They spin, they lift, they actuate linear motion. The operation of power screws often seems simple. But making it appear easy is a result of good, thorough mechanical analysis and design, and making the right selection.
Power screws fall into two basic categories: lead (or machine) screws, which have sliding contact between the nut and screw, and ball screws, which operate on rolling contact. The generic term "Acme screw" is often sweepingly used to denote lead screws. But an Acme screw is actually a specific type, with a particular thread geometry.
Sliding contact assemblies typically use nuts made of internally lubricated plastic or bearing-grade bronze. Plastic nuts usually travel on stainless steel screws, with bronze nuts riding over carbon steel screws; if the nut is a medium grade bronze, a stainless steel screw is also an option.
Ball screw assemblies, which come in carbon or stainless steel, use recirculating ball bearings that roll along the helical grooves in the screw and nut. In this way, the nut can move along the screw without having to contend with sliding friction.
Which screw will do
The advantages of each kind of screw can be outlined with respect to some common design parameters.
Load capacity is often a top priority. Ball screws generally achieve equal or better loading than comparable lead screws.
The choice of nut material greatly affects a lead screw's load capacity. Bronze nuts have considerable strength, depending on the grade, while plastic nuts are most often used to carry 100 lbs or less, although plastic nut designs for 300 lbs and beyond are possible.
Efficiency varies greatly between rolling and sliding screw assemblies because there is a huge difference in friction between the two. Lead screws are typically no higher than 70% efficient, and can be as low as 20%. The efficiency of a lead screw is determined solely by its helix angle and by the friction coefficient between screw and nut. Ball screw efficiency, on the other hand, is very consistent regardless of helix angle and is typically better than 90%.
Irreversibility, or self-locking, is often desirable in a power screw assembly. The low friction of ball screws facilitates reverse driving; in fact, when the screw isn't active, a brake is usually needed to support vertical loads.
Lead screws can be made to selflock relatively easily. However, they too can be reverse-driven if steep helix angles and low friction are present. For self-locking, lead screws require a friction and helix angle such that their efficiency is less than 50%.
Constant motion at load is readily supplied by ball screws. Because they generate low frictional heat, their duty cycle is practically unlimited.
Plastic and stainless lead screw assemblies, with their limited load capacity, typically run at a duty cycle of 50% under the rated load. Lead screw assemblies that use bronze nuts have substantial load capacities, but these heavier loads increase frictional heat, so their duty cycles can be less than 10%.
Speed of the linear output motion is a function of the lead angle and the screw speed. Lead screws can't rotate all that quickly due to the frictional heat buildup. However, lead screws come in a wide range of leads, from under 0.050 to as high as 2.000 in./rev. In spite of the limited screw speed, the higher leads can deliver jog speeds to 70 in./sec.
Ball screws offer a different slant. They can spin quite fast, but they generally only come in medium- range leads, around 0.200 to 0.500 in./rev.
Position accuracy, including resolution and repeatability, is affected by the lead and the backlash. Lead screws with extremely low leads are good candidates for slow, high-resolution travel. Again, ball screws are generally available only in moderate lead ranges.
As for backlash, standard ball and lead screw configurations can have between 0.002 and 0.010 in. Power screws typically address the backlash problem by preloading the nut portion of the assembly, using one of several methods.
Forcing two nuts away from each other along the screw axis induces axial preloading. This reduces backlash in lead screws by keeping internal threads flush against the screw threads; in ball screws, the ball bearings are held tight against the sides of the grooves. The overall nut assembly, which delivers the linear motion, therefore moves only in strict conjunction with the screw rotation.
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Among the methods of axial preloading, a common approach uses a spring (called a "compliant spacer") to press the two nuts in opposite directions. This axial take-up scheme relies on a spring force that exceeds any driven load opposing it. In sliding- action power screws, such high spring rates often mean strong continuous pressure between nut and screw threads, which compounds the sliding friction and increases drag torque.
Ball screws, however, have highly efficient contact conditions, and this preload mechanism yields far lower drag torque; a similarly preloaded ball screw recovers nearly all of the spring force exerted on the forward nut through backdriving in the secondary nut.
Another effective axial preload setup uses a solid spacer between the nuts. This provides a stiff assembly, but with less drag torque than the spring method, since the spacer is only wedged in enough to tightly fill the gap. On a ball screw, this configuration can be rather large and costly. Spacer-preloaded plastic nut assemblies on lead screws do just fine for light loads; they are cost effective, their repeatability can be better than 0.0005 in., and the size is only marginally larger than a standard plastic nut. Since there is gradual wear in sliding-action nuts, the spacer has to adjust to keep the threads in continuous tight contact.
Radial preloading is an option only for lead screws. This method can be accomplished by axially sectioning the nut into "fingers." The sectioned nut can then be tightened in the radial direction, pressing inward and meshing tightly with the screw threads. This is often done using a ramped sleeve to bear on a ramped nut exterior. The sleeve is axially loaded by a compression spring.
Radial preloading relies on angled threads; pressing square threads together this way doesn't generate the right contact. This inexpensive method offers good anti-backlash performance, but with limited load capacity; due to the thread angle, heavier loads may force the fingers of the nut to slide out and away from the screw. Plus, the sectioned nut has a lower integrity, and may be prone to excessive strain, especially in the torsional direction.
Lubrication is pretty much required for ball screws if they are to have a reasonable life. Lead screw mechanisms involving bronze nuts need lubrication as well, and it is often a thick, damping grease. Lead screw assemblies with plastic nuts can run well with no oil or grease, due to the internal lubricants in the nut material. However, additional lubrication can increase the load capacity and extend the life.
Noise is usually not an issue with lead screws, although stiction (frictional impedance to sliding) sometimes produces chatter or squeal in the absence of lubrication.
Ball screws are prone to a certain amount of noise because of the ball bearing recirculation.
The operating environment can sway the choice of material and style for the power screw. Ball screws, when made of carbon steel, (a common choice), may be sensitive to certain corrosive environments.
Lead screw configurations typically come in a stainless steel screw and plastic nut combination, or with a carbon steel screw and bronze nut. Medium grade (and medium capacity) bronze nuts can be used with stainless as well as carbon steel screws, although the heavy grease used with bronze nut assemblies protects well against corrosion whether or not the screw is stainless.
The stainless and plastic combination, which can do without lubrication, is highly corrosion resistant regardless, withstanding clean rooms, outdoor moisture, and so on. Temperature is a different story. The plastic nut may be limited to temperatures between 30° and 120° F.
Custom design might be the best, if not the only way to go under certain circumstances. Lead screws have relatively straightforward geometry and performance, and lend themselves to design adjustments fairly easily.
With custom ball screws, you may have to work within stricter parameters to satisfy your design, due to the more complicated geometry necessary to the recirculating ball scheme.
Robert Lipsett is Engineering Manager with Ball Screws & Actuators Co., San Jose, Calif.