The perfect marriage of Gears and Motors

Alan Feinstein
Vice President
Bayside Motion Group
Port Washington, N.Y.

Electromechanical motion-control systems are typically configured with parts from different suppliers. Users often buy servo-motors and drives from one company, gearheads and mechanical components from another, and possibly controllers from a third. That's changing as single-source suppliers gain favor. Through corporate acquisitions or internal R&D efforts, many motion-control companies now provide a wider range of products. Nonetheless, one-stop shopping doesn't guarantee a truly integrated motion system.

Take servo-gearmotors as an example. Clearly, the designs of brushless dc motors and servo gearheads can each be individually optimized. But combining the two in a servosystem can produce unintended effects that hurt performance. Integrated gearmotors provide a better solution. In these units, the brushless dc motor and helical planetary gearing are designed and manufactured as a single system, not as individual components. Benefits include:

• Smoother and quieter operation.
• Lower operating temperatures.
• Higher reliability and efficiency.
• Reduced wear.
• Smaller size.

The primary advantage is an integrated design eliminates parts that must be joined and aligned, the most common source of failure. Conventional motors and gearheads require a mechanical coupling that introduces the potential for misalignment. This usually leads to accelerated wear and premature failure. The latest integrated servo gearmotors, in contrast, feature a single-piece rotor assembly and input gear, supported by precision bearings. The design ensures alignment and minimizes wear on gear teeth and bearings. The end result is smoother operation and longer life.

Another benefit is quieter operation. Traditional mechanical connections with even minor misalignment generate noise. For example, tests revealed a typical combination gearhead and motor operated at 70 dB while the same-size integrated gearmotor ran below 65 dB.

Single-piece construction improves reliability as well. Integrated designs have fewer parts and eliminate potentially weak connections. In traditional designs, clamps, thermal shrink fits, and tapered fittings typically join gears and motor. Unlike one-piece designs, these methods have the potential for dynamic failure.

Integrated designs also tend to be smaller and run cooler than motor/gearhead combinations of equal rating. In any servo/gearhead assembly, both motor and gearhead generate heat. Traditionally, however, each is designed on a stand-alone basis to optimize thermal dissipation. This presents a problem when motor and gearhead are joined and each transfers heat to the other. The result can be higher than expected operating temperatures that adversely affect the motor, bearings, seals, and lubricants.

An integrated design considers heat generation and dissipation throughout the entire assembly, not of individual parts. Viewing the system as a single heat generator often lets designers lower overall temperatures and improve operating conditions for critical components.

Thermal-resistance is one indicator used to compare the thermal behavior of an integrated gearmotor versus a traditional motor and gear-head. Thermal resistance determines the heat transfer and, thus, temperature difference between any two points. The higher the thermal resistance, the greater the temperature differential between a heat source and sink. For instance, if comparing winding temperature to ambient, a lower thermal resistance means a lower winding temperature for a given dissipation.

One such study involved 90-mm frame-size gearheads with a 10:1 helical gear ratio and a 1.5-in. stack, running at 3,000 rpm. Results showed the thermal resistance of the integrated gearmotor winding was 1.23°C/W, while thermal resistance in the motor-and-gearhead winding was 1.57°C/W. The integrated gearmotor's 22% lower winding-to-ambient thermal resistance permits higher power throughput for the same winding temperature. Motor efficiency is higher in the gearmotor assembly and the same size motor provides higher torque. In thermal tests, temperature rise of the gearmotor housing (over the motor section) was 28°C lower than that of the conventional motor mounted to a gearhead.

In addition to improved dynamic performance, another benefit is smaller size. Applications that demand gearing in tight spaces often require right-angle gearboxes which are more compact than conventional inline units. An integrated design changes that because the typical gearmotor is 27% shorter than a comparable motor/gearhead configuration. This opens the opportunity to apply inline gearmotors in spaces that previously demanded more costly right-angle products.