Leadscrew

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Three types of screw thread used in leadscrews:
3 & 4: buttress thread
5: round thread
6: square thread
DVD drive with leadscrew and stepper motor.
Floppy disc drive with leadscrew and stepper motor.

A leadscrew (or lead screw), also known as a power screw[1] or translation screw,[2] is a screw used as a linkage in a machine, to translate turning motion into linear motion. Because of the large area of sliding contact between their male and female members, screw threads have larger frictional energy losses compared to other linkages. They are not typically used to carry high power, but more for intermittent use in low power actuator and positioner mechanisms. Common applications are linear actuators, machine slides (such as in machine tools), vises, presses, and jacks.[3]

Leadscrews are manufactured in the same way as other thread forms (they may be rolled, cut, or ground).

A lead screw can be used in conjunction with a split nut.

Types[edit]

Power screws are classified by the geometry of their thread. V-threads are less suitable for leadscrews than others such as Acme because they have more friction between the threads. Their threads are designed to induce this friction to keep the fastener from loosening. Leadscrews, on the other hand, are designed to minimize friction.[4] Therefore, in most commercial and industrial use, V-threads are avoided for leadscrew use. Nevertheless, V-threads are sometimes successfully used as leadscrews, for example on microlathes and micromills.[5]

Square thread[edit]

Square threads are named after their square geometry. They are the most efficient, having the least friction, so they are often used for screws that carry high power. But they are also the most difficult to machine, and are thus the most expensive.

Acme thread[edit]

An Acme screw

Acme threads have a 29° thread angle, which is easier to machine than square threads. They are not as efficient as square threads, due to the increased friction induced by the thread angle.[3]

Buttress thread[edit]

Buttress threads are of a triangular shape. These are used where the load force on the screw is only applied in one direction.[6] They are as efficient as square threads in these applications, but are easier to manufacture.

Characteristics[edit]

A leadscrew nut and screw mate with rubbing surfaces, and consequently they have a relatively high friction and stiction compared to mechanical parts which mate with rolling surfaces and bearings. Leadscrew efficiency is typically between 25 and 70%, with higher pitch screws tending to be more efficient. A higher performing but more expensive alternative is the ball screw.

The high internal friction means that leadscrew systems are not usually capable of continuous operation at high speed, as they will overheat. Due to inherently high stiction, the typical screw is self-locking (i.e. when stopped, a linear force on the nut will not apply a torque to the screw) and are often used in applications where backdriving is unacceptable, like holding vertical loads or in hand cranked machine tools.

Leadscrews are typically used well greased, but, with an appropriate nut, they may be run dry with somewhat higher friction. There is often a choice of nuts, and manufacturers will specify screw and nut combination as a set.

The mechanical advantage of a leadscrew is determined by the screw pitch and lead. For multi-start screws the mechanical advantage is lower, but the traveling speed is higher.[7]

Backlash can be reduced with the use of a second nut, or a tensioning spring, to create a static loading force known as preload; alternately, the nut can be cut across its diameter and preloaded by clamping that cut back together.

A leadscrew with a sufficiently high helix angle can back drive: forces on the nut applied parallel to such a leadscrew will cause the leadscrew, if it is not otherwise held in place, to rotate. Such a tendency to backdrive depends on the thread helix angle, coefficient of friction of the interface of the components (screw/nut) and the included angle of the thread form. In general, a steel acme thread and bronze nut will back drive when the helix angle of the thread is greater than 20°.

Advantages & disadvantages[edit]

Leadscrews are used to raise and lower the front door of the Boeing 747-8F Freighter aircraft.

The advantages of a leadscrew are:[2]

  • Large load carrying capability
  • Compact
  • Simple to design
  • Easy to manufacture; no specialized machinery is required
  • Large mechanical advantage
  • Precise and accurate linear motion
  • Smooth, quiet, and low maintenance
  • Minimal number of parts
  • Most are self-locking

The disadvantages are that most are not very efficient. Due to the low efficiency they cannot be used in continuous power transmission applications. They also have a high degree for friction on the threads, which can wear the threads out quickly. For square threads, the nut must be replaced; for trapezoidal threads, a split nut may be used to compensate for the wear.[4]

Alternatives[edit]

Alternatives to actuation by leadscrew include:

Mechanics[edit]

Diagram of an "unwrapped" screw thread

The torque required to lift or lower a load can be calculated by "unwrapping" one revolution of a thread. This is most easily described for a square or buttress thread as the thread angle is 0 and has no bearing on the calculations. The unwrapped thread forms a right angle triangle where the base is \pi d_m long and the height is the lead (pictured to the right). The force of the load is directed downward, the normal force is perpendicular to the hypotenuse of the triangle, the frictional force is directed in the opposite direction of the direction of motion (perpendicular to the normal force or along the hypotenuse), and an imaginary "effort" force is acting horizontally in the direction opposite the direction of the frictional force. Using this free-body diagram the torque required to lift or lower a load can be calculated:[8][9]

T_{raise} = \frac{F d_m}{2} \left( \frac{l + \pi \mu d_m}{\pi d_m - \mu l} \right) = \frac{F d_m}{2} \tan{\left(\phi + \lambda\right)}
T_{lower} = \frac{F d_m}{2} \left( \frac{\pi \mu d_m - l}{\pi d_m + \mu l} \right) = \frac{F d_m}{2} \tan{\left(\phi - \lambda\right)}
Coefficient of friction for leadscrew threads[10]
Screw material Nut material
Steel Bronze Brass Cast iron
Steel, dry 0.15–0.25 0.15–0.23 0.15–0.19 0.15–0.25
Steel, machine oil 0.11–0.17 0.10–0.16 0.10–0.15 0.11–0.17
Bronze 0.08–0.12 0.04–0.06 - 0.06–0.09

where

  • T = torque
  • F = load on the screw
  • dm = mean diameter
  • \mu\, = coefficient of friction (common values are found in the table to the right)
  • l = lead
  • \phi\, = angle of friction
  • \lambda\, = lead angle

Based on the Tlower equation it can be found that the screw is self-locking when the coefficient of friction is greater than the tangent of the lead angle. An equivalent comparison is when the friction angle is greater than the lead angle (\phi > \lambda).[11] When this is not true the screw will back-drive, or lower under the weight of the load.[8]

The efficiency, calculated using the torque equations above, is:[12][13]

\mbox{efficiency} = \frac{T_0}{T_{raise}} = \frac{Fl}{2 \pi T_{raise}} = \frac{\tan{\lambda}}{\tan{\left(\phi + \lambda\right)}}

For screws that have a thread angle other than zero, such as a trapezoidal thread, this must be compensated as it increases the frictional forces. The equations below takes this into account:[12][14]

T_{raise} = \frac{F d_m}{2} \left( \frac{l + \pi \mu d_m \sec{\alpha}}{\pi d_m - \mu l \sec{\alpha}} \right) = \frac{F d_m}{2} \left( \frac{\mu \sec{\alpha} + \tan{\lambda}}{1 - \mu \sec{\alpha} \tan{\lambda}} \right)
T_{lower} = \frac{F d_m}{2} \left( \frac{\pi \mu d_m \sec{\alpha} - l}{\pi d_m + \mu l \sec{\alpha}} \right) = \frac{F d_m}{2} \left( \frac{\mu \sec{\alpha} - \tan{\lambda}}{1 + \mu \sec{\alpha} \tan{\lambda}} \right)

where \alpha\, is one half the thread angle.

If the leadscrew has a collar in which the load rides on then the frictional forces between the interface must be accounted for in the torque calculations as well. For the following equation the load is assumed to be concentrated at the mean collar diameter (dc):[12]

T_c = \frac{F \mu_c d_c}{2}

where \mu_c is the coefficient of friction between the collar on the load and dc is the mean collar diameter. For collars that use thrust bearings the frictional loss is negligible and the above equation can be ignored.[15]

Coefficient of friction for thrust collars[15]
Material combination Starting \mu_c Running \mu_c
Soft steel / cast iron 0.17 0.12
Hardened steel / cast iron 0.15 0.09
Soft steel / bronze 0.10 0.08
Hardened steel / bronze 0.08 0.06


Running speed[edit]

Safe running speeds for various nut materials and loads on a steel screw[16]
Nut material Safe loads [psi] Speed
Bronze 2500–3500 Low speed
Bronze 1600–2500 10 fpm
Cast iron 1800–2500 8 fpm
Bronze 800–1400 20–40 fpm
Cast iron 600–1000 20–40 fpm
Bronze 150–240 50 fpm

The running speed for a leadscrew (or ball screw) is typically limited to, at most, 80% of the calculated critical speed. The critical speed is the speed that excites the natural frequency of the screw. For a steel leadscrew or steel ballscrew, the critical speed is approximately[17]

N = { (4.76 \times 10^6) d_r C \over L^2}

where

  • N = critical speed in RPM
  • dc = smallest (root) diameter of the leadscrew in inches
  • L = length between bearing supports in inches
  • C = .36 for one end fixed, one end free
  • C = 1.00 for both ends simple
  • C = 1.47 for one end fixed, one end simple
  • C = 2.23 for both ends fixed

See also[edit]

References[edit]

  1. ^ Ball Screws & Lead screws, retrieved 2008-12-16 .
  2. ^ a b Bhandari, p. 202.
  3. ^ a b Shigley, p. 400.
  4. ^ a b Bhandari, p. 203.
  5. ^ Martin 2004, p. 266.
  6. ^ Bhandari, p. 204.
  7. ^ Bhandari, pp. 205–206.
  8. ^ a b Shigley, p. 402.
  9. ^ Bhandari, pp. 207–208.
  10. ^ Shigley, p. 408.
  11. ^ Bhandari, p. 208.
  12. ^ a b c Shigley, p. 403.
  13. ^ Bhandari, p. 209.
  14. ^ Bhandari, pp. 211–212.
  15. ^ a b Bhandari, p. 213.
  16. ^ Shigley, p. 407.
  17. ^ Nook Industries, Inc. "Acme & lead screw assembly glossary and technical data"

Bibliography[edit]

External links[edit]