Transmission Sensitivity Testing

Southwest Research Institute (SwRI) operates a worldclass transmission test facility providing uniquely accurate measurements to identify variations in transmission performance. Engineers need this information to optimize transmission configurations to improve performance. To date, SwRI has tested 60 transmissions and transaxles, amassing the largest transmission efficiency database in the world.

The energy wasted in transmissions varies widely due to manufacturing tolerances and in-service environmental factors. Because torque losses have not been quantified through research, they are poorly understood. Using the detailed information generated by SwRI's precision analytical equipment, manufacturers better understand the variability inherent to their product and what factors can be better controlled to create a more uniform product.

With extremely sensitive measuring equipment and specially designed test stands, SwRI offers precision transmission testing not previously possible. These tests identify critical production and environmental factors that can be improved, refined, or accommodated.

SwRI evaluates transmission components and assemblies, including:

• Pumps
  • Design
  • Variability
  • Cold operations
    
    
• Gearboxes
  • Friction material
  • Square seals
  • Fluid evaluations
    
    
• Torque converters
  • Variability
  • Vibration dampers
    
    
• Transmissions
  • Variability
  • Temperature gradients
  • Noise

Pumps

Design

Pumping systems consume disproportionately high levels of energy within the transmission. To support pump design optimization programs, SwRI quantifies the performance of fixed and variable displacement pumps, such as:

• Internal-external
• Gerotor
• Hypocycloid
• Duocentric
• External-external
• Vane

Evaluations quantify pump:

• Torque
• Delivery
• Volumetric efficiency
• Mechanical efficiency
• Overall efficiency

Variability

Variability in the pump dimensions greatly affect pumping torque and discharge flow. For example, leakage flow and internal friction are functions of gear and housing clearances. However, improvements in leakage flow and internal friction are diametrically opposed. Determining an optimal value and its corresponding performance tradeoff is critical to understanding the ramifications of production process variability. SwRI conducts controlled testing to isolate and quantify the dependent relationship between various pump parameters.

Cold Operations

Pumping automatic transmission fluids at temperatures below 0 degree F requires special cooling systems, as fluid viscosity increases exponentially as temperatures approach -40 degrees F. SwRI tests pump performance at low temperatures using specially designed test stands that eliminate problems that occur during cold testing, especially when evaluating the pump outside the transmission case.

SwRI cold tests transmission pumps at -40 degrees F to study problems that can occur during start-up, including erratic or interrupted flow and inadequate bearing lubrication.

Torque Converters

Variability

Manufacturing process control is particularly important for torque converters, which have more than 100 parts. Performance variations are associated with:

• Blade angle
• Blade thickness
• Blade profile
• Blade attachment
• Overall housing length

Using precision instrumentation, SwRI tests the variability associated with converter:

• Torque ratio
• K-factors
• Efficiency
• Phase change point

Using statistical techniques, the Institute designs experiments to extract high quality data about performance variability from the smallest possible experiment run and sample size, reducing research time and costs.

Because they contain numerous parts, torque converters are particularly susceptible to slight production variances. Using statistically sound experiments, SwRI collects data about performance variability associated with torque converters.

Vibration Dampers

Reciprocating engines produce torsional fluctuations that propagate through the vehicle frame, producing unacceptable vibrations. To supplement torsional damping effects, manufacturers mount compression springs in the lockup clutch to reduce these undesirable vibrations. Using special test rigs, SwRI quantifies deflection characteristics and hysteresis effects associated with these springs and determines static and dynamic lockup clutch breakaway torques.

Gearboxes

Friction Materials

Wasted energy associated with clutch pack drag is affected by friction material configuration, fluid temperature, and running clearance. For example, torque losses associated with some grooved friction materials are relatively unaffected by speed while smooth friction material torque losses are highly dependent on speed.

To investigate these phenomena, SwRI configured a test stand to provide independent control of any parameter associated with the operation of a clutch pack, such as clearance, fluid temperature, fluid pressure, fluid flow, differential input-output speeds, and rotation directions. To meet the high-speed requirements of future clutch packs, the stand is equipped with motors capable of 15,000 rpm operation with accuracies of ±1 in-lb, a capability unmatched by any other known facility in the world.

Square Seals

To determine drag losses associated with transmission square seals, SwRI tests organic and ferritic seal materials at various temperatures and pressures. Designed to simultaneously operate a large number of seals, the test stand determines both out-of-range and average values.

Fluid Evaluations

As manufacturers increase transmission efficiency, parallel efforts seek to improve efficiency by modifying the viscosity of automatic transmission fluids to reduce parasitic drag associated with rotating components. SwRI evaluates different fluids through a temperature range of -40 degrees F to 250 degrees F. Testing in this dynamic range allows SwRI engineers to collect important data on component and assembly performance. Fluid interactions are quantified under extreme conditions to determine deterioration in transmission efficiency.

To increase transmission efficiency, SwRI evaluates the viscosity effects of transmission fluids over temperatures ranging from -40 degrees to 250 degrees F.

Transmissions

Production Variability

Automatic transmissions contain upwards of 600 components. The sheer number of parts, and their associated tolerances, can produce significant performance variations between transmissions produced on the same assembly line. Using test stands equipped with precision in-line torque transducers, SwRI measures torque losses to ±1 in-lb when spinning the transmission under no-load conditions and under loads across the entire operating range.

Temperature Gradients

To better understand transmission performance, SwRI measures thermal gradients in more than 20 axial and radial positions around the transaxle case. The transmission is operated at specific steady-state torque and speed conditions while incrementally increasing sump temperatures from ambient temperatures to 250 degrees F. By measuring input and output speeds and torques, SwRI can determine efficiency values and trends. Temperature gradients in the transaxle can dramatically affect performance and shift quality. The high-temperature measurements provide information about specific component deterioration.

Noise

Noise and vibration are subjective factors consumers evaluate when buying a car. Transmission noise -- gear whine, fluid pumping, and cavitation -- is particularly troublesome because the transmission case acts as a resonator, amplifying the sound. The Institute conducts sound testing on individual operating components and complete transmission assemblies by positioning microphones in various locations. At steady-state conditions, engineers subtract the background noise to isolate the test article sound values. The noise is measure in A-rating or 1/3 octave scales, among others. Frequencies are analyzed to determine whether the noise is speed dependent or structure borne.

To help manufacturers reduce noise associated with powertrains, SwRI isolates the noise associated with various components and determines if the noise is structure borne or speed dependent.

This brochure was published in January 1996. For more information about transmission sensitivity testing, contact Douglas Fussner, Phone (210) 522-3972, Engine, Emissions and Vehicle Research Division, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510.

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