FEATURE LINEAR MOTION


Thomson provides high load ball screws that are commonly used in injection moulding machines, metal pressing and forming, test rigs, straightening machines and tube bending machines


Technological advances mean it is now possible to specify high


load ball screws in applications where you


would have traditionally chosen a roller screw, as Markus Brändle.


product line specialist, EMEA region, screws and linear bearings & guides at Thomson Linear, explains


Can you use a ball screw in high load applications? R


oller screws are often promoted as the only technology for handling


large loads when size is a constraint. So, if you needed to actuate a 500kN axial load along a 1500mm stroke, you would probably choose a roller screw. In actuality, however, advances in


ball screw technology make special versions viable candidates for high load applications as well – and this is important because a high load ball screw is typically less than half the cost of a comparable roller screw at equivalent performance points.


FEATURES A ball screw consists of a threaded metal shaft with a nut that rides along its length as a motor rotates the shaft. The nut connects to a table, an arm or other load as determined by the industrial process that is being automated. Re- circulating ball bearings, packaged within


Standard L10 Bearings are often specified to give an ‘L10


life expectancy calculation of a ball screw assembly ’ life. This is the life at which 10% of the ball screw assemblies in


that application can be expected to have failed due to classical fatigue failure or, alternatively, the life at which 90% will still be operating. The L10


life of the ball screw assembly is a theoretical life and may not


represent service life of the assembly. It can be calculated according to the following formula: L10


= (Cam/Load)3 Where: L10


= Life expectancy (expressed in millions of revolutions) Cam = Dynamic capacity of the ball screw


16 FEBRUARY 2019 | DESIGN SOLUTIONS


the nut, contact the threads and provide the load bearing path. The coefficient of friction between the mating components is very low, providing a system efficiency which is typically greater than 90%. The load capacity of the ball screw is then a


Thomson ball screws offer gentle recirculation of the balls and precise positioning


function of the ball bearing diameter, number of ball bearings, and the surface contact area. The combination of these parameters then defines the load capacity of the assembly and therefore the resultant life of the components. For higher loads, roller screws have


been used in many applications. In a roller screw, the load bearing mechanism is a set of recirculating rollers instead of balls. As the rollers have more total surface contact area than balls, this increases the load carrying capacity and service life. However, roller screws require extensive upfront precision machining and more complex assembly, meaning a higher cost and larger footprint for final installation.


A NEW DEVELOPMENT Thomson has developed a high load ball screw by implementing a thread form that increases load bearing capacity four to eight times over traditional ball screws. This gives the application engineer the option of deploying a ball screw in almost any application that previously required a roller screw. Or, because the life of the assembly is a direct function of the load capacity (see L10


Life Expectancy


Calculation), using a high load ball screw in a low load application would extend the life of the product. Consider, for example, an automotive


equipment assembly application, which is running a load of 100kN, 24/7. Based on the standard life calculation, the product may last three to four months according to the fatigue failure analysis and statistical life expectancy. Implementing a high load ball screw, however, would double the load capacity and yield eight times the life. So there is the option of going for higher load capacity, longer assembly life, or a combination of the best of both.


/ DESIGNSOLUTIONS


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