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
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GERWAH® Product Line

The GERWAH® line of products consists of magnetic couplings, metal bellows couplings, servo-insert couplings, line shafts, RING-flex® couplings and safety couplings. These couplings are available in a range of sizes and torque capacities to 3,800 lb-ft. The low mass of the lightweight construction helps increase machine performance and reduce energy costs.

Ringfeder Power Transmission USA Corporation markets a range of power transmission components and keyless shaft/hub technology.  Other power transmission products include shock absorbing devices, flexible elastomeric couplings, flexible disc couplings and torque limiters along with other specialty and custom made products.

RINGFEDER

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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

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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.)

Corrosion-Resistant Couplings Suitable For Multiple Applications

A full line of shaft collars and couplings for pump drive and structural systems in water treatment, pollution control, and similar facilities is available from Stafford Manufacturing Corp. of Wilmington, Massachusetts.

Stafford Corrosion-Resistant Collars and Couplings are offered in 303 and 316 stainless steel, brass, bronze, and other materials for various power transmission and structural system requirements. Featuring a wide range of sizes and styles, they are suitable for use in pump drive systems, mixing equipment, flow control instruments, and other applications exposed to water, harsh chemicals, solvents, and detergents.

Developed for water treatment, pollution control, pulp and paper, chemical plants and related facilities, Stafford Corrosion-Resistant Collars come in 1-pc, 2-pc and set-screw styles in sizes up to 16″ I.D. and the couplings in 1-pc, 2-pc, and 3-pc styles up to 6″ I.D. All can be modified with special bores, keyways, mounting holes, flats, hinges, threads, and more.

Safety Coupling Catalog

Zero-Max has published a 12-page Zero-Max catalog of Torq Tender overload safety couplings. These overload safety couplings disengage motor drives for overload protection. They protect motor and drive systems from jam ups and excessive loading.

System designers will find this new catalog helpful when examining overload protection options for a particular system design. Available in models that will disengage torques from 2 to 3,000 in. lbs., mechanically operated Torque Tenders often are the best design option because they are tamper proof and do not require costly and potentially risky calibration procedures. The torque value is controlled by the Torq Tender part number ordered and preset at the factory.

Torq Tender operation in most mechanical power transmission systems is simple. When the load exceeds the Torq Tender rating preset with precision tempered torque springs, the unit’s drive key pivots out of a slot, disengaging the Torq-Tender coupling. Once the overload is removed and the speed reduced, the Torq Tender resets itself automatically. This automatic resetting action engages the system’s drive shaft and puts the entire drive system back into motion at its original position.

Torq Tenders are available in many standard sizes. The smallest is the TT1X with a 2 to 60 in. lbs. torque rating. The largest is the TT4X with a 750 to 3000 in. lbs torque rating. Special Torque Tender designs are also detailed in the new catalog for handling various mountings, bore sizes and housings.

Inside the new brochure is complete selection and ordering information. Also, you can now size, select and see the right Torque Tender solution with Zero-Max Partstream, the configurable 3D CAD downloads.

Handbook for Mechanical Torque Limiters

As a part of a series of technical handbooks developed for the European manufacturing industry, R+W has released a new handbook for the design and application of safety and overload couplings.

Covering the very basics of what torque limiters are, the varieties available, and why they are used, all the way though complex theories and advanced sizing formulas used to determine optimum coupling selection, this handbook includes information for all levels of design engineers with a potential need for torque overload protection. This is the second of a two part series, with the first book handling the design and application of precision couplings and line shafts. Contact R+W America for more information.

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New: R+W Series SK2 Safety Coupling

Completely corrosion proof for outdoor and wash-down applications, the R+W SK2 can be a failsafe solution to overload problems.

Torque overload protection is often overlooked in machine design until something jams resulting in expensive damage and down time. The R+W Series SK2 safety coupling was developed specifically for this purpose. It uses a patented ball detent system that is absolutely backlash free and will disengage within 1 to 3 milliseconds when an overload is detected.

Four versions are available including a single position, multi-position, load holding, and free wheeling. These couplings are preset in the factory to a customer specified disengagement torque and can be adjusted in the field after installation.

These unique couplings are manufactured with a stainless steel bellows exhibiting very high torsional rigidity with zero backlash. The moment of inertia is also very low due to its low mass and low weight, making them ideal for servo driven systems.

The SK2 couplings are available for a torque range of 0.2 to 1800 Nm (1.8 to 15,930 in. lbs.). Both shaft hubs are custom bored for shaft sizes of 4 to 80 mm (0.25in to 3.125in.).

More details at http://www.rw-america.com/torque-limiters/index.html

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