Home Page
Product Finder
Technical Information techquestions@techno-isel.com 19 5)    Ball and Acme Screw Drive Mechanisms This section will introduce most of the more common types of drive mechanisms found in linear motion machinery.  Ideally, a drive system should not support any loads, with all the loads being handled by a bearing 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 system will be accompanied by a diagram and useful equations when applicable.  Some of the terms used with screws, the most common drive component, are as follows: lead —  advance of the nut along the length of the screw per revolution pitch —  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 shaft outer 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 to provide for a more heavy-duty screw (the threads look “stubby”) critical shaft speed —  operating speed of spinning shaft that produces severe vibrations during  operation.   This  is  a  function  of  length,  diameter,  and  end supports. maximum compressive load —  maximum  load  that  can  be  axially  applied  to  the  screw  before buckling or permanent deformation is experienced.  Also referred to as column strength. end bearing supports —  the screw must be supported at one or both ends with thrust type bearings.  Depending upon the application, it may also be desirable to  provide  for  a  stiffer  system  by  incorporating  angular  contact bearings (fixed support). Although  shafts, gear trains, belt and pulley, rack and pinion, and chain and sprocket drives are practical in other applications, they require special consideration when used in CNC machinery.  This is because there is typically backlash associated with these types of drives, which increases the system error.  Thorough technical descriptions of these types of drives can be found in the Stock Drive Components Library. Lead screws are threaded rods that are fitted with a nut. There are many types of threads used, but the most prevalent in  industry  is  the  ACME  lead  screw.    Because  the  ACME thread is an industry standardized thread style, it is easily interchanged  with  parts  from  various  manufacturers.   The basic function of a screw is to convert rotary input motion to linear output motion.  The nut is constrained from rotating with  the  screw,  so  as  the  screw  is  rotated  the  nut  travels back and forth along the length of the shaft.  The friction on the nut is a function of environment, lubrication, load, and duty 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 silent operation, manufacturing ease, and a wide choice of a materials.  Disadvantages of ACME screws include lower  efficiencies  (typically  30-50%,  depending  on  nut  preload)  which  require  larger  motor  drives,  and unpredictable service life. Lead Screw System Lead Screw 3 Lead Nut 3