The basic stepper motors are rotated by a magnetic rotor core that interacts with the pulsating stator electromagnetic field generated by the stator. The linear motors combine a screw and a nut to convert the rotary motion into a 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.
Classification of screw
Power screws fall into two basic categories: ball screws, which operate on rolling contact, and lead screws, which have sliding contact between the nut and screw.
The ball screw assembly is usually made of carbon steel or stainless steel, using a recirculating ball bearing 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.
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.
Which screw to choose?
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 100lbs or less, although plastic nut designs for 300lbs and beyond are possible.
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.