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Engineering roundtable: Picking couplings, shafts, and locking devices

Aug. 1, 2002
What use are good components if their performance gets garbled in transmission? Couplings, shafts, and locking devices smooth even brute-force inputs for safer, more consistent output. However, protecting other costlier components does take its toll. Here we discuss connector issues, and where clever gains can be made.
Materials — When complicated geometry proves more trouble than it’s worth, the deceptively simple function of transmitting torque can be improved by even small changes in chemical makeup. The core material and finish of a connecting device are its dominating characteristics.

Jim/Lovejoy: There is always a trade off between cost and convenience. Many times older materials have not been surpassed in terms of usefulness for a job. Rubber, for example, continues to be the best material for hysteresis damping and a leader for resilient couplings.

Tom/Climax Metals: Special high-tech platings and surface treatments are being tested for possible release in the near future.

Rob/Thomson Industries: In the linear motion industry, poor steel quality and incorrect surface finishes, as well as incorrect Rockwell hardnesses and OD tolerances can severely limit bearing and shaft life, causing premature failure. Materials must meet specific application requirements.

Andrew/R+W: As far as extending the life of a coupling, several options exist. Using a doublewalled stainless steel bellow can make for a much stronger flexible element than helical or beam cutting of one continuous piece of stock. In addition, using a welded connection between a bellow and hubs rather than a traditional swedged or bonded connection can make the coupling less likely to shear in the case of excess lateral misalignment. Beyond using stronger materials and assembly methods, other adaptations can be made on an application- specific basis.

Tom/Climax Metals: Corrosion-resistant shaft collars in Type 316 stainless steel or yellow zinc dichromate plated steel are ideal for wash down and highly corrosive environments in mining, fertilizer, construction, pharmaceutical, and food processing industries.

Rob/Thomson Industries: AISI 1566 is good for common linear bearing applications, AISI 52100 for lightweight applications (for example, running wires or coolant through a tube as in aerospace applications), 440C hardened stainless for increased corrosion resistance in medical, and 316 non-hardened stainless for very corrosive atmospheres in food packaging and processing.

Jim/Lovejoy: We see more coupling types made in powder metals as presses become more suitable for short runs; these couplings are low cost and strong. We also see more materials of the visco-elastic type for higher load carrying with resilience.

Paul/Servometer: An electrodeposited nickel bellows coupling exhibits about one quarter the windup of hydroformed bronze or stainless steel bellows; its elastic windup is typically less than 1 arc-sec per N-cm.

Jim/Lovejoy: We are always trying to increase coupling life via new materials and designs; new materials with promise include the viscoelastic urethanes and powder metal technology.

Tom/Climax Metals: Our nonmetallic shaft collars are made from USDA and FDAapproved nylon and acetal. They are ideal for washdown environments found in food processing and pharmaceutical applications because they resist solvents, chemicals, wet, and caustic environments.

Jim/Lovejoy: We see more non-lubricated high power couplings in the future. Right now the coupling with the greatest power density capability is the all-steel gear coupling. But it requires a grease lubrication to reduce tooth wear.

Maintenance — Connecting devices often function as “safety valves” in the systems where they’re used. By absorbing abnormal forces, they protect more costly components from damage. However, this puts their accessibility and interchangeability at a premium.

Andrew/R+W: When machine jams and collisions occur it can be very costly, not only in terms of damage, but also in terms of down time and lost production.

Tom/Climax Metals: Our new adjustable-width collars are designed for fluid power or robotics requirements, where linear slides are common. They provide for quick and accurate adjustments that allow the end-user to make precise stroke adjustments.

Carl/Ringfeder: Keyless locking devices were designed to replace traditional keyed, welded, and press fit connections in power transmission. The inherent mechanical advantage of multiple screws in generating clamping power for transmitting torque on shafts is exploited to make component mounting and dismounting simpler.

Galen/Rockwell: Torque wrenches are a valuable asset for mounting couplings and bushings to shafts. Proper alignment is also critical.

Jim/Lovejoy: Our jaw in shear is an example of a coupling that allows maintenance with a minimum of tools, and little or no disassembly of the coupling when replacing the wearable parts.

Carl/Ringfeder: Our system allows the technician to easily install or remove the locking device — as during mounting a component on a shaft — with hand tools, eliminating the need for heating/cooling or press or welding systems to install a component.

Tom/Climax Metals: Any time a component part is replaced, replace the shaft collar or rigid coupling. This assures that any excessive wear resulting from metal stress from the mating component, or even standard use, won’t have an adverse effect on the new component. All metals do become subject to stress fracture over time and eventually crack and fail.

Andrew/R+W: To a large extent shaft couplings are maintenance free. If a coupling failure is to occur, perhaps due to conditions and events not foreseen by the machine OEM, replacing it with the identical part is not always the best option. End users should contact coupling suppliers with as much specific information about the application as possible to find out if other alternatives may be more suitable.

Galen/Rockwell: Given that the weakest link of a machine should be the least expensive and the easiest to replace, sometimes the best thing a coupling can do is fail. When this design concept is adopted, it usually applies to an easily replaceable elastomeric element. Upon system overloads or extreme abuse, the element may fail, thus protecting motors, shafts, and other more expensive machinery from failure and costly downtime. This scenario uses the coupling to act as a preventative maintenance technique.

Andrew/R+W: On other couplings, a patented ball-detent system can detect torque overloads within 1 to 3 milliseconds. When torque spikes beyond a level preset in the factory according to customer specifications, this coupling will disengage the drive from the driven elements, putting an immediate halt to the overload.

Misalignment — The components that connectors engage are never lined up perfectly. Couplings provide room for error ... within reason. It’s best to be familiar with a machine’s behavior when selecting connectors for it.

Andrew/R+W: The flexibility of zero backlash bellow couplings protects bearings from lateral loading, which can occur as a result of thermal changes in shafting, as well as slight misalignments in the drive components.

Galen/Rockwell: Tyre-type couplings can transmit full load torque with as much as 4° of angular misalignment combined with 1/8 in. of parallel misalignment. This design provides the end user with additional room for error and less time during installation focused on alignment. This type of coupling, mounted with tapered bushings, provides a well-balanced product.

Douglas/Zero-Max: Composite disc couplings are superior to the bellows style because bellows fatigue over time due to its extremely rigid design. It also cannot take any misalignment. On the same token, beam styles wind up, so they’re not torsionally stiff and do not provide zero backlash.

Paul/Servometer: Rigid couplings provide high torque capacity and stiffness without windup or backlash, but they are not very forgiving of misalignment. With angular or parallel misalignment, or with axial compression or extension, rigid couplings and their associated shafts and bearings wear and fail prematurely.

Jim/Lovejoy: Usually high misalignment capability means high reactionary forces on the connected equipment bearings. In the diagnostics realm we have seen electronic transfer of information in pictures as a great improvement in providing information for fast analysis of breakdowns. We expect that embedded devices in the more expensive sophisticated couplings will provide additional means to prevent breakdowns or analyze the reasons for such breakdowns.

Paul/Servometer: We make flexible bellows-type shaft couplings that tolerate deliberate motion or assembly misalignment. The flexible element is an electrodeposited nickel bellows soldered to stainless steel or adhesive-bonded to aluminum hubs.

Jim/Lovejoy: It is usually the flexing device that wears in a coupling. In some cases, wearing is actually the cause of flexibility. Depending on how you want to define productivity, if the coupling provides power transmission under misaligned conditions, it is helping productivity; however, if it wears at the same time, it limits it. But that’s when ease of maintenance can tip the scale back in the other direction.

Andrew/R+W: Shaft misalignments beyond the specified run-out on the coupling can cause even high strength couplings to tear or break. A failure due to fatigue almost always impacts the flexible center section of the coupling rather than the mounting hubs. Flexible couplings extend machine service life by absorbing the stress normally placed on bearings due to misalignment, but do not eliminate that stress entirely.

Reversing — For some applications (for example, during advancing and retracting motion) a shaft must spin one way and then another. Coupled with the higher-frequency reversing associated with any rotation, this movement takes its toll on connecting devices, which eventually fatigue from stress-reversal hysteresis. Life can be extended by eliminating extreme geometries such as keyways, where the first tiny cracks form.

Douglas/Zero-Max: Keyways and tapers weaken from reversing and backlash applications, and cause excessive wear to shafts and components.

Carl/Ringfeder: When a connection is not properly designed or the application parameters are not fully understood by the customer or the application engineer, then an unsuitable device can be selected. Parameters such as nominal torque versus peak torque required, bending loads, axial loads, high or low temperature environments, moisture or frequency of removal of the component all have an input into which device is selected. Reversing or shock loads are not of primary concern to a locking device unless these loads exceed the device’s capacity.

Paul/Servometer: But for a drive coupling, quick reversing or frequent start-stop actions, such as the short stepping moves of an index table, add loading. With a complete definition of the duty cycle, coupling applications engineers can allow for torque spikes and other conditions.

Douglas/Zero-Max: Zero backlash spider elements will fatigue and create backlash over time due to the constant compression of the element in reversing applications.

Paul/Servometer: We make flexible bellows-type shaft couplings that tolerate deliberate motion or assembly misalignment. The flexible element is an electrodeposited nickel bellows soldered to stainless steel or adhesivebonded to aluminum hubs.

Selection —Where do connectors appear? Most often, they are used in industrial environments as components in plant machinery. Taking the time to find its real specifications will help in picking the right connector and continuing operations uninterrupted.

Tom/Climax Metals: Shaft collars and rigid couplings are MRO (maintenance, repair, and operation) items in many factories. When mating components wear out and need to be replaced there is production downtime, which is very expensive. Products that last longer in harsh operating environments need less frequent replacement, which results in greater equipment utilization and more profit.

Carl/Ringfeder: With industrial users making every attempt to squeeze more production from their machines to reduce costs, the machinery components must be able to survive the extra requirements. Higher speeds, larger loads and continuous operation are only a few of the ways machinery is pushed to its limits. It is therefore necessary that the components and materials used in the equipment be properly engineered and designed.

Andrew/R+W: The most important factor in determining the most appropriate coupling for an application is the peak torque that will be transmitted through it. If the nominal operating torque in a system is determined to be x, but a peak torque of 2x is occurring in spikes several times per minute, a coupling with a torque rating of x may not be strong enough.

Paul/Servometer: Maximum instantaneous torque from a clutch start or brake stop should be obtained. Accurate torque estimates are essential for reliable coupling operation. For a low inertia motor with a gear reduction drive, multiply the starting torque of the motor by the gear speed reduction ratio. When bellows couplings operate under compression, they can run at a maximum of 75% of the rated torque. When extended, the normal torque rating applies.

Galen/Rockwell: The basic purpose of a coupling is to connect two shafts and transmit torque. Beyond those two primary functions, couplings damp vibrations, correct misalignments, eliminate backlash, and reduce maintenance, to name a few.

Paul/Servometer: Bellows are well proven in rotary couplings. However, not every application needs the precision and reliability that flexible bellows couplings provide.

Galen/Rockwell: Application productivity is maximized by recognizing and selecting the proper coupling that best complements the rigors and demand of the application.

Jim/Lovejoy: Pick couplings for the job to be done without adding unnecessary bells and whistles, which could become a maintenance problem in the end.

Galen/Rockwell: Designers can optimize productivity by selecting the proper components and following the manufacturers published product data.

Application — Connecting devices can be replaced more often than more expensive components. Still, extended life is always a welcome feature. The benefit of extra features must be weighed against other factors, such as cost.

Carl/Ringfeder: All coupling designs have typical failure modes. Coupling failures occur when a coupling is utilized above its nominal torque or misalignment capacity. However, the speed at which couplings fail depends how

Tom/Climax Metals: Our clamping collar products feature a groove on the bearing face that has a square-tobore tolerance, resulting in longer bearing life between replacements.

Andrew/R+W: As many machine builders increase their use of linear actuators, linear guides, linear potentiometers, and linear motors, the importance of a quality linear coupling grows as well. Linear actuators can heat up, bind, and jam as a result of a rigid connection between drive and driven elements. Rod ends provide one possible solution, but are not backlash free, which is a key issue with these drive components.

Douglas/Zero-Max: Productivity comes from the fast adjustment and increased life of keyless devices over old keyway-style connections. A simple screw adjustment can be used, possibly hundreds of times, without loss of component life.

Jim/Lovejoy: Couplings that do not wear or have infinite life may not provide the other attributes such as resilience or tuning or have low cost.

Douglas/Zero-Max: Our hydraulic keyless connections don’t shift axially on the shaft when mounted, as many mechanical keyless models do. Also, the simple design features fewer adjusting screws; one model features just one radial adjusting screw for a closer shaft fit without the need for axial adjusting screws.

Galen/Rockwell: Locking devices, such as bushings, provide greater end user flexibility by minimizing inventory, providing greater radial and torsional holding power, minimizing vibration from eccentricity, and decreasing installation and removal time.

Jim/Lovejoy: Many times the best coupling is the simplest because it does not include unneeded expensive features. Zero backlash, low inertia, reduced moment, and high-speed balance are all features that add cost but perhaps do not add utility to the system. Yet they are often specified when not needed.

Meet the coupling and locking-device experts

Carl Fenstermacher, Ringfeder Corp., Westwood, N.J.
Expertise: Keyless locking devices; torsionally rigid, flexible disc couplings

Douglas Moore, Zero-Max Inc., Minneapolis, Minn.
Expertise: Keyless shaft connections with hydraulic shaft clamping; composite disc couplings

Tom Fabek, Climax Metal Products Co., Wickliffe, Ohio
Expertise: Shaft collars and rigid couplings; standard and custom designs

Rob Welling, Thomson Industries Inc., Port Washington, N.Y.
Expertise: Hardened, ground, and polished shafting for use with linear bearings; steels and stainless steels

Jim Mahan, Lovejoy Inc., South Haven, Minn.
Expertise: Flexible couplings used for power transmission; variable speed V-drives and tensioning devices; shaft-locking devices

Paul Hazlitt, Servometer Inc., Cedar Grove, N.J.
Expertise: Flexible bellows-type shaft couplings

Galen Burdeshaw, Rockwell Automation, Greenville, S.C.
Expertise: Flexible and rigid couplings; shafts for motors, conveyor assemblies, and gearboxes; locking devices; low to high taper bushings

Andrew Lechner, R+W America LP, Bensenville, Ill.
Expertise: Servo-class flexible shaft couplings and torque limiting safety couplings; torque limiters

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