When it comes to motion for handheld and miniature medical device designs, a standard electrical mini motor may seem like a good starting point. But while a miniature motor generates rotary movement, applications like medical pipettes or syringes, and mesotherapy devices often move loads linearly instead of rotationally. For these medical tools as well as some industrial devices, engineers must develop their own system to translate rotary motion into linear motion. Here are some of the basics to understand when considering your linear motion options.

Rotation can be converted into linear motion via a screw-and-nut system assembled on the motor shaft. There are two main types of screw-and-nut systems: ball screw and lead screw.

A ball screw operates on rolling contact between the nut and a screw. The ball is recirculated along a helical groove. The rolling components keep friction low, while allowing high efficiency greater than 90 percent along with a high load capability.

A lead screw is typically composed of a stainless steel screw and a plastic nut. Both components are in direct contact, generating more friction than the ball screw system. This can be a good, economical option if cost is a concern. The nut material generally affects the life and maximum load capability of the system. However, with two preloaded nuts, axial play can be eliminated. There are typically two types of linear options to consider:

Option 1: Motor With Integrated Lead Screw

Standard linear actuators, which are often called digital linear actuators (DLAs), are fully integrated linear mechanisms that use a can stack stepper motor. This is a generally cost-effective choice. Inherent with stepper technology, the motor itself is a positioning system, so the control does not need position feedback. The DLA can be driven in full steps, half steps or micro steps, depending on the required resolution.

Option 2: Custom Motor Assembly

For applications requiring high performance in a limited package, consider a custom assembly. Most custom assemblies are built with either a brush DC, brushless DC or stepper disc magnet motor. Each technology has its own benefits and advantages over can stack steppers. For example, high-acceleration applications are well-suited for a low inertia motor such as a disc magnet stepper. For higher power in a small package, the best option may be a combination brushless DC, gearbox and lead screw. And, for high efficiency, a coreless brush DC motor can be especially desirable for battery powered applications.

Figure 6 Motor and lead screw (1)  

A custom assembly also offers flexibility when selecting the lead screw. The R&D team can choose whether a ball screw or regular lead screw is preferable, suggest different pitches, adapt the material or even optimize the dimensions.

Partner To Define Both Device and Motor Requirements

For linear applications, motor suppliers can support the development team by offering standard linear motors or by creating a linear, custom motorized design. In addition to defining the device’s technical requirements, both the motor supplier and application designer should also determine the requirements at the motor level. With a full understanding of the project’s needs, design teams can find the best compromise between the tool’s technical and commercial requirements.

For more information about miniature motors for handheld medical devices, visit our applications page