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This article takes an in depth look at lead screws.
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A lead screw is a kind of mechanical linear actuator that converts rotational motion into linear motion. Its operation relies on the sliding of the screw shaft and the nut threads with no ball bearings between them. The screw shaft and the nut are directly moving against each other on a large contact area, so higher energy losses due to friction are produced. However, the designs of lead screw threads have evolved to minimize friction.
The lead screws are a cost-effective alternative to ball screws in low power and light to medium-duty applications. Since they have poor efficiency, their use is not advisable for continuous power transmission. Unlike ball screws, they operate silently with no vibration and have a more compact size. They are typically used as a kinematic pair (linkage) and actuation and positioning in equipment such as lathe machines, scanners, recorders, wire bonders, and disk drive testers. They are used to transmit forces in testing machines, presses, and screw jacks.
The components of a lead screw are the following:
The screw shaft is a cylindrical rod that has a single or series of grooves running helically around its length; this is referred to as the external thread.
The thread is the structure responsible for converting rotational motion into linear motion as the screw shaft and the nut slide with each other.
The lead screw nut is a cylindrical section that has an internal thread that matches the external thread of the screw shaft.
Lead screws may be operated in two possible ways. One mode of operation is either the screw shaft or the nut rotates and moves linearly while the other component is fixed. This mode is commonly seen in printers and helical pairs. The other mode of operation is either the screw shaft or the nut rotates but does not move linearly. This mode is commonly seen in presses and lathes.
The design aspects of lead screws are the following:
The major diameter is the largest diameter of the thread. The major diameter of the screw shaft is the distance between two opposite crests, while the major diameter of the nut is the distance between two opposite roots.
The minor diameter is the smallest diameter of the thread. The minor diameter of the screw shaft is the distance between two opposite roots, while the minor diameter of the nut is the distance between two opposite crests.
A crest is the raised helical structure in an external thread (screw shaft) and the recessed helical structure in an internal thread (nut).
A root is the recessed helical structure in an external thread (screw shaft) and the raised helical structure in an internal thread.
The thread depth is the distance from the root to the crest, measured radially.
The flank is the surface that connects the root to the crest.
The pitch diameter, or the effective diameter, lies concentrically and approximately halfway between the major and minor diameters. It is the diameter of the imaginary cylinder whose circumference intersects half of the thread pitch.
The pitch is the axial distance between two adjacent threads measured parallel to the axis. It is equivalent to 1/number of threads per inch.
The lead is the linear distance traveled by the screw shaft or nut along its axis in one complete revolution ( rotation). As the lead increases, the linear speed also increases, but the load capacity of the lead screw decreases.
The number of starts refers to the number of independent threads running around the length of the thread. The lead of a screw is determined by multiplying the number of independent threads by the pitch.
Most lead screws have one, two, or four starts. A single-start lead screw has a lead that is equivalent to its pitch. Multiple-start lead screws are used when higher speeds and higher load capacities are desired. The higher the number of starts, the longer the linear distance traveled for a single rotation. In double start lead screws, for example, the lead is equivalent to twice its pitch, which means that the axial distance covered by one rotation is two-pitch units.
The helix angle is the angle formed between the helix of the thread and the line perpendicular to the axis of rotation. Generally, a lead screw with a higher helix angle has lesser frictional losses and therefore has higher efficiency. This is because the number of revolutions to rotate such a screw is lower than a screw with a lower helix angle for the same linear distance covered. However, it requires more torque to rotate the screw.
The lead angle is the complementary angle of the helix angle. It is the angle formed between the helix of the thread and the line parallel to the axis of rotation.
The thread angle is the angle formed between two adjacent threads.
Screw handedness refers to the direction in which the thread runs along the length of the screw. A lead screw may be right-handed or left-handed. In right-hand and left-hand screws, the thread runs around the screw length in a clockwise direction and counterclockwise direction respectively.
The following are the types of lead screw threads based on their profile:
The square thread has its flanks at right angles to the axis of the screw. No radial or bursting pressure is acting on the nut since square threads have a 00-thread angle. Square threads have less resistance to motion and less friction.
Square threads are typically used in power transmission. Typical applications of square threads are in lathe machines and jackscrews. However, they are difficult and costly to manufacture. They are manufactured by using a single-point cutting tool. Their load capacity is also the lowest since the areas of the tooth at the crest and root are similar.
The acme thread has a 290-thread angle. This modification of square threads was developed in the mid-s. Acme threads have a higher load capacity than square threads because the tooth has a wider base. Another advantage of this type is their low number of threads per inch, which increases the lead. The wear of the threads can be compensated. However, they are less efficient than square threads due to friction introduced by the thread angle.
The angled flanks of the acme threads allow them to be manufactured easier than square threads by a multi-point cutting tool. Typical applications of acme threads are in bench vices, clamps, valve stems, lathe machines, and linear actuators.
There are three types of acme thread: the General Purpose, the Centralizing, and the Stub acme threads. Both General Purpose and Centralizing acme threads have a depth thread equivalent to half of its pitch diameter. Centralizing acme threads have tighter tolerances between the external and internal threads to prevent wedging when a radial load is applied. The stub acme thread has a thread depth of less than half of its pitch while adapting the features of the General Purpose and the Centralizing acme threads.
The trapezoidal thread is similar to the acme thread, except that the thread angle is 300. It is manufactured in metric dimensions; that is why it is referred to as "metric lead screw" or "metric acme screw."
The buttress thread is designed to handle high axial loads and transmit power in one direction; the direction depends on the orientation of the weight-bearing and trailing flanks. The weight-bearing flank of a standard buttress thread makes a 70 slant while its trailing flank makes a 450-angle slant. The tooth of the buttress thread has a wider base, which gives the screw about twice the shear strength of the square thread. The efficiency is almost equal to the square thread due to its low frictional losses.
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Buttress threads are used in large screw presses, jacks, vertical lifts, turning equipment, and milling equipment. However, it is only ideal for unidirectional thread application and performs poorly if the axial load is applied in the opposite direction.
The commonly used methods in manufacturing lead screws are the following:
In thread rolling, the metal rod (the blank material) is compressed between two roller dies containing a thread profile. The dies deform the surface of the blank after multiple passes; this transfers the thread profile to the workpiece. Thread rolling is a metal cold forming process; hence, the thread achieves higher strength and hardness. The products from thread rolling are "rolled threads".
In thread whirling, the metal rod is clamped in the whirling head and titled to achieve its desired helix angle. The whirling head rotates the rod at high speeds while pushing it slowly against a single cutting tool. Thread whirling enables the production of threads in only a single pass. It is capable of making deeper and more accurate threads. The products from thread whirling are "cut threads."
Linear actuators are devices that move loads in a single-axis straight path. They can be driven by lead screws. There are two types in which lead screw actuators can be operated:
The nut pushes and moves the load linearly. To increase stability and load capacity, it is typically supported by a linear guide which consists of guide rails, bearings, and a carriage. The nut is equipped with an anti-backlash mechanism to ensure the accuracy and repeatability of linear motion.
Lead Screw Actuator from Del-Tron Precision, Inc.A lead screw table is an enhancement of lead screw actuators that features a wider platform in which the load can be mounted. The lead screw is typically positioned in parallel between two guide rails and the nut is housed inside the carriage of the linear guide. Lead screw tables are used in positioning systems of larger-sized objects.
Crossed Roller Tables from Del-Tron Precision, Inc.Lead screw stages are actuators used in precise positioning systems. They have high torsional stiffness and are equipped with a friction locking mechanism which makes them compatible for both horizontal and vertical applications. Some types of lead screw stages can move on more than one axis. Lead screw stages are available in X, X-Y, and X-Y-Z configurations.
Lead Screw Stages from Del-Tron Precision, Inc.The efficiency and wear of lead screws are greatly influenced by the coefficient of friction between the mating surfaces of the nut and the screw shaft. The coefficient of friction is an intrinsic property of a material. Both materials for the screw shaft and nut must be compatible with each other. Hard materials are avoided as they significantly contribute to the wear of the lead screw. The other desirable properties of lead screw materials are high tensile and compressive strength, fatigue resistance, rigidity, and corrosion and chemical resistance.
The materials commonly used for the screw shaft include carbon steel, stainless steel, aluminum, and titanium. Meanwhile, the material for the nut is replaced by plastic or bronze:
Lead screw materials are commonly coated with engineered composite materials with self-lubricating properties to further reduce their coefficient of friction as well as up their anti-corrosion and thermal resistance properties to further protect them from harsh environments. The coating eliminates the need for lubrication. Coating materials include polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK), Torlon®, and Vespel®.
Backlash is defined as the axial movement of the screw shaft and nut without the rotation of either one. It is considered an inherent property of lead screws. It occurs when there is unwanted clearance and insufficient fitting of the internal and external threads; this causes lost motion of the lead screw components.
Backlash can affect the accuracy and repeatability of the lead screw. A certain amount of backlash is tolerated in some applications such as presses, jacks, clamps, and vices. However, it can degrade the performance of positioning, dispensing, and assembly systems. It can also increase the wear of the lead screw.
An anti-backlash nut keeps the screw shaft and the nut closer to each other. The categories of anti-backlash nuts are the following:
The axial anti-backlash nut requires the use of a spring placed between two nut halves. The spring compresses the opposing flanks of the internal and external threads and eliminates the unwanted axial clearances between them. However, axial anti-backlash nuts require greater torque to drive the lead screw and induce more frictional losses, making the lead screw less efficient. To keep backlash in the system to a minimum, the spring force must be greater than the load.
Radial anti-backlash nuts compress the screw shaft and the thread via radial force and eliminate the unwanted radial clearance between the crest and the root. The backlash is eliminated constantly regardless of the load applied. The radial force also compensates for the wear of the threads. The reduction of radial backlash is accomplished in two ways. The first method uses a nut body with flexible fingers that are pushed down by an axial spring. The other type of radial anti-backlash nuts feature a spring wrapped externally around the nut body that can be adjusted to meet preload and clearance requirements.
Another method of eliminating axial backlash is use of a spacer between two nut halves bolted together. The bolt holds the two opposing flanks together, and the spacer prevents the two nuts from being over-tightened and consequently applies additional compressive force to the screw shaft.
Backlash can be predicted and compensated for electronically by software. This is possible in more automated and high-tech systems.
Other than backlash and materials of construction, consider the following aspects during lead screw selection, operation, and maintenance:
PV rating gives the highest combination of axial load and revolutions per minute (rpm) the lead screw can withstand and is based on the heat generated during movement and wear of the lead screw. The PV value is the product of the contact surface pressure and sliding velocity, which are two independent parameters of a lead screw.
The PV curve defines the safe operating limits of the lead screw and describes the inverse relationship between the contact surface pressure and sliding velocity in order to operate safely. When carrying a heavier axial load, the rotation speed of the screw should be reduced. The same principle should be applied whenever the axial load or rpm must be altered.
The PV value is affected by the type of material used in constructing the lead screw and also its lubrication conditions.
End fixity refers to how the lead screw is supported. It affects the rigidity, critical speed, and buckling load of a lead screw. The types of end fixity are fixed-free, floating-floating, fixed-floating, and fixed-fixed.
The critical speed of a lead screw is the maximum revolutions per minute it can rotate; this is based on the natural frequency of the screw. If the operational rpm exceeds the critical speed, excessive vibrations will occur and eventually damage the lead screw. The critical speed is based on its minor diameter, length, shaft straightness, assembly alignment, and end fixity. It is recommended that the operational rpm does not exceed 80% of the evaluated critical speed.
Buckling load, or the column strength, refers to the maximum compressive force that a lead screw can withstand before bending or buckling. It is an important consideration in lead screw sizing. For the same end fixity type, the buckling load increases with increasing minor diameter and decreasing length between bearing supports.
Lead accuracy is the deviation of the actual linear distance covered by a lead screw from the theoretical distance calculated based on its pitch and leads. It is included in the specifications disclosed by the lead screw manufacturer. A screw with a lower value for lead accuracy definitely has better accuracy and precision.
The advantages of lead screws are the following:
The disadvantages of lead screws are the following:
A lead screw assembly is a simple design with few parts. With so few parts, the nuts are easier to customize. Custom sizes and designs can be created quickly to match unique applications. Nuts can be made of custom polymers or bronze. Nuts can be molded, tapped or machined
Internally
lubricated plastics and optional PTFE coatings reduce or eliminate
service requirements. 300 Series stainless steel screws and plastics are
good choices for clean room or instrument-grade applications.
Many lead screw choices are self locking. This reduces the need for a brake on vertical application or to hold a load.
This all adds up to a high-performance-to-price ratio (value).
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