A Brief Analysis of Swing Bending Tests on Robot Flexible Cables

Addtime： 2025-07-05 Readings： Author: admin

The swing bending test is a key method for evaluating the bending resistance of cables. There are two common test methods:

Test 1: Follows the robot special cable test technical specification T/SZRCA 002-202;

The swing bending test project is an experimental project formulated in accordance with the relevant provisions of the Electrical Appliance Safety Law. The test method is shown in Figure 1:

Take a test sample with a length of 1.1m from the finished product. Connect all the core conductors in series. One end is fixed at one end on the swing test machine according to the requirements shown in the figure above, and the other end is suspended with corresponding weights according to the table below. Select an appropriate bending radius, and perform bending tests at a swing angle of ±90 degrees at a constant rate. After swinging to the number of cycles specified by the standard (one round trip counts as one cycle), no short circuits or open circuits shall occur in any core wire of the cable, and no abnormal cracking or damage shall occur in the insulation or sheath. The bending radius, load weight, and rate are shown in Table 1.

Test 2: USB cable plug network tail test method and equipment

Take a test sample of 1.1m in length from the finished product. Connect all the core conductors in series. One end is fixed at one end on the fixed clamping plate of the oscillation test machine according to the requirements shown in the figure below. The other end is suspended with corresponding weights as specified in the table below. The bending test is conducted with a bending radius, using the fixed clamping plate as the bending center, at a swinging angle of ±90 degrees, and at a certain rate. The test is performed until the specified number of cycles is reached as per the standard requirements. (one round trip counts as one cycle). After the test, no short circuits or open circuits shall occur in any of the core wires, and the insulation and sheath shall not exhibit any abnormal cracking or damage.

The two tests exhibit significant differences in equipment details and operational requirements, which directly impact the accuracy, reproducibility, and severity of test results.

Analysis of core differences and their impact on test results:

1. Cable fixation and bending starting point:

Test 1 (standard method): The cable fixation point is located above the swing axis. After the cable is secured by the fixture, the cable remains relatively stationary between the fixed point and the bending radius fixture (typically a replaceable semi-circular/arc-shaped guide wheel or fixture). The actual bending occurs on the curved surface of the bending radius fixture. The fixed point does not participate in the bending motion and is not subjected to bending stress.

Test 2 (Specific Method): The cable fixation point (fixation clamp) serves as the bending center. After the cable is clamped, the bending action occurs directly at the exit edge of the fixation clamp. The fixation point is both the fixation point and the starting point of the bending (stress concentration point). .

Impact on results:

1. Test 1 simulates a scenario where the cable undergoes long-term bending at a point far from the fixed end (e.g., the cable path at a robot's moving joint), with the stress concentration zone located on the bend radius fixture that can be designed.

2. Test 2 simulates a scenario where the cable undergoes bending adjacent to the rigid fixed end (e.g., the connector tail), i.e., the “cable tail” test. The edges of the fixed clamping plate become unchangeable, sharp stress points.

3. In Test 2, due to repeated bending at the root of the cable where it is rigidly clamped, the stress concentration effect is significantly stronger than in Test 1, typically leading to faster cable failure (especially of the conductor), resulting in more severe (or stringent) test outcomes.

II. Test Result Reproducibility (Operator Dependency):

Test 1 (Standard Method): Since the fixed points do not move or bear any force, and the bending radius is ensured by a specialized fixture, the operator only needs to correctly install the cable and fixture. The primary function of the fixture clamping force is to prevent the cable from slipping. As long as the minimum clamping force required to prevent slipping is achieved, the pressure applied to the cable is small and relatively constant. Therefore, test results exhibit high reproducibility regardless of the operator or when the same operator performs multiple tests.

Test 2 (Specific Method): Clamp locking force is the critical variable in the test. To prevent the sample from slipping or being pulled out during testing (especially when the load is heavy), the operator must apply sufficient locking force. However, excessive clamping force can directly compress, flatten, or even break the conductors or insulation layers inside the cable, especially at the edges of the clamping plates (i.e., bending points). The “degree” of clamping force (i.e., “as tight as possible while ensuring no slippage”) highly depends on the operator's experience and judgment.

Impact on results:

The results of Test 2 are significantly influenced by the operator's subjective factors (tightening force control). Different operators, or even the same operator performing different tests, may experience variations in tightening force, leading to fluctuations in test results (failure counts, failure modes) that cannot be reproduced. Test 1 nearly eliminates this human operational influence.

III. Bending stress contact surface and stress distribution:

Test 1 (standard method): Bending stress primarily acts on the entire semi-circular arc surface of the specialized bending radius fixture. This fixture can be selected based on cable diameter to ensure the contact arc surface is sufficiently long (i.e., the bending radius R meets standard requirements). The large contact area results in lower pressure per unit area, leading to relatively uniform and smooth stress distribution.

Test 2 (Specific Method): Bending stress is concentrated at the edge of the fixed clamping plate. This edge is typically a small arc (e.g., a quarter circle) or a sharp angle, with a fixed, non-adjustable bending radius that is often small. The contact area is extremely small (approximating point or short-line contact), resulting in extremely high pressure per unit area and highly concentrated stress.

Impact on results:

1. The stress distribution in Test 1 is closer to the ideal bending state, more accurately reflecting the fatigue resistance of the cable under a gentle bending path, and avoiding atypical failures caused by localized stress concentration (such as the conductor being directly cut by the fixture edge).

2. In Test 2, stress is highly concentrated in an extremely small area at the edges of the fixed clamping plates. Especially when testing cables with larger diameters, higher hardness, or complex structures, this point/line contact stress concentration effect far exceeds real-world application scenarios. It resembles an accelerated stress test or a targeted test for specific stress points (such as cable ends), and its severity may not directly represent the cable's overall performance under standard bending paths. For cables with large cross-sectional areas, Test 2 conditions are too severe and non-representative.

4. Stress Release and Conductor Deformation Mechanism:

Test 1 (Standard Method): When the cable is bent on a bending radius fixture, the tensile or compressive stress on the conductor can be relatively smoothly transmitted, deformed, and released along the conductor's stranding direction over a longer arc surface. During bending, the conductor has space for minor relative slippage to accommodate deformation.

Test 2 (specific method): Since the bending point is rigidly fixed at the clamp exit, when the outer conductor needs to elongate, the elongation deformation is restricted to an extremely short span (near the clamping point), and the conductor cannot effectively release stress through slippage along the stranding direction. It is forced to undergo severe localized tensile deformation at the edge of the fixed point (“hard drawing”). The inner conductor is subjected to severe compression. This repeated, severe localized plastic deformation is highly likely to cause rapid fatigue fracture of the conductor, with the fracture point typically occurring at the edge of the clamp.

Impact on results:

1. The failure mode in Test 1 typically aligns more closely with conductor fatigue fracture caused by long-term flexible bending (fracture surfaces exhibit neckdown and fatigue cracks) or fatigue cracking of insulation/sheath materials.

2. Test 2 is highly likely to cause severe localized tensile deformation, flattening, and ultimately fracture of the conductor at the clamp edge. This fracture mode may exhibit cutting or shearing characteristics (related to the shape of the clamp edge), differing from the fatigue fracture mode under actual gradual bending. It focuses more on evaluating the cable's resistance to extreme stress at fixed points.

Test objectives and practical application scenarios.

Therefore, the choice of swing test method should be based on the test objectives and actual application scenarios.

Test 1 (T/SZRCA 002-2022) is a standard, general method for evaluating the overall bending fatigue resistance of cable bodies. It has good reproducibility and better represents the performance of cables in dynamic bending paths.

Test 2 is more suitable for evaluating the bending resistance of the cable-connector interface (cable end) under severe fixed-point stress conditions. It is more rigorous but results are highly influenced by operational factors, and may be overly stringent or non-representative for cables with large cross-sectional areas.

The test results from the two methods are not directly comparable.

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