Agricultural tractors. Test procedures - Rear three-point linkage lifting capacity
1Key Takeaways
1 Scope This document specifies test procedures for determining the lifting capacity of rear-mounted three-point linkage systems: a) the maximum vertical force which can be exerted by the hydraulic lift at the lower hitch points throughout their full range of movement; …
2Expert Interpretation
This article provides an in-depth analysis of the BS ISO 789-2:2018 standard for the test method for the lifting capacity of rear three-point hitch systems on agricultural tractors. It covers key technical points such as test conditions, measurement accuracy requirements, and hydraulic system performance evaluation, helping manufacturers and testing agencies accurately implement standardized testing procedures.
Analysis of the core content of the standard
BS ISO 789-2:2018 is the UK's national standard transformation of ISO 789-2:2018, replacing the 1983 edition of BS 6347-2. This standard specifically targets two lift capacity test methods for the rear three-point hitch system of agricultural tractors:
| Test type | Measuring point | Key parameters |
|---|---|---|
| Lower hitch point lift | Center of horizontal connecting rod | ±1% force accuracy |
| Frame coupling lift | 610mm behind the hitch point | 90% safety valve pressure |
Technical evolution and test requirements
Compared with the third edition in 1993, the main technical changes in the fourth edition include:
- The number of test points has been increased from 3 to 6 evenly spaced measurement points.
- A new requirement for hydraulic oil temperature control (65±5°C) has been added.
- The test report is now required to include a rear axle torque calculation.
Typical Application Case
During testing on a certain model of tractor, it was found that when the suspension frame was raised to its highest position, the hydraulic system pressure dropped by 12%. Using the standard's 90% relief valve pressure correction formula, the final measured effective lifting force was 23.5kN, meeting the ISO 730 standard for Category 3 suspension.
Implementation Recommendations
- The run-in run specified by the manufacturer must be completed before testing (Clause 5.2.1)
- The viscosity of the hydraulic oil should be classified and labeled in accordance with ISO 3448:1992
- The ambient temperature should be controlled within the range of 16-30°C (Clause 5.7.2)
Standard Limitations
The standard specifically emphasizes that the test results are only used for tractor performance comparison and cannot be used as a basis for recommending the size of matching implements (1 Scope Note 1). When the tractor cannot reach the power range specified in ISO 730, this should be clearly stated in the report (Clause 6.1.2.2).