Power screws convert rotary motion into controlled linear motion and are widely used for lifting, positioning, clamping and force transmission. Common arrangements include square, Acme and buttress threaded screws, recirculating ball screws and roller screws.
The appropriate type depends on load, speed, efficiency, positioning accuracy, backlash, stiffness, duty cycle, manufacturing requirements and whether the mechanism must resist back-driving.
A power screw is a linear actuator in which relative rotation between a screw and nut produces axial movement. Depending on the arrangement, either the screw or the nut may rotate while the other component translates or remains stationary.
Power screws are commonly used:
Sliding-thread power screws can provide substantial mechanical advantage in a compact mechanism, but sliding friction reduces efficiency and produces heat and wear. Recirculating ball and roller screws replace much of the sliding contact with rolling contact and can therefore provide substantially higher efficiency.
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The square thread is a traditional power-transmission thread used for applications such as screw jacks and clamps. Its thread flanks are approximately perpendicular to the screw axis, so the thread geometry does not introduce the radial component associated with inclined thread flanks.
This gives the square thread low frictional losses compared with conventional sliding-thread alternatives and makes it useful where efficiency is important. Its principal disadvantage is manufacture: square threads are more difficult to machine accurately and are less convenient for arrangements such as split nuts.
Acme threads are widely used for power transmission and linear movement, including machine-tool lead screws. Their inclined flanks make them easier to manufacture than square threads and provide a stronger thread root.
The form is also well suited to split-nut arrangements. The inclined thread angle, however, increases the normal force between the mating thread surfaces and therefore affects friction, operating torque and efficiency.
Efficiency should consequently be calculated from the actual lead angle, thread angle and coefficient of friction rather than assigned a single general value.
A buttress thread is intended principally for transmitting high axial load in one direction. One flank is relatively steep and carries the main working load, while the opposite flank is more inclined.
This arrangement combines good load-carrying capability with relatively favourable friction characteristics in the principal loading direction. Buttress threads are therefore useful where the dominant axial force acts consistently in one direction. They are less appropriate where similarly high loads must be transmitted in both directions.
A recirculating ball screw uses balls rolling between helical grooves in the screw and nut. As relative rotation occurs, the balls carry the axial load while moving along the grooves. At the end of their working path they are returned through a recirculation system and re-enter the loaded region.
Replacing sliding thread contact with rolling contact greatly reduces friction. Ball screws can therefore achieve efficiencies above 90% under suitable operating conditions and are widely used for high-speed and precision linear motion.
A roller screw uses a number of threaded rollers arranged around a central threaded shaft. The rollers provide multiple load-carrying contact regions between the screw and nut.
Roller screws can provide high axial load capacity, high stiffness and good efficiency in a relatively compact arrangement. They are particularly suited to demanding high-load or high-duty linear actuator applications, although their complexity and cost are generally greater than those of conventional sliding screws.
| Type | Principal Characteristics | Typical Use |
|---|---|---|
| Square thread | Low sliding friction and no thread-angle radial component, but relatively difficult to manufacture | Jacks, clamps and traditional high-force power screws |
| Acme thread | Strong thread form, easier manufacture and suitable for split nuts | Lead screws, machine tools and general linear motion |
| Buttress thread | High load capacity primarily in one axial direction | Heavy unidirectional loading |
| Ball screw | Rolling contact, high efficiency, low friction and high positioning capability | CNC machines, automation and precision positioning |
| Roller screw | Multiple rolling contacts, high load capacity and high stiffness | High-load and high-duty electromechanical actuators |
Selection should not be based on efficiency alone. Consider the required axial load, linear speed, stroke, positioning accuracy, stiffness, backlash, duty cycle, life, lubrication, contamination, temperature and whether the load can drive the screw backwards.
Calculate screw lead and helix angle, raising and lowering torque, thread and collar friction, efficiency, self-locking and overhauling conditions.
Power Screw Torque & EfficiencyCheck thread bearing, bending and shear stresses together with screw direct stress, combined torsion and axial loading, and column buckling.
Power Screw Strength Equations