is looking only at torsional stiffness specs and picking the highest one. Often these numbers are much higher than what is truly needed for system accuracy or tuning needs. When the coupling is too stiff, fatigue becomes an issue.

Often overlooked is a coupling's design durability and its ability to transmit a load. These factors are best examined through visual inspection. For example, a disc coupling that transmits shear loads though strong fastener connections, not flex elements, increases machine productivity.

Darrin/Gerwah: One of the most common mistakes is to use inconsistent application parameters when specifying components from different manufacturers. In particular, some servo coupling applications are specified using the motor or gearbox's peak torque and matching to the nominal torque rating of the servo coupling, leading to oversizing. Not only does this raise cost, it can cause an unnecessary increase in inertia, resulting in a higher daily operating cost of the machine.

Other mistakes include overlooking the operating environment for situations like extremely high or low temperatures, vacuum conditions, and exposure to caustic chemicals. These are factors that, if unaccounted for in the application, can cause significant operating problems or servo coupling failure.

Chris/Diequa: The most common mistake is mounting couplings to shafts that are outside the misalignment parameters. Because many couplings seem very flexible, installers believe more misalignment is possible. However, high material stress can lead to quick failure if alignment specs aren't maintained. A second mistake is unaccounting for momentary peak loads. Without moving parts, the torque capacity is based on the material and connecting strength.

Paul/Servometer: The most common error when choosing a coupling is not fully understanding the application for which it is being matched. Spatial and dynamic forces acting on the coupling, as well as its range of abilities, must be considered.

What are some of the common mistakes end users make with regard to servo couplings, and in what way do these mistakes affect productivity?

Darrin/Gerwah: Most commonly, end users improperly align servo couplings to manufacturers published specs. An improperly aligned servo coupling is likely to suffer from fatigue, and ultimately, premature failure. Another common mistake during installation is not tightening clamp screws to the correct torque. Each coupling has a manufacturer specific tightening torque, and this value should be found by using a properly calibrated torque wrench.

  • If too much torque is applied, it can overstress parts of the hub, causing it to break during installation, or at peak moments in the operation.

  • Too little torque, on the other hand, causes the coupling to slip around the shaft during operation, leading to shaft and coupling hub scarring, serious control issues, and possible damage to machine components.

Chris/DieQua: Once an initial installation is executed properly, there are rarely any problems. However, issues do arise when replacing drive components or altering electrical torque limits. For instance, shaft alignments must be considered when replacing motors or gearheads. When changing motor torque parameters, users must determine the effects on mechanical elements. Sometimes designers limit torque electronically to reduce mechanical costs.

Blair/Ruland: The most common mistake is when an end user looks at a coupling failure as a problem with the coupling, rather than a problem within the system. For example, an end user uses a particular coupling for some time and notices it fails more often. Rather than reviewing the entire system, a “bigger is better” approach is usually adopted, and a larger or stronger coupling is specified. While this approach may solve immediate problems (usually at an increased cost), the long-term effect is that components such as bearings and gears deteriorate further, potentially damaging even more costly components (such as servomotors). Consulting with the coupling manufacturer at the design selection stage or when a coupling problem arises is the best approach. The way a coupling fails is an indication of larger problems.

Paul/Servometer: The most common error end users make is assuming a coupling can do more than it is designed for, which can be attributed to a lack of understanding of the application, as well as the coupling's capabilities and limitations.

Robert/Zero-Max: An end user has little control over the servo system once it has been delivered and setup. Working with the system builder is the best way to ensure a fast and productive machine down the road. If a servo coupling is marginal, the machine's productivity is greatly affected.

Darrin Hartel
Gerwah Drive Components
Forest Park, Ga.
(800) 211-3968

Paul Hazlitt
Director of Engineering
Servometer Corp.
Cedar Grove, N.J.
(973) 785-4630

Vic Jha
Rimtec Corp.
Addison, Ill.
(630) 628-0036

Randy Kuper
Gam Enterprises Inc.
Chicago, Ill.
(708) 887-5000

Robert Mainz
Applications Engineer
Zero-Max Inc.
Plymouth, Minn.
(800) 533-1731

Blair Morad
Product Engineer
Ruland Mfg.
Marlborough, Mass.
(508) 485-1000

Charlie Mudra
Product Manager
Lovejoy Inc.
Downers Grove, Ill.
(630) 852-0500

Chris Popp
Diequa Corp.
Bloomingdale, Ill.
(630) 980-1133

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