Lead Screw: Common Industrial Applications And Design Types
With high stiction and self-locking properties
, lead screws do not backdrive when holding vertical loads or when used in other motion control applications. Unlike other power tools, lead screws can be easily manufactured without any specialized machinery.
Design Considerations
Common parameters significant to the design of lead screws include critical speed, thrust, physical characteristics, thread arrangement, and resolution. Lead screw diameter for a linear motion application depends on the amount of thrust required. High thrust application requires the screw diameter to be larger, as the lead screw will be subject to high tension and compression.
The velocity limit of a lead screw dictates its maximum speed. A lead screw's critical speed (rpm) denotes its maximum speed limit after which the screw shaft becomes dynamically unbalanced. The input rpm and the linear velocity of lead screws are always directly proportional, depending on the lead length. In high-speed linear motion applications, lead screws are designed with large lead shafts to lower the input rpm of the screw.
The critical speed of lead screws depends on the length, diameter, and configuration of the screw. A lead screw's critical speed value can be calculated from the following equation:
Critical Speed, rpm = (KC x 10^6)(d / D^2)
where K = 4.76, C = end fixity value (0.36 for one end fixed, one end free; 1.00 for both ends simple; 1.47 for one end fixed, one end simple; 2.23 for both ends fixed), d = root diameter of screw, D = length between bearing supports
Design Types
According to their thread geometry, lead screws can be classified into square thread, acme thread, and buttress thread forms.
Square Thread
Square threads are the most commonly used forms of lead screws available in a range of industrial load positioning applications. They have greater intrinsic efficiency than trapezoidal or acme threads. The thread angle of square threads is 0. Hence, they do not exert radial or bursting pressure over the nut while positioning loads. This, in turn, increases the service life of lead screw nuts. Large thread roots make the square threads unsuitable for large load positioning applications. Another downside of square threads is that they are difficult to manufacture.
Acme Threads
Acme threads, generally available with a thread angle of about 29, are simple and easier to machine than square threads. They are best suited for applications that warrant large load bearing capacity and high accuracy. Standard variations of acme thread lead screws include tooth shaped multi-start threads, self-centering threads, and left-hand threads. The tooth shaped configuration of acme threads allows the use of split nuts that prevent nut wear. Large acme thread lead screws are used for motion control applications in aircraft flaps, machine tools, conveyors, and jacks, while short length threads are used in hose connectors, valve stems, bonnets, and steering mechanisms.
Buttress Threads
Buttress threads offer the benefits of both acme and square threads. They are widely used in sealing type machines and oil field tubing applications. When used as pipe thread in oil field tubing, buttress threads offer a tight hydraulic seal. Another major application area of buttress threads is in the artillery construction field, as the screw-type breechblock withstands high axial loads during the explosion of powder charge. In machinery applications, buttress thread forms are used to handle elevated axial thrusts in one direction. The thread face of a buttress thread is either at right angle to the axis of the screw or slanted to nearly 45. At this angle, the strength of the thread is twice that of the square thread form.
Industrial Applications
Lead screws with their different types find applications in various industries. Generally, lead screws are available with self-lubricated nuts. However, in applications that use plastic nuts, lead screws can be run dry with relatively greater friction. Lead screws can produce sensitive as well as smooth displacement. Hence, they are used as essential elements in almost all measuring devices. Lead screw assemblies find their usage in manual as well as power driven applications that comprise of:
Machinery slide drives
Valve stems
Scaffolding support legs
Shop & office table lifts
Clamps
Presses
Farm machinery
Optical system
General purpose jacks
Trailer jacks & jackstands
Medical device drives
Screw press drives
Vises
by: Ron Bargman
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