Maintenance free couplings for heavy engineering

Extreme temperature ranges, high pollution levels, high vibration levels, and 24/7/365 operation call for engineered drive components which can take the abuse. Extending component maintenance intervals is a priority, since industrial gearboxes, electric motors, bearings, and drive shafts typically require at least partial disassembly and replacement of wear parts. A new solution involves zero maintenance, stainless steel bellows couplings for up to 100,000 Nm, engineered to run smoothly for decades.

One challenge in the development of the BX series was the design of the metal bellows itself. The bellows serves to alleviate misalignment loading between adjacent bearing sets while transmitting rotary power. Traditional sizing calculations were used at first, having proved successful for high speed servo drive systems, which tend to be some of the most demanding of the customary applications for bellows couplings. After a short time it became clear that a widely marketable high torque bellows coupling could not be designed without FEA, as the first installed models were two to three times larger than the final product design. As life testing later confirmed, the software revealed some errors in the initial interpretation of the sizing, demonstrating that the traditional methods did not scale properly. In collaboration with a major bellows hydroformer, R+W reviewed and updated these traditional bellows sizing methods to better reflect the realities of what the formed bellows can tolerate in this physical size range.

A special welded joint between the bellows, intermediate tubing, and mounting flanges was also selected, involving a combination of TIG (perfect root) and MAG (low heat generation). This welding method is a recent development of RWTH Aachen University IME, a world renowned leader in mechanical and electrical engineering research, and has demonstrated superior fatigue characteristics in vibration and life testing. With these two important features in place, hubs, flanges, and intermediate tubing, fabricated from high strength steel, were configured to round off the complete product line. R+W has been able to launch the all new “BX” series, with a variety of standard and custom mounting options available, and presently broken down into three specific series.

BX1 with Flange Mounting

The BX1 is the basic model from which the complete BX series is configured. Mounting flanges are welded directly to the bellows, which are then used for adaptation to shaft mounting systems in the cases of the BX4 and BX6. Power is therefore transmitted through the BX1 by means of a frictional flange connection. A common misconception is that the torque is transmitted through the bolts themselves. This can have two negative consequences in that the coupling would exhibit backlash, and that ultimately the bolts would fatigue and fracture. In case of reversing applications or those with frequent load variation this failure would occur within a very short amount of time. Basically torque ratings of flanges are calculated based on the pressure generated by the bolt circle arrangement, the diameter into which they have been arranged, and the coefficient of friction of the flange surfaces. A number of safety and application factors are normally added to ensure a pure high-pressure frictional connection results.

BX4 with Keyway Mounting

The BX4 is described as a low backlash coupling, which transmits torque purely though the form fit of a shaft key or spline. Some potential for backlash always exists due to the clearance required to fit the hub over the shaft. However reduced, this is even in the case of an interference fit, and especially in cases where only set screws are used for the axial retention of the hub. The main advantage of a keyway connection is the quick and easy installation and removal of the coupling hubs. In lieu of keyway connections, spline connections provide additional security and surface area contact with superior stress distribution characteristics. In consideration of some extra cost, this can be a good solution for a more compact design, as the necessary shaft engagement length can then be reduced. Generally speaking, the BX4 is well suited to unidirectional drive applications, which must accelerate, even to high speeds, and maintain those speeds for prolonged periods. Drive systems with shock loading, reversals, and high duty cycles are better served by the BX1 and BX6 designs.

BX6 with Conical Clamping Ring

The BX6 makes use of a high pressure, keyless, frictional clamping connection. The conical clamping ring system consists of two components: the clamping ring and the hub. The clamping ring is drawn over a slotted tapered section of the hub by means of a set of fastening bolts. Sliding the two tapered surfaces against one another creates a high level of surface pressure in the vertical direction, compressing onto the cylindrical shaft. The resulting contact force eliminates the need for keyways and is suitable for transmitting extremely high torque levels depending on the diameter of the shaft. Since the BX6 has many planes of symmetry across its length, it is also well suited to very high rotational speeds, as a result of the low residual imbalance.

Variable Length with Internal Support

The intermediate section of the BX series consists of high strength steel tubing with adapter flanges common to the full product line. The length of the intermediate tubing section can be specified according to customer requirements and interchanged for machinery modifications as needed. The adapter flanges are made with a gimbal system which extends through the inside of the bellows and engages a concave receptacle on the opposite side. This support system causes the entire mass of the intermediate section to be carried by the shafting rather than the flexible bellows. It also serves as a safety feature in the event of a machine crash. Should the bellows or its connection be caused to fail by extreme overload, the tubing would still not be allowed to escape from the hubs, eliminating the need for coupling guards in some cases. This safety feature does not exist in flex disc or universal drive shaft products.

Summary and Outlook

The all new “BX” series offers standard torque ratings of up to 100,000 Nm and bore diameters of up to 280 mm. This technology provides a viable alternative to steel disc couplings, with improved torsional stiffness, balance, and safety features. Flexible design configurations allow for the BX to be made to suit applications with specialized flanges, hubs, materials and surface treatments for even the harshest of environments. The internal support system provides for a high degree of security unavailable in other similar types of couplings. The symmetrical design of the series BX1 and BX6 allows for an optimal balance quality in highly dynamic drive systems and test stands, with ISO Grade 2.5 certification available upon request.

The principle of the formed metal bellows as an equalizing element offers many unique performance advantages for today’s advanced drive technology. Designs capable of transmitting up to 2,000,000 Nm are currently in development.

R+W America
www.rw-america.com

 

Compact safety coupling

Making use of the existing EKL line of compact elastomer couplings, the  SLE safety coupling offers a compact, flexible and vibration damping option with integral torque overload release. Designed for aggressive servo driven applications, the SLE is backlash free, relying on preloads and frictional connections, along with a precision molded, wear resistant, polyurethane insert to damp vibration and relieve bearing loads resulting from misalignment. In the event of a machine crash, power surge, or any other cause of unanticipated overload, the SLE will disengage the motor or gearbox shaft from the load in less than 3 milliseconds. Re-engagement takes place automatically with slow speed rotation of the shaft subsequent to the overload occurrence. Available in four sizes, the SLE handles a range of disengagement torque values from 5 to 700 Nm, and English or metric shaft diameters from 12 to 60 mm.

R+W America
www.rw-america.com/torque-limiters/torque-limiter-sle_t.php

 

 

Compact stainless steel bellows couplings

The BKS series of stainless steel, welded bellows couplings are up to 1/3 shorter in length, reducing moment of inertia and saving space. Manufactured in Germany, the BKS makes use of 1.4301 (304) stainless steel hubs, precision machined to a high level of concentricity. Laser welded while mounted onto a common shaft to guarantee straightness, the couplings compensate for shaft misalignment and precisely transmit motion with zero backlash. Allowable temperatures range from -40 to +300° C. The BKS coupling is available in 6  sizes, with torque ratings from 15 to 500 Nm and English or metric bores from 12 to 75 mm.

R+W America
www.rw-america.com/bellows_couplings/index.html

 

RR Fisher & Co Releases R+W Safety Couplings For Particle Measuring Systems

RR Fisher & Co. Ltd. the released its ATEX certified R+W safety couplings for Sympatec’s particle measuring systems. These safety couplings offer an ATEX design with high-temperature steel plug segment.

Sympatec’s particle measuring systems are specifically designed to detect particles in a nano range up to 10mm. The systems identify application in quality assurance in pharmaceutical manufacturing, cement production, coffee production and print toner manufacturing.

A representative sample is first taken from the production system before measurements can be made. The materials that will be tested are conveyed on air via a production process in pipes with 50mm-800mm inside diameter.

A SK5/10/F/XX type of R+W safety coupling was used in the drive mechanism of the sampling tube. The coupling has ATEX certification for zones 2/22 and 1/21. This is important because samples are taken in an ignitable dust or explosive atmosphere.

Aside from operating and installation instructions, a required declaration of conformity and ATEX certificate is enclosed with the R+W Safety Coupling when it is delivered.

Specified by the customer, the SK5 pluggable solution is necessary for certain industrial environments where a wide variety of products are often designed on the same equipment line.

The components utilized in processing the previous products must be cleaned when converting the line for the production of a different product. Removing components for cleaning is made easy with its plug-in capability.

Another problem at this stage is about the temperature range. The range of operating temperature specified for the coupling is up to 150ºC.

Since plastic plug-in segments are rated for use up to 120ºC, the material for the said parts was changed. Built similar to the plastic part, the new steel plug design can handle an extended temperature range.

www.rrfisher.com.au

www.sympatec.com

Explosion proof couplings

Flexible couplings are a critical component for applications involving potentially explosive materials found in automotive paint and cleaning stations, chemical plants, and powder mixing areas.  A lack of radially flexible elements in shaft linkage can result in high radial loads on shaft bearings, eventually leading to heat generation and bearing failure, making flexible shaft couplings essential to machine drive design in these cases.  Even more critical is that the potential for sparks must be eliminated.  For use in explosive environments R+W has developed a full range of ATEX certified “explosion proof” couplings in accordance with the European directives, ATEX 95 and ATEX 137.

These special couplings are precision machined with a thermally and chemically stable, wear resistant, polyurethane insert press fit between the two for zero backlash.  A smooth fit between the insert and the hubs helps the insert to compensate for lateral, angular and axial shaft misalignment.  The insert is impregnated with graphite, giving it electrically conductive properties, eliminating the potential for any charges arcing from one hub to the other.  Official serialized markings including the part number are required by the directive and are clearly visible on each unit.

These precision couplings are available in a variety of mounting configurations, and can include torque overload protection.  There are nine total sizes ranging from torque ratings of 2 – 2150 Nm (17 to 19,000 in-lbs).  Both English and metric bore diameters are available in a range from 3 – 80 mm (1/8 to 3.125 in.) with or without keyways.

R+W America
www.rw-america.com

2009 BK Series Catalog from R+W

Stainless steel bellows couplings have grown more prevalent, driving the development of a wide variety of mounting configurations, wall thicknesses, lengths, torque ratings, etc.

The 2009 BK series catalog from R+W represents the latest collection of standard bellows coupling designs for applications requiring torque ratings from 15 Nm – 10,000 Nm. Flanges, shafts, hollow bores and nearly any other mechanical interface are accommodated.

R+W America
www.rw-america.com

Elastomer couplings with higher torque handling capacity

The growing popularity of curved jaw (elastomer) style couplings for precision applications has driven the need for couplings that handle more than the traditional torque capacity of 2,150 Nm up to a maximum torque of 25,000 Nm.

Available with split clamping collars or keyway and set screw connections, the three new body sizes allow for backlash free, vibration damping power transmission, paired with strong torque density. Dual flexture and jack shaft versions are also available for spanning longer distances and compensating for larger misalignments. Unlike the pre-existing range of R+W elastomer couplings, which use a single spider element between the new hubs, the new larger sizes will use individual vibration damping compensation elements to fit between each mating set of coupling teeth. These couplings are available in English and metric bore diameters up to 170 mm.
R+W America
www.rw-america.com

6 ways to assess torque needs for safety couplings

Safety couplings that operate on the ball detent principle primarily suit disengagement torque applications. But, with some modification, they can suit highly dynamic applications with resonant frequencies and torsional rigidity. Here is a brief examination of common equations used to calculate the following torques for safety coupling design in a drive system: disengagement torque, acceleration torque, acceleration and load moment, thrust force, resonant frequency, and torsional rigidity.

Disengagement torque. The disengagement torque must be greater than routine torque moments within a drive train. First, determine torque requirements within the drive train. In practice, a multiplication factor of 1.5 times the nominal operating torque is often adequate to accommodate acceleration moments and other influencing factors. To calculate minimum torque ratings for a drive train, use the following equation:

T_KN ≥ 1.5 x T_AS

Where:

T_KN = torque in the drive train (Nm)

T_AS = Peak torque in the drive train (Nm)

Peak torque is usually taken from the rating plate on the given drive mechanism.

You can use the number 9,550 as a constant value to convert power into Nm. Thus:

T_KN ≥ 9,550 x P_AN/n x 1.5

Where:

P_AN = Power of the driving side (kW)

n = speed (rpm)

Acceleration torque. The acceleration torque method is a more accurate technique. In addition to angular acceleration, it makes allowances for peak torque on the driving side, the mass distribution, and the moments of inertia inherent to the driving and driven ends. With the help of a correction factor (surge or load factor) established according to the machine and application, acceleration torque can be determined using this method. Normally, a distinction is made between three types of surge or load factors:

S_A = 1 (harmonic strain)

S_A = 2 (periodic strain)

S_A = 3-4 (non-periodic strain)

The following equation reflects these relationships:

T_KN ≥ α  x J_L ≥ (J_L/J_A + J_L) x T_AS x S_A

α = Angular acceleration (s^-2)

J_L = Moment of inertia on the load side (kgm²)

J_A = Moment of inertia on the driving side (kgm²)

S_A = Surge or load factor

Acceleration and load torque. The most accurate but complex assessment of torque for the evaluation of safety couplings is the acceleration and load torque method (start-up under load). This approach simulates an application in which constant acceleration and deceleration under load conditions takes place. Load torque is used as an additive factor to acceleration torque.

The following equation, with differentiation of individual variables, describes this relationship:

T_KN ≥ a x J_L + TAN ≥ [(J_L/J_A + J_L) x (T_AS – T_AN) + T_AN] x S_A

T_AN = Peak torque for the load side (Nm)

These three design methods are based on manufacturer data for the drive and the load components. In addition to torque moments, only moments of inertia and potentially incurred acceleration are included.

Thrust force. Another option for assessing application torque is the thrust force method. This method can be applied to spindle and lead screw drives as well as toothed belt drives, depending on the design of the drive system.

In addition to overall thrust force for the entire unit, thread pitch and efficiency play substantial roles in the proper design of spindle and lead screw drives. Here is the equation for the applied torque:

T_AN = (s × F_v)/2000 × ∏ × η

s = thread pitch (mm)

F_v = thrust force (N)

η = efficiency

∏ = pi

If the drive and load are not linked by way of a spindle or lead screw, but by a toothed belt drive, use the following equation to calculate the incurred torque:

T_AN = (d₀ × F_v)/2000

d₀ = pinion diameter of the toothed pulley (mm)

Resonant frequency. Each body and component in the drive train has its own natural frequency. The resonant frequency of the coupling and the entire drive system can be approximated with the following equations. A prerequisite for the calculations is the summation of mass moments of inertia of the individual components to determine the total mass moment of inertia. The torsional rigidity of the entire drive train also has a big influence on oscillation. The equation for calculating the coupling’s resonant frequency in Hz is:

ƒ_e = 1/2p x  √C_T x ((J_A + J_L)/(J_A x J_L))

The equation for calculating the natural oscillation in speed is:

n_e = 30/p x  √C_T x ((J_A + J_L)/(J_A x J_L))

ƒ_e = resonant frequency of the system (Hz)

C_T = Torsional rigidity of the coupling (Nm/rad)

n_e = Natural oscillation term of the system (rpm)

Torsional rigidity. Whether a machine is designed to be rigid or damping depends on the respective application. The rigidity of all individual components, including the coupling, should always be taken into account. In theory, if a body twists by a defined angle if it is subjected to a certain load (torque). The degree of twist depends on the rigidity of the body (countering the torque). This relation is expressed:

φ= 180/p x T_AS/C_T

R+W America

www.rw-america.com

Compact Precision Torque Limiters from R+W America

Based on a compact and simple design, the ESL series torque limiters from R+W offer accurate performance at a reasonable cost. Unlike traditional ball-detent torque limiters, the ESL spring loads two series of ball bearings against one another to create a rolling effect at overload.

The rolling effect reduces wear and at the same time lets the clutching interface serve as the bearing support during overload disengagements, saving space and cost. This torque limiter uses a specially developed “digressive spring characteristic,” so sensitivity to overload and torque disengagement accuracy are not compromised. Disengagement takes place within 3 milliseconds of overload, and at a value within +/-5% of the disengagement torque setting. The basic design mounts with a keyway and set screw; customized mounting attachments are also available. Technical specifications, solid models, and video are available at:

R+W America
http://www.rw-america.com/elastomer_couplings/torque-limiting-coupling-esL-t.php

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

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