PMDC gearmotor solution by Groschopp met the positioning and load requirements of a medical table.

Highly specialized medical procedures such as joint replacement and trauma surgery require automated operating room tables that can be positioned with extreme accuracy. Just as important is high torque output and quiet operation in a compact gearmotor solution that can drive up to a 500-lb load.

Sioux Center, IA-based Groschopp, Inc, recently helped meet these design challenges when it teamed with a leading maker of medical tables. For this particular table, two hinged arms had to be driven so that their positional alignment and movement would be completely synchronized at all times. The traditional solution of link chains, belts, and pulleys have greater margins of operational error, making it more difficult to synchronize the movement and alignment of the two arms. A typical solution would involve one motor for each hinged arm, but using a single motor with two outputs provides greater accuracy, and is more cost effective.

Compounding the design challenge was the 24-volt power supply specification. Groschopp’s design team chose to use a 24-volt permanent magnet DC gearmotor to provide the needed output given the intermittent operation. The motor was customized for high torque, and because the same motor drives both planetary speed reducer outputs, there is both mechanical and electrical ‘linkage.’ If two separate motors were used, a more complex and expensive, common control scheme would be required to provide this ‘linkage.’ The high efficiency of the planetary reducers made it possible to achieve the needed torque while staying below the 20A current limit. Once the dual planetary dc gearmotor solution was proved out, the team pursued extensive prototyping and testing to reduce noise, and then worked to reduce noise from the motor, gearbox, and gearmotor assembly. Customizations include:

• Use of high-resolution imaging techniques to characterize the dc motor’s commutator; special manufacturing processes can be employed to reduce brush noise.

• Use of low-noise planetary speed reducers, with a strategic combination of non-metallic and metallic gears to deliver high torque and minimize sound.

• Critical evaluation of the alignment of the two speed reducers, and engineering mounting/hardware to minimize noise that could result from sub-optimal gear teeth engagement with the motor shaft, resulting in reduced operational efficiency.

The prototyping, testing, and customization efforts paid off in the form of a design solution that met the customer positioning, high-torque, and low-noise requirements.