FEAT RE MACH NE BU L FEA ATURE HINEI
UILI DIIN
NG & FRAME
MEWOR K S
The top 10 factors to consi de r when selecting linear system s
Bob Love, business devel pment manager at Schaeffler UK, outlines the top 10 factors engineers should consider when specifying linear modules for single and multi-axis handling and positioning systems
Bob Love, business development manager at Schaeffler UK, outlines the top 10 factors engineers shoul odules for singl
positioning system
n 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 ballscrew driven systems, belt-driven linear actuators, ballscrew- driven linear actuators, linear tables and linear motors. When a linear supplier receives a linear enquiry, the factors below need careful
I
consideration before a suitable linear module for a single-axis, two-axis or three-axis positionin g system can be selected .
1. MASS MASS & CEN RE OF GRAV CENTRE OF GRAV TYVIITY
The mass (and geometry) of the object to be moved and the position of its centre of gravity as it moves relative to a coordinate 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 and worst-case scenarios and then averaging these is sufficient for most app lications .
2. CONF GURATION MOUN ING CONFIGURI ATION & 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 sy stem require a movin g carriage or a moving rail? Are the axes vertical, horizontal or inclined?
3. STROKE ENG HS ROKE LENGTHS
The ‘effective’ and ‘total’ stroke length for each axis is also critical. With ballscrew driven linear actuators, for example, the stroke length is
32 32 MAR MARCH 201 2017 | AU AUTOMA MAT ATION
limited to the length of the ballscrew itself. Therefore, maximum stroke lengths tend to be around three metres. But with belt-driven systems, there are no such restrictions and so stroke lengths can be higher, up to 20 metres if required. If linear motors are specified, in theory
application with loads and forces, this is ver y useful to the supplier.
7. CYCL 7. CYCLE TIMESIMES
Cycle times dictate the life of a linear system. If the customer needs the systemto last for a
stroke length is unlimited, but in reality, length s minimumof 10 years, changing the tool on a above 10 metres are rare .
4. ACCUR
machine, actuator accuracy
CCURACY & REPEAT REPEA ABIL TY ATABILITY
Depending on the application, accuracy and repeatability will differ greatly between applications. Most customers know what their requirements are. For example, if the is for an automated pick-and-place then it is likely that high repeatability
and accuracy are required. Typically, the
accuracy of a ballscrew-driven linear actuator i s 0.16mmper metre with repeatability of +/- 0.01mm. For belt-driven actuators, typical accuracy is around 0.5mm per metre, with repeatability of +/- 0.10mm .
5. LINEAR SPEED INEAR SPEED
Acceleration itself is not normally the defining issue inmulti-axis positioning systems. It is the loads due to these accelerations in these systems that are critical. The highest acceleration of a
although linear act
typically they aremuch less than th uator to date is around 50m/s2,
is , ny
often between 0.5m/s2 and 5m/s2. Deceleration is also important, particularly if there are emergency stops required in the system. Apart fromacceleration and deceleration forces, required speed can also dictate the type of linear systemchosen, generally to amaximumof 1 0m/s .
6. EXTERNAL EXTERNAL LOADS ADS & FORCES 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
10. T 10. TOTAL C ST O OWNERSHIP L COST OF OWNERSHIP
Although the initial purchase price of a fully protected and sealed linear systemis relatively high compared to a standard one the potential savings that can be achieved in the formof increased productivity, longer operating life and often outweigh the of the system.
,
initial higher purchase price reducedmaintenance costs,
Schaeffler www
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info.uk@schaeffl
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machine five times every hourmay not be an issue. But if the tool is changed 10 times per hour, a different type of linear systemmay be required in order to guarantee a 10-year operating life .
8. ENVIRONMENTAL ENVIRONMENTA AL FACTORS ORS
Environmental factors such as temperature, humidity and contamination (i.e. dust, oil, water, washdowns, chemicals and 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 fromthe operating environment. Linear
actuators can be protected fromthe environment by incorporating special sea resistant materials and coat
ings, special greases ls, corrosion-
or by using plastic parts where necessary . 9. ELEC RICAL CONSIDERATIONS 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 ca n add mass to the axes.
consi er w en speci ying l near and m
axi handl ng and
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