Ballscrew linear actuator selection made clear with Matara

(See Matara at MachineBuilding.Live, 2 October 2024, on stand 46)

Linear motion systems generally consist of a controller (either a computer or a stand-alone electronic device), a motor and the actuator which moves the load. The common choice of actuators is between ballscrew linear and belt driven models.

Ballscrews are usually selected for their high-precision linear motion control and their ability to safely move heavy loads across a distance in a smooth motion at speed, whereas belt driven linear actuators perform better when it comes to lower loads and high speed/long stroke applications.

A ballscrew linear actuator converts rotational motion into linear movement using a threaded shaft and ball bearings. The shaft of the ballscrew includes a helical path around which the ball bearings move, behaving like a screw but with very little friction. The ball assembly behaves as the nut, with the threaded shaft acting as a screw.

Manufactured to higher tolerances than some of the other linear motion drive options, ballscrew linear actuators are a popular choice for high precision applications where accuracy and control are critical. They are also a good option for high load applications and for where thrust force is required. The main drawback of ballscrew linear actuators is that, due to the need for the balls to be recirculated, the overall size of the device is larger and bulkier than other options.

Types of linear ballscrews

There are three main types of linear ballscrews to select from, each with their own pros and cons.

As the name implies, external return ballscrews feature a ball return that is external of the nut, usually by the means of a tube. This ballscrew type is able to deal with high speed and/or high load industrial applications such as process automation or CNC. On the down side, it has a relatively large footprint, requires internal lubrication and performance can be affected by the ingress of dirt.

If size is an issue, then an internal return ballscrew might be preferable. The contained ball return allows the balls to exit the track and directs them into the next thread, returning the ball. It keeps the overall size smaller than other solutions and provides higher accuracy. However, this design also makes for a less robust linear ballscrew, so should generally not be used for high load or high-speed applications.

The other option is an end cap return ballscrew. Here, the balls are deflected by an end cap though a route bored transversely down the nut body where the cap at the other end deflects the balls back into the threads. This type of ballscrew is generally quieter and cleaner than other designs and is often used in food preparation and medical applications.

Aspects to consider when selecting a linear ballscrew

Here we look at the key aspects to take into consideration when selecting a ballscrew driven linear actuator.

Accuracy versus cost: There are two different methods of manufacturing ballscrews; rolling or grinding. Rolled screws use rotating dies to deform metal bars and create the helical thread needed for a ballscrew. Although there are ways that this manufacturing method can be more tightly controlled, this solution is generally considered a cost-effective option that produces a less accurate final product.

Ground ballscrews, however, are made by cutting threads into hardened material which produces a much more precise thread with tighter tolerances and lead accuracies, but you can expect to pay up to 30% more than rolled linear ballscrews.

Backdriving risk: Particularly relevant if your load is to be carried vertically, backdriving happens when the motor shuts off maybe as a result of a power cut or problem with the motor itself - and the ballscrew free-falls as a result. Brakes must be fitted as a safety precaution in this case, as the weight of the ballscrew in free fall could damage not only the machine but also operators.

Critical speed: The maximum speed of a ballscrew depends on a combination of the screw diameter, the end fixing type and the screw length. There are standardised ways of calculating this value.

Load: The maximum weight of load that can be carried is dependent on the ballscrew diameter, the end fixing and the free screw length.

Environmental conditions: in more demanding applications, ballscrews made from hardened steel will provide greater strength, stiffness and resistance to wear. Where they will be exposed to dirt, debris or chemicals, it may be prudent to protect them from these contaminants. A coating such as Armoloy can be used to improve corrosion resistance. A wiper to prevent contaminants from entering the ball nut is a further option, and bellows boots can be used to cover the screw and provide protection that way.

Lubrication: Operating temperature and frequency of use both impact the need for lubrication. During operation, friction generates heat within the ballscrew assembly. If it exceeds the recommended limits, it can cause lubricant degradation, reduced viscosity, and increased wear. Lubricant can also be impacted with high frequency use, leading it to be consumed or gradually depleted. Insufficient lubrication can lead to increased friction, wear, and reduced performance.

Regular lubrication maintenance is required in order to facilitate an efficient output and extend the lifespan of your ballscrew driven linear actuator; that not only means maintain lubricant at the correct levels, but also cleaning the system and replacing the lubricant to remove any accumulated contaminants.

Maintenance: This should be a consideration when selecting the right ballscrew for use. For applications where minimal maintenance is a priority, preloaded ball nuts offer reduced wear and extended maintenance intervals thanks to a slight axial load applied to the ball bearings which ensures consistent contact between the balls and the screw.

In harsh or dirty environments, sealed ball nuts are ideal as their protective seals prevent contaminants from entering the ball nut assembly, making for less frequent cleaning and lubrication.

For sites that really want to avoid regular lubrication, then lubrication-retention ball nuts and ballscrew systems incorporating long-life lubrication systems are the best option. Lubrication-retention ball nuts are designed to allow the lubricant to stay within the ball nut assembly for an extended period. Those with long-life lubrication systems, such as grease-filled or oil-filled units, go one step further by providing a continuous supply of lubrication to the ball nut and screw assembly.