High Torque Motor Turns At Speeds Up To 3,500 RPM

Today’s turning centers can be economical without sacrificing power or precision. Demonstrating this is the Lynx 300, a new turning center from Doosan that’s just now entering the United States.

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

Torque Limiters that Keep Overload in Check for Heavy Equipment

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.

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.

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.

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.

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

www.ringfeder.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

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

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.)

Pneumatic Torque Limiter Provides Overload Protection

Nexen announced their TL Series pneumatically engaged, single-position torque limiters, delivering overload protection for industrial machinery. The TL Series uses a ball/detent interface and proximity sensor to immediately disengage the machine shaft when excessive torque or a machine jam occurs, effectively protecting downstream equipment and product from damage and decreasing downtime. Upon detecting the overload condition, the sensor instantly sends a signal to the torque limiter’s control valve, exhausting the air and disengaging the unit for a clean disconnect of power to the driven components.

By utilizing pneumatic actuation, TL Series units facilitate remote trip-out torque adjustment via an air regulator, allowing operators to optimize overload protection while the machine is in use-thus eliminating the need for inconvenient onsite adjustments. Each torque limiter’s interface has five ball/detents arranged in an asymmetrical pattern, assuring each engagement occurs in the same position and affirming machine components are accurately synchronized. The unique hard-chromed detent interface decreases drive-ring wear when the balls are pressed against the face during jog-to-position engagement, extending operational life as well as ensuring complete disengagement every time-without premature re-engagement.

Semi-open and totally enclosed, nickel-plated TL Series units are available. Semi-open units are sealed to keep contaminants out of the ball/detent area, while enclosed units are designed to protect the ball/detent and spline areas in washdown or other manufacturing environments where liquids are present. A variety of ball/detent sizes are available to provide a range of torque capacities, making the TL Series well suited for a broad application base, including packaging machinery, food processing, bottling, material handling and newspaper presses.

Several additional features of the TL Series ensure its long-lasting, dependable operation:

• The air chamber is sealed with o-rings to eliminate air leakage and minimize repairs, with backup rings to prevent o-ring dislocation, decreasing wear and extending the component’s operational life.

• Internal springs separate the ball and detent interfaces, preventing the ball from being forcefully removed from the detent-extending life and eliminating detent distortion, as well as preventing the torque limiter from re-engaging until the machine stops.

• The proximity sensor features an LED that illuminates to indicate when the torque limiter is engaged.

• Single-flex or double-flex couplings are available to deliver high shaft misalignment protection with zero backlash and excellent torsional rigidity.

http://www.nexengroup.com/nexen/index.jsp

Servo Torque Limiters

Protection of an electromechanical system from torque overload is often assumed to be guaranteed by the current limits set in the drive parameters. But in the case of hard stops, impacts and other situations where overload occurs very rapidly, sufficient energy to do damage often exists in the rotating inertia driving the load.

R+W servo-rated torque limiters are accurate to +/-5% release torque and positively disengage the motor or gearbox shaft in 1-3 milliseconds. Patented pre-loads and a specially designed spring system remove backlash and guarantee a very low profile for higher release torques.

Slip Systems for Wind Turbine Generator Protection

CENTA produces an enhanced version of its Torque Hub, the slip clutch which offers a simple and low-cost design for torque overload generator protection.

In wind power plants, electrical circuit problems are known to cause short-term torque peaks. In this event, slip clutches protect the costly gearbox against overload by slipping at a defined maximum torque in order to temporarily interrupt the drive.

The slip process takes place not on the generator shaft surface, but inside the CENTA Torque Hub, which is mounted on the generator shaft in the coupling’s clamping set.  CENTA’s latest development in slip systems-”CENTA Torque Set”-now positions the slip unit to the middle section of the shaft. By relocating the slip function, manufacturing tolerances at the generator shaft no longer cause variations in the slip torque, resulting in improved accuracy of the slip function.

Another advantage of the new design is that the slip system is made to be pre-mounted as a complete unit. The maximum torque is set on a certified test bench according to the manufacturer’s requirements and documented in a test report, eliminating the need for on-site adjustments.  The low-maintenance slip system is also designed to allow for multiple slip processes (> 200) without having a major impact on the slip torque setting. Depending on the size, a torque range of up to 120 kNm is covered. The new slip system can also be provided as a low-cost system component which does not include the coupling unit.

Servo Torque Limiter from R+W America

Protection of an electromechanical system from torque overload is often assumed to be guaranteed by the current limits set in the drive parameters. But in the case of hard stops, impacts and other situations where overload occurs very rapidly, sufficient energy to do damage often exists in the rotating inertia driving the load.

R+W servo-rated torque limiters are accurate to +/-5% release torque and positively disengage the motor or gearbox shaft in 1-3 milliseconds. Patented pre-loads and a specially designed spring system remove backlash and guarantee a very low profile for higher release torques.

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

Next Page »