An engineer usually has several options when selecting screws best for movement-centric applications. The decision to make is always centered on whether a ball screw assembly or lead screw assembly will work best in a particular application. Even though lead screws are a cheaper alternative to ball screws, it is not always easy to determine which screw assembly to work. How do Lead Screws and Ball Screws Differ? Generally, most linear motion control systems can use either of the screws, but there are critical differences between the two that will help you determine which one to use. The two types of screws differ in the way a load is carried between the moving surface. Ball screws have recirculating ball bearings within the screw that reduce friction between the surfaces, thereby maximizing efficiency. Lead screws, on the other hand, utilize helical threads and a nut and primarily depend on low friction coefficients between the sliding surfaces.
The rolling nature of a ball screw helps eradicate the sliding friction associated with lead screws. Ball screws have a much intricate design compared to lead screws. While ball screws need a hardened surface for precision bearing, lead screws are more compact and provide excellent design flexibility. They make less to no noise when properly applied and can self-lock during vertical applications. What Are the Uses and Benefits of Lead Screws and Ball Screws?Lead screws are cheaper than ball screws and do not need a braking system because of their self-locking nature. On the other hand, lead screws perform better in vertical applications and are mostly utilized in OEM applications. Compared to ball screws, they are less noisy, with a flexible configuration, and need no upfront testing. Ball screws have a high efficiency and speed levels suitable for many applications. Engineers prefer these screws because they are more reliable and replaced less frequently than the lead screws. They require less torque, making cheaper motors ideal for running them. A ball screw assembly is ideal for applications that need smooth motion due to their limited friction. What Factors Should You Consider When Choosing Ball or Lead Screws? As much as they can accomplish the same tasks, some strengths and weaknesses make each one of them suitable in one application over another. Let us explore some factors that should influence your decision in choosing the right screws for your application.
1. Material
Engineers often prefer lead screws for operations in challenging environments, including conditions that ball screws will find impossible to withstand. This is because they lack recirculating elements, which are vulnerable to debris and contamination. Lead screws can be made from various materials like bronze and plastics. Such materials can work in just any kind of environment. The shafts can be made of aluminum, carbon, or stainless steel for additional protection.
2. Load Capacity
For heavy loads, ball screws are a better option compared to lead screws. They have a high load capacity due to their recirculating steel balls that support the load. While lead screws with bronzed nuts can drive heavy loads, friction increases as the load increases, thereby lowering their duty cycle. Ball screw’s life is also predictable based on the L10 bearing equation. With their wear characteristics, it is hard to predict lead screws life.
3. Vertical Operations
Thanks to a self-lock mechanism, lead screws usually have a lower chance of back driving than ball screws, which are very likely to allow the load to fall if there is a loss of power to the motor. Although the low efficiency of a lead screw is usually considered a negative factor, they are very effective in vertical applications. Lead screws can prevent the load from back driving when the motor is powered off. This comes in handy in applications where the load must remain on top of its strokes as other processes happen, such as tray stacking. Lead Screws do not need a braking device to perform this task, unlike the ball screws, which require a holding or a braking device.
4. Lubrication
Oils and greases cause contamination in cleanroom environments. Ball screws require frequent lubrication, unlike the stainless steel lead screws. This kind of lead screw comes with a self-lubricating polymer nut that makes it more cleanroom-friendly. Medical laboratory and electronic applications mainly utilize this concept to maintain the integrity of the cleanroom.
5. Duty Cycle
The critical speed of the screw limits the maximum speed of both ball screws and lead screws. Inherently, ball springs have low friction, thus producing low heat, which means they can withstand higher duty cycles than the lead screws. The duty cycle is only taken into account in ball screw selection when ascertaining the travel amount the screw will attain in its calculated life. Utilizing a high lead ball screw provides great carriage speed for applications needing fast moves. Designers will opt for ball screws because of their long life and low wear.6. Power SteeringWhen choosing screws for automotive applications, ball screws are the most preferred, especially in the automotive steering system. Their primary function is to simply convert rotary motion into linear motion in the power steering system, making them feasible and economical. For this reason, a rolled ball screw is quite useful in motion translation.
7. Washdown Applications
Lead screws are perfect for washdown environments due to a wide selection of materials and coatings for screws and nuts and the lack of lubrication to be flushed out. Mostly, they are used in food and beverage processing and packaging applications where equipment must be thoroughly washed and sanitized. They are also suitable for water jet applications due to their ability to operate even when fully submerged in water.
8. Low Noise Applications
For applications that require low noise, like laboratory automation or medical imaging equipment, an engineer will opt for lead screws. Unlike ball leads that generate much noise due to their steel on steel rolling elements, lead screws produce little noise owing to their sliding motion. Printers, scanners, and appliances are some of the consumer devices requiring low noise components, thus utilizing lead screws. ConclusionChoosing the wrong product, such as an oversized lead or ball screw, will result in unneeded costs, recurring replacements, and design rework, or even worse, failure. Use this guide to make the appropriate choice for a ball screw or lead screw assembly.
