Reliability Demonstration Testing For Miniature DC Gearmotors

A DC gearmotor is an assembly of a brushed or brushless DC motor and gearbox that specifically fulfills an application’s torque, speed and efficiency requirements. Adding a gearhead to the motor reduces the output speed while simultaneously increasing the torque. Gearmotors play an important role in applications because they can achieve higher torque than an individual motor in a smaller diameter with additional length.

In addition to meeting torque, speed and efficiency requirements, reliability is a fourth crucial parameter. To this end, miniature motor suppliers must ensure their motors meet certain reliability targets — which is where Reliability Demonstration Testing comes in.

What is Reliability Demonstration Testing?

Reliability Demonstration Testing (RDT) verifies whether a product has met certain reliability requirements with a stated confidence level. This pass/fail test is usually performed at a system or product level, and the process can be divided into three stages:

• Planning. This stage involves setting the reliability goal and structuring the RDT. The next step is to select the distribution and acceleration models to calculate the sample size, test time and confidence level.
• Testing and monitoring. This stage involves performing the RDT and monitoring the gearmotor’s performance each day — e.g., measuring voltage, current, speed, temperature and performing visual inspections.
• Data analysis. This stage involves compiling the test data and writing a report based on the results. The report compares the achieved results to the goal and outlines how the data was calculated.

RDT can be completed under both the actual application conditions, as well under accelerated stress conditions. If the required time for the reliability target is very high, the accelerated life test method can shorten the test time for gearmotors using stresses like power, torque, speed and temperature.

RDT in Practice

A medical device manufacturer developed an infusion system and engaged its motor supplier to validate the reliability of the DC gearmotor via RDT. In this case, the supplier needed to validate the B10 life — or time at which 10% of units in a population will fail — at a 95% confidence level. Since the required B10 life was significant at 10,000 hours, which is over a year of actual run time, the reliability engineer opted to perform the RDT in an accelerated manner.

After calculating the acceleration factor and test time using the parametric binomial method, the engineer executed the test plan. The engineer set up 10 gearmotors and test instruments on a test bench, along with a power supply, test fixtures, couplers, brakes, wiring harness, multimeter and torque gauge. The reliability engineer maintained the required torque level using hysteresis brakes and torque limiters, then performed functional tests on each motor prior to RDT.

During the RDT, the engineer recorded daily performance parameters, including voltage, current, speed and temperature. In this example, all 10 motors completed the required test time of 1,081 hours without failure, and no abnormalities were observed. Therefore, the test validated the required B10 life of 10,000 hours with a 95% confidence level.

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RDT can help us understand the target reliability of DC gearmotors in various applications. To explore this method in greater detail, including how it validated the medical infusion system, download our white paper.