Ball Screws, Lead Screws Face Off

Ball screws and lead screws often are not interchangable, and there’s always a trade-off between precision, rigidity and load capacity. I’m also told that specs don’t always predict performance. I’d like some experienced views on their real-world differences.

—from November ’08 Control Design

ANSWERS

More Than You Might Think
The three types of commonly used screw mechanisms are planetary roller screw, ball screw and acme or lead screw. There are significant differences between the three.

Lead screws use a typically triangular thread in the nut and on the screw shaft. The result is high contact area and a high load-carrying capacity and high stiffness, but with very low efficiency. The constant sliding friction and low efficiency limits the rotational, and thus linear speed available, and also limits the duty cycle to typically far less than 100%.

ADVERTISEMENT MORE CONTACT Comparing a similar size ball screw to the planetary roller screw design illustrates many more contact points on the roller screw, which can result in a load-carrying capacity up to 15 times greater than ball screws, as well as improved stiffness. Source: EXLAR

Ball screws offer high efficiency due to the rolling nature of the ball bearings. This allows reasonable speed and typically 100% duty cycle when used within the device ratings. Stiffness is much less than an acme screw, and, due to much lower contact area, the load-carrying capacity is much less.

Planetary roller screws offer the best of both worlds. Roller screws provide high thread-area contact through the use of threaded rollers. This equates to much higher load-carrying capacity and life than a ball screw. This also equates to much higher rigidity than a ball screw. Because the threaded rollers are captured in a planetary fashion, they are not subject to the conflicting friction of the ball bearings within a ball screw, which allows for higher rotational acceleration and the same or higher rotational speeds. The rolling nature of the threaded rollers provides the high efficiency of a ball screw, in contrast to the low efficiency of an acme screw.

John Walker, vice president, sales and marketing,
Exlar

This One’s Just Right
There is a general distinction between the applications for a ball and lead screw. Lead screws are applied in OEM applications where a “just right” solution is required. Lead screws can be tailored easily to provide the required performance for the right applications at the lowest cost. This sometimes requires lifecycle testing in the design phase, but for an OEM the extra upfront work is well worth the product cost savings. The ability of ball screws to carry much higher loads and achieve faster speeds with continuous duty cycles can be well worth their added cost. For end users, the predictability of ball screws makes them the best choice for fast integration and reliability. Factory automation relies heavily on ball screw technology for instance. Of course there are many OEM applications where a ball screw is required such as in the machine tool industry. For OEMs, performance and cost, not analytical predictability, ultimately dictate the technology.

The key difference between a ball screw and a lead screw is in the way the load is carried between the moving surfaces. A ball screw uses recirculating ball bearings to minimize friction and maximize efficiency while a lead screw depends on low coefficients of friction between sliding surfaces. A lead screw therefore typically cannot achieve the efficiency of a ball screw, which is around 90%. A quick review of tribology—study of wear and friction—leads one to conclude that sliding friction is inherently less predictable than power transmission using recirculating ball technology. The fatigue life equations, e.g. L10 life, are fairly reliable in their range of applicability. Thus, there is a fundamental difference in application of a ball screw and a lead screw due to the ability to predict performance and life.

With all the advantages of a ball screw—load capacity, rigidity, efficiency, duty cycle, predictability—there is a price to be paid. Although their performance-to-cost ratio is very high when compared to other means of translating linear motion, a ball screw design is more complex, requiring hardened precision bearing surfaces and a ball recirculation mechanism. On the other hand, a lead screw is very compact, offers great design flexibility, is quiet when properly applied, is generally corrosion-resistant and can be made to self-lock for vertical applications. They are very capable in many applications, but they do have their limitations as well.

Robert  Lipsett, engineering manager,
Thomson BSA/Danaher Motion

Let’s Compare
A fundamental difference is that ball screws have circulating balls, which eliminates the sliding friction of lead screws. Here’s a tally of the ball screw’s pluses and minuses.

• Less heat buildup due to friction
• More efficient running due to the lower friction (90+% efficiency, depending on lead angle)
• High efficiency promotes smaller motor size
• Lower system cost of ball screw, bearings, motor and drive vs. lead screw
• Ball screw is higher in cost, component to component
• Better suited for applications where high throughput, high speeds and continuous or long cycle times are required, due to the lower friction and lower heat buildup
• Not self-locking, susceptible to back-driving due to the high efficiency
• Can be a problem in vertical application
• Requires brake to keep the load from falling
• Typically, lead and positioning accuracy are much better with ball screws
• No stick-slip.