Agricultural tractors — Test procedures — Part 6: Centre of gravity
1Key Takeaways
This document specifies a method of determining the position of the centre of gravity of agricultural tractors. The method is applicable to agricultural tractors having at least two axles fitted with wheels or tracks.
2Expert Interpretation
This article provides an in-depth analysis of the technical requirements for center of gravity testing for agricultural tractors in the ISO 789-6:2019 international standard, including core content such as test equipment specifications, three-dimensional coordinate measurement methods, and suspension system locking requirements. It also provides differentiated testing solutions and implementation recommendations for wheeled and crawler tractors.
Interpretation of the Standard's Core Content
This international standard specifies the method for determining the center of gravity of agricultural tractors. It is applicable to agricultural tractors equipped with at least two wheeled or tracked axles. This standard uses the suspension-ground reaction method to establish a three-dimensional coordinate system (longitudinal x-coordinate, vertical h-coordinate, and horizontal y-coordinate) to accurately locate the center of gravity.
Test Equipment and Preparation Requirements
| Equipment Type | Accuracy Requirements | Special Instructions |
|---|---|---|
| Scale/Load Sensor | ±0.5% | Pre-calibration Required |
| Crane | Safety Load ≥1.5 Times Vehicle Weight | Suspension Line Must Be Vertical |
| Platform with Knife Edge | Levelness ≤0.5° | Special for Crawler Tractors |
Implementation Example: When testing a certain wheeled tractor model, the fuel tank must be fully loaded, the hydraulic tank fluid level must be normal, and a 75kg counterweight must be placed under the driver's seat. The suspension system must be locked at normal operating height, and tire pressure must be controlled within ±5% of the manufacturer's recommended value.
Comparison of Three-Dimensional Coordinate Measurement Methods
| Coordinate Dimensions | Wheeled Tractor | Crawler Tractor | Calculation Formula |
|---|---|---|---|
| Longitudinal (x) | Direct Measurement of Axle Load | Requires Knife-Edge Platform | x = d·F₂/m |
| Vertical (h) | Measurement at an Incline of 15-25° | Contact Grouser Positioning | h = (F₃·d)/(m·tanθ) |
| Lateral (y) | Left and right wheel load difference method | Track gauge as the lever arm | y = dₜ/2 - (F₅·dₜ)/m |
Technological evolution and implementation recommendations
Compared with the 1982 version of the standard, the main updates of the 2019 version include: New suspension system locking requirements, Supplementary vertical coordinate alternative measurement method (Clause 6.3.8), and Improvement of the term definition system, etc. Special attention should be paid during implementation:
- Crawler tractors must ensure that the grounding teeth are located on the contact line BB'
- The tilt test angle must not be less than 15°
- Tables 1-4 added in the new version must be used for calculations using the substitution method
Typical problem: When a laboratory was testing an articulated tractor, it was found that simultaneous measurement of the front and rear axles would lead to data deviation. According to clause 5.5 of the standard, the articulated joint should be locked in a straight position or tested in sections at a specific angle.