The gap is narrowing between applications that require ball screws and those that are suitable for lead screws, and it’s becoming more difficult for engineers and designers to determine which technology is best for their machine. Advances in lead screw designs and materials have given them higher load capacities and better positioning accuracy, making them a sound choice for more than just the low-cost, low-precision applications that they were used for in the past.
Lead screws incorporate a screw and a plain nut (with no rolling mechanisms). They rely on the angle of the thread and friction between the nut and the screw to produce motion. The screw is typically made of either carbon steel or stainless steel, while the nut can be bronze, plastic, or polymer.
Ball screws also incorporate a screw and a nut, but in this case, the nut houses recirculating balls to carry the load. This design minimizes friction and provides high efficiency.
Despite the growing overlap in their technical capabilities, however, there are still best-fit applications for each technology. In some cases, the application requirements simply exceed the capabilities of a lead screw, and only a ball screw will perform as needed. Below are four common application criteria that (in most cases) require a ball screw.
With recirculating steel balls to support the load, ball screws have a higher load capacity than comparably sized lead screws with plastic or polymer nuts. Lead screws with bronze nuts can drive heavier loads, but as the load increases, so does friction. And higher friction means lower duty cycle. (More on duty cycle below…)
Also related to load, ball screw sizing is based on the L10 bearing life equation, which provides a statistically proven estimation of the screw’s life, in meters or rotations traveled. The wear characteristics of a lead screw make life nearly impossible to predict. Lead screws with plastic nuts can be selected based on their PV value, but this gives a range of pressures and velocities that the screw can withstand; not an estimation of life.
High accuracy requirements
Unlike lead screws, which have backlash between the nut and the screw, ball screws can be, and often are, preloaded to remove backlash. This is typically done by using balls that have a slightly larger diameter than the space between the raceways of the screw and the nut. (Some lead screws are offered with nut designs that eliminate backlash, but these typically add friction and reduce efficiency.)
In addition, ball screws are classified by ISO, DIN and JIS standards regarding their lead deviation, so choosing the proper accuracy class ball screw is straightforward.
Rolling elements, such as the balls in a ball screw assembly, rely on point contact to support a load, whereas sliding elements, like the screw and nut of a lead screw assembly, rely on line contact. Point contact produces less friction, which gives the assembly higher efficiency. Case in point: ball screw efficiency is almost always 90 percent or better, while lead screw efficiency is normally below 50 percent. High efficiency translates to lower required motor torque, which means that a smaller motor (and related components) may be suitable for the application.
Demanding duty cycle
The maximum speed of both lead screws and ball screws is limited by the critical speed of the screw. However, the lower friction inherent in ball screw designs means they produce less heat, and therefore, can withstand higher duty cycles than lead screws can. In fact, duty cycle is only considered in ball screw selection when determining the amount of travel that the screw will achieve in its calculated life. On the other hand, duty cycle and heat generation must always be taken into account when selecting a lead screw.
Like plain linear guides with sliding friction, lead screws with plastic nuts must adhere to their maximum PV (pressure x velocity) value.
Choosing the wrong product—whether it be an oversized ball screw or a lead screw that doesn’t provide sufficient life—can lead to unnecessary costs, design rework, frequent replacement, or even failure. Here we’ve reviewed common requirements that call for the use of ball screws over lead screws. To see when lead screws are the more appropriate choice, check out this article.
Feature image credit: NSK Ltd.