Our motors are built with either precious-metal brushes or graphite brushes. We do offer some of our motors in specific sizes in both versions. On the smaller motors, the terminal resistance can be the same for both precious metal and graphite. On the larger motors, the resistance of the graphite brushes might be slightly higher. It is also important to note that the contact resistance of the graphite brushes changes depending on load. We list our specs based on 25°C ambient.

I first want to point out the difference between a rheostat and a potentiometer. A rheostat is a simple, two-terminal variable resistor, while a potentiometer is a three-terminal variable resistor. There are two main terminals that, if measured, represent the maximum value of the resistor. The third (center) terminal is called the wiper. The wiper, when in the center position, splits the difference of the resistor with respect to each of the main terminals.

Potentiometers are used as voltage dividers and are much more common than rheostats. They can also be used as rheostats, but in such cases, one main terminal and the wiper terminal would be used.

We don’t sell rheostats or potentiometers, but many of our controllers have built-in potentiometers. We do have motors that can operate on very low voltages, and you can certainly vary the speed by adjusting the voltage. With brushed motors, it is completely linear.

There can be what might seem as an overwhelming number of factors associated with the life of a motor and/or gearbox in any specific medical application. Life is entirely application dependent; the life of a motor can vary from quite short to very long—potentially tens of thousands or even hundreds of thousands of hours. Factors such as current draw and electrical load, speed, type of operation (continuous- or intermittent-duty cycle), temperature, humidity, vibration, type of installation, brush type, lubricants used, thermal management and heat sinking, and axial and radial loading are typical factors that will come into play. Brushless motors have the longest potential life, since the life of a brushless motor is generally limited to the life of the ball bearings. If properly sized, a brushless motor can reliably run for many years continuously. In contrast, a brushed motor generally has a life span limited to the life of the brushes. If a gearbox is used in combination with either a brush or brushless motor, it may become the life-limiting factor.

It is nearly impossible to theoretically determine motor life in any specific application; every application and the related performance required are unique. However, many things can be done to maximize motor life and reliability, which is critically important for medical products. First, consider only the highest-quality motion control products that have a strong presence in the medical market and ISO 13485 quality certification. Design the device using brushless rather than brushed motors, and be sure that the motor and gearbox are rated conservatively for the application’s loading and performance. Gearbox input speeds and axial/radial loading should be minimized, and all potential environmental factors should be considered. Theoretical ball bearing calculations can be performed, but they often do not produce realistic figures, since they do not take such factors as lubricant deterioration, lubricant contamination, or the type of lubricant into account.

Ultimately motor life can only be determined confidently through actual and extensive life testing—most importantly, by simulating application circumstances and environments that are identical to real conditions. In combination with the motor manufacturer’s engineering support and destructive analysis of motor and gearbox parts following life tests, the actual wear of potential life-limiting parts such as bearings and brushes can be measured. By comparing this information with the relative number of test hours, motor life can be estimated most accurately.

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