Top 10 factors to consider when specifying linear systems

In this article, 'linear modules' refers to linear rails, linear actuators, driven linear systems or linear X-Y tables. These types of systems typically incorporate a number of different linear drives and actuators, including ball screw-driven systems, belt-driven linear actuators, ball screw-driven linear actuators, linear tables and linear motors.

When a linear supplier receives an enquiry, the factors below need careful consideration before a suitable linear module for a single-axis, two-axis or three-axis positioning system can be selected. In reality, not all the technical data below will be available and so certain assumptions may need to be agreed between supplier and customer when specifying a system.

(1) Mass and centre of gravity

The mass (and geometry) of the object to be moved and the position of its centre of gravity as it moves relative to a co-ordinate or datum point on each axis must be calculated. As a mass is accelerated or decelerated along multiple axes of travel, the position of its centre of gravity relative to each axis will change. This needs careful consideration so that the moment loads at multiple points in the system can be established. Often, calculating the best-casse and worst-case scenarios and then averaging these is sufficient for most applications.

(2) System configuration and mounting

System configuration, including the number of axes of motion, needs careful thought. The most common are two-axis (X-Y) configurations, but less complex single-axis applications and the more complex three-axis configurations are also possible.

System orientation and mounting are important. In a single-axis system, this is fairly straightforward, but in multiple-axis systems, this becomes more complex. Factors to consider here include the direction of travel of each axis and the distance between the rails. Does the load need to be moved simultaneously in multiple axes or does each axis move individually? Does the system require a moving carriage or a moving rail? Are the axes vertical, horizontal or inclined?

(3) Stroke lengths

The 'effective' and 'total' stroke length for each axis is also critical. With ball screw-driven linear actuators, for example, the stroke length is limited to the length of the ball screw itself. Therefore, maximum stroke lengths tend to be around 3m. But with belt-driven systems, there are no such restrictions and so stroke lengths can be higher, up to 20m if required. If linear motors are specified, in theory stroke length is unlimited but, in reality, lengths above 10m are rare.

(4) Accuracy and repeatability

Depending on the application, accuracy and repeatability will differ greatly between applications. Most customers know what their accuracy requirements are. For example, if the actuator is for an automated pick-and-place machine, then it is likely that high repeatability and accuracy are required. Typically, the accuracy of a ball screw-driven linear actuator is 0.16mm per metre with repeatability of +/-0.01mm. For belt-driven actuators, typical accuracy is around 0.5mm per metre, with repeatability of +/-0.10mm.

(5) Acceleration, deceleration and linear speed

Acceleration itself is not normally the defining issue in multi-axis positioning systems. It is the loads due to these accelerations in these systems that are critical. The highest acceleration of any linear actuator to date is around 50m/s2, although typically they are much less than this, often between 0.5m/s2 and 5m/s2. Deceleration is also important, particularly if there are emergency stops required in the system. Apart from acceleration and deceleration forces, required speed can also dictate the type of linear system chosen, generally to a maximum of 10m/s.

(6) External loads and forces

What are the positions and magnitudes of the loads in the system, including external impact forces such as stops or human interventions? Is something pushing or pulling on the load to be moved or does the load need to be brought quickly to a stop at the end of its travel? For example, a drive may bring the linear actuator to a stop or a 'home' datum position. How this is achieved and how this affects the loads on the mass to be moved are key considerations. If the customer can provide a 2D line sketch of the application with loads and forces, this is very useful to the supplier.

(7) Cycle times

Cycle times dictate the life of a linear system. If the customer needs the system to last for a minimum of 10 years, changing the tool on a machine five times every hour may not be an issue. But if the tool is changed 10 times per hour, a different type of linear system may be required in order to guarantee a 10-year operating life.

(8) Environmental factors

Environmental factors such as temperature, humidity and contamination (ie dust, oil, water, washdowns, chemicals, coolants, etc) will also affect the choice of linear system. A dusty working environment may require the customer to implement external bellows or dust extraction devices for the linear system. The two biggest causes of failures of linear modules are lack of lubrication or re-lubrication, and contamination from the operating environment. Linear actuators can be protected from the environment by means of special seals, corrosion-resistant materials and coatings, special greases or by using plastic parts where necessary.

(9) Electrical considerations

For multi-axis positioning systems, drives and other electrical systems are often complex and therefore require careful consideration. A multi-axis linear module is likely to incorporate electric motors, controllers, geared drives, cables, grippers, limit switches, encoders, brakes and other control devices. All of these features can add mass to the axes.

(10) Total cost of ownership

Although the initial purchase price of a fully protected and sealed linear system is relatively high compared with a standard linear system, the potential savings that can be achieved in the form of increased productivity, higher operating life and reduced maintenance costs, often more than outweigh the initial higher purchase price of the system, ie the total cost of ownership of the system is significantly lower.

To learn more about specifying linear modules please visit www.schaeffler.co.uk.