Posted by admin | Posted in Woodworking | Posted on 05-06-2010
Tags: cnc, cnc linear motion, cnc linear motion components, cnc linear motion systems, hardware, linear, mechanical, motion
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Lead screw vs. Ball Screw: Determining the Best Design for your Application
Lead Screw:
Lead screws are transmission components that transform radial or rotary motion into rectilinear motion. Typically, they are available complete with screws, nuts, and bearing supports. Specific application dictates the type of material best suited for a lead screw assembly. Common lead nut materials include polymer composites, brass, and steel. Polymer-based lead nuts offer the flexibility of easy modification whenever needed. Another significant feature of the mating nuts is that they do not possess any internal moving components like ball screws. Depending on screw geometry, lead screws are available in square thread, acme thread, or buttress thread forms.
In the past, lead screws were considered as inexpensive alternatives to ball screws in less-demanding applications. Today, advancements in motion control technology have extended their use in various applications, including valve actuators, automated manipulators, bridge and gantry systems, and machine tools. One such technological advancement is the use of anti-backlash nuts along with stepper motors in lead screws. Acme lead screws are most economical for linear motion applications. They provide higher accuracy and require much lower drag torque when compared to ball screws. In motion control applications, acme lead screws with self-lubricated nuts are subject to more extensive wear and friction than the ballscrews. The wear on the nuts can be prevented using additional lubrications like light grease. The selection of an appropriate lubricant will also reduce the friction between nuts and screws as well as the required torque. Acme lead screws with proper lubrication offer longer service life and noiseless operation than ball screws.
Precision lead screw, another type of advanced lead screw, is best suited for motion control applications that warrant high accuracy. These assemblies are available with precision lead screws, preloaded nuts, self-aligning thrust bushings, thrust bearings, and optional lubricators. Typically, a phosphor bronze split nut is used as the preloaded nut that increases the axial stiffness and suppresses the backlash. The optional lubricator functions as an external support to the screw preventing dust and offers better lubrication to the screw. Lubrication oil can be changed or re-filled when needed as it extends the service life of the lead screws. Precision lead screws can be coated to enhance their resistance to acids, bases, solvents, and other mechanical damage. Solid film based lubricants called polytetrafluoroethylene (PTFE) are now used as self-lubricating systems in precision lead screws to eliminate the need for surface coating and optional lubrication.
Ball Screw:
A ball screw, also called as a ball bearing screw or recirculating screw, translates rotational motion into linear motion with minimum friction. Ball screw assemblies consist of screws and nuts with matching circular grooves that act as spiral raceways for ball bearings. Ball nuts are retained in place on the threaded shafts by a train of balls riding within the grooves. In contrast to lead screws, ball screws are bulky, as they need a mechanism to recirculate ball bearings. Commonly used screw materials include carbon and stainless steel, while the ball bearings are usually fabricated from steel. Recirculated ballscrews are commonly used in the aerospace and defense industries. High efficiency and predictable life expectancy make ball screws perfect for machine tooling and robotic applications.
The recirculation mechanism is critical to the high efficiency of ball screws. Some of the commonly used recirculation methods include external recirculation, internal recirculation, and endcap recirculation. In the external return system, balls travel within the raceway between the ball nut and screw spindle. External return tubes attached to the nuts serve as return systems that divert the balls to the thread raceway. The endcap return system is similar to an external recirculation design, except the return tube is bored within the nut body. Ball bearings recirculate only once within the internal return type ball screws. Ball return caps within these ball screws facilitate the cross-over of balls to adjacent raceways. With large load carrying capacity, ball screws have higher mechanical efficiency compared to lead screws. Backlash and bearing support are the two important parameters significant to the design of ball screws. Backlash generally occurs when the direction of the load is altered resulting in misassembly of the parts. This can be prevented by introducing a preloaded nut to increase the stiffness, which consequently improves the alignment of the parts. The screw shaft must be maintained at a constant temperature, as thermal expansion of the shaft is probable. The rate at which wear occurs is lower and depends on the positioning of the assembly. In addition, the sliding friction produced during the operation can be replaced by revolving motion of the balls. The high cost of ball screws is compensated by its low power requirements.
Ball screw assembly eliminates backlash, as they are provided with a group of grooves and preloaded nut that helps in accurate positioning. They are available in a variety of sizes that are widely used for linear motion applications. These screws are specifically used in CNC machinery, precision machine tools, and industrial equipment, such as printing and textile machines. In addition, they are used in loading equipment. Commonly used lubricants in ball screws include grease, oil, or dry film lubricants. Typically, a coating of black oxide is used to surface finish ballscrew assemblies. This secondary operation offers low coefficient of friction, resulting in higher accuracy and better efficiency than the acme lead screws. However, ball screws do not require any anti-friction coatings as they have the ability to prevent formation of rust.
About the Author
Ron Bargman, president of Zycon.com, has been fascinated and involved with the engineering and manufacturing processes required to turn ideas into products for over 30 years. Mr. Bargman is a regular contributor of manufacturing theme articles, and his rich industry history provides insight into manufacturing and engineering events and changes that are timely, poignant, and relevant. Through Zycon, he is able to transfer his passion for the industry by assisting engineers, designers and inventors find the services, parts and components that they need to succeed.
With Zycon, you can easily contact and quickly get quotes for a ball screws and lead screws direct from the manufacturer.
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