Couplings for higher speed applications

ServoClass Couplings feature high torsional stiffness, zero backlash and low hysteresis to ensure repeatable precise positioning at speeds up to 10,000 rpm. Their robust design ensures precise system operation in automated 24/7 applications. That is especially true where stresses occur at increased speeds in a servo system. Designed with 304 stainless steel disc packs and 7075-T6 aluminum hubs, the couplings are inherently strong, durable and precisely balanced. To ensure precise alignment of the assembled components, ISO 4762 CL12.9 corrosion resistant socket head cap screws are used with a carefully controlled assembly process. The couplings are available in single and double flex models in inch and metric bore sizes from 0.157 in. (4mm) to 1.750 in. (45mm). All models and sizes feature clamp style hubs and operate in temperatures from -22° to +212° F (-30° to +100° C).

Zero-Max
www.zero-max.com

 

Miniature couplings for medical applications

These miniature rigid couplings handle a variety of medical device applications, including test and measurement systems, imaging, patient positioning, and liquid handling such as chromatography.

Rigid couplings are zero-backlash, torsionally stiff, and have high torque capacity. They should be used when shaft misalignment is neither present nor desired, with the rigid coupling often used to help establish the alignment. These couplings feature precision honed bores on straight bore couplings for improved fit and straightness as well as Nypatch treated hardware that resists loosening in vibratory conditions. Rigid couplings with bore sizes as small as 3 mm are available in carbon steel, high grade aluminum for low inertia and 303 stainless steel.

These couplings are RoHS and REACH compliant.

Ruland Manufacturing Co., Inc.
www.ruland.com

 

VULKAN Couplings unveils New Integrated Shaft Coupling Design

VULKAN Couplings (www.vulkan.com) has unveiled a new innovation of the Rato DS coupling that is directly connected to a composite shaft.

Aside from being a torsional coupling, the combination of high radial stiffness and low bending and axial stiffness makes the Rato DS ideal to work as an integrated misalignment coupling when it is rigidly connected to an intermediate shaft. A torsionally stiff misalignment coupling at the intermediate shaft’s rear end creates the second bending flexible pivot, which offers a double cardanic design. This unique combination of the intermediate shaft and the Rato DS is known as the VULKAN Integrated Shaft Coupling (ISC).

Even at high misalignment levels, the angular deflection of the Rato DS causes a minimal level of strain against the torque load, which results to a lower power loss. Thus, it effectively eliminates the need for a misalignment coupling between the shaft and Rato DS. The intermediate shaft can be connected directly to the coupling’s inner ring where the Rato DS is not radially supported.

The new ISC design’s advantages include the major reduction of parts, less design of the bearing and lighter weight, which leads to reduced alignment and installation efforts. The new design of the ISC also has no noise transmission path over bearing or metallic parts between the engine and gearbox. The Rato DS rubber element of the assembly provides outstanding sound isolation against structure borne noise.

Depending on the speed of the shaft the ISC design based on RATO DS can be offered with composite shafts to a maximum length of 7 m. All RATO DS coupling elements whether dual couplings or single row couplings can be utilized for the ISC design.

Stafford Large Bore Shaft Collars and Rigid Couplings

Custom large bore shaft collars and couplings for drive train applications in wind power generation systems are available from Stafford Manufacturing Corp.

The company’s shaft collars and rigid couplings are custom engineered for applications up to 16” and 8” I.D., and come in one- and two-piece configurations that can be modified to meet a variety of special requirements. Suitable for wind power generator drive systems, they  incorporate steel targets that can be read from both the face and the O.D. to add speed sensing capability to shafts.

These products can be manufactured from aluminum, steel, stainless steels, titanium, brass, and other alloys with dimensional tolerances of <0.001” and run-out and concentricity tolerances of <0.001” TIR, depending on the part configuration. Modifications can include keyways, slots, cam surfaces, special threads and other features.

Stafford Mfg.

www.staffordmfg.com

Six factors to remember about couplings in a motion system

Physical values such as torque, torsional rigidity, spring stiffness, moment of inertia, imbalance, and zero-backlash play a major role in coupling design. Here are a few facts to keep in mind when you design your motion system.

Torque (Nm): is the product of an acting force and the effective length of the acting force’s lever arm.

T = Fxr

T = Torque (Nm)

F = Force (N)

r = Lever arm (m)

With a force of 100 N and a 1 m long lever arm, you can generate a torque of 100 Nm. Or, you can generate a torque of 100 Nm with a force of 1000 N and a 0.1 m long lever arm. For couplings, a specific amount of torque can be achieved with a large outer diameter of the coupling and a correspondingly low acting force or with a small outer diameter and a correspondingly high acting force.

Torsional rigidity (Nm/rad): refers to the rigidity of a coupling when it is subjected to a torsional load. If the torque exceeds the maximum torsional value of the coupling, the coupling will no longer be strong enough to transmit the acting rotational force. Ex: If a coupling with a torsional rigidity of 10 000 Nm/rad is subjected to 10 Nm, the connection element will twist by 1/1000 rad. That is equal to an angle of twist of about 0.057 degrees (1 rad = 57°17’44.8”). For a torsionally rigid or vibration damping coupling, this angle of twist may still be within the admissible range.  In practice, torsionally rigid couplings normally have a maximum angle of twist of less than 0.05 degrees and vibration damping couplings have a maximum angle of twist of less than 5 degrees.

Spring Stiffness (N/mm): is the counterforce exerted by the coupling in case of differentiated position of the axes in an axial, radial, and lateral direction. Ex: If the axial spring stiffness of a coupling is 30 N/mm, the coupling will exert a force of 30 N in the case of an axial displacement of 1 mm. These forces are important in a design with couplings, particularly when selecting bearings or other drive system components.

Moment of inertia: is the moment resistance when the rotational speed is changed. Normally, the lower the total weight and the smaller the outer diameter of the coupling body, the lower the moment of inertia. The reverse is also true, the higher the weight and larger the outer diameter, the higher the moment of inertia. This feature is important in highly dynamic applications because the drive has to generate sufficient torque to overcome a body’s moment of inertia to accelerate and decelerate.

Imbalance: in a drive system, imbalance should be as low as possible for smooth operation. Caused by asymmetries in the drive system where mass is distributed unevenly, it affects centrifugal forces on the entire drive system. It can be rectified by “balancing bores,” which are normally drilled directly into the location of the disproportionally high concentration of mass.

Zero backlash: is a lack of empty space or “play” when the rotational speed, direction of rotation, or torque changes. It does not mean that there is no angle of twist. Backlash is an important factor in predicting bearing life.

Information courtesy of R+W America

Rw-america.com

For safety, electronics may not be the best choice

The trend of replacing mechanical systems with electrical systems continues. Even developers of hydraulic and pneumatic systems are following it. But, as is becoming evident through the latest unintended acceleration issues, electronic components can have a few drawbacks that should not be overlooked in a design.

When in comes to designing a system for safety, specifically when considering whether to choose a mechanical component such as a coupling, or to go electronic, remember this: Electronic safety components have two major disadvantages compared to mechanical safety components.

1. Reaction time. Assume a machine crashes and causes an overload. According to engineers at R+W America, a signal from the monitoring circuit does not reach the motor controller until 5 to 7 ms following a sharp increase in torque. During this period of latency, the controller attempts to further increase torque to reach the setpoint value. Most likely, another 10 ms will pass before the motor is shut off. Depending on the drive train’s moments of inertia, more time can pass before the electronics brings the whole system to a stop.
2. Multiple potential failure sources. Electronic monitoring systems need multiple sensors for data. Between the monitoring system and all of its sensors and other components, you have a system with multiple possible points of failure.

A mechanical safety coupling, on the other hand, completely disconnects the drive from the load within 3 to 5 ms; 1/3 of the time needed by an electronic cut-off. Noted engineers at R+W America, “electronic machine monitoring is not suitable for high speeds due to the large centrifugal mass of the rotating parts.”

Also with a mechanical safety coupling, you have one component per axis, reducing the number of possible points of failure.

Safety couplings must demonstrate two clear behaviors:

1. Upon overload, separation of drive train and load should occur within a few milliseconds.
2. After the coupling has disengaged, residual friction should not be excessive so as not to damage coupled components that continue to be driven due to mass moments of inertia.

According to R+W, safety couplings can be subdivided into five classes:

1. Rigid safety couplings used in indirect drive applications.

2. Torsionally rigid safety couplings for use between two shafts or flanges. These couplings resist twisting and can be subdivided into two groups.

A. Single-piece torsionally rigid safety couplings.

B. Press-fit couplings.

3. Vibration-damping safety couplings are fitted with an elastomer insert that damps incurred drive vibration.

4. Economy safety couplings suit applications requiring simple overload protection and functions as a variation of the ball-detent principle.

5. Torque-limiting line shafts, which span long distances between shafts.

(Some material, courtesy of R+W America.)

Multipurpose W-Style Couplings

Line of W-Style multi-purpose couplings are compatible with most spiral, braided and industrial hoses to handle a wide range of hydraulic applications. Designed for non-skive SAE 100 R12 and all wire braided hose applications, these couplings are available in 562 different end types and in sizes 4-32, non-skive all sizes of R12, non-skive 4SH 12-20 sizes, and a full line of metrics. They have corrosion resistant ROHS compliant plating and are compatible with most model crimpers.

Ruggedly designed to eliminate leaks in hydraulic systems, they suit industrial and commercial applications including construction, agricultural, mining, off highway vehicle, and plant maintenance equipment. Field proven in vibration and shock conditions, they are designed to handle sub-zero through high temperature applications. Kurt couplings meet SAE specifications and are quality manufactured in accordance with ISO 9002/QS 9000 quality processes and systems.

Kurt Hydraulics
www.kurthydraulics.com

Stafford Stepped Couplings

Stafford Manufacturing Corp. offers a line of rigid, clamp-type reducer couplings that are stepped down to mate large and small shafts, inch-to-metric shafts and smooth to keyed bores.

Stafford Stepped Couplings include one-, two- and three-piece styles with reducer bores for connecting shafts from 1/4” to 2-1/2” I.D., including inch-to-metric and smooth-to-keyway. Featuring eight hex type clamp screws each to provide the holding power necessary for high-load applications, these rigid reducer couplings will not mar expensive drive components.

Suitable for any shaft step-down requirement, Stafford Stepped Couplings are available in steel, stainless steel, and aluminum. A full range of standard combinations are offered and specials can be provided to meet custom requirements. Applications include maintaining and upgrading drive systems in all types of processing, mixing and conveying equipment.

Silicone Insert Couplings from Sterling Instrument

New Hyde Park, NY — A new series of silicone insert couplings from Sterling Instrument (ISO 9001:2000+AS9100B Registered Manufacturer) features electrical isolation and no backlash. These metric couplings, identified as the S54HSAM… (clamp type) and S5PSAM… (set screw type) Series are stocked in 5 different bore sizes ranging from (6 mm to 16 mm).

These couplings have aluminum hubs with either set screws or clamps for fastening to shafts. The insert is silicone 40 ShA. Operating temperature ranges from -50°C to +150°C. They range in length from 26.5 mm to 57 mm. Their maximum speed is 5000 rpm.

They can be used in various applications and are especially able to accommodate tight or skewed connections. Quotes, online orders, available stock, and 3D CAD Model downloads are available at our new eStore at: www.sdp-si.com/eStore. SDP/SI offers over 1000 different types of couplings including inch and metric: magnetic, flexible, rigid, oldham, bellows, flexible shaft, spider type, Fairloc® shaft type, helical, slit-type, and neoprene flexible type couplings.

Sterling Instrument
www.sdp-si.com

Tips to simplify coupling selection

For a coupling in a servo application to work properly, you need to satisfy a number of application factors including: torque, shaft misalignment, stiffness, speed, and space requirements.

Here’s a look at the available types of servo couplings and what you need to consider for each of them during the selection process.

Beam couplings
Beam type couplings are manufactured from a single piece of material, usually aluminum, and use a system of spiral cuts to accommodate misalignment and transmit torque.  For many applications, beam couplings are a good economical and maintenance free choice.

The single piece design transmits torque with zero backlash. Two basic variations exist: a single beam style and a multiple beam style.

The single beam style has one long continuous cut that usually consists of multiple complete rotations. It is very flexible and accommodates light bearing loads.

For many applications, flexible beam couplings are a good economical and maintenance free choice.

It is able to manage all types of misalignment, but works best with angular misalignment or axial motion. It is not well suited to parallel misalignment because the single beam must bend in two different directions simultaneously, creating larger stresses in the coupling that could cause premature failure.

Under misalignment conditions, the long single beam allows the coupling to bend easily. But the relatively large amount of windup under torsional loads adversely affects the coupling’s accuracy.

Single beam couplings are an economical option best used in lower torque application and in connections to encoders and other light instruments.

Multiple beam couplings, which usually consist of two or three overlapping beams, attack the problem of low torsional rigidity. The use of multiple beams lets the beams be shorter without sacrificing much of the misalignment capabilities.

The shorter beams make the coupling torsionally stiff. Overlapping them so the beams work in parallel increases the allowable maximum torque making them suitable for use in light duty applications with connections, such as from a servo to a leadscrew. A drawback is that bearing loads are increased by a sizeable amount over the single beam variety but, in most cases, remain low enough to protect bearings effectively.

Some manufacturers take the multiple beam concept to another level. Instead of using a single set of multiple cuts, they use two sets. The use of multiple sets of cuts gives the coupling additional flexibility to accept more misalignment, including parallel misalignment. With parallel misalignment, one set of beams bends in one direction and the second set bends in the other direction.

Most commonly, these couplings are made of aluminum, but they also come in stainless steel. Stainless protects against corrosion, and increases coupling torque capacity and stiffness to sometimes double that of aluminum versions. The increase in torque and stiffness, though, is offset by a dramatic increase in mass and inertia. Keep in mind that in applications using smaller motors, a large percentage of the motor’s torque is used to overcome the inertia of the coupling.

Oldham couplings
The Oldham coupling is a three piece coupling comprised of two hubs and a center member. The center disk, which is usually made of a plastic or, less commonly, a metallic material, transmits the torque. On the center disk, mating slots are located on opposite sides and oriented 90 degrees apart. Drive tenons are located on the hubs. The slots of the disk fit on the hub tenons with a slight press fit that allows the coupling to operate with zero backlash. Over time, the sliding of the disk over the tenons will create wear to the point where the coupling will experience backlash. The disks are inexpensive items easilyreplaced, so a new insert will restore the coupling’s original capability.

The choice of materials for Oldham couplings depends on requirements for backlash, stiffness, vibration, and noise.

In operation, the center element slides on the hub tenon to accommodate misalignment.

The only resistance to misalignment is the frictional force between the hub and disk, Oldham couplings have bearing loads that do not increase as misalignment increases. Unlike other types of couplings, there are no bending members that cause bearing loads to increase as the shafts get out of alignment.

These couplings only allow a small amount of angular misalignment (less than one-half a degree) and axial motion (less than 0.005 in.), and are limited to speeds of 4000 rpm. Larger amounts of angular misalignment cause the coupling to lose its constant velocity characteristic, and axial motion is limited by the three-piece design of the coupling, which does not allow for use in push-pull types of applications. Because the center disk is a floating member, both shafts must be supported to keep the coupling from falling apart.

Bellows couplings easily bend under loads that result from angular, parallel, and axial motion.

Oldham couplings can handle relatively large amounts of parallel misalignment, from 0.025 in. to 0.100 in. or more depending on coupling size. Coupling manufacturers generally provide smaller misalignment ratings to obtain longer life ratings. These ratings can be surpassed at the expense of coupling life.

These couplings are available in a range of disk materials. The choice depends on requirements for zero backlash, high torsional stiffness and torque, or vibration absorption and low noise. Nonmetallic inserts are electrically isolating and can act as a mechanical fuse. When the plastic insert fails, it breaks cleanly and does not allow transmission of power, preventing other damage from occurring to machinery components.

Zero backlash jaw couplings
Jaw couplings are either conventional straight jaw or curved jaw zero backlash versions. Conventional straight jaw couplings are not typically well suited to servo applications that require the accurate transmission of torque. Zero backlash jaw couplings, on the other hand, are well suited to servo applications. The curved jaws help to reduce deformation of the spider and limit the effects of centrifugal forces during high-speed operation.

Jaw couplings handle high-speed applications well, but are less able to handle large amounts of misalignment.

Zero backlash jaw couplings consist of two metallic hubs and an elastomer insert commonly referred to as a “spider.” The spider is a multiple lobed insert that fits between the drive jaws on the coupling hubs with a jaw from each hub fitted alternately between the lobes of the spider. As in the oldham coupling, there is a press fit between the jaws and the spider for the coupling to deliver zero backlash.

In contrast to the oldham coupling, where the torque disk is in shear under torsional loads, the jaw coupling’s spider operates in compression. Be careful not to exceed the manufacturer’s rating for maximum torque, which can be significantly below the physical limitations of the spider. The spider can be compressed so that there is no longer a preload and backlash will occur.

Jaw couplings are well balanced and able to handle high-speed applications, 40,000 rpm or more. They do not handle very large amounts of misalignment, especially axial motion. Large amounts of parallel and angular misalignment cause loads on bearing to be higher than those of most other types of servo couplings.

If a spider fails, the coupling will not disengage. The jaws from the two hubs will mate similar to teeth on two gears and continue to transmit torque with metal-to-metal contact. Depending on the application, such action may be desirable or it could cause problems in the overall coupling system.

An advantage of the jaw coupling is the ability to mix and match spiders based on the application. Manufacturers of zero backlash jaw couplings offer multiple materials with different hardnesses and temperature capabilities that let you choose exactly the insert that meets the application’s performance criteria.

Disk couplings
At minimum, disk couplings have two hubs and a thin metallic or composite disk that transmits the torque. The disk is fastened to the hubs usually with a tight fitting pin that eliminates any play or backlash between the parts.

Torsionally stiff, disc couplings can accept up to 5 degrees of misalignment with some of the lowest bearing loads available.

Some manufacturers offer disk couplings with two disks separated by a rigid center member attached to a hub at each end. The rigid center member is usually metallic, but plastic versions are available and can be used to electrically isolate the coupling. This configuration will reduce torque capacity and torsional stiffness.

The difference between the two variations is similar to the difference between the single beam style coupling and the multiple beam coupling with two sets of cuts. The single disk coupling is not adept at accommodating parallel misalignment due to the complex bending of the disk. The two-disk style allows each disk to bend in opposite directions to harness the parallel offset. The properties of this type of coupling are similar to those of bellows couplings. They transmit torque in a similar manner. The disks are very thin, allowing them to bend easily under misalignment loading, which allows the coupling to accept misalignment up to 5 degrees with some of the lowest bearing loads available in a servo coupling.

Torsionally, the disks are very stiff. The disk coupling has stiffness ratings slightly lower than that of bellows couplings. A downside to these couplings is that they are delicate and prone to damage if misused or installed improperly. For proper operation, take care to insure that the misalignment is within the coupling ratings.

Bellows couplings
The Bellows coupling is an assembly of two hubs and a thin walled metallic bellows. In most cases, welding or an adhesive marry the hubs to the bellows.

Although other materials can be and are used, the two most common materials for the bellows are stainless steel and nickel. Nickel bellows are made using an electrodeposition method. It involves machining a solid mandrel in the shape of the finished bellows. The nickel is electrodeposited onto the mandrel, which is then chemically dissolved leaving behind the finished bellows. Manufacturers can precisely control the wall thickness of the bellows, creating thinner walls than is possible with other methods of bellows forming.

Rigid couplings can suit servo applications, especially if misalignment is tightly controlled.

The thinner walls give the coupling greater sensitively and responsiveness, which makes them suitable for precise small instrumentation applications. However, thinner walls also reduce the torque capacity of the bellows putting a limit on useful applications.

Stainless steel bellows are stronger than nickel versions and usually manufactured through hydroforming. A thin walled tube is placed into a machine and hydraulic pressure is used to form the convolutions of the bellows around specialized tooling.

The uniform thin walls of bellows allow it to bend easily under loads caused by the three basic types of misalignment between shafts: angular, parallel, and axial motion. Generally, bellows allow for up to 1 to 2 degrees of angular misalignment and 0.010 in. to 0.020 in. of parallel misalignment and axial motion.

The thin, uniform walls result in low bearing loads that remain constant at all points of rotation, without the damaging cyclical high and low loading points found in some other types of couplings. All of this is accomplished while remaining rigid under torsional loads.

Torsional rigidity is a key factor in the accuracy of the coupling. The stiffer the coupling, the more accurately it translates motion from the motor to the driven component. In the area of servo couplings, bellows type couplings are some of the stiffest available, making them ideal in applications that require a high degree of accuracy and repeatability. Some manufacturers offer bellows couplings with stainless steel hubs, which can be useful in applications requiring corrosion resistance, but their mass can be a factor in their operation. A coupling with aluminum hubs has very low inertia, a feature important for highly responsive systems. Some manufacturers balance their couplings to suit high-speed applications of more than 10,000 rpm.

Rigid couplings
These couplings were not often considered for servo application. Recently, however, smaller sized rigid couplings, especially in aluminum, operate in motion control applications because they offer high torque capacity, stiffness, and zero backlash. Torsionally rigid with virtually zero windup under torque loads, they are also rigid under loads caused by misalignment.

If misalignment is present in the system, however, the shafts, bearings or coupling will fail prematurely. Thus, the couplings cannot be run at extremely high speeds because they cannot compensate for thermal changes in the shafts from heat buildup in high-speed use.  However, in servo applications where misalignment can be tightly controlled rigid couplings perform admirably.

Ruland Manufacturing Company, Inc.
www.ruland.com

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