We examine the factors that affect drive train selection and characteristics of the three types of power screw drives used in linear actuators.
By Igor Glikin, Senior Mechanical Engineer, Tolomatic, Inc.
Editor's Note: This article is adapted from a comprehensive white paper, "Screw-Driven vs. Belt-Driven Rodless Actuators: How to select drive trains for reliability, efficiency and long service life." Visit http://goo.gl/TMEOzn to download the free, full white paper with in-depth information about application parameters and timing-belt drive functions, materials and tooth geometry.
Most electromechanical rodless actuators commonly use one of two main drive trains to convert rotary motion of the electrical motor to linear motion of the actuator's load-carrying device: a power screw drive or a timing belt drive. While both screw and timing belt drives offer efficiency, reliability and long life while requiring very little maintenance, each has its limitations. This article examines the fundamentals of how the power screw drive works.
Key Factors that Affect Drive Train Selection
Power screw drives and timing belts carry a dual function in linear motion. They're used for linear positioning and therefore need to provide acceptable accuracy and repeatability. At the same time, they transmit power, which requires them to possess sufficient strength. A screw mechanism produces linear motion by rotating either the screw or the nut in an assembly. Similarly, timing-belt drives transmit torque and linear motion from a driving pulley via the belt to a driven pulley.
The motion-control application determines which drive train to select. Basic to any motion control system selection are duty cycle, life cycle and cost.
More pertinent to drive-train selection are length of stroke, linear velocity and acceleration and orientation of move. Drive trains vary in capacity, so thrust of the actuator, as well as load and force of the actuator carrier, will also affect drive train choice. Shock loads and noise are important considerations for some types of processing.
Additional factors that determine the selection of drive train technology include:
- Accuracy of a positional move: the ability of the actuator to achieve a commanded position, or the difference between the actual position and a commanded one.
- Repeatability: the degree to which an actuator can return to a reference position in multiple attempts. Unidirectional repeatability is measured by approaching the position from a single direction; bidirectional repeatability is measured by approaching it from opposing directions.
- Permissibility of backdriving: ability of an electromechanical actuator to lose its carrier's position under axial load at loss of power.
Power Screw Drives
Power screw drives are known for high-thrust capacity, accuracy and repeatability. Relatively low system inertia and predictable service life (ball and roller screw drives) also are benefits. These parameters make power screw drives ideal for a variety of applications such as machine tools, assembly and packaging equipment, and robots.
Three primary types of screws are used in linear actuators: lead (or Acme), ball screws and, less common, roller screws. The differences are in the design of the thread shape along with the design and operation of a matching nut.
1. Lead Screws. Lead screws (also called Acme screws) are known for their relatively low cost and smooth, quiet operation. The trapezoidal threads of lead screws are made of carbon, alloy or stainless steel, and are available in a variety of diameters and leads. Solid nuts used with lead screws usually are made of composite materials (most commonly acetal resins) or bronze.
A solid nut sliding along a lead screw's threads creates a line contact between the surfaces of the two parts. As a result of the friction losses of this motion, drive efficiency is less than 60%. Drive efficiency is a function of (a) coefficients of friction between lead screw and lead nut; and (b) the lead screw's helix angle.
Energy lost caused by friction is dissipated as heat, which limits the application's duty cycle. Heat generation prevents actuators driven by lead screws from being used at high speeds or with high axial loads.
Bidirectional repeatability is mainly affected by the amount of axial backlash or free play of a lead nut on the screw. Unidirectional repeatability is typically affected by component wear. Typical rolled-lead nuts will produce up to .010 in. (.254 mm) of backlash. In many cases, anti-backlash, spring-loaded, self-adjustable lead nuts eliminate this problem.
Because of low efficiency, most lead screws can't be backdriven.
2. Ball Screws. Ball screws are power screws that work as helical raceways for high-grade, chrome steel ball bearings. They recirculate inside a nut and normally are manufactured of high-carbon, alloy or hardenable stainless steel. Ball screws with ground threads cost significantly more than rolled screws, but can be manufactured with a high lead accuracy grade that some applications require.
Ball-screw assemblies offer two main advantages compared to lead screws:
1. Balls move along the screw threads in a rolling motion based on a point of contact between the surfaces. This significantly reduces friction and increases mechanical efficiency, which for most ball screw drives is typically no less than 90%.
Because ball nuts essentially are rolling bearings, their dynamic load ratings can be readily calculated according to ISO standards. This makes expected service life highly predictable.
As with lead nuts, different methods control backlash and improve repeatability of ball nut assemblies. Spring-loaded ball nuts are available, as well as ball nuts that are custom fit with balls of selected sizes that minimize backlash or eliminate it altogether, effectively creating negative clearance or preload.
2. Because of their high efficiency, properly lubricated ball screw drives offer higher speeds and higher duty cycles compared to lead screws. Ball nut assemblies can deliver a high thrust. Because of their high efficiency, most ball nuts can be backdriven.
While offering a smooth linear motion, ball screw drives often are noisier than lead screw drives.
3. Roller Screws. Roller screw drives aren't commonly used in rodless actuator applications. Information is given here primarily for reference purposes. Rodless actuators typically have a built-in bearing system, and as a result are designed to “carry” loads as opposed to “pushing” them. Because of this, thrust requirements are usually much lower.
In most rodless applications, ball screw drives can handle the force and precision and don't require the higher thrust capacities or the higher cost of roller screw drives. Planetary roller screws use threaded rollers, instead of balls, as the load transfer elements between nut and screw.
Roller screws have significantly higher service life than ball screws. Roller screw actuators have high dynamic load ratings. Roller nuts have lower efficiency than ball nuts and are prone to overheating in high duty-cycle applications. Roller nuts, however, are invaluable from the standpoint of “force density” and life.
Editor's Note: This article is adapted from a comprehensive white paper, "Screw-Driven vs. Belt-Driven Rodless Actuators: How to select drive trains for reliability, efficiency and long service life." Visit http://goo.gl/TMEOzn to download the free, full white paper with additional information about application parameters and timing-belt drive function, materials and tooth geometry.
Tolomatic, Inc., based in Hamel, Minnesota, is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™. Tolomatic designs and manufactures electric and pneumatic linear actuators.