Choosing the correct drive screw type is critical to getting the best actuator performance, accuracy, and repeatability.
Chris Maupin • Program Manager and Engineer | Hunt Valve Actuator Division
Screw drivetrains are the most common types of electromechanical linear actuators. A screw drivetrain acts as a linear force generation system, converting a rotary torque input from a motor into linear thrust and motion. Since electromechanical actuators are also used for linear positioning, they must be able to ensure accurate and repeatable levels of linear motion, while still being capable of providing the required force for the application. A screw mechanism produces linear motion by rotating either a nut or, more typically, a screw in an assembly to generate motion.
While the basic principle is the same for all screw drivetrains, there are a variety of types and designs, each with their own characteristics and benefits. Determining the appropriate drive screw is dependent on the intended application, and its inherent requirements. The system’s load, desired travel speed, duty cycle, surrounding environment, and operating temperatures will all impact the performance of a drive screw. Choosing the wrong type can lead to operational inefficiencies, or worse, premature failure. As a result, finding the best solution for a specific application is crucial to achieving optimal results.
It starts with understanding the relative strengths and weaknesses of the various screw drivetrain technologies available. Each of the three major screw drivetrain types – ball, roller, and ACME/trapezoidal – has its own advantages and disadvantages. Here we offer a high-level overview of these three screw technologies to help you make the right choice for your system.
Ball screw drivetrains
Ball screw drivetrains are widely used in linear motion applications because they combine a high level of efficiency with high load-life characteristics and a predictable product life. Well-engineered ball screws are ideal for high-duty cycle applications that demand high thrust levels.
Mechanically speaking, a ball screw is composed of a metal screw and a nut. Metal ball bearings in the nut act as the mating interface between the threads of the nut and the screw. Depending on the design, these ball bearings roll and recirculate through a single circuit, or a series of circuits, either within the nut or in external return tubes. This happens while the screw or nut itself rotates, causing one of these components to move, providing the necessary linear motion.
There are two main types of ball screw drivetrains – the single ball nut and the dual nut styles. The first type incorporates a single ball nut, which will typically have some inherent level of backlash – the level of free movement between the nut and the screw – due to the gaps between the ball bearings as they travel along the screw and circulate through the nut. The second type of ball screw-driven unit is a dual nut style, with nuts that are pre-tensioned against each other. This helps reduce backlash by mitigating the gap between the ball bearings and the threads, providing greater accuracy and repeatability.
The level of backlash within a ball screw design is important. Backlash has an impact on the linear drive unit’s repeatability, meaning its capability to repeatedly, precisely, and continuously reach the same position that it reached before under the same operating conditions. Ball nuts typically have anywhere from 5 to 25 thousandths of an inch of inherent backlash. Beyond choosing a dual nut style design, backlash can be mitigated in a ball screw system by loading each circuit with ball bearings of a larger than nominal diameter. This tightens the gap between the ball bearings and notably mitigates the screw assembly’s inherent backlash.
In most cases, a ball-screw-driven unit’s accuracy, or ability to hit a targeted linear position, is a product of the quality of the manufacturing process used to create the screw and the consistency of the screw thread profile over the length of the stroke. The repeatability attributable to the nut(s) and the accuracy attributable to the quality of the screw must both be considered when evaluating the overall precision of the unit.
When to choose a ball screw drivetrain: Ball screws are great options for applications that call for a high duty cycle, high speeds, and high loads. They are also good choices for when you’re looking for a bit of a torque advantage. The rolling elements inside the ball nuts typically have lower friction than roller and acme screws, which allows them to have a mechanical efficiency of up to 90 percent. This high efficiency makes them perfect for operations where a higher level of performance is required and where screw wear is an issue.
While they offer good performance capabilities, a potential drawback for ball screws in certain applications can be the noise they create in use. This is due to the sound produced by the balls colliding in the return tubes and circulating through the nut.
Roller screw drivetrains
Similar to a ball screw design, roller screws are comprised of a screw and an interfacing nut, with their thread form often being triangular in design. However, rather than using ball bearings to interface with the thread form, the roller screw design uses small, rotating rollers within the nut to provide contact between the nut and the screw itself. This means that the roller nut has multiple sets of rollers, providing a significantly greater amount of contact points with the screw compared to ball nuts. This configuration allows for a line of contact between the rollers, the nut, and the screw, offering shock, load, and overall stiffness advantages over ball screw designs.
With the similarities of their design compared to ball screws, it’s no surprise that they share similar efficiencies. Since they have an increased contact area, their efficiency drops slightly lower than that of the ball screw, averaging around 85%. This efficiency can vary notably and largely depends on screw diameter and screw lead.
There are two main types of roller screws; standard roller screws and inverted roller screws. A standard roller screw consists of a hardened threaded shaft and either a planetary or recirculating roller nut, with planetary nuts being more common. In this arrangement, the shaft is connected to a motor or gear train, and the nut translates up or down the screw to create linear motion, much like ball and lead screws. Although manufacturing techniques vary, typically the final thread form is ground into the shaft post heat treat. This allows the nuts to be matched to the screws for a high precision, long life assembly. Inverted roller screws use a threaded tube instead of a shaft, which is basically a long version of the standard roller screw nut. The planetary nut is usually fixed to a shaft, and the tube or shaft can be spun to create the linear motion. If the tube is spun, the nut translates up and down inside the tube; if the shaft fixed to the nut is spun, the tube extends or retracts. While this type of screw can result in a more compact overall assembly, it is more limited in the size and overall lengths that can be manufactured. These tubes are also hardened, but the thread inside the tube is generally not ground afterward, so the final assembly cannot be matched as precisely.
Precision grinding and machining, combined with geometries that allow for more contact points in the same envelope, give roller screws a high dynamic load rating, or DLR. This results in a longer product life for a similar sized screw assembly.
When to choose a roller screw drivetrain
Machine designers often choose roller screws for complex applications where a moderate to high degree of precision is required. They’re fit to handle more challenging applications as they offer high efficiency, large load-bearing capabilities, and high duty-cycle capabilities. They’re also good options where less system maintenance is desired or for when you don’t want to have to worry about replacing parts as frequently, as roller screws offer an extended product life given the greater contact area on the screw threads.
Roller screws generally produce notably less noise than ball screws during operation. Their noise only comes from the planetary rollers within the nut. Because these planetary rollers make constant contact with the screw surface, the noise level associated with use is less than that of ball screws, whose ball bearings have freedom of movement.
Note that the tight machining tolerances essential to manufacturing these screws have the potential to add to the overall cost of the system. For simpler systems, non-critical functions, or for applications that are less demanding, the higher functionality of roller screws can be cost-prohibitive.
ACME and trapezoidal screw drivetrains
ACME and trapezoidal screw drivetrains are best used in lower precision applications with low speeds and duty cycles. While loose machining tolerances may allow ACME and trapezoidal screws to be effectively interchangeable, their geometries actually differ by 1 degree. ACME screws have a 29-degree included angle, while trapezoidal threads have a 30-degree included angle. As the screw turns, the threads transmit linear force to the nut. These drivetrains use a thread form with a trapezoidal tooth shape that’s typically rolled into a steel shaft. This creates a strong thread form, which transmits a linear force to a solid nut from the sliding surfaces on the flanks of the thread form. The sliding surfaces are the cause of the inefficiency, where much of the energy required to turn the screw is lost to heat.
ACME screw efficiencies depend on the nut material (often made of plastic, polymers, brass, or bronze), lead of the screw, and the type/amount of lubrication used. Their efficiencies are typically much lower than ball or roller screws, ranging from approximately 20 to 80 percent. Lower efficiencies can prevent loads or external forces from back-driving the assembly, which can be an advantage for some applications, and detrimental to others. It’s important to note that vibration can allow any ACME or trapezoidal screw to back-drive. The inherent energy losses mean these types of screws require more torque than other screw types to provide the same thrust.
When to choose an ACME/trapezoidal screw drivetrain
Of the three screw types, ACME screws cost the least and are often the most readily available. They’re suitable for low-speed applications or in systems where there are not high duty cycle demands.
However, ACME screws are not suitable for applications requiring high duty cycles or high travel speeds. That’s why ACME screws shouldn’t generally be used for more complex applications, especially ones with variable operating conditions. Their product life is often unpredictable compared to the other two screw types, meaning that screw maintenance and replacement are more dynamic with these types of screws.
What this brief survey tells us is that there’s no screw drivetrain that’s objectively better than the others in every instance. Each of the three main screw types has its own advantages and disadvantages that must be taken into consideration. Finding the right screw drivetrain will entirely depend on the specifics of your unique application and the motion/action desired.
Hunt Valve Actuator Division