Key Questions to Ask When Ordering Lead Screw and Screw Nut

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.

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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.

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.

When engineers come to me looking to utilize a lead screw to actuate their application, I get them thinking about four main data points that can help determine the majority of questions I need to ask to help them utilize lead screws and polymer nuts properly. Those data points are payload, speed, duty cycle and stroke.

In this blog post, I'll dive deeper into each of those data points and discuss additional factors that you should consider when choosing the right lead screw for your application. 

Payload & Speed

The first step is determining the quantified axial payload. This figure helps determine how much force is required to drive the application. 

Second, you want to take the application's speed into account. I usually ask an engineer a certain set of questions to help determine what the goal is for the application. It could be as simple as how many seconds do you want to complete this stroke in? Or, how quickly does it need to get from point A to point B?

The RPM (feed rate / lead of desired screw) is important but is typically determined after you have figured out the application&#;s linear feed rate, also known as the linear distance traveled per minute. Once you have that figure, you can determine the RPM of the application by taking a look at your desired lead screw and noting the distance traveled per revolution. 

Duty Cycle & Stroke

When working with a polymer lead screw nut, one of the more important factors that's going to determine the lifetime of the part is the application&#;s duty cycle. How often is it going to be running per minute, per hour, per day?

This is extremely important for a polymer part because if the application has a higher load and a higher speed, the wear is going to be higher for the part. So, to properly spec a lead screw size, you need to take duty cycles into account to make sure that the PV values are in check.

Also, make sure you ask yourself how long is the application stroke and how long does the overall screw need to be?

Additional Considerations

Payload, speed, duty cycle and stroke are four extremely important data points; but that's not all that should be considered.

You should be asking yourself whether or not you need a lead screw that's going to be self-locking or if you want a part that is able to back drive. You should be thinking about the types of linear guides that you're using for an application. Are you going to be using a sliding guide? Or, are you going to be using a rolling guide, like a reciprocating ball bearing? Those types of questions are going to help determine whether or not you can use a smaller or a larger lead screw in your application. 

You also want to take into account how you're going to drive this application. Are you going to be using a DC motor or a stepper motor? Are you going to be hand cranking this application? These are some of the secondary factors that you want to consider when you're trying to narrow down what lead screw you use in your application. 

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