The advanced method uses a direct fit between the screw and the motor. In this scenario, the screw is inserted into a hollow shaft within the motor and then affixed with a laser weld on the motor’s back. This method ensures:
- fewer components
- maximum engagement between screw and motor
- highest accuracy alignment possible
- least amount of runout
- low maintenance and extended life
Selection of motor type and design
Linear actuators come with a choice of motor options, with the most common being open loop steppers, closed loop steppers containing either a board mounted control or smart stepper, or brushless DC (BLDC) motors. Each has its own performance proposition regarding speeds and load capabilities, and each comes with their own set of pros and cons around cost, integration, and control.
Typical general-purpose motors use a wavy washer to hold bearings and the assembly in place. While they are usually adequate for rotary applications, wavy washers can cause small amounts of axial or linear play, resulting in inaccuracies of linear position. To alleviate this, one or both of two elements can be modified in the design. Larger bearings can be inserted to increase the thrust load capability of the assembly, and a spanner nut can be added and adjusted to a predetermined torque specification to take the play out of the system.
Choice of control options
The last advancement in the modern manufacturing model is how the physical linear motion is directed and controlled. Traditionally, these need multiple separate pieces including an amplifier and controller. Each needs a cabinet and the associated hardware, wiring, encoder, and sensors for feedback, making them complicated and cumbersome to install, troubleshoot, and operate.
Off-the-shelf smart motor solutions have simplified the wiring and have reduced the number of connectors and sensors associated with gaining step-servo type performance and control. This provides cost savings, thanks to a lower component count and less installation time and labor. These motors also come in preassembled industrialized packages that seal and protect the board and control from abuse or contamination. For applications requiring specific parameters, a custom unencapsulated IP20 motor-mounted board control is a useful option. This is especially true for large-volume applications placed in stylized housings or equipment. These actuators utilize the advantages of smart motors, where control is right at the motor for more efficient communication with the master or PLC.
To find more in-depth information and application examples on this topic of lead screw assemblies, please read the Design World article: Linear motion systems: Only as strong as weakest link.