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Measuring Transfer Case Spin Loss: Practical Application of SAE J2985
The SAE J2985 recommended practice provides a standardized method for measuring speed-dependent parasitic loss (spin loss) in transfer cases used in passenger cars and light trucks. With increasingly stringent fuel economy regulations, accurate component-level data is essential for vehicle-level modeling and analysis. This article covers key elements of the standard, including the critical metric of Average Skin Temperature (AST), test conditions, and common pitfalls to avoid.
The Foundation: Average Skin Temperature (AST) and Thermocouple Placement
AST is the arithmetic average of seven strategically placed soldered-tip thermocouples adhered with thermally conductive adhesive. This metric was validated through internal/external thermocouple studies to accurately represent the core temperature of the unit under test (UUT).
| Thermocouple | Location Description |
|---|---|
| 1–4 | As close as possible to the outer race of input and output shaft support bearings (4 total) |
| 5 | Near the vent, but not inside it |
| 6 | Top of transfer case housing, center of main input shaft |
| 7 | Top of case above torque multiplying gear set (multi-speed); for single-speed, near the transfer gear driving the front output |
🔍 Engineering Insight: Using soldered-tip thermocouples with thermally conductive adhesive ensures readings truly represent the metal temperature, not just surrounding air. This method was developed from extensive internal/external thermocouple array studies and provides a robust measurement of core temperature.
Controlling Test Conditions: Soak, Chamber, and Stabilization
Soak Time: Achieve thermal equilibrium with an initial 4-hour pre-soak at the target AST, followed by a 45-minute additional soak before data acquisition. Between runs, bring the AST back within ±1°C of the target with all individual thermocouple readings within ±2.5°C, then hold for another 45 minutes.
Temperature-Controlled Chamber: The chamber must achieve an AST range of –7°C to +65°C ±1°C using convective conditioning without directly warming or cooling the oil. Avoid localized hot or cold spots (e.g., a heat gun blowing directly on the UUT). Liquid nitrogen can be used for low temperatures but must never contact the UUT directly.
Stabilization Time: At each speed step, hold the input speed constant long enough to eliminate inertia influence but not so long that temperature rises. Typically, trim 0.2 seconds before and after the speed change, then average torque over a 5-second window.
⚠️ Common Mistake: Insufficient soak time is a frequent error. Ensure the UUT reaches complete thermal equilibrium before testing. Also verify that all seven thermocouple locations are within ±2.5°C of the target AST before starting a run.
Frequently Asked Questions
Why is AST used instead of a single temperature point?
AST accounts for temperature gradients across the housing and provides a repeatable, robust representation of the core temperature, as confirmed by internal/external thermocouple studies.
Can I use a standard oven or freezer as the temperature chamber?
Not directly. The chamber must condition by convection without directly conditioning the oil and without creating hot/cold spots. Direct impingement from a heat gun or cooling nozzle can lead to inaccurate AST readings.
What is the purpose of stabilization time?
It ensures that torque measurements are free from inertial effects due to speed changes. The control capability of the dynamometer influences the required time, but the goal is a steady-state measurement without thermal drift.
How is sump temperature used in the test?
Sump temperature is recorded separately (via a thermocouple in the drain plug) and used alongside AST to fully characterize the thermal state, but it is not part of the AST calculation.
