Ball screw assemblies convert rotary motion into linear motion.
They are widely used to guide, support, position, and precisely move components and products in a variety of automation applications.
In order to ensure the machine's accuracy, repeatability, and service life, while reducing costs, it is essential to choose the appropriate ball screw.
This article summarizes nine factors to consider when selecting a ball screw assembly:
1.Determine load and speed requirements.
Although you may end up modifying the load or velocity specifications based on considerations of available component options, these are always application dependent.
2. Determine the Application Life Expectancy
The life expectancy should be determined based on the operating profile (how many hours per day, days per week, and weeks per year the ball screw operates) and the total life requirements of the ball screw.
For more complex applications, you can also choose to construct a complete motion profile. Each segment of the motion profile requires the starting speed, ending speed, operating time, and torque during that segment.
All ball nuts whose curves pass through or exceed the plotted points are applicable to this example.
The applicable life expectancy shown in this figure must not exceed the maximum static load capacity given in the ratings table for individual ball nut assemblies.
3. Determine Accuracy Requirements
Lead accuracy is the most commonly used metric for ball screw accuracy.
Lead of a Ball Screw refers to the distance that the nut travels along the screw's axis for each full rotation of the screw.
Lead accuracy is measured as the allowable travel variation (actual position versus theoretical position) per foot (imperial ball screws) or per 300 mm (metric ball screws).
Ball screws are available in two tolerance levels: transport (T-Grade) and positioning (P-Grade.)
T -Grade is the lower grade, typically with an accuracy of ±50 µm/300 mm or higher. Designers primarily use it for applications that require simply translating a load from one point to another.
When applications require the absolute position of the load at every point along its travel without the use of external feedback components, the higher-precision P grade is often specified, typically offering an accuracy of ±23 µm/300 mm or lower.
4. Determine Repeatability Requirements
Repeatability measures the system's ability to return to a specified position during operation.
The factor that influences repeatability is the free play between the nut and the screw, known as "backlash," which can be measured axially or radially.
The backlash of a ball screw is largely dependent on the diameter of the ball bearings.
To measure axial backlash, ensure the screw is stationary, then push and pull the ball nut axially while measuring its movement with a dial indicator.
Alternatively, place a dial indicator on the ball nut in the system, advance it 1 inch, and then return it to its original position. The deviation from zero is the backlash.
Repeatability is a quantified measure of this deviation, and ball screws are known for their low backlash and excellent repeatability.
Preload nuts can control backlash.
When using cross-lead preload, the lead is offset within the ball nut to provide a precise preload force.
This type of preload is often used in applications requiring both repeatability and high stiffness.
Dual-nut adjustable preload uses a compression spring to axially load the two ball nuts against each other. It is typically used in positioning applications where repeatability is critical.
Without preload, there is axial play between the screw and nut, typically 0.002 to 0.008 inches (0.06 to 0.2 mm), depending on size.
Preload is not typically used in transportation or vertical applications. Unpreloaded ball nuts have internal play between components, which means backlash.
Preloaded ball nuts, on the other hand, have no axial play, eliminating backlash and increasing stiffness.
Preload also increases the torque required to rotate the screw, measured as a percentage of the preload to the dynamic capacity.
A ball nut with a dynamic capacity of 1500 lbs and a preload rating of 10% has an internal preload of 150 lbs.
Precision threaded ball screws are typically used without preload. While preloaded ball screws can improve repeatability by eliminating backlash, they do not compromise accuracy.
Some precision screws are available with optional preloaded ball nuts.
Due to the complexity and additional machining, assembly, verification, and measurement, these costs are higher than non-preloaded nuts.
Ball screw assemblies can be preloaded with either a double nut or a single nut configuration.
There are three main types of preload:
single nut with oversized balls (4-point contact), single nut with skipped lead (2-point contact), and double nut (2-point contact).
Whether the screw is spun or ground also affects repeatability.
Rolled ball screws with standard preload are available with static loads up to 400 kN and dynamic loads up to 130 kN, lengths up to 8 m, diameters up to 80 mm, and accuracy grades up to P3.
Larger sizes and higher precision applications typically require spun or ground screw shafts.
5. Evaluate Mounting Issues
Mounting flexibility significantly impacts the available ball screw options.
The configuration of the end supports and the travel distance will determine the load and speed limitations of the ball screw.
Ball screw mounting options include supported and fixed.
Supported ends secure the ball screw at a single focal point and are not subject to bending moments.
Supported ends are generally easier to align and install than fixed ends, and therefore typically offer lower installation costs.
Fixed ends resist bending moment loads because they are typically based on two bearings spaced sufficiently apart to maintain the ball screw perpendicular to the plane of the rotating bearing.
Fixed ends offer higher column strength and higher critical speeds.
Figure 5 illustrates four mounting options. Components can be fixed at both ends, fixed at only one end, supported at the other, supported at both ends, or fixed at one end with the other end free.
Components should also be properly aligned with the drive system, bearing supports, and load for optimal performance and life.
Geometric orientation is another critical mounting factor.
Horizontal loads are equal to the payload weight multiplied by the coefficient of friction.
Vertical loads are equal to the weight. Position requirements determine which grade of ball screw is suitable for the application.
6. Determine whether the ball nut should be used in tension or compression mode.
Tension loads stretch the screw axially, causing it to elongate and potentially break.
Compression loads apply axial pressure to the screw and may cause it to bend.
A ball screw in tension can withstand loads up to the nut's rated capacity, so when using a ball nut in tension mode, simply check the nut's rated capacity.
A ball screw performs best when subjected only to axial loads.
The screw must withstand an axial load equal to and opposite to the load applied to the ball nut by the motor torque.
7. Consider Vibration and Harmonics
The limiting speed of a ball screw is typically its tendency to vibrate based on its natural frequency.
The critical speed is the speed at which the ball screw resonates.
The critical screw speed depends on the screw's root diameter, unsupported length, and end support structure.
For example, a 100-inch-long ball screw with a 1-inch root diameter, when fixed at both ends, has a natural frequency of approximately 18 Hz.
If both ends are supported rather than fixed, the natural frequency of the same ball screw decreases to approximately 8 Hz.
If the screw's rotational frequency matches its natural frequency, even slight imbalances in the screw can cause resonance.
Excessive bending and bowing can cause the screw to malfunction.
If the load, life, and speed calculations confirm that the selected ball screw assembly meets or exceeds the design requirements, proceed to the next step.
If not, a larger diameter screw will increase the load capacity and rated speed.
A smaller lead will reduce linear speed, while a larger lead will increase it.
8. Explore Other Design Possibilities
Other factors to consider when selecting a ball screw include the ball return system, application interface, environmental compatibility, and imperial vs. metric measurement scales.
The choice of ball return system can impact cost, installation, noise, and other application-specific factors, depending on the application.
Common return types include:
• External return tubes, commonly used with imperial screws, are cost-effective and easy to install, maintain, and repair.
• Internal button return systems, commonly used with low-lead screws, are compact, have no external radial protrusions, and are less complex to install. They also produce less noise and vibration than external return systems.
• Internal button return systems, commonly used with four-point contact, single nut, and preloaded assemblies.
• Internal end cap return systems, commonly used with high-lead screws, are compact, have no external radial protrusions, and are less complex to install. They also produce less noise and vibration than external return systems.
In a typical application interface, a ball nut flange connects the ball nut to the load. Threaded and cylindrical ball nuts are a common alternative, but many other options exist.
Most configurations are available with bearing supports and custom end machining to meet application interface requirements.
Environmental suitability is another factor influencing ball screw selection.
Some units offer standard wiper kits to protect components from contamination and provide lubrication.
There are many options for materials and coatings that can withstand different environmental conditions.
The choice between inch or metric scale can depend on the application, so making this choice too early in the process can significantly limit your options.
For example, if you decide to use an inch scale from the outset, you won't be able to meet high-precision needs, as inch scales are only suitable for transportation-grade applications.
However, since high-precision metric-scale ball screws can be used in applications that only require transportation-grade accuracy, you may find yourself weighing the pros and cons of each scale.
If these are your options, an inch-scale ball screw may be a better choice, as it can use an external ball return system to circulate the ball bearings. While external recirculation systems tend to be noisier, run rougher, and take up more space than internal recirculation systems, they are cheaper to manufacture and can use more ball bearings, increasing their load-carrying capacity or lifespan, depending on the application or user preference.
9. Understand Handling and Maintenance Requirements
Ball screws must be handled carefully before proper installation.
Shock to ball bearings can cause indentations or cracking of the bearing races, damaging the bearing races.
High loads or screw bending can cause the screw to bend.
Keep the assembly well-packed, well-lubricated, and stored in a clean, dry location, as debris and contaminants can clog the recirculation tracks, and high humidity or rain can cause corrosion.
Proper lubrication is critical to the performance and life of ball screws. Lubricants maintain the low-friction advantage of ball screw assemblies over other motion control technologies by minimizing rolling resistance between the balls and raceways and sliding friction between adjacent balls.
Lubricants are often taken for granted, but choosing the right lubricant for the specific application can ensure the proper operation of the ball screw throughout its expected life.
Lubricant can be sprayed directly to the area where lubrication is needed in a controlled flow rate, and as the lubricant flows through the ball nut, it removes contaminants.
Lubricant also provides a cooling effect. On the other hand, because oil can also contaminate the process fluid, a pump and metering system are required for proper filling.
Grease is less expensive than oil, requires less frequent application, and does not contaminate the process fluid.
On the other hand, grease is difficult to retain inside the ball nut and tends to accumulate at the ends of the ball nut's travel, where debris and wear particles can accumulate.
Incompatibility between old grease and relubricated grease can also cause problems, so careful compatibility checks are crucial. Load-bearing grease helps extend component life, but the total load rating does not change.
Noise is also a factor to consider. Larger ball screws use larger ball bearings and are therefore inherently noisier. External return systems are also inherently noisier than internal return systems.
Using a washer ball can reduce ball nut noise, but this reduces load capacity.
Selecting and applying grease correctly can also reduce overall noise levels. Minimizing or even eliminating backlash can also reduce component noise.
Maintenance teams should regularly inspect ball screws to avoid downtime and maximize service life.
Performing the following inspections will help identify indicators of failure:
Check for metal fragments that could cause damage, which could be signs of ball fracture or component wear.
• Measure clearances to verify component wear.
• Inspect raceways for wear, spalling, indentations, and contamination.
• Check to ensure the screw is lubricated and free of contamination and corrosion.
• Check that all connections are tight and free of vibration.
• Inspect the drive system to ensure that the drive torque is constant and the ball screw is running smoothly and quietly.
• The noise level should be the same as on the first day of operation.
The ball nut should move smoothly and regularly. Any change in noise level or "feel" indicates internal damage.
Conclusion
Ball screws provide an excellent method for converting rotary motion to linear motion in many applications, including those requiring high loads and close tolerances. To select the correct ball screw type for a specific application, design engineers must consider the advantages and performance of each type. Choosing the right technology can reduce design complexity, improve performance, and lower the overall cost of the assembly.
If you're ready to optimize your machinery with the perfect ball screw assembly, contact us today! Our team of experts is here to help you choose the right solution based on your specific needs. Don't let precision, repeatability, or efficiency become a challenge-let us guide you through every step of the selection process. Reach out now and get started on improving your operations with high-quality, reliable ball screw assemblies!
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1. Determine load and speed requirements.
2. Determine the expected life of the application.
3. Determine accuracy requirements.
4. Determine repeatability requirements.
5. Evaluate installation issues.
6. Determine whether the ball nut should be used in tension or compression.
7. Consider vibration and harmonics.
8. Explore alternative design options.
9. Understand operating and maintenance requirements.
