HAP 0921

www.hpmag.co.uk HYDRAULICS & PNEUMATICS September 2021 33 a gearbox for optimised torque performance. Designing the optimal motor assembly means understanding the application’s power demand as well as the motor’s power generation. The desired output force and linear speed vary depending on the application’s requirements. Power is generated by the motor’s torque and rotational speed and it can be calculated by using the expected output power and by taking into account motor efficiency and the lead screw parameter, including the efficiency and pitch. Let’s take an example with the development of a laboratory medical device for low volume liquid transfer, a single motor package limited to a maximum diameter of 20mm controls a multi-pipette channel. The filling stage must take less than 2.5 seconds and the pipettes then travel 50mm in 4 seconds where they are emptied in 30 sub-steps. The application requires a high-resolution system and a good repeatability to consistently provide the same amount of liquid for each sub-step. For this kind of application, a standard digital linear motor with a lead screw will usually fulfil requirements with no special development necessary, beneficial to keep costs down. A can stack stepper motor enables pipette filling control as a result of the multi-step resolution over liquid delivery into sub-volumes, and thanks to an optimised ball bearing assembly, the axial play is removed, ensuring high repeatability. Lightweight and compact In an alternative application, a recent example of a battery-powered medical device handled by a doctor during an operation demands efficient power usage. It also has to be lightweight and compact, requiring a solution with a maximum diameter of just 13mm and for this application, coreless brush DC motors ensure high efficiency. For size optimisation, the mini motor should also be paired with a gearbox. For the geared motor selection, the engineer will take in account the duty cycle and in this case the medical device will be used over several minutes in continuous duty. To determine the required input power (torque and speed) generated by the motor, some calculations are necessary. First the conversion of the linear motion (force and linear speed) requested by the application into rotative motion (torque and rotational speed). This depends on the lead screw parameters (pitch and efficiency). To know the necessary power at the motor level, you will need to consider the ratio and efficiency of the gearbox. To ensure that the motor is powerful enough in continuous use, the required motor torque should be lower than the rated torque specified by the manufacturer. When the motor and gearbox demands have been ascertained, the power requirement and efficiency of the solution can be calculated. Supporting linear motion application design, Portescap can support engineers with standard and customised solutions. Defining the technical requirements for the application, it’s vital that the miniature motor is correctly specified and sized to ensure optimum integration and consequently, the application’s maximum performance. www.portescap.com Motor and lead screw example. Lead screw example. Ball screw example.