4 Advice to Choose a Lead Screw And Nut Assembly Supplier

When determining if an application calls for a lead or ball screw, your decision should be guided through evaluating the inherent advantages and disadvantages of each. This evaluation process can have a significant impact on the performance and economic benefits of linear motion projects.

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The first step comes from understanding how they differ. Both ball and lead screws use threaded rods to convert from rotary to linear motion. Ball screws, as the name would imply, use recirculating ball bearings to minimize friction. Lead screws, by contrast, rely on a low coefficient of friction from sliding surfaces.

A ball screw (left) uses ball bearings to minimize friction and maximize efficiency, while a lead screw (right) depends on low coefficients of friction between sliding surfaces to transfer motion.

Functionally, ball and lead screws differ according to the following characteristics: motion profile, degree of precision, environmental resilience, level of customization, predictability, operating life, and ROI. Evaluating how these characteristics apply to an application will help to determine the right solution.

1) What is the motion profile of your application?

For applications of high speed or load (those greater than 100 lbs), ball screws should be considered first. Their multi-ball design provides more surface contact, which enables them to carry higher loads faster, while achieving higher application speeds with a continuous duty cycle.

For applications of lower speed or load, lead screws have a slight advantage. This is also true for applications with a vertical configuration, as lead screws can self-lock and counter the forces of gravity.

Unlike belt-driven, ball screw, or rack-and-pinion systems, lead screws allow IV pumps to dispense fluids with total consistency and not be affected by pushback.

2) To what degree is efficiency a consideration?

Ball screws provide a lower coefficient of friction than sliding contacts, so they have higher efficiency. Lead screws generally have efficiency of 30-70%, where ball screw efficiency ranges from 85-95%.

This increase in efficiency means that for ball screw applications, consideration for the braking of high inertia or overhung loads may be required.

It is also important to view efficiency from more than just an energy consumption perspective, and consider conservation. More efficient transfer of energy can mean a smaller, more compact prime mover.

3) What level of accuracy and repeatability is required?

Accuracy is the potential variation in travel distance within a specified screw length. It is an expression of many factors, including diameter, stroke, production process and measurement scale.

All ball screws require a recirculation system for the ball bearings. Return systems, such as this one shown, are optimized for each diameter and lead combination to maximize load capacity, minimize footprint and guarantee smooth operation.

Ball screws tend to be more accurate, although this is not as significant a differentiator between ball and lead screws as is efficiency. Where a metric rolled ball screw might be accurate to 23 microns over a 300 mm span, a rolled lead screw would have an accuracy of 75 microns over that same span.

Repeatability is the ability to consistently achieve the desired accuracy. Like accuracy, it is a function of many variables. Ball screws tend to have greater repeatability due to their adjustable nut design through preloading the ball bearings. Preloading a ball nut tightens the spacing between the ball bearings, providing less ‘play’ or backlash in the nut-to-screw movement. This, in turn, results in improved accuracy and repeatability. Lead screws are available with anti-backlash designs, but unlike ball screws with circulating bearings, the lead screws’ sliding friction design can only have limited pressure to reduce its tolerances before preventing the lead screw from rotating.

4) What are the environmental considerations?

For applications that would benefit from a quieter, smoother operation, lead screws have the advantage. With fewer moving parts, they don’t have the noise that results from ball recirculation.

Applications where cleanliness and corrosion resistance are important also play into the lead screws’ advantage. Often not requiring lubrication, lead screws can result in a simpler design that also benefits from being more tolerant of contamination. For applications where the use of a ball screw can’t be avoided, means for mitigating the egress of contaminants should be put in place.

5) How will you determine ROI?

The relative simplicity of lead screws makes them easier to manufacture, which is why they are typically less expensive. System cost, however, cannot be judged independently of the application context.

For the OEM producing lighter-load medical equipment such as insulin pumps and medical analyzers, smooth and quiet operation are often valued over high accuracy, efficiency or repeatability. Thus, lead screws are likely the most economical solution.

For those designing high-precision systems for aircraft, power steering, robotics, semiconductors, and many factory automation applications, achieving core functionality will require paying the premium for ball screws.

The long-term cost of ownership to the end-user should be factored as well. Comparatively, ball screws have a slight advantage over lead screws in lifecycle cost. This is due to reduced friction, heat and wear.

For more information, please visit Lead Screw And Nut Assembly Supplier.

On the other hand, the lubrication-free feature of lead screws provides a definite economic benefit for end-users, who would otherwise have to factor maintenance costs into their ROI.

In Summary

When making a determination between lead or ball screw for a given application, the answer lies in the nuance of the application itself. The heavier the application load, faster the speed requirements, tighter the efficiencies, higher the degree of accuracy and/or repeatability the more likely it is that your application calls for a ball screw. If, on the other hand, you are moving a light load and require a clean, quiet, vibration-free or customizable operation, then the more economical lead screw is probably the best fit. Engaging your in-house engineer or third-party specialist will be key, but in the meantime, your answers to the above questions will get you started in the right direction.

In the realm of engineering and mechanical design, the choice of ball screws plays a pivotal role in ensuring the smooth and efficient functioning of various systems, from general machinery to high-precision CNC equipment. Ball screws, with their ability to convert rotary motion into linear motion with minimal friction, are indispensable components in a wide array of industrial applications.

To make an informed decision during the selection process, it is essential to grasp the key parameters that influence the performance and capabilities of ball screws. These parameters, including the nominal diameter, lead, length, nut type, and precision, are pivotal factors that determine load capacity, linear speed, and positioning accuracy. Understanding these parameters in-depth is crucial for engineers and designers seeking to optimize the performance and reliability of their mechanical systems.

This comprehensive guide aims to provide a detailed understanding of the critical parameters involved in the selection process of ball screws. By delving into the significance of each parameter and its implications for specific applications, this guide seeks to empower engineers and industry professionals with the knowledge necessary to make informed decisions, ensuring the seamless integration of ball screws in their designs for optimal performance and longevity.

During the selection process of ball screws, the main parameters to consider for the ball screws themselves are as follows:

1. Nominal Diameter

Refers to the external diameter of the screw, with common specifications including 12, 14, 16, 20, 25, 32, 40, 50, 63, 80, 100, 120. It's important to note that manufacturers generally stock products in the range of 16-50, while other diameters are mostly made to order with lead times approximately ranging from 30 to 60 days. Nominal diameter is generally proportional to the load capacity, with larger diameters accommodating heavier loads. For specific values, consult the manufacturer's product samples. It is advisable to select a diameter between 16 and 63.

2. Lead

Also known as pitch, it represents the linear distance the nut travels with one complete rotation of the screw. Common lead options include 1, 2, 4, 6, 8, 10, 16, 20, 25, 32, 40. Stock products generally offer 5 and 10 for medium and small leads, while larger leads such as , , , , are typically made to order by most manufacturers. Lead is related to linear speed, with larger leads resulting in higher speeds. It is recommended to choose leads of 5 or 10.

3. Length

There are two concepts of length: overall length and threaded length. Some manufacturers only consider the overall length, while others need to provide the threaded length. Threaded length comprises the total length of the thread and the effective stroke.

The length of the thread is calculated as the sum of the effective stroke, nut length, design margin (considering the compression length of the protective cover, generally calculated as 1/8th of the maximum length of the protective cover). During the design phase, the total length of the screw can be approximately calculated by adding the effective stroke, nut length, design margin, the length of support at both ends (bearing width + lock nut width + allowance), and the length of the power input connection (if using a coupling, approximately half the length of the coupling + allowance). 

It is particularly important to note that if the length exceeds 3 meters or the length-to-diameter ratio is greater than 70, it is advisable to consult the manufacturer's sales personnel in advance to confirm the possibility of production. Generally, conventional products from domestic manufacturers have a maximum length of 3 meters and special products of 16 meters, while foreign manufacturers offer conventional products of 6 meters and special products of 22 meters. It is recommended to select lengths of 6 meters or less; for lengths exceeding this, it is more cost-effective to use gear racks.

4. Nut Type

Product samples from various manufacturers present multiple nut forms, with the initial letters of the model number representing the nut type. Flange forms include circular flanges, single-cut edge flanges, double-cut edge flanges, and no flanges. Based on nut length, there are single and double nut types. It is essential to note that single and double nuts do not differ in terms of load and rigidity, contrary to what may be suggested by sales personnel.

The primary difference between single and double nuts is that the latter allows for preload adjustment while the former does not. Additionally, the price and length of the latter are roughly twice that of the former. It is advisable, whenever possible, to select standard forms during the selection process to avoid issues with spare parts lead time during maintenance. For applications involving frequent motion and high precision maintenance, choose double nuts, while for other applications, choose double-edged single nuts. It is recommended to opt for an internal circulation double-cut edge flange single nut form whenever possible.

5. Precision

Ball screws are classified into P and T classes according to GB standards, representing transmission and positioning classes, with precision levels ranging from 1, 2, 3, 4, and so on. Foreign products generally do not distinguish between transmission and positioning and are represented uniformly by C0 to C10 or specific numerical values. Generally, for general machinery or ordinary CNC machinery, select C7 (positioning error within any 300 strokes is ±0.05) or below.

For high-precision CNC machinery, choose C5 (±0.018) or higher, and for optical or inspection machinery, choose C3 or higher. It is essential to note that there is a significant correlation between precision and price. Moreover, the concept of precision pertains to both assembly and maintenance, indicating that the error of the lead screw cannot fully represent the error of the entire ball screw assembly. The fact that the factory precision is qualified does not guarantee the maintenance of this precision within the rated service life. This is a matter of reliability and is related to the manufacturer's production process.

The aforementioned parameters are the primary ones to consider during the selection process. Additionally, the following parameters need to be taken into account: the number of ball circuits, ball diameter, manufacturing method code, and preload level.

6. Number of Ball Circuits

This parameter is generally indicated after the lead in the model number, for instance, -4, where the "-4" signifies the number of ball circuits. Discussing the concept of cycle method may be complex; however, users can understand it simply as the number of cycles the rolling elements complete. A larger value signifies a greater load-bearing capacity and longer nut length.

7. Ball Diameter

This parameter refers to the diameter of the rolling elements. While it may not be apparent in the model number, it is indicated in the technical parameter tables of various manufacturers. It is generally related to the nominal diameter and lead, and users need not concern themselves with it. However, for some users, if improper use causes the ball to fall out of the nut and requires reinstallation, this parameter becomes important to consider. The dimensions are generally precise up to 0.001.

8. Manufacturing Method Code

Ball screws are primarily manufactured using two methods: rolling and grinding. The former is also known as rolling manufacture or transformation and is generally denoted by "F," while the latter is known as grinding manufacture, denoted by "G." Due to differences in the manufacturing process, the two methods achieve different levels of precision. Currently, the highest precision that can be achieved by the rolling method is C5. As far as I know, only Han Yi can achieve this precision. Grinding can produce even higher precision products. However, it is essential to note that the two manufacturing methods do not have a direct causality with precision and performance. In other words, if you choose a precision of C7, it does not matter how it was manufactured.

Rolling belongs to batch production, while grinding belongs to precision manufacturing. The production efficiency of the former is much higher than that of the latter, but the manufacturing equipment cost of the former is also much higher than that of the latter. In other words, the entry threshold for grinding screws is lower, while the entry threshold for rolling production is higher. Manufacturers that can produce rolling screws can generally also produce grinding screws, but manufacturers that can produce grinding screws may not necessarily produce rolling screws. Therefore, if the same precision product is available in the rolling version, it is advisable not to choose the grinding version for the simple reason that it is more cost-effective. Additionally, it should be clarified that rolling and grinding only refer to the lead screw, while the nut is entirely manufactured by grinding.

9. Preload

Also known as pretension, users do not need to know the specific preload force and preload method. It is sufficient to select the preload level according to the manufacturer's samples. The higher the level, the tighter the fit between the nut and the screw, and the lower the level, the looser the fit. The principle to follow is to select a slightly higher preload level for scenarios involving a large diameter, double nuts, high precision, and large driving torque, and a lower level for the opposite scenarios.

Conclusion

Selecting the right ball screws for a particular application involves a careful consideration of various parameters, ranging from nominal diameter and lead to nut type and precision. Each of these factors contributes significantly to the overall performance, load capacity, and precision of the system. By understanding the nuanced relationship between these parameters and the specific requirements of a given application, engineers and designers can make informed choices that lead to the optimal functioning and longevity of their mechanical systems.

Moreover, the importance of consulting with manufacturers and understanding their product offerings, lead times, and customization options cannot be overstated. Collaboration with knowledgeable sales personnel can provide valuable insights and ensure the smooth integration of the selected ball screws into the overall design.

In an ever-evolving industrial landscape, where precision, efficiency, and reliability are paramount, a comprehensive understanding of ball screw parameters is essential for achieving desired performance outcomes. By applying the knowledge gained from this guide, engineers can navigate the complexities of ball screw selection and pave the way for the successful implementation of these crucial components across various mechanical applications.

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