Ball screws for efficient, dynamic linear motion
01 May 2012
Ball screws are precise mechanical components that turn rotary motion into linear motion. Their function is efficient and they offer high dynamic performance. However, their application can be complex because of the need to consider factors such as speeds, loads, accuracy, duty and lifetime. Geoff Spear runs through the basic information that a machine design engineer should have at his or her fingertips in order to make educated choices.
With performance on offer up to 2m/s, with 90% efficiency, high dynamics and zero backlash, it is worth getting to know ball screws. They comprise two basic components: the ball screw itself, which is a threaded shaft, and a ball nut, which has an internal thread that echoes that of the screw. Within the nut are contained a quantity of balls whose rotation allows the nut to move linearly along the screw.
In most cases, the rotation of the screw achieves linear movement of the nut, but rotation of the nut on a fixed screw can achieve the same. The ball screw and the thread or raceway within the nut are machined to high levels of precision defined by international standards. The efficient action of the balls in the raceway of the nut causes axial displacement and they are returned to their original position after one or several revolutions using diverting elements called liners.
Balls screws offer certain performance advantages. They can be used for both fast (up to 2m/s) and slow running applications, maintaining around 90% efficiency, and their dynamic load ratings range from 10 to 150kN (though operating factors will usually reduce these figures). Screws can be up to 5m and more in length without compromising the high dynamics and rigidity. Compared with hydraulic and pneumatic systems, ball screws offer high efficiency (no power loss when not operating), no air or hydraulic fluid leakage, and ease of control with electric motor drives.
The international standards BS ISO 3408 and DIN 69051 are a big help in the selection and application of ball screw systems. They define performance ratings, accuracy grades and the dimensions of screws and nuts. As a general guide, the performance of one system manufactured to these standards will match that of another, although components will not be interchangeable.
The standards define accuracy grades and the most common of these are grades 3, 5 and 7. These relate to thread errors over a 300mm length and even the lowest of these three levels has a tight limit of 52 microns travel error. Class 7 is the least accurate yet it suits about 80% of industrial applications. Class 5 reduces errors by half and is more appropriate for precision positioning applications. Class 3 reduces by half again to give the extremely high precision required, for example, by machine tools. Ball screws are machined from alloy steel with a final hardness of 58-61Hrc.
Ball nuts are also machined from alloy steel, with a hardness matching the screw, and these are hand assembled using steel balls available at nominal size, also three levels up and down with variations of a few microns.
There are two designs of ball recirculating systems. With a radial liner, the balls travel for one revolution of the raceway and are then deflected back to the start point. There are typically three or four raceways with liners (plastic or steel according to operating temperature) located in the wall of the nut.
The alternative ball recirculating system is known as the frontal deflector where a single raceway takes the ball in several revolutions down the length of the nut. A deflector (liner) then diverts the balls into a return tube machined in the nut body. They pass axially back up the length of the nut and a second deflector returns them to the start position. Selecting the optimum recirculating system is a rather complex task, best left to the manufacturer.
Ball nuts can have standard dimensions to DIN 69051 or special to customer requirements. Single nuts suit rolled or whirled screws but have a small amount of backlash typically below 0.1mm. The backlash can be removed using preloaded single nuts where raceways inside the nut have a micron shift in the thread lead. This preloads the balls with a consequent small decrease in load capacity of the nut. Double preloaded nuts restore the load capacity by using extra length and balls. Generally backlash free preloaded nuts are used with whirled screws of accuracy grade 5 and 3 for positioning applications.
The first rule in ball screw selection is that radial loads should be zero or very low. Ball screws are designed for axial loads and radial loads will sharply decrease lifetime. Selection calculations are best left to the supplier who can optimise the design using features such as multi-start threads, ball recirculating systems; the supplier then takes responsibility for the sizing, according to. speed profiles and duties; the maximum/minimum loads; mounting fixtures and orientation; operating environment, and expected lifetime.
Other practical considerations include ensuring that the ball screw is mounted in good quality angular contact bearings to accept the thrust loads. Ball screws should be ordered with ends customised (keyways, grooves, etc) to suit the mounting. Choice of lubricant is best left to the supplier. Oil is best to maintain cooling and achieve long life, but many customers opt for grease. And remember, ball nuts should not be disassembled from their screws as re-assembly requires specialist skills.
Servomech ball screw systems are available in the UK from Techdrives, which offers engineering support to optimise selection. Screws are manufactured by rolling or whirling with diameters 14 to 100mm and are stocked in lengths to 5.7m. Accuracy classes from 7 to 3 are offered and screw pitches can be from 5 to 40mm. Single, double and preloaded nuts to DIN 69051 or special dimensions are available.
Geoff Spear is with Techdrives
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