When it comes to linear actuators, selecting the right drive technology can be a precise balancing act as there is no ‘one size fits all’ solution.
Due to the breadth of applications – from automated packaging lines and pick-and-place operations to complex machines such as 3D printers – making the correct choice is less about concentrating on a single aspect than finding the optimum balance of performance from a variety of different factors.
Most electromechanical linear actuators rely on one of five common drive train types: ball screws, lead screws, timing belts, rack and pinion tracks and linear motors.
Common drive trains
Ball screws are ideal for high duty cycle applications and where high force density, precision and repeatability are required. The rolling ball bearings reduce friction and deliver high mechanical efficiency, even in continuous use. Ball screws can achieve moderate speed.
Lead screws are suitable for low duty cycle applications, or those requiring small adjustments. They typically only offer about half the efficiency of ball screws, so require twice the torque to achieve the same thrust output. However, lead screws provide cost-efficient and compact solutions for high-force applications.
Timing belts are simple, robust mechanisms for high-speed applications requiring long life and minimal maintenance, where precision greater than 100 microns is sufficient. They are efficient and easy to operate and can run at 100 percent duty cycle. Timing belts are available in longer lengths than screw drives.
Rack and pinion systems are useful for very long travels requiring high speed but are not known for their precision. They offer high force density but require regular system lubrication. In addition, removing system backlash from this type of drive train is not always possible, and they can be quite noisy in operation.
Linear motors offer high speed, acceleration and precision. Cost is the principal drawback, while force density is also less than other drive systems. The absence of a mechanical connection between the moving and static components of linear motors makes their use difficult in vertical applications.
The PETS principle
The selection options for a linear drive can be grouped into the following categories: precision, expected life, throughput and special considerations (PETS).
For precision, always start with an understanding of needs relative to resolution. The other considerations are repeatability and velocity control. Linear motors and ball screws are typically best in terms of precision characteristics.
With lifespan, mechanical efficiency is the primary consideration, unless the requirement is for a dirty or harsh operating environment. High drive train efficiency is synonymous with long life and reduced energy consumption. Factors such as wear resistance, dirt resistance and maintenance requirements are also important. Due to their high efficiency and limited maintenance needs, timing belts are the go-to option in this category.
Throughput can be considered by first scrutinising the speed and acceleration or deceleration characteristics of each technology – depending on the length of linear travel required. If the need is for longer travel where more of the cycle time is spent at top velocity, speed is the most important. If shorter moves are required, acceleration and deceleration characteristics will take precedence. Linear motors are unparalleled when it comes to throughput.
Some other considerations to take into account when looking at each technology include material and implementation costs, while force density is a further increasingly important factor to bear in mind as machine designs continue to miniaturise, particularly when specifying end effectors or tooling mounted to an axis.
For more information about the four key performance characteristics to consider when choosing a linear drive train from our white paper click here to download.
Article contributed by Olaf Zeiss, product manager, Actuators Electromechanical & Drives Division Europe.
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