Dynamic gear tester

 

A new gear tester from Promess Inc., Brighton, Mich. (promessinc.com), measures tooth-to-tooth contact variations in both directions at each degree of rotation, and can take 10,000 samples/sec.

This lets it monitor and control torque-to-turn requirements of the assembly being tested. The device can also be used to monitor and apply axial loads.

The test station uses high-resolution angular encoders on the input and output shafts to measure angular position. Both shafts are also driven by a torque functional tester with built-in torque sensors. The input shaft puts torque on the assembly being tested while the output shaft applies braking torque. The tester also measures backlash by reversing the direction of torque being applied by both shafts and measuring the resulting displacement. Software collects and analyzes data, including minimum, average, and maximum backlash, and puts it in graphical form.

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Compression ignition comes to gas engines

What many consider the Holy Grail of combustion technology is one step closer to prime time.

In the race to field energy-efficient vehicles, don’t count out internal combustion technology just yet. GM recently debuted its homogeneous-charge-compression-ignition (HCCI) system in two drivable concept cars, a production-based 2007 Saturn Aura and Opel Vectra.

“HCCI was a dream of engine designers when I was an engineering student years ago,” says Tom Stephens, group vice president, GM Powertrain and Quality. “Today, using mathbased predictive analysis and other tools, we are beginning to make this technology real.“

HCCI is the capstone of an integrated suite of engine technologies that includes central direct-fuel injection, variable-valve lift, mechanical camshaft phasing, and individual cylinder pressure sensing. HCCI engines are said to use 15% less gas than conventional port-fuel injected engines, and meet current emissions standards.

Unlike spark-ignition gas engines or compression-ignition diesel engines that have a combustion process characterized by growth of a flame front from a single point in the combustion chamber, HCCI produces a flameless, simultaneous release of energy throughout the entire combustion chamber. Lack of a flame and hot zones lowers combustion temperature and NOx emissions.

Fuel-air mixtures are comparatively lean, which helps the engine approach the efficiency of a diesel, but without the need for costly lean-NOx after-treatment systems. Burning less fuel at lower temperatures also cuts the amount of heat energy lost during combustion, boosting efficiency. HCCI engines have a compression ratio of 12:1 (similar to that of a conventional direct-injected gas engine), so they can run on regular pump gas and E85. GM says HCCI engines will cost less to build than diesels because the latter need stronger components to withstand compression ratios greater than 20:1.

“Perhaps the biggest challenge of HCCI is controlling the combustion process,” says Uwe Grebe, executive director for GM Powertrain Advanced Engineering. “With spark ignition, you can adjust the timing and intensity of the spark. But with HCCI’s flameless combustion, you must change the mixture composition and temperature in a complex and timely manner to get comparable performance.”

Having ample heat in the combustion chambers is key to making HCCI work. The engines use a conventional spark ignition for cold starts and when HCCI is disengaged. Fuel comes from conventional injectors located in the center of each combustion chamber. A controller uses special algorithms and feedback from the cylinder pressure sensors to adjust cam timing and fuel injection in the milliseconds between combustion events. “Going to HCCI mode from conventional spark ignition signals the fast-mechanical cam phasers and a variable-valve-lift mechanism to close the exhaust valves early in the exhaust stroke, trapping some of the hot residual combustion gases in the combustion chamber,” explains GM Global HCCI Program Manager Matthias Alt. “This helps maintain a high cylinder temperature to facilitate auto-ignition when the fresh airfuel charge is added next cycle.” Operation at cold ambient temperatures necessitates trapping more hot gas in the combustion chamber (earlier exhaust-valve closing), for example.

Currently, the GM demonstration prototypes can run in HCCI mode to about 55 mph, going to spark ignition at higher vehicle speeds and under heavy engine load. A goal of the program is to extend HCCI’s operating envelope through refinements to the control system and engine hardware. GM says HCCI will work on any gasoline engine in its inventory and could combine with hybrid technology. No release date has been set for production HCCI-engine cars.

 HCCI first drive

Machine Design Editors Lawrence Kren and Robert Repas recently drove an HCCI-engine Opel Vectra and Saturn Aura at GM’s Milford Proving Grounds in Milford, Mich. Driving the cars at modest speeds and accelerations automatically engages HCCI mode, accompanied by a diesel-like clatter from the engine. The clatter was less pronounced in the Opel Vectra, however. GM engineers credit a special diesel noise-abatement package in the European-spec Vectra with the lower cockpit sound levels. The package includes an insulated engine cover and additional firewall soundproofing. Transitions to HCCI mode from conventional spark ignition were abrupt and gave a noticeable shudder. GM says such transitions will be imperceptible in production vehicles, similar to the deactivation performance of the company’s production Active Fuel Management system. AFM in GM V8s runs the engines on four cylinders under low loads to save fuel.

TwinForce squeezes V8 performance from a V6

Direct fuel injection and turbocharging have long been used in diesel engines for power and fuel economy.

Direct fuel injection and turbocharging have long been used in diesel engines for power and fuel economy. Ford Motor Co., Dearborn, Mich. (ford.com), is now taking that concept, calling it TwinForce, and using it in various consumer cars and trucks. Vehicles with a TwinForce Duratec 35 V6 engine should have 415 hp and 400 lb-ft of torque, numbers usually associated with a 6-liter V8. The TwinForce V6 would also get 15% better fuel economy than a similarly powered V8, according to Ford.

Compared to conventional port-fuel injection, TwinForce direct injection more precisely controls how much and when gas is sent into the cylinders. This means combustion is more efficient. Meanwhile, twin turbochargers deliver more air to the cylinders and boost power. It also reduces emissions and the engine can burn E-85 ethanol. The Ford says its new technology will be on future Ford and Lincoln vehicles.

Fundamentals of ultraprecision machining

 

Ultraprecision machining (UPM) comes from the optics industry so not many designers are familiar with the process. However, the technology has the potential to revolutionize the way manufacturers, in general, finish parts or make fine-featured patterns.

First, recall that “high precision” in traditional machining generally refers to tolerances in the single-digit micron range. In inch units, machinists talk about holding “tenths” (ten-thousandths of an inch, or 0.0001in.). And the best conventional machining and grinding machines typically get Ra values no better than 0.1μm. In contrast, ultraprecision machining provides accuracy an order of magnitude better by holding submicron tolerances. UPM also obtains surface finish Ra values better than 0.5nm. When using diamond tooling on nonferrous materials, UPM produces yet more-impressively smooth finishes. Ametek Precitech, in Keene, N.H., manufactures UPM equipment and has provided the benchmark for this technology. Precitech’s machine layout resembles that of standard equipment, but the details make all the difference. Programming input resolution, the precision level of the machine inputs, is 0.01nm for linear and 0.026arc-sec for angular position. Work piece spindle speeds hit 18,000rpm and milling spindles rotate at 15,000 or 50,000rpm. Work piece positional accuracies of 1micron linear and ±2arc-sec are standard and — because these errors are repeatable — software compensation can be used to reduce them by a factor of 10.

CENTRIFUGAL CASTINGS

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A NEW ERA FOR NATURAL GAS TURBINE

     Many older, decommissioned coal power plants are being replaced with highly efficient natural gas turbines. In some places where fuel is imported, such as in Europe and Asia, highly efficient turbines are critical for power plants to continue operations because of high fuel costs. In the U.S., where natural gas prices are lower, highly efficient natural gas turbines are not only being selected as replacements for their cost savings, but also for their ability to meet stricter ambient emission regulations.

     Twenty years ago, land-based electrical power gas turbine (GT) efficiencies crept upward from 15%, as E-class turbines matured into F-class. A decade ago, turbines were approaching combined-cycle efficiencies of 50%. The large, high-capacity G, H, and J class electrical GTs of today produce hundreds of megawatts of power. Used in combined-cycle power plants along with heat-recovery steam generators and steam turbines, they can achieve greater than 60% combined-cycle efficiency.

     Large GT manufacturers have taken different approaches to increasing efficiency, including increasing overall mass flow, increasing compressor pressure ratio, and increasing the temperature of the air entering the turbine. The latter, increasing the firing temperature, is the most common and most technically challenging.

Hotter Is Better

     As the temperature of the air entering the turbine has increased to be higher than the melting point of some of the materials that come in contact with hot gases inside the turbine, manufacturers have not only had to develop specialty turbine blade materials, but also new blade designs, coatings, and cooling technologies specifically to enable robust, reliable operations at these “ultra-high” temperatures.

     According to Carlos Koeneke, vice president of project engineering and quality assurance at Mitsubishi Heavy Industries, many of these types of advancements have led the way for operation at turbine inlet temperatures of 1,600 degrees C, achieved during the last year.

     “The biggest challenges with increasing the temperature were the materials exposed to the hot gases, which have historically limited the temperature in the past," he says. "We’re getting to the point where we can’t further improve the materials, and are now using thermal barrier coatings as facilitators. Also, internal cooling is now needed, which ‘steals’ some energy from the system but enables operatio.”

     As a result of higher firing temperatures, mega turbines not only produce higher amounts of power, but higher power density as well, so fewer turbines are needed to generate the same amount of power. On a dollar-per-kilowatt/hour basis, the price is lower for mega turbines, since power plants actually spend less in construction, footprint, and installation.

Increased Flexibility

     Despite having high efficiency, these new GTs in combined-cycle power plants are challenged when it comes to operating efficiently under variable loads. Flexibility, which can refer to the ability to start and stop frequently or rapidly, is becoming more important as more generating plants receive power from sources such as wind and solar.

     Achieving high efficiency in a combined-cycle plant requires operation at constant base loads or at “full load,” or peak operating conditions. “It’s not that long times are needed to achieve high efficiency, but they don’t react well to big changes,” Koeneke says.

     Single-cycle GT systems that are faster to react are now able to operate more efficiently when “parked” at lower loads, and are also maintaining emissions compliance over wider load ranges. Some power plants are adopting multiple systems, both combined and single mode, for more flexible operations, since a single system cannot perform well in both modes.

     Efficiency has long been the goal in designing and operating gas turbines. Having achieved greater than 60% efficient emissions-compliant operation, the new-era turbines are entering what could be referred to as “flexible efficiency,” as the focus shifts toward the ability to adjust to new types of demands for efficient operations.

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Vision and Multisensor Inspection Goes Mainstream

                Software advances make vision and multisensor technology an everyday tool for inspecting and analyzing mechanical components.

              Miniaturization and advanced materials are cutting costs and improving the utility of all sorts of mechanical and electromechanical products. Examples include handheld digital devices, medical implants, miniature plastic-gear drives, diesel-fuel injectors, and compressor blades. Manufacturing engineers are thereby looking for ways to measure and analyze such components quickly and accurately during product development and production.

               The exclusive use of contact-inspection systems is no longer an option for many kinds of parts. Conventional CMM probes, for example, even with 1-mm tips, cannot access small blind holes or tiny features. In other instances, complex geometries prevent probes from reaching critical points. In addition, soft, pliable, and dual-durometer materials that easily deform, as well as mirror finishes that may be damaged by contact, also make poor candidates for tactile inspection.

             In the past, when these sorts of components were a rarity, measuring microscopes were a suitable choice. However, the increasing number of parts with small and inaccessible features along with requirements in some industries for 100% inspection have turned microscope inspection into part-validation bottlenecks. 

              Fortunately, current vision and multisensor systems, which might include devices such as microprobes, laser scanners, and chromatic white lights, let users rapidly collect vast amounts of dimensional information for design analyses and subsequent part validation. The systems use CAD-based programming and inspection software to operate in 2, 2.5, and 3D modes, collecting data that is useful not only for validating dimensions, but also for analyzing designs and manufacturing processes.

                During the past five years, inspection-equipment developers have invested a lot of time in developing software that includes proprietary algorithms for accurately capturing images and transforming them into discrete data points that can be automatically compared to nominals in CAD models. These efforts have pushed vision and multisensor equipment onto the shop floor and away from the dedicated inspection laboratory. The advanced systems are as easy to use as a typical CMM.

Algorithms augment optics
                   A big barrier to the primary use of vision and multisensor devices in advanced metrology has been the perception that adjusting systems for appropriate lighting, contrast, and edge-detection sensitivity took specialized knowledge beyond that of average users. While this once may have been true, it is no longer the case. Many powerful new software algorithms effectively automate these important adjustments to provide consistent inspections from part to part and one vision machine to another.

               A legitimate concern has been the subjectivity of making manual adjustments to set contrast. Optimized contrast substantially improves inspection accuracy by improving the vision system’s capability to detect edges and compensate for the tendency of light to bend around the edges of cylindrical surfaces, thereby shortening measured distances. Today, special algorithms automate the adjustment of contrast levels. At the touch of a button, the algorithm makes a series of rapid iterative adjustments until it reaches the best contrast.

                Also, differences in light sources (for example, halogen or LED) used to illuminate parts and ambient lighting in different locations was another source of vision-measurement variability. However, it is now straightforward to correct for these variations. Current inspection software lets users compensate for these effects just as they would calibrate a probe on a CMM.

            Additionally, because camera probes do not touch the edge they are measuring, edge detection must rely on the accurate interpretation of the data the vision software receives from the camera. Advanced vision-inspection software can fine-tune algorithms to account for both the part surface and illumination. This lets the software accurately find each feature edge.

                Generally, inspection software uses a dominant-edge algorithm to select the edge of a part —especially when using a device containing built-in illumination — and this approach works well. But when measuring top-lit parts with a high-surface finish, this method is problematical. In these cases, a specific-edge algorithm is preferable. It detects features of interest based on contrast, shape, and location. Another example: Grind marks on the part might confuse a camera using top lighting. Here, the software might apply another type of algorithm that chooses the most dominant edge out of possible candidates in the camera’s field of view.

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