CONTROL & AUTOMATION SPECIFYING A LINEAR ACTUATOR
MATARA UK LTD
By Japh Humphries , UK Sales & Commercial Manager , Matara UK Ltd
Linear motion systems are business critical across an extensive range of industries , from food processing plants , to semiconductor manufacturers and packaging producers to name but a few ; in fact , virtually anywhere that requires loads to be cost-effectively moved in a straight line , safely , securely and precisely . These systems have improved automation whilst simultaneously lowering production costs , making them invaluable .
Linear actuators are at the heart of a linear motion system , combining linear guides and power-transmission components in a single unit . Whilst machine builders can opt to design and produce these elements in-house , the majority opt for off the shelf , ‘ ready-made ’ linear actuators as it reduces overall machine design and fabrication costs and , as they feature application-tested and optimised construction , are highly reliable , accurate and provide repeatability ; all of which are core to production machine and automation systems .
Whilst it ’ s important to consider all components of a linear motion system separately and as a whole , choosing the right linear actuator – belt driven , ball screw driven or electric rod type actuator - is fundamental . There are a number of factors that should be taken into consideration , but three stand heads and shoulders above the rest and will ‘ make or break ’ a system .
KEY FACTOR 1 : SPEED
The speed of the process at hand will impact the actuator ’ s longevity and efficiency .
A ball screw linear actuator will usually perform at around 0.35 to 1.5m / s at stroke
lengths under one metre , although higher speeds and longer lengths can be achieved by adding supports to the assembly . However , this will only take you so far as a ball screw actuator may be susceptible to ‘ screw whip ’ at high speeds , where the screw vibrates and bows as it turns . Over time this can negatively impact the performance of the actuator , leading to premature wear and shortening its lifespan .
Whilst there isn ’ t a universal cut off point at which a ball screw linear actuator is no longer an option – it ’ s very much dependent on a range of factors including the dimensions and material of the screw and the actuators ’ use and environment – if you want to achieve higher speeds , up to 3m / s , then a belt actuator should be considered . Belt actuators perform better at higher velocities and also in applications that require a long stroke . Which brings us to our next key factor .
KEY FACTOR 2 : STROKE LENGTH
Stroke length refers to the distance the linear rail actuator is required to move a load in one direction .
Linear ball screw actuators are usually designed at around 1000mm stroke length , although smaller and larger lengths can be accommodated by , for example , using different diameter balls . However , we would never recommend a ball screw actuator for stroke lengths over 54000mm . For longer stroke lengths , belt driven actuators are a better option . At Matara , we are able to manufacture linear actuators from 100mm to 6700mm stroke lengths .
One of the most important considerations when looking at stroke length , and one that all too frequently gets overlooked , is the safety stroke . The safety stroke is an allowance that provides room for the actuator to coast to a stop if an emergency stop of the system is necessary . This prevents the bearing table ( and the load ) from hitting the ends of the actuator , which can cause damage . Even if emergency stops are rare in your particular process , it ’ s still good practice to factor in a safety stroke as linear actuators aren ’ t designed to constantly be run into a hard stop as this can cause wear and tear to the inner workings of the actuator over time .
To calculate the safety stroke , for belt driven actuators specify a distance equal to two turns of the motor on each end of the actuator ( or four turns in total ). For ball screw actuators , base it on twice the pitch of the ball screw . Other factors may have to be taken into consideration , but this calculation is a good starting point .
KEY FACTOR 3 : LOAD
Stroke length impacts on speed and vice versa , but load has a direct effect on both .
Anyone involved in an engineering capacity will appreciate the importance of load
54 PECM Issue 52