I. Reliability Metrics
Reliability metrics include MTBF and MTTR. According to industry standards, core stability indicators comprise Mean Time Between Failures (MTBF)—calculated as total uptime divided by the total number of failures—and Mean Time To Repair (MTTR)—calculated as total repair time divided by the total number of failures.
Availability is defined as MTBF divided by the sum of MTBF and MTTR (e.g., an MTBF of 236 hours and MTTR of 4 hours yields ~98.3% availability, while 117 hours and 3 hours yields 97.5%). Conclusions should be drawn only after analyzing the actual data.
II. Process Capability (Cpk)
Evaluation of the Process Capability Index (Cpk). Industry guidelines state that Cpk measures the ability of the entire production line (encompassing Man, Machine, Material, Method, and Environment) to consistently produce conforming products over the long term.
A Cpk of 1.33 indicates that the process is four standard deviations away from the nearest specification limit, representing the minimum capability requirement generally accepted in manufacturing.
While CMK is used to measure the equipment itself, Cpk accounts for variables such as operators, material batches, and environmental fluctuations; stability evaluation requires calculating process capability based on continuous data sampling rather than relying on the performance of a single test run.
III.
Tiered Testing Process
Tiered testing and evaluation process.
Industry specifications dictate that stability evaluations be conducted using load gradients: first, continuous no-load operation for two hours to check motor speed, transmission noise, and control system response (touchscreen latency should not exceed 0.5 seconds), ensuring there are no jams, abnormal noises, or error alerts; next, operation at 30% of rated load for four hours to test precision and conveying fluidity; finally, a gradual increase to full load followed by extreme condition testing—with data recorded and compared against the baseline at each stage.
IV. Quantitative Standards
Quantitative standards for vibration, noise, and temperature rise.
According to industry testing standards, accelerometers are used to measure vibration on the bearing housing and machine frame: frame vibration must not exceed 4.5 mm/s RMS (ISO 10816-3), and bearing vibration must not exceed 2.8 mm/s RMS (high-frequency vibration indicates bearing defects); noise levels measured at a distance of 1 meter must not exceed 85 dB (A-weighted); thermal imaging is used to monitor temperatures, with bearing temperatures stabilizing within 70°C above ambient temperature and gearbox oil temperature rise not exceeding 20°C—exceeding these limits constitutes non-compliance.
V.
Continuous Monitoring Under Full Load
Continuous monitoring during full-load operation. Industry practice dictates that full-load testing covers five areas: power consumption and efficiency;
temperature rise of critical components (cooling system performance); mechanical vibration and noise levels; continuous monitoring of operational parameter fluctuations (cycle time, positioning accuracy drift); and safety protection functions (overload protection, power-loss recovery mechanisms). Stability is fundamentally defined by minimal parameter fluctuation: stable cycle times, controlled accuracy drift, and extended mean time between failures (MTBF)—all of which must be verified through recorded data rather than verbal assurances.
Conclusion
How is the stability of a custom assembly line evaluated?
The conclusion is “three sets of tools plus continuous data”: reliability is assessed via MTBF (total operating time divided by number of failures), MTTR (total repair time divided by number of failures), and availability (MTBF divided by the sum of MTBF and MTTR), with the industry generally requiring availability above 95%; process capability is evaluated by ensuring a Cpk of at least 1.33 and a 4-sigma margin for critical dimensions; and operational testing proceeds in stages—2 hours at no-load, 4 hours at 30% light load, followed by full-load and extreme operating conditions—maintaining frame vibration below 4.5 mm/s, noise below 85 dB, and bearing temperature rise within 70°C above ambient.
In accordance with industry practice, stability is a value to be calculated rather than merely observed.
The proper way to evaluate the stability of a custom assembly line is to require the supplier—at the time of acceptance—to provide a log detailing the number of faults, accuracy drift, and noise levels during 72 hours of continuous operation, and then to determine compliance based on MTBF and Cpk data.
