Ball Screw End Fixity Introduction
End fixity refers to the method by which the ends of a ball screw are supported within a system.
When using a ball screw system, in any application, choosing the correct fixing method is going to be critical to achieve the highest performance and longest lifetime.
A ball screw assembly consists of three main components: the screw, the ball nut and the end support bearings. The end support bearings have a significant impact on the rigidity, buckling load and maximum speed of a ball screw system.
There are two main types of supporting a ball screw, floating (often referred to as simple) and fixed. A floating end support has a single radial ball bearing which provides support against radial loads. Fixed-end support uses an angular contact thrust bearing, which takes axial and radial forces on the screw.
Understanding these types of support methods and deciding which end support fixity arrangement to use is key when selecting your ball screw configuration. Let’s have a look at the different arrangements for supporting your ball screw.
Kiwi Motion stocks and supplies complete rolled ball screw systems to the whole of the UK & Ireland. As linear experts, we can help you select the correct screw, nut, and end support bearings for your machine design.
Estimated reading time: 10 minutes
Table of contents
End Fixity Arrangements
There are four different combinations of ball screw support arrangements to choose from. These are fixed-fixed, fixed-floating, floating-floating, and fixed-free. Each combination supplies a different level of rigidity to the screw assembly and each one will vary in suitability for varying machine designs.
Fixed – Fixed
The fixed-fixed arrangement stands as the pinnacle of rigidity among end fixity configurations for ball screw systems. This unparalleled stiffness is primarily attributed to the presence of angular contact thrust bearings on both ends, which collectively contribute to the exceptional performance of the system.
Angular contact thrust bearings are engineered to withstand high axial loads while maintaining minimal deflection, thereby ensuring excellent stiffness throughout the operation. This stiffness translates into enhanced precision, reliability, and stability, particularly in applications where precision machining or high-speed operations are paramount.
Moreover, the robust support provided by angular contact thrust bearings significantly bolsters the system’s ability to withstand buckling loads, which is crucial in scenarios where heavy loads or dynamic forces are involved. This capability not only enhances the safety and reliability of the machinery but also extends the operational lifespan of the ball screw assembly.
However, it’s important to acknowledge that the fixed-fixed configuration does come with a caveat. Unlike other configurations that allow for some degree of axial movement to accommodate thermal expansion, the fixed-fixed setup lacks this flexibility.
In environments where temperature variations are prevalent, this limitation may pose challenges, potentially leading to binding or increased stress on the components.
Despite this drawback, the benefits of the fixed-fixed configuration far outweigh its limitations, particularly when compared to configurations such as fixed-free, which offer significantly lower rigidity and performance capabilities.
Furthermore, an interesting aspect of the fixed-fixed arrangement is its impact on the unsupported length of the ball screw. With thrust bearings positioned on both ends, the maximum distance between the ball nut and either fixed bearing occurs precisely when the nut is at the midpoint of its stroke.
Consequently, the unsupported length is effectively halved, minimising the risk of deflection or instability during operation.
Fixed – Floating (Simple)
The most widely adopted method for end fixity, known as the fixed-floating (simple) configuration, stands out due to its exceptional balance in providing both axial and radial support for ball screw systems. By strategically incorporating angular contact thrust bearings on one end and a single radial ball bearing on the other, this configuration effectively distributes forces along the length of the screw, ensuring stability and smooth operation.
The utilisation of angular contact thrust bearings facilitates the absorption of axial forces, allowing the ball screw to efficiently transmit and withstand axial loads. Meanwhile, the presence of a radial ball bearing provides crucial radial support, enhancing the overall rigidity of the system and minimising deflection under load.
One of the key advantages of this configuration lies in its ability to accommodate thermal expansion effectively. As machines operate under varying temperatures, thermal expansion can exert significant stress on components. However, the fixed-floating configuration mitigates this concern by allowing for controlled axial movement, thereby reducing the risk of binding or damage due to thermal expansion.
Moreover, this configuration boasts impressive performance metrics, including high critical speeds and robust support against buckling loads. The combination of axial and radial support, coupled with its versatility in accommodating thermal expansion, makes it a preferred choice across a wide range of machine designs and applications.
Whether in precision machining, robotics, aerospace, or automotive industries, the fixed-floating configuration offers engineers and designers the flexibility and reliability needed to achieve optimal performance and longevity in their machines. Its balanced support characteristics and adaptability to varying operating conditions make it a cornerstone in the realm of ball screw technology.
Floating – Floating
The floating-floating configuration offers a straightforward and easily mountable end fixity solution for supporting ball screw systems. By employing this setup, engineers benefit from simplified installation processes, making it particularly attractive for applications where ease of mounting is prioritised.
In this configuration, both ends of the ball screw are equipped with radial ball bearings, which excel in providing radial support to the screw shaft. This radial support ensures smooth and stable rotation of the screw, allowing for precise positioning and motion control in various machine designs.
However, it’s essential to note that while the floating-floating configuration excels in radial support, it may exhibit some degree of axial compliance. This means that under axial loads, there may be a slight amount of deflection or movement along the axis of the screw. While this axial compliance may be acceptable in applications where strict rigidity requirements are not critical, it’s crucial to carefully evaluate its implications, especially in high-demand scenarios.
In environments where precision, accuracy, and stability are paramount, the floating-floating configuration may fall short of meeting stringent performance expectations. High-demand applications, such as precision machining or high-speed automation, may require greater rigidity and resistance to deflection to ensure optimal performance and reliability.
Therefore, while the floating-floating configuration offers simplicity and ease of installation, it’s essential to weigh its benefits against its limitations, particularly in terms of rigidity and performance capabilities. Engineers must carefully assess the specific requirements of their applications to determine whether the floating-floating configuration is suitable or if an alternative configuration with higher rigidity and stability is warranted.
In conclusion, while the floating-floating configuration provides radial support and ease of mounting, its limitations in terms of rigidity and axial compliance must be carefully considered, particularly in high-demand scenarios where precision and stability are critical. Engineers should conduct thorough evaluations to ensure that the chosen configuration aligns with the performance requirements of their applications.
Fixed – Free
The fixed-free configuration represents the most basic approach to supporting a ball screw system, characterised by one end of the screw being “free” from any support mechanism. While this setup offers simplicity and ease of implementation, its suitability is limited to specific scenarios where the trade-offs are acceptable.
In applications where short screw lengths and low-speed operations are prevalent, the fixed-free configuration may suffice. For instance, in compact machinery or systems with limited space constraints, the absence of support at one end allows for easier integration and reduced complexity.
However, it’s important to recognise the inherent drawbacks of the fixed-free configuration. The “free” end of the screw significantly diminishes the system’s ability to withstand buckling loads – a critical consideration in applications subject to heavy loads or dynamic forces. Without adequate support, the unsupported portion of the screw is susceptible to bending or deflection under load, potentially compromising positional accuracy and overall system stability.
Furthermore, the fixed-free configuration imposes limitations on the system’s speed capabilities. The absence of support at one end introduces a degree of instability, especially at higher speeds, leading to vibrations, resonance, and reduced precision in motion control applications. This can adversely impact productivity, efficiency, and the overall longevity of the system.
How End Fixity Affects Ball Screw Performance
The significance of the end support bearing arrangement in ensuring the longevity and optimal performance of a ball screw assembly cannot be overstated. It serves as a critical determinant of the system’s ability to withstand dynamic forces, maintain stability, and achieve precision motion control. Two key parameters of ball screw performance – critical speed and buckling load – are directly influenced by the chosen end support method.
The relationship between the end support method and ball screw performance can be clarified by a factor that varies based on the rigidity of the configuration. This factor is at its lowest for the least rigid option, namely the fixed-free configuration, and reaches its peak for the most rigid configuration, the fixed-fixed setup. This underscores the pivotal role that rigidity plays in enhancing the critical speed and buckling load capabilities of the system.
By opting for fixed-end support on at least one end of the screw, engineers can effectively mitigate the risks associated with excessive unsupported length. This reduction in unsupported length minimises the likelihood of failure under buckling load and critical speed conditions, thereby enhancing the overall reliability and durability of the assembly.
Moreover, the presence of fixed-end support facilitates the accommodation of heat build-up and thermal expansion within the system. This is particularly crucial in environments where temperature variations are common, as it helps prevent binding, premature wear, and other detrimental effects associated with thermal stress.
Our Capabilities
At Kiwi Motion, we understand the importance of precision and reliability in ball screw applications. That’s why we offer tailored machining services for ball screw ends, ensuring correct tolerances for concentricity, axial run-out, and radial run-out of bearing surfaces. Whether it’s machining to specific drawings or standard end support machining details, we prioritise precision to meet the unique requirements of each application.
As a leading supplier of linear motion solutions across the UK & Ireland, Kiwi Motion provides comprehensive support beyond just ball screws. We offer expertise and supply solutions for the entire linear motion system, including linear rails, guides, shafts, bushings, and crossed roller bearings. With our extensive range of products and industry knowledge, we’re committed to helping our customers achieve optimal performance and reliability in their motion control applications.
End Fixity Conclusion
In conclusion, selecting the right end fixity configuration for your ball screw system is paramount to achieving optimal performance, longevity, and reliability. Each configuration offers its own set of benefits and limitations, catering to specific requirements and preferences in various machine designs and applications.
Whether you opt for the unparalleled rigidity of the fixed-fixed configuration, the balanced support characteristics of the fixed-floating setup, the simplicity of the floating-floating arrangement, or the basic functionality of the fixed-free method, careful consideration of factors such as rigidity, thermal expansion tolerance, and performance requirements is essential.
At Kiwi Motion, we stand ready to assist you in navigating the complexities of ball screw end fixity. With our expertise in linear motion solutions and tailored machining services, we ensure precise tolerances and optimal performance for your machine design needs.
By understanding the impact of end fixity on critical parameters like critical speed and buckling load, engineers can make informed decisions to enhance the reliability and longevity of their ball screw systems. With the right end fixity configuration in place, you can confidently propel your projects forward, achieving precision motion control and operational excellence in every application.
Shop for Ball Screws and Supports
