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Extensive transmission powerpath analysis and design capabilities have been developed at Southwest Research Institute (SwRI). Transmission powerpath technology is complemented by testing of complete transmissions as well as their major components through the small subcomponent level.

Additionally, numerous analytical design and modeling tools are used to further expand transmission performance projections. These capabilities apply to both manual and automatic transmissions and transaxles. The majority of this work and the discussion herein applies to automatic transmissions.

We are continuously evaluating various transmissions and is improving on the technologies involved in analyzing transmission powerpath variables. All aspects of the powertrain are reviewed and integrated into the SwRI powerpath analysis process.

In addition to being able to perform comprehensive powerpath analysis services, SwRI is a world leader in terms of the number of transmission tests conducted and the broad scope of transmissions that have been analyzed. This testing experience allows us to provide a significant amount of additional insight and engineering content into its analysis and design.

Our experience allows for recommendations to be made to the manufacturer that dramatically improve the performance and efficiency of their transmission. SwRI evaluates and improves the following aspects of transmission design and configuration during its powerpath analysis programs:

• Rotating inertia values of transmission components

• Absolute and relative speeds of transmission components

• Type, size, and preload of bearings within the transmission

• Type and size of shaft seals within the transmission

• Number and location of planetary gearsets within the transmission

• Use of overrunning clutches

• Performance of transmission during shifting—synchronous vs. non-synchronous shifts

• Use of clutches vs. band brakes

• Clutch pack quantity

• Clutch pack clearance

• Clutch pack size and groove pattern design

• Gear tooth size and shape

• Shaft size and stiffness

• Case stiffness because of ribbing and other design features

• Torsional wrap-up of shafts during loading

• Use of noise abatement procedures in gear tooth design, shaft location, case design and bearing support systems

Rotating Inertial Values

Engineers perform detailed inspections, measurements, and analyses for all rotating components within the transmission. A transmission is disassembled, and the various rotating components are separated by respective speeds. Most of the component inertia values are precisely measured. Some of the component inertias, however, are analytically determined. This procedure requires a complete analysis of the rotational speeds of the various components.

The combination of inertial properties and the rotating speeds of the components are input into a computer model that provides an output of the various combinations of component inertia as well as the complete transmission rotating inertia value for each driving gear and neutral. Quantifying these rotating inertias is critical for the proper modeling of both the vehicle and the transmission.

Manufacturers strive to minimize the rotating inertia of the transmission components to impart a lively performance feel to the driver. This improvement in performance also results in improved vehicle fuel economy. An even bigger concern to transmission designers now is the improvement in transmission shift feel. By minimizing and or optimizing the transmission inertia, the transmission designer can significantly improve transmission shift feel.

Bearing Effects

Engineers use internally developed computer modeling techniques to evaluate the effects of different types of bearing systems. These effects vary from simply determining the type of transmission efficiency losses that exist because of the bearings, the effects of these bearings on the life of the transmission, or the effect that the bearings support system has on the mechanical integrity of the transmission. Finite element models are used to determine both the stiffness requirements and the potential effects on transmitted noise from the bearings.

Shaft Seals

We evaluate the type of seals used in the transmission with a focus primarily on the effect of the seal on transmission spin losses. SwRI has a comprehensive database of seal drag losses versus seal type that can help the transmission designer choose the right seal for each application.

Planetary Gearset Design

Engineers routinely analyze the effect of the planetary designs and location on the transmission efficiency as well as the total vehicle performance. SwRI has found that many transmission manufacturers use more planetary gearsets than necessary or may make compromises on the location of these gearsets that have significant effects on transmission performance.

It has been observed that the location and type of planetary gearsets drastically reduce the efficiency of the transmission. In addition, the location of these gearsets may cause the transmission manufacturer to use excessively large gears, clutches, or numbers of clutch plates to transmit the required torque load.

The tooth counts observed and the resultant gear ratios may significantly reduce vehicle performance and fuel economy. We have developed computer models that can predict vehicle performance and fuel economy using various combinations of planetary gears ratios.

Overrunning Clutches

An important element in transmission design is the use of overrunning clutches, which are often required to obtain various combinations of gears from a gearset. For instance, an overrunning clutch may hold during application of reverse gear and allow overrunning in first gear. Using an overrunning clutch instead of a band or clutch brake may significantly reduce the amount of transmission efficiency losses.

Overrunning clutches are also used extensively by sophisticated transmission designers to achieve a smoother nonsynchronous shift sequence rather than requiring multiple clutch pack engagements (synchronous shift) to obtain a gear shift.

The proper use of overrunning clutches can significantly improve the customer's perceived shift feel. Engineers routinely evaluate the use of overrunning clutches in the transmission to provide the transmission manufacturer with an improved transmission product.

Clutches Versus Bands

SwRI has significant experience in evaluating the differences between the performance of a clutch and band brake in a particular application. Manufacturers are designing more and more bandless transmission configurations to improve transmission shift feel. SwRI has shown that this often results in significantly increased transmission efficiency losses. SwRI has suggested to transmission manufacturers that they consider using a band for the application of gears where shift feel is not as critical or noticeable, such as for first or reverse gear.

Clutch Pack Variables

We routinely provide transmission manufacturers with an analysis of their product in terms of its clutch pack configuration and can compare this configuration with the industry state-of-the-art designs. SwRI is an industry leader in the evaluation of clutch pack components and has developed this capability based on empirical and analytical procedures. SwRI pioneered and developed a revolutionary new test stand and procedure for testing clutch packs under conditions that accurately simulate vehicle operating conditions.

In addition, SwRI has developed a dynamic clutch pack modeling computer program that evaluates most clutch pack design variables available to the transmission designer. This program can perform numerous iterations of various clutch packs for variables such as the number of clutch disks, groove pattern, pack drainage criteria, pack clearance, clutch lining material and clutch engagement pressures and cycles to determine the efficiency losses of the clutch while disengaged. This model also has the capability to dynamically engage the clutch and evaluate factors such as apply time, facing temperature, and more important, the rate of change of acceleration, known as "jerk," felt by the vehicle driver.

Gear Tooth Noise Considerations

Engineers evaluate transmission gear tooth design considerations such as torque carrying capacity, windage, torque losses, and noise abatement procedures. SwRI has developed noise control procedures on gear teeth and has run noise evaluation tests on both manual and automatic transmissions. By using an order tracking method of noise analysis, we are able to isolate and determine if a particular gear may be a high contributor to noise within the transmission.

SwRI has recommended using gear noise reduction procedures to transmission manufacturers. Finite element analysis as well as AGMA analysis procedures are used to determine the performance characteristics of transmission gear teeth.

Transmission Component Stiffness

We have developed test procedures for evaluating the relative stiffness of numerous components within the transmission. These procedures complement the inspection and design criteria obtained from various transmission components. SwRI has found that some manufacturers use a very robust transmission design to improve both the life and efficiency of the transmission under high load conditions.

SwRI has also found that other transmission manufacturers use relatively light components to minimize transmission losses under primarily lightly loaded conditions. Shaft size, gear size, and transmission case design features can result in significant changes in performance of the transmission during operation.

Finite element modeling is used to accurately predict and determine the effects of transmission shaft design and its surrounding components. The design and physical properties of these components is also critically analyzed to determine their effect on transmission noise generation.

Future Directions for Powerpath Analysis and Design

SwRI is aggressively pursuing its own internally conceived and developed comprehensive powerpath modeling tool called TransCAD. This stand-alone graphical user interface, computer modeling program allows the transmission designer to make important powerpath decisions early in the design phase and provides a means to quickly run numerous transmission design iterations.

For the designer, powerpath variables such as drive line kinematics, gear ratios, inertias, torsional stiffness and transmission efficiency losses can be quickly evaluated and optimized. The model will also provide data about the torque carrying capacity of the transmission and can significantly help the transmission designer to design in improved shift feel.

The ability to design shift feel characteristics will drastically reduce the amount of time and cost that will be required to bring a transmission to market. Designed-in shift feel will reduce the time and amount of effort necessary to perform expensive and difficult on-the-road transmission shift calibration procedures. This tool has the capability to revolutionize the design of transmissions in the future by allowing a family of transmissions to be optimally designed for specific applications while using basic proven transmission building blocks.

For more information about our transmission powerpath analysis and design capabilities, or how you can contract with SwRI, please contact Douglas Fussner at dfussner@swri.org or (210) 522-3972. 

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Southwest Research Institute^® (SwRI^®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 11 technical divisions.

December 28, 2012