The use of mechanical torque limiters is often considered to be outdated by those who prefer to control torque overload through electronic current limitation.  While this is effective in many cases, as machinery becomes more dynamic, the inertia of moving parts becomes more critical.  It is indeed possible to abruptly decelerate a rotating mass through unintentional blockage or application of a dynamic braking system at a faster rate than the drive would normally accelerate.  This creates torque overload through reflected inertia which is completely independent of the electronic system, and can easily exceed the peak torque rating of the motor.  While older and bulkier designs may be out of the question, these modern mechanical torque limiters offer a high level of sensitivity and accuracy, with increasingly smaller impact on the size, mass, balance, and power consumption of the drive system.

The SL Series uses the proven spring loaded ball detent system, along with a previously patented preload for zero backlash operation.  But to achieve its target of 50% weight reduction, R+W embarked on a two year collaborative effort with local universities, designing the product from the ground up rather than simply redesigning or optimizing existing products.  The result is a torque limiter constructed from state of the art materials with unique surface treatments and innovative assembly technology – surpassing weight reduction targets and simultaneously reducing its footprint.  One example of this newly achieved size reduction is a torque limiter rated to disengage at 160 Nm, which in the past would have had at best a mass of 1.3 kg and a moment of inertia of 1.6 * 10^-3 kgm², now weighs 370 grams with a moment of inertia of 0.8 * 10^-3 kgm².  What that amounts to is an automatic torque limiter with unparalleled power density on planet earth.

In addition to custom material specifications, specially designed spring systems, and some improvements to the ball detent configuration, resulting in a 40% increase in torque capacity for a given size, the weight reduction was also achieved through the compression of individual components. This, of course, is without negative impact on the precision or service life of the torque limiter.  The SL Series, just like the previously existing R+W torque limiter designs, can handle in excess of 10,000 disengagement events, depending on rotational speed.

The four sizes (Series 30 / 60 / 150 / 300) cover disengagement torque values from 5 Nm to 700 Nm, and involve various mounting options, including both direct and indirect drive versions.   Models SLN (clamping) and SLP (keyway) attach by flange to sprockets, sheaves, pulleys, and gears, and include an integral dual bearing system to support belt and chain tension when properly located over the shaft.  Models SL2 (bellows coupling) and SLE (servo insert coupling) mount inline between two independently supported shafts, such as motor to ball screw connections, and compensate for the small but inevitable misalignment which exists in this type of machine layout.  All four types are field adjustable, and come with both English and metric bores according to customer specifications.

The SL Series gives everyone who has overlooked this type of device due to size or weight considerations a chance for a second look.  Contact R+W today with your requirements and realize the benefits of working with the ultimate coupling, worldwide.

R+W America
www.rw-america.com

The flexible misalignment coupling is used to connect the drive from the gearbox to a lead screw and had to be able to resist any current leakage.

Schmidt Offset couplings deliver precision for parallel offset shafts. They transmit constant angular velocity and torque in a range of parallel shaft misalignments. They are used in a range of machine applications including printing, embossing, paper converting, pharmaceutical, and automated assembly systems.

The new Schmidt sealed bearing coupling feature incorporates needle bearings with internal micro-poly lubrication. There are no lube fittings making for a cleaner coupling setup. Compact in design, Schmidt Offset couplings with the new zero maintenance bearing option keep out contaminants and foreign matter, enabling the couplings to be used in less than ideal operating environments.

Schmidt Offset couplings are designed to handle high amounts of parallel offset up to 17 in. and are available with torque capacities up to 459,000 in.-lbs. They impose no side loads on shafts or bearings and eliminate radial shaft vibrations.

Zero-Max
www.zero-max.com

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

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

Typically, they are used in applications where vibrations and crushes may appear. The dampening attributes of those couplings are caused by the spider element which is positioned between the two hubs. This spider element is available in different shore hardnesses. Depending on the requirements it is recommended customers use a spider element with low dampening properties for applications with low vibrations and crushes (higher torsional stiffness of the coupling) and to use a spider element with high dampening properties for application of high grades of vibrations or crushes (lower torsional stiffness of the coupling).

A further positive property of the servo insert couplings is that they are pluggable, so they can be assembled quickly and easily even under difficult assembly conditions. The many different varieties of the hub style include shaft fixing with set screws, with clamp hubs and with outer conical hubs. Customers will find that the most popular type, as always, is with the clamping hubs as they ensure the fastest assembly/disassembly without leaving marks on the shafts. Torque range varies from as low as 1.2Nm up to 940Nm on the largest sizes. Other advantages include an extremely compact design and minimum mass and inertia as standard. They offer high resistance against environmental influences and temperatures as well as being non-wearing and therefore maintenance free.

A misalignment of the shafts in axial, lateral and angular directions can be compensated by a servo insert coupling as well but it should only be a minimum of misalignment because relatively high reset forces are caused by the coupling which do negatively influence the lifetime of components such as bearings in this environment, the company states.

Typical applications include -

Stepping motors, servo drives, machine tools, CNC machines, wood working and packaging machines, factory automation machinery, printing machines, sheet metal forming machines, industrial robots, textile machines and control and feedback control systems.

www.ondrives.com

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.).

www.victaulic.com

The Lynx’s 20 Hp, high torque motor provides the power for heavy cuts and the speed to produce near-mirror finishes. Its 10″ diameter 3-jaw power chuck turns 3″ bar stock at speeds up to 3,500 rpm, and its recommended price is well below others that do less and cost more.

The Lynx’s bed is a one-piece Meehanite casting to absorb vibration and help dissipate heat, while its heavy ribbing resists twisting and distortion. A 30° slant bed maintains a minimal and constant distance between tool tip and guideway. This maximizes rigidity while eliminating distortion under heavy loads. The headstock is mounted on the same plane as the tailstock to assure perfect alignment and center height regardless of the bed temperature. Widely-spaced linear motion guideways allow high-speed rapid traverses – 945 IPM on the X-axis, and 1,181 on the Z.

Each axis is driven by a large diameter, high precision ball screw that is selected for its outstanding combination of accuracy, rapid traverse speeds, and high feed thrust. The ball screws are mounted directly to the A.C. servo motors. This design minimizes backlash for superior machining accuracy. Ball screws on each axis are protected in case of a crash by an electric torque limiter. Upon impact, the torque limiter senses the load and immediately reverses the servo motor and stops the axis movement.

Lynx’s 12-station turret features an 8.26″ diam. curvic coupling and 11,465 lbs of hydraulic clamp force. This configuration provides high rigidity for superior accuracy and surface finish, long boring bar overhang ratios, and extended tool life. Indexing repeatability is +/-0.0005°. Turret indexing is non-stop and bi-directional, with 0.15 sec. next-station index time. A rotary encoder determines the turret position, and a proximity switch confirms the clamp.

Additional standard features of importance include a waylube separation system, a tool setter that eliminates skim cuts and manually entering tool offsets, automatic forced lubrication, coolant system with a 42 gal. coolant tank, and a part catcher. The controller is a Fanuc 0iTD.

www.doosaninfracore.co.kr

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.