Misumi Now Offers Comprehensive Line Of Couplings
MISUMI Coupling types include high precision Single Disc and Double Disc Clamping models, many of which may be used with servomotors. Also offered are Oldham, Slit and N Couplings, as well as Jaw, Sleeved, Bellows, and Resin types, in a variety of sizes and configurations.
The SCXW is a Double Disc type precision disc clamping coupling with high torque but no backlash. Its torsional rigidity is up to 26% higher than conventional (standard) disc clamping couplings. It suits applications requiring fast positioning precision. All the bolts are trivalent chromate plated and suitable for use in clean environments.
The MFJGWK is available as a high rigid Oldham coupling, set screw type. The MFJCGWK is a high rigid Oldham coupling, clamping type. Both products feature an aluminum bronze spacer and have a keyed bore. These couplings have allowable torque 2X higher compared to resin spacers.
The SCOC is a short Oldham coupling, clamping/spacer type. This space-saving version works with miniature devices because it is 17% shorter than conventional types.
Most MISUMI Couplings ship in six days, with the exception of the new High Rigid Oldham Couplings, which ship in eight days. Some couplings offer optional express shipping for faster delivery.
MISUMI USA, Inc.
Http://us.misumi-ec.com
Zero Backlash Coupling Ideal for Belt Driven Linear Actuators
August 9, 2010 by CouplingTips
Filed under Elastomer, Featured
Due to greater demands in modern machine design and construction, R+W has developed its EK7 series of SERVOMAX Zero Backlash Elastomer Couplings.
This coupling was specifically designed to mount to hollow bores with an expanding tapered clamping element, making it ideal for belt driven linear actuators. The EK7 provides design engineers with a number of advantages, including:
· Most belt driven linear actuators require a shaft adapter in order to couple the pulley to a motor or gear head. The EK7 eliminates the need for this additional hardware.
· In designs where motor shafts are typically mounted directly into the pulley, customer specified products often provide shafting that is too large. The EK7 can couple to the large shaft with a smaller expanding element to link the two together.
· Assemblies involving a motor, gear head and coupling used to drive the actuator can become quite large and cumbersome. The EK7 plays a small part in reducing that system size by reducing the coupling adapter flange by the length of one hub.
In addition to providing these and other novel mechanical linkage solutions, the EK7 also offers the benefits of all SERVOMAX couplings, particularly in its zero backlash torque transmission. The moment of inertia is also very low due to its low mass and low weight; ideal for high-speed servo applications where rapid acceleration/deceleration cycles exist.
The couplings are manufactured with precision machined jaws and an elastomer insert press fit between them for vibration damping and zero backlash transmission of torque. The coupling hub is custom bored and can accommodate shaft diameters from 4 to 60 mm (3/16 to 2.25 in.) and the expanding hub can accommodate bores from 12 to 70 mm (1/2 to 2.75 in.). Sizes are available for torque capacities up to 450 Nm (4,000 in. lbs.).
Torque Limiters that Keep Overload in Check for Heavy Equipment
July 7, 2010 by CouplingTips
Filed under Featured, Torque Limiters
A different approach is needed when designing mechanical torque limiters for high horsepower drives.
The basic principles of mechanical torque limiter design are similar to those that have been known since some of the first machines were built, yet it remains a dynamic field. Function, space restrictions, safety considerations and continuously changing machinery design drive the need for these components to evolve.
Typically, safety element torque limiters are supplied as a pre-set and self-contained package for integration into timing sprockets, sheaves and cardan shafts, like the one shown here.
In particular, high horsepower drives often call for mechanical design to be approached from different perspectives. As motors, gearboxes, and machines increase in size, power density can become disproportionate from one driveline component to the next, emphasizing the need for more rugged, robust and compact equipment. Precision mechanical components used in the packaging and light manufacturing automation industries, for example, may not be adequately scalable, and so be outsized quickly as drive requirements reach into the thousands of horsepower.
This disparity is seen in the design of modern torque overload release devices, the majority of which have torque release values inappropriately low for use on heavy equipment requiring operation and disconnect at torque levels beyond 10 KNm, such as large recycling equipment, gas turbines, windmill test stands, and industrial crushers. While market demand may be greater for smaller torque limiters, the availability of heavy-duty devices is critical as mass, inertia and destructive forces increase in high-powered machinery.
One exception to the rule of disproportionate size increase is perhaps the oldest and most rudimentary form of torque overload release device; the shear pin coupling. In this case, one or more pins link two rotating bodies with known yield strength located at a pre-defined radius from the center of the rotational axis. At some torque level near the calculated maximum, the pin(s) will break for a complete separation of the driving and driven shafts and fail to transmit the excessive torque.
Shear pins have protected rotating equipment for centuries, but they lack accuracy and can require much time to repair after overload. To maximize plant uptime and improve the accuracy of release torque, vendors have developed a variety of torque overload release devices with integral bearings and simple mechanical reset features. A limited number of these torque overload release devices have been reconfigured for high horsepower.
Spring tensioned torque limiters
The first widely used modern overload release devices came about in the 1930s for use in the steel industry where downtime can be expensive, and replacement of shear pins time consuming and dangerous. These parameters led to the development of the spring-tensioned form-fit torque limiter, which uses the same fundamental principle of a set release force located at a specific center distance.
In spring tensioned torque limiters, ball or roller bearings are precisely loaded into detents machined into an output flange that will break away quickly and accurately at a predefined torque level. This type of torque limiter will either ratchet or free wheel during and after overload, depending on size considerations and the rotational speed of the axis.
A slightly more sophisticated form of torque limiter is the ball-detent design. After overload release it reengages quickly.
The shear pin coupling is a rudimentary form of torque overload release device. It links two rotating bodies with known yield strength located at a pre-defined radius from the center of the rotational axis. It will break at a specific torque level and separate the driving and driven shafts so as not to transmit excessive torque. The problem with shear pins is that they lack accuracy and take time to repair.
In general, you can adjust the torque of these overload release devices by turning a single screw or spanner nut. Their ratcheting features represent a very fast and convenient means of recovery from overload, since all they require is either low speed operation or manual back driving of the axis after the blockage has been cleared. Since their initial development, hundreds of designs of “ball-detent” and “pawl-detent” mechanical torque limiters have been introduced, with a variety of adaptations made for high speed, high accuracy, light weight, and backlash free operation.
Higher horsepower needs
But, however convenient, these torque limiter designs tend to fall off at torque levels any greater than a few thousand Neutonmeters. The basic problem is that overload breakaway devices rely almost exclusively on torque as a measurable component of power.
Practical implementation of high horsepower drive systems normally involves a slow steady increase in the rotational speed of an axis, where the torque required for instantaneous acceleration would be overwhelming. Drive shafts and gearboxes, therefore, are not typically required to handle the severe peak torques associated with rapid acceleration and deceleration of the load inertia, as might be found in lighter manufacturing systems. As a result they tend not to be as large as a proportionate size increase might require in terms of pure torque capacity. This situation poses a torque density problem for mechanical overload devices.
Beyond 10 KNm common overload release designs become impractically large in outside diameter; the primary limiting factor being the spring set used to load the components together. Since industrial gearboxes, motors, and pumps tend to grow in diameter at a much slower rate than these types of torque limiters, as power increases there comes a certain point at which a traditional single spring form fit torque limiter makes no sense at all, and would tower over the equipment it was designed to protect. Clearly the lever arm component of the torque limiter design must be addressed. The simple answer is to substantially increase the force by which the individual transmission elements are loaded into the output.
There are two widely accepted approaches to overload release devices for torque in excess of 10 KNm, both of which seek to increase force over a reduced lever arm distance. One is a compact, simple design involving hydraulic pressure applied between the two otherwise free spinning surfaces. The other is based on a modified spring tensioned device similar to those previously addressed. Each has their advantages depending on the desired result.
Hydraulic versions
Hydraulic torque limiters basically apply hydraulic pressure between the two otherwise freely spinning surfaces. One or more chambers are inflated by hand to the desired pressure level, calculated as a function of release torque and based on charts provided in the manufacturer documentation. Special fluids guarantee a constant coefficient of friction throughout various operating conditions. These chambers let you apply a high level of force over a very small surface area. When the desired release torque is reached, the output will begin to slip against the input, causing the hydraulic valves to shear off, purging the fluid and fully releasing the input and output components of the torque limiter. Through an integral bearing, the load inertia coasts to a stop without further damage to the machine components or the torque limiter itself. Reconnection involves replacing the valves, refilling the chambers, and resetting the pressure.
Compared with shear pins, hydraulic torque limiters let you maintain strict control over the disengagement torque setting, which can be unpredictable with shear pins. They otherwise represent a compact choice for accurate torque overload release at tremendously high torque values, handling as much as 10,000 KNm. What they do not offer is a major reduction in the time required to recover from an overload event.
Modified spring tensioned device
For maximum plant uptime, a slightly more sophisticated form of the ball-detent design still offers the fastest means of re-engagement after overload release. Several decades ago, torque limiter manufacturers developed self-contained tangential force modules based on a plunger design. The torque density problems associated with traditional ball-detent torque limiters are then addressed through the use of one or more of these individually spring tensioned elements, which can tolerate very large tangential forces.
Spring tensioned torque limiters contain ball or roller bearings that are precisely loaded into detents machined into an output flange that will break away quickly and accurately at a predefined torque level. This type of torque limiter will either ratchet or free wheel during and after overload.
Since the individual torque transmission elements provide their own back stop for the spring tension, an array of small blocks are used, which are forced outward to clear the way for the plunger core to retract into the housing after sufficient tangential force actuates the system. The result is a “snap action,” which causes the plunger to quickly retract into the housing within a few milliseconds of overload. Once again, an integral bearing enables the load inertia to coast to a stop without further damage to the machine components or the torque limiter itself.
The key advantage to this design is the quick reloading of the individual elements into the output flange with either a gentle blow from a mallet or light pressure from a pry bar. Once the driving and driven components of the torque limiter are rotated back into the necessary orientation, re-engagement takes place quickly and easily. Depending on practical considerations, you can use pneumatic actuation systems to automate re-engagement, though future designs are likely to incorporate a more widely applicable, self contained and fully mechanical reset function.
As with traditional ball-detent torque limiters, spring tension is adjusted through the rotation of a nut, only in this case the elements are individually adjusted to the desired tangential force value, and a torque calculation is made based on the number of elements and their distance from the center of the rotational axis. While the earlier designs of safety element torque limiters involved special datasheets used in conjunction with measurements taken from the spring height, increasingly manufacturers indicate the correct nut location with a marked scale. You can make a coarse adjustment by adding or removing safety elements, which is made more plausible by torque limiter designs with the maximum number of receptacles pre-machined into the base element and with simple covers installed to guard them from contamination. The ability to make such adjustments means you do not need to ship the torque limiter back to the manufacturer for rebuilding in the case of gross miscalculation of the torque requirement.
Because of the modular design, safety element type torque limiters can be used for almost any torque release value, depending on the size and number of elements used, and limited by the maximum diameter allowed by adjacent equipment. For this reason, individual safety elements are normally made available for use into existing machinery designs or for custom coupling systems, including some used for linear force limitation.
For the most part, safety element torque limiters are supplied as a pre-set and self-contained package for integration into timing sprockets, sheaves and cardan shafts. Some manufacturers provide them as fully integrated flexible safety couplings, such as jaw, gear, and disc pack types to name a few. Custom options often include special materials, integral brake discs, high temperature felt seals, and added bearing support. As is the case in any field of design, manufacturers are driven to improve reliability and ease of use, while simultaneously reducing weight and space requirements for installation.
ServoClass® Couplings deliver high torsional stiffness
March 25, 2010 by llangnau
Filed under Featured, Industry News, Servo
ServoClass® Couplings suit applications using ac and dc servomotors that need precise positioning and the ability to handle high reverse loads. These couplings have zero-backlash and low hysteresis.
Three new sizes have been added to the ServoClass coupling line. They handle bore diameters from 0.875 in. (20 mm) to 1.378 in. (35 mm) and operating torque from 3937 to 9843 lb-in., (100 to 250 Nm). Additional sizes are available starting with the smallest bore size 0.157 in. (4 mm) and larger.
All ServoClass couplings are manufactured of RoHS compliant materials. They are lightweight and designed with 304 stainless steel disc packs and 7075-T6 aluminum hubs and center members. They are available in single and double flex models in inch and metric sizes. All models and sizes feature clamp style hubs with corrosion resistant socket head cap screws.
“ServoClass couplings provide a better option compared to beam or bellow style couplings,” reports Robert Mainz, Zero-Max sales manager. “As the cycle becomes faster, they outperform beam couplings, which experience harmful windup. Their robust design also outperforms the fragile design of bellows couplings.”
Zero-Max
www.zero-max.com
Schmidt 5D Couplings Handle Axial, Angular And Parallel Shaft Misalignments
March 19, 2010 by llangnau
Filed under Featured, Flexible, Industry News
Schmidt 5D Couplings suit applications requiring large axial, angular and parallel shaft misalignments. This “all-in-one” coupling design will handle all shaft displacements providing low backlash for precision high torque applications such as roll forming and similar heavy duty fabricating equipment.
Designed to fill an important need in the Zero-Max family of torque-rigid couplings, these couplings allow for easy adjustment to any possible misaligned shaft position without imposing heavy side loads on shafts, bearings or other machine equipment. The coupling can accommodate up to 5 degrees of angular misalignment and as high as 1.5 in. parallel misalignment while maintaining undisturbed power transmission at constant angular velocity. Acting forces within the coupling can be precisely calculated, assuring reliable, trouble-free system operation. This unique design will tolerate high shock and reversing loads with minimal or no maintenance required.
Additional features include: space-saving design and easy installation — couplings can be mounted to shaft hubs or directly to existing machine flanges (no need to reposition either shaft being coupled). Available in standard and inverted hub configurations in bore sizes from 1.500 in. to 6.375 in. or 38 mm to 160 mm. Custom designs can take this coupling design beyond the catalog specifications. The ten different model sizes handle speeds up to 1000 RPM and torque from 2800 to 500,000 in-lb. Special design modifications are available.
“The 5D coupling has very robust design features for use in applications such as roll drive systems used in converting machinery. The unique and flexible design allows for a range of movement to improve the quality of the end product.” reports Robert Mainz, Zero-Max sales manager. “They do a very good job of handling shaft misalignments and protecting drive train components in these high performance systems.”
Zero-Max
www.zero-max.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.
- 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.
- 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:
- Upon overload, separation of drive train and load should occur within a few milliseconds.
- 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
Corrosion-Resistant Couplings Suitable For Multiple Applications
February 25, 2010 by CouplingTips
Filed under Featured, Safety
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.
High Misalignment, Low Inertia Servo Couplings
January 25, 2010 by CouplingTips
Filed under Bellows, Featured
The trend in industry to process more material, faster and more efficiently calls for a high torque, high misalignment, low inertia servo coupling, with minimal compromise to torsional stiffness. Stainless steel bellows couplings have the highest torsional stiffness of commercially available flexible couplings, making them the best choice for aggressive servo driven applications. But in cases where the two shafts to be connected are mounted to different bearing surfaces, it can be difficult to maintain the precise alignment tolerances normally required.
Utilizing a special, high stiffness bellows and new, high strength connection method, the BKZ handles an average of 2.6x the traditional torque rating at a given outside diameter, and an average of 2.9x more lateral misalignment, opening up the benefits of precision bellows couplings to a whole new segment of machine design and servo motion control.
Accepting bore diameters ranging from 15 to 60mm and torque ratings from 20 to 1000Nm, the BKZ range is available in 4 sizes, with a variety of materials, finishes and the optional self-opening clamp system. View online at http://www.rw-america.com/bellows_couplings/bellow-coupling-bkz-t.php
R+W America
www.rw-america.com
Why Couplings Need Lubrification
January 18, 2010 by CouplingTips
Filed under Featured, Industry News
In an ideal world, multiple components could be produced in a single piece, or coupled and installed in perfect alignment. However, in the real world, separate components must be brought together and connected on-site.
Couplings are required to transmit rotational forces (torque) between two lengths of shaft, and despite the most rigorous attempts, alignment is never perfect. To maximize the life of components such as bearings and shafts, flexibility must be built in to absorb the residual misalignment that remains after all possible adjustments are made. Proper lubrication of couplings is critical to their performance.
Figure 1. Types of Misalignment
MISALIGNMENT
Misalignment can occur as either an offset or angular displacement on two of the three possible axes (Figure 1). The third axis, in the longitudinal direction, is not commonly measured, though errors in this direction can result in excessive thrust loads in a system. For major installations, such as large compressors, wire alignment methods are used. Smaller applications have traditionally used rim-and-face dial indicator readings to quantify and correct misalignment, though optical laser indicators have grown in popularity due to their ease of use and accuracy.In pace-setting maintenance organizations, efforts are also made to compensate for thermal growth that occurs in equipment during operation. All materials (except water) expand a small amount when heated; the amount by which they do so is governed by the material’s coefficient of thermal expansion and the degree to which it is heated. A machine that is brought into alignment at ambient temperature will creep into a position of misalignment as the machinery materials climb or fall to operating temperature.
Attempts are made to preheat or cool equipment to normal operating conditions before performing alignment checks. Alternatively, calculations of anticipated thermal growth can be used to intentionally misalign the drivetrain at ambient temperature so that it may grow into alignment. Whatever precautions are taken to make alignments as precise as possible, some amount of residual misalignment will inevitably remain. Misalignment forces rigid machine components such as shafts to deflect in order to effectively become aligned. This deflection stresses the components, causes vibrations, and distributes higher and uneven loads on the structures that support these elements, such as bearings. These stresses waste energy and can dramatically reduce equipment life and reliability.
Designed properly, couplings can absorb misalignment forces so that more expensive, critical and sensitive components may be saved. While rotating shafts appear sturdy, the bearings which support them are some of the most sensitive precision components in the drivetrain.
Figure 2. Gear Couplings
TYPES OF COUPLINGS
Coupling designs may be divided into four principal categories, each having several specific designs. Solid and magnetic couplings do not require lubrication, but are included here for completeness. Solid couplings are fundamentally rigid structures that do not compensate for misalignment, but do allow two shafts to be joined for the purpose of transmitting torque. Bolted hubs keyed onto shafts are an example of a machine with magnetic couplings. Magnetic couplings allow shafts not in direct contact to be driven together using powerful permanent or electrical magnets. A sealless magnetic drive pump is a common example.Other coupling types are flexible couplings and fluid couplings. Many flexible couplings use fixed-position flexible metallic, rubber or plastic elements, such as discs or bushings, that rotate with the shafts and absorb misalignment. Designs of this type do not require lubrication. Others such as geared, chain, grid and universal joints do require lubrication for performance and longevity. Fluid couplings include torque converters and torque multipliers. These couplings are filled with lubricating fluids that rely on the fluid to transmit torque.
Figure 3. Chain Couplings
FLEXIBLE COUPLINGS
Gear couplings (Figure 2) compensate for misalignment via the clearance between gear teeth. Shaft-mounted external gear teeth on both shafts mate with internal gear teeth on a housing that contains a lubricant. Other designs mount external teeth on only one shaft, mating with internal teeth mounted to the other shaft. Acceleration or deceleration can result in impacts between gear teeth due to backlash from the clearance being taken up on opposite sides of gear teeth. Misalignment will result in sliding relative motion across mating teeth as they pass through each revolution.Chain couplings (Figure 3) operate similarly to gear couplings. Sprockets on each shaft end are connected by a roller chain. Clearance between components and clearance in mating the chain to the sprockets compensate for the misalignment. Loading is similar to that of geared couplings.
External grid couplings (Figure 4) use a corrugated steel grid that bends to compensate for loading induced by misalignment. Grooved discs attached to the ends of each shaft house the grid, which transmits torque between them. Low-amplitude sliding motion develops between the grid and grooves as the grid deforms under load, widening in some locations and narrowing in others over each revolution.
Universal joints are used for maximum allowable misalignment up to 20 to 30 degrees, depending upon the design. They are used extensively for the drive shafts of vehicles to allow the wheels to move with the suspension system. Universal joints use a four-spindled component called the spider to connect two shafts terminating in yokes or knuckles at right angles (Figure 5). Each of the four spider journals is supported by a bearing or bushing contained in one of the knuckles, which allow articulation.
Figure 4. Grid Coupling
FLEXIBLE COUPLING LUBES
Both lubricating oils and greases can be selected to lubricate flexible couplings. Unless specifically noted by the coupling designer, couplings for the majority of industrial components are grease lubricated. Coupling components are protected primarily by an oil film which bleeds from the grease thickener and seeps into the loading zone.Lubricated flexible couplings require protection from the low-amplitude relative motion that develops between components. Other concerns include centrifugal stress on the lubricant (particularly grease), which causes premature separation of the oil from the thickener, poor oil distribution within the housing and oil leakage from the housing.
The motion’s low amplitude, articulation speed and tendency toward a sliding rather than rolling action inhibits the development of hydrodynamic (full-film) lubrication. Greases made with high-viscosity base oils, anti-scuff (EP) and metal-wetting agents are recommended to overcome the boundary (mixed-film) conditions that often exist in flexible couplings. High oil viscosity also slows the leakage rates.
Centrifugal forces in flexible couplings can be extreme, becoming greater with increased distance from the rotational axis. Even moderately sized couplings can generate forces thousands of times greater than gravity (referred to as Gs). Grease makers put a high priority on formulations that resist premature separation of oil and thickener due to the high G forces.
Figure 5. Universal Joint
FLUID COUPLINGS
Fluid couplings transfer momentum from the input shaft to a fluid and then to the output shaft when transmitting torque. Misalignment is accommodated solely by clearances between the moving parts. The small clearances don’t provide much room for error in alignment. However, it is possible to effectively compensate for shock loading and high-torque starting loads as there is no solid connection between input and output shafts.In fluid couplings, an impeller attached to the input shaft accelerates fluid within the coupling as it spins, much like in a centrifugal pump. This fluid then hits the vanes of the output shaft’s runner, transferring its momentum as the runner accelerates. It will accelerate until it approaches the speed of the input shaft, but will never actually reach it. The difference in speed between the input and output shafts is known as slippage. Of course, frictional and viscous drag must be overcome before the output shaft can rotate. The minimum input speed required for this condition is known as the stall speed. Equipment with large static loads, such as a steam or gas turbine, would use a fluid coupling to minimize the initial stress on the driving shaft.
Shock loads on the input side, such as starting torque, are never created. The speed of the input shaft is never restrained. When the stall speed is exceeded, the output shaft will begin to accelerate, but will do so at a constrained rate due to its moment of inertia (resistance to angular acceleration). Slippage is created as the runner accelerates to the speed of the input, dissipating excess energy through viscous heat generation in the fluid. Output side shock loads will be similarly dissipated, even if the output shaft should completely stall.
Torque converters and multipliers are special applications of fluid couplings that allow the input torque to be modified before transmission. These designs operate fundamentally by the same principles, but are mechanically much more complex.
FLUID COUPLING LUBES
The dissipation of energy that makes fluid couplings so tolerant of shock loading creates the potential for rapid and extreme increases in fluid temperature. The energy dissipated during stall and slip is converted to heat through the viscous shearing of the fluid (fluid internal friction). In extreme applications, the fluid temperature can rise above the normal 200-degree Fahrenheit operating temperature in less than a minute.Oxidation and thermal degradation resistance are important qualities of oil used for fluid couplings because of the potential for drastic temperature increases. Similarly, a high viscosity index (VI) is also useful to prevent severe decreases in operating viscosity at temperature spikes and excessively high operating viscosity at low-temperature conditions.
Low-viscosity fluids are ordinarily used in these applications to reduce the power lost to heat due to fluid friction. Fluid coupling viscosities may fall between 2.5 to 72 centistokes (cSt) at 40 degrees Celsius. For fluid couplings designed to operate at high temperatures, viscosity limits may be given at 100 C.
These fluids must also resist foaming due to the severe agitation caused by the impeller’s movement and its impact upon the runner vanes. Rust-protective properties help preserve the coupling’s metal components. Hydrocarbon-based fluids are superior in this regard to other fluids, but their performance can be improved through rust-inhibiting additives. Seal compatibility is also important for long-life usefulness.
RECOMMENDATIONS
Acceptable life can be expected from any of these devices only if proper maintenance is performed. Lubricant levels and quality must be verified through periodic checks. Additional lubricant may be needed to compensate for leakage. Periodically flush and change the lubricant to remove harmful by-products of lubricant breakdown, to replace oil-depleted grease or to refresh the additive population. Gear couplings require perhaps the most maintenance. Typical relubrication intervals are six months to one year, depending upon application severity and experience.All maintenance tasks must be performed with attention paid to contamination control. The sliding contact suffered by many couplings indicates that abrasive three-body wear caused by particulate contamination could be particularly damaging. Improper removal of solvents used to clean couplings during inspections and flushing operations can lead to significant viscous thinning of the lubricant in operation or detrimental reactions with grease-thickening materials.
Couplings will endure when the demands placed on them are reduced. Consider the first line of defense to be a minimization of shock loading, including hard starts and sudden load reversals. Sometimes operational demands make this impossible. The principal source of loading in coupling systems can be controlled to a great extent, however. Proper alignment is considered a high-priority, precision maintenance functions. Use vibration analysis or thermography during operation to identify couplings that are not in alignment, as even the sturdiest foundations shift over time. Certainly, check for proper alignment whenever intrusive maintenance or repairs are performed on the coupled components.
