Selecting the best linear drive technology for your additive manufacturing (3D Printing) machine, or any other application, should be a process driven by the balancing of cost with performance, and not with blinders focused on the latest technology. Linear motion is created when electrical power is supplied to a coupled device through a drive mechanism (drive train). There are several different types, each with different capabilities and requirements that help determine which is most suitable for your 3D printing application.
The most commonly used technology for 3D printers, it consists of a timing belt with teeth that is used to connect to a moving carriage which guides a payload. A rotating pulley with mating teeth is connected to a motor. Rotating the motor shaft rotates the pulley that pulls the belt in the direction it needs to go. There are several benefits belt drives provide in this application:
The greatest risk is that belts can stretch over time, producing a ringing/oscillatory effect which increases the difficulty of maintaining tight tolerances. To mitigate this stretching, system accelerations and decelerations must not overtax the belt.
A higher precision and better performing system, the screw drive couples a rotary motor to a linear spindle (ball screw or lead screw). As the motor turns, a nut rigidly fixed to a carriage (where the printer head or payload is located) moves either toward or away from the motor based on the shaft’s rotation. There are multiple types of screw technologies, but we will focus on two; lead screws and more efficient ball screws.
A benefit of the lead screw design is that it is relatively inexpensive because it is simpler and easier to produce. Because of its inefficiencies (this design is from 20-70% efficient) and design, the screw can be self-locking. What this means is that in a situation where you lose power, the load will not move because the screw is unable to rotate. Another benefit is size; a lead screw can generate much more axial force than can a belt drive because of the mechanical advantage of the screw over that of a pulley. Risks include incorrect alignment or installation and sliding and wear on the screw surface with continued use, wearing down the nut and creating more axial play. Another risk in using lead screw designs is incorrect alignment or installation.
The ball screw uses a similar screw but with a nut integrated with rolling ball bearings, greatly increasing efficiency and reducing power consumption. It is a more robust design because it eliminates a sliding element, but it also requires periodic lubrication for the metal-to-metal contact that occurs between the rolling ball bearings and the metal screw spindle. The lead screw also has a thrust advantage over a belt drive system. A risk of the ball screw system is that the increased thrust that leads to higher acceleration and faster throughput in 3D printing systems can cause the system to be perceived as too loud and needing service, when in fact it does not.
Table: Drive Train Technologies Summary
This design eliminates the mechanical drive train and directly couples the motor to the carriage by re-designing the rotary servo motor to lay flat on the table, which gives it a much higher response time. Directly coupling the motor eliminates any mechanical windup you may have with the other two drive train designs. This results in higher precision printing capabilities and faster throughput. The major drawback to linear motors is their cost, and the major risk comes from one of the drive system’s moving parts –the cables. The selected cables must be able to withstand the number of bends and moves that the system undergoes in a given time frame.
Watch this video for an introduction to a special presentation by Parker's Ben Furnish and Jim Monnich to help you understand the development risks associated with your motion application. Download "Mitigating 5 Key Risks in Precision Linear Motion System Development" to help you maximize your design for performance and cost.
This is Part One of a Three-Part Discussion. In our next blog post on this topic, we’ll look at how we can select various bearing technologies to reduce risk in building 3D printing systems.
Ben Furnish, Market Development Manager, Automation Group, Parker Hannifin Corporation
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