Feature Drives & Controls Your guide to selection
Mike Hughes, applications engineer at Schaeffler UK, provides some useful tips and guidance on how to select the most suitable linear actuators for single and multi-axis positioning systems
hen it comes to selecting suitable linear motion systems, many engineers still lack confidence. Some avoid linear motion systems altogether and revert to their traditional comfort zone of specifying rotary alternatives. However, in reality specifying linear motion systems is not such a ‘black art’.
W
Usually potential issues can be resolved and a suitable solution found via discussion with a linear systems sup- plier. The supplier can inform the cus- tomer exactly what is and what isn’t possible in terms of linear motion sys- tems and can provide valuable advice and guidance. Terminology can also cause confusion. Many engineers refer to linear actuators as ‘linear modules’, ‘driven linear sys- tems’ or ‘linear X-Y tables’. These types of systems, which we will refer to as linear modules, normally incorporate a number of different linear drives and actuators, including belt driven linear actuators, ballscrew driven linear actua- tors, linear tables and linear motors. The factors below need careful consid- eration when selecting a suitable linear module for a single-axis, two-axis or three-axis positioning system: System configuration, including the number of axes of motion, is often the first factor that needs careful thought. The most common are two-axis (X-Y) configurations, but less complex single-axis applications and three-axis configurations are also possible. System orientation and mounting are also key factors. In a single axis linear system, this is fairly straight- forward, but in multiple axis sys- tems, this becomes more complex. Factors to consider here include the direction of travel of each axis. Does the load need to be moved simultane- ously in multiple axes or does each axis move individually? Mass and centre of gravity: The mass (and geometry) of the object to be
26 Above and below:
there is no real need for engineers to lack confidence when it comes to selecting suitable linear motion systems for an application, as it is not the ‘black art’ that many
engineers think it is
moved and the position of its centre of gravity as it moves relative to a coordi- nate or datum point on each axis must also be calculated. Clearly, 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, cal- culating the best and worst case scenar- ios and then averaging these is sufficient for most applications. Stroke lengths: The ‘effective’ and ‘total’ stroke length for each axis is also critical. With ballscrew driven linear actuators, for example, the stroke length is limited to the length of the ballscrew itself. Therefore, maximum stroke lengths tend to be around three metres. However, with belt driven systems, there are no such restrictions and so stroke lengths can be higher than this - up to as much as 20m if required. If linear motors are specified, in theory, stroke length is unlimited, but in reality, lengths above ten metres are rare. Traverse speeds: The limiting factors for traverse speeds and traverse times are the ballscrews and/or the bearings. Typically, with ballscrew driven actua- tors, maximum speeds of 3m/s are possi- ble. For belt driven actuators with track roller guidance systems, the maximum speed is around 8-9m/s. If recirculating linear bearings are used with belt driven actuators, maximum speeds are similar to their ballscrew equivalents (i.e. 3m/s). Acceleration itself is not normally the defining issue in multi-axis posi- tioning systems. It is the loads due to these accelerations in the system that are critical. The highest acceleration of any linear actuator to date is around 40m/s2
. Deceleration is also important, particularly if there are emergency stops required in the system. Cycle time requirements: Cycle
, although typically accelera- tions are more likely to be much less than this, often between 0.5m/s2 5m/s2
and
times dic- tate the life
of a linear
system. For example, a positioning system such as a tool changer on a machine tool might change the tool five times per hour. How is this cycle time going to vary from day to day and how will this affect the fatigue life of the linear components within the system? External loads and forces: This includes external impact forces on the system such as stops or human inter- ventions. 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?
Environmental factors: Environmental factors such as temper- ature, humidity and contamination will also affect the choice of linear system. A dusty working environment may require the customer to imple- ment external bellows or dust extrac- tion devices for the linear system. Linear actuators can be protected from the environment by incorporating spe- cial seals, corrosion resistant materials and coatings, special greases or by using plastic parts where necessary. In medical applications, the overall noise of the system may be a factor that needs addressing. A lower speed linear actuator may be the solution here, but if high speeds need to be maintained, spe- cial components, materials or coatings may need to be specified in order to minimise noise levels. Electrical considerations: For multi- axis positioning systems, drives and other electrical systems are often com- plex and therefore require careful con- sideration. A multi-axis linear module is likely to incorporate electric motors, controllers, geared drives, cables, grip- pers, limit switches, encoders, brakes and other control devices.
Schaeffler (UK)
www.schaeffler.co.uk T: 0121 313 5870
Enter 213 APRIL 2013 Electrical Engineering
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52