Techno Linear Motion Catalog11Technical Information4) Drive MechanismsThis section will introduce most of the more common types of drive mechanisms found in linear motionmachinery. Ideally, a drive system should not support any loads, with all the loads being handled by abearing system. Topics discussed will include, but not be limited to, the mechanism of actuation, efficiency,accuracy, load transfer, speed, pitch, life cycle, application and maintenance. Each type of drive systemwill be accompanied by a diagram and useful equations when applicable. Some of the terms used withscrews, the most common drive component, are as follows:lead advance of the nut along the length of the screw per revolutionpitch distance between corresponding points on adjacent thread forms(pitch = lead / # of starts)# of threads number of teeth found along a unit length of the screw (1 / pitch)# of starts number of helical grooves cut into the length of the shaftouter diameter largest diameter over the threaded section (at top of threads)root diameter smallest diameter over the threaded section (at base of threads)stub specific type of ACME thread where the root diameter is larger toprovide for a more heavy duty screw (the threads look stubby)critical shaft speed operating speed of spinning shaft that produces severe vibrationsduring operation. This is a function of length, diameter, and endsupports.maximum compressive load maximum load that can be axially applied to the screw beforebuckling or permanent deformation is experienced. Also referred toas column strength.end bearing supports the screw must be supported at one or both ends with thrust typebearings. Depending upon the application, it may also be desirableto provide for a stiffer system by incorporating angular contactbearings (fixed support).Although shafts, gear trains, belt and pulley, rack and pinion, and chain and sprocket drives are practical inother applications, they require special consideration when used in CNC machinery. This is because thereis typically backlash associated with these types of drives, which increases the system error. Thoroughtechnical descriptions of these types of drives can be found in the Stock Drive Products Components Library.Lead screwsare threaded rods that are fitted with a nut.There are many types of threads used, but the most prevalentin industry is the ACME lead screw. Because the ACMEthread is an industry standardized thread style, it is easilyinterchanged with parts from various manufacturers. Thebasic function of a screw is to convert rotary input motion tolinear output motion. The nut is constrained from rotatingwith the screw, so as the screw is rotated the nut travelsback and forth along the length of the shaft. The friction onthe nut is a function of environment, lubrication, load, andduty cycle; therefore, practical life cycle is difficult to quantify.Lead screw/nut drive systems are available in a variety of sizes and tolerances. Contact is primarily sliding,resulting in relatively low efficiency and a wear rate proportional to usage. Advantages include the self-locking capability in back drive mode which is good for vertical applications, low initial costs, near silentoperation, manufacturing ease, and a wide choice of a materials. Disadvantages of ACME screws includelower efficiencies (typically 30-50%, depending on nut preload) which require larger motor drives, andunpredictable service life.Lead Screw Lead Nut Lead Screw System