Common mistakes to avoid when selecting ball screws

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

In a linear motion system, a ball screw converts rotational motion into linear movement. Ball screws are both highly efficient and economical devices for motion control, ideally suited to applications where the performance required exceeds those of a lead screw.

Ball screws consists of a threaded shaft with a helical path around which ball bearings move; this is the screw element of the unit. The ball assembly behaves as the nut. On first appearance then, ball screws seem relatively straightforward; one of the easiest parts of a linear motion system to specify. But looks can be deceiving.

Product specification

Let's start with the two key ways of manufacturing ball screws: rolled and ground. The former is a lower cost method, resulting in an inexpensive, less accurate product. Grinding, on the other hand, produces a more precise thread with tighter tolerances and lead accuracies, but with a higher price tag to match. Both rolled and ground ball screws will do the job at hand, but if accuracy is key to your application don't make the mistake of basing your decision on price; choose ground ball screws.

The next key decision to be made should be the type of return you want: external, internal or end cap return. This is where things get a little more complicated as there are a number of factors that will determine which is right for you. If you have a high speed and/or high load industrial application, where the process doesn't generate a lot of dirt, an external return ball screw will generally be a good option. If high load or speed isn't a factor, but where there is limited space, an internal return ball screw could well be preferable.

Lastly, there's end cap return ball screws which are mostly quieter and cleaner than other designs, so are particularly popular in the pharmaceutical and food industries. Choose wisely.

Once you have narrowed down the field by deciding on the type of ball screw that's right for you, you need to get down to details and select individual ball screws that will work with the load to be carried and at the expected speed. This is frequently where things go wrong as people often miscalculate one or the other. Both speed and load calculations are based on the ball screw diameter, the end fixing and the free screw length. There are standardised ways of calculating these values but, if in doubt, ask the manufacturer/supplier. It may well save you time and money.

Ball screws are designed to meet various applications and budget, but one thing I would stretch the budget for is a ball screw that has been manufactured with the ball nut fitted to the screw during machining. This avoids any possibility of the balls falling out of the assembly and ensures that the ball nut and screw assembly remains within factory settings. It also means that your ball screw can be preloaded to meet your exact requirements. It doesn't add much to the overall cost of the ball screw and it's well worth that small added investment.

A good manufacturer will be able to provide you with additional options for your ball screw to meet operating conditions, especially environmental factors. Ball screws can be coated to prevent damage for example, or be supplied with a wiper to prevent contaminants from entering the ball nut, or bellows boots can be used to cover the screw and provide protection that way.

These are important considerations for more demanding environments such as chemical processing or where there is considerable dirt and dust. Investigate these options with your supplier to ensure ball screw longevity and reliable operation.

Product installation

Now you have the right ball screw for your application, but there are still some pitfalls to avoid at the installation stage.

The one we witness most frequently and that is most frustrating all round is not allowing enough space for the ball screw. This generally comes from lack of experience with ball screws as lead screws are more frequently used in linear motion systems due to their lower cost. The overall size of a ball screw device is larger than a comparable lead screw due to the need for the balls to be recirculated so be sure to factor this in.

Another regular error is to misalign the ball nut to screw, leading to side loading or eccentric loading which may cause the ball bearings to wear flat in places or even break out of the return tubes. It is critical that the ball nut only experiences axial loading, so take the time to correctly align the ball nut to screw. Where any bending moments are acting on the ball screw assembly, we recommend the use of linear rails.

Lastly, be sure to include braking mechanisms to eliminate the risk of back driving. Back driving occurs when the motor shuts off and the ball screw free-falls as a result. Brakes must be fitted as a safety precaution in this case. This is particularly pertinent if the load is to be carried vertically. Failing to fit braking mechanisms is a far too common mistake we see and doesn't end well.

Product maintenance

So, your ball screw has been appropriately selected, correctly installed, and is working away. But like all moving parts, it needs to be maintained.

The number one maintenance error by far is inadequate lubrication, causing collision of metal, which leads to an increase of friction and detrition, leading to premature ball screw or ball nut failure. This is easy to address though. Check the manufacturer's product information to gauge the correct amount of lubrication and guidance on the frequency of application.

When choosing lubricant type, be aware that oil is generally deemed the better option when speed exceeds 3-5 m/min. Using consistent grease will lead to increase of dynamic friction torque linearly along with increase of rotating speed. Correct lubrication will help dissipate heat and reduce metal-to-metal wear of components.

As part of the maintenance process, check to see if the ball screw is being operated at the correct speed it was designed for. Exceeding the critical speed will cause the screw to whip or vibrate and cause misalignment which can also lead to pickup fingers breaking, and the ball bearings coming out of the nut.

In a similar vein, do not remove the ball nut from the screw without using a mandrel as all the balls will fall out and you will need to replace the entire unit; a small mistake but one that is both costly and inconvenient.

Mistakes and can do happen, but learning from others' common mistakes mean you don't have to make the same ones.