Name: screw jack self-locking behavior
Uploaded: 2023-09-22T11:49:29.000Z
Duration: 1 min 16 s
Channel: JoVE

Consider a square-threaded screw jack carrying a load. If the screw retains its position even after the moment is removed, it is called self-locking.
This happens if the frictional force direction reverses and the reaction force acts on the other side of the thread&#39;s normal.
Here, the static friction angle can be larger or equal to the lead angle.
To wind a self-locking screw downwards, a moment has to be applied to the screw in the opposite direction. By applying the equilibrium equations for the forces, the magnitude of the moment can be determined.
If both angles are equal, the reaction acts vertically, balancing the load such that the screw is on the verge of winding downwards.
If the friction angle is smaller than the lead angle, the screw is not self-locking, and a moment in the opposite direction is applied to stop the downward winding of the screw.
This moment creates a horizontal force that prevents the sliding of thread on the inclined plane. From the free-body diagram, the magnitude of this moment can be determined.

self-locking screw

A square-threaded screw jack is a mechanical device widely used for lifting heavy loads or applying considerable force. One of the key features that can make a screw jack more effective and reliable is its self-locking capability.
A square-threaded screw jack carrying a load is considered self-locking if the screw retains its position even after the moment applied to it is removed.
<img alt="Static equilibrium diagram, forces on inclined plane; includes W, Ff, θ, φs, N, and R vector symbols." src="/files/ftp_upload/14320/14320_PT_Figure_1v2.svg" style="text-align: center; height: 350px;" data-altupdatedat="1761830439000" data-alt="Figure 1">
This occurs when the frictional force direction reverses, and the reaction force acts on the other side of the thread's normal. In this situation, the static friction angle is larger or equal to the lead angle of the screw. To wind a self-locking screw downwards, a moment must be applied to the screw in the opposite direction.
<img alt="Static equilibrium diagram, ΣF=0, illustrating forces and angles on an inclined plane." src="/files/ftp_upload/14320/14320_PT_Figure_2v2.svg" style="text-align: center; height: 350px;" data-altupdatedat="1761830439000" data-alt="Figure 2">
By applying the equilibrium equations for the forces acting on the screw, the magnitude of the moment required to initiate downward winding can be determined.
<img alt="Static equilibrium equation, \( \frac{M''}{r} - R \sin(\phi_s - \theta) = 0 \), for research analysis." src="/files/ftp_upload/14320/14320_PT_Equation_1v2.svg" style="text-align: center; height: 29px;" data-altupdatedat="1761830439000" data-alt="Equation 1">
<img alt="Static equilibrium equation, Rcos(ϕs−θ)−W=0, depicting force balance, mathematical formula." src="/files/ftp_upload/14320/14320_PT_Equation_2v2.svg" style="text-align: center; height: 13px;" data-altupdatedat="1761830439000" data-alt="Equation 2">
<img alt="Static equilibrium equation: M''=rWtan(ϕs−θ); mathematical expression, mechanics principle." src="/files/ftp_upload/14320/14320_PT_Equation_3v2.svg" style="text-align: center; height: 13px;" data-altupdatedat="1761830439000" data-alt="Equation 3">
If both the static friction angle and the lead angle are equal, the reaction force acts vertically, balancing the load such that the screw is on the verge of winding downwards. In this case, the screw is in a state of equilibrium, where it can maintain its position without any additional moment applied to it.
However, if the static friction angle is smaller than the lead angle, the screw is not self-locking. In this scenario, a moment in the opposite direction must be applied to stop the downward winding of the screw. This opposing moment generates a horizontal force that prevents the sliding of the thread on the inclined plane of the screw jack.
<img alt="Inclined plane diagram, static equilibrium concept with forces W, Fh1, R, angle θ." src="/files/ftp_upload/14320/14320_PT_Figure_3v2.svg" style="text-align: center; height: 350px;" data-altupdatedat="1761830439000" data-alt="Figure 3">
The magnitude of the required opposing moment can be determined by analyzing the free-body diagram of the screw jack under these conditions.
<img alt="Static equilibrium equation: M'=rWtan(θ−ϕs); concept in mechanics and physics diagram." src="/files/ftp_upload/14320/14320_PT_Equation_4v2.svg" style="text-align: center; height: 13px;" data-altupdatedat="1761830439000" data-alt="Equation 4">
This information is crucial for designing and operating a screw jack effectively, as it allows engineers to select the appropriate parameters for the specific application.

Self-Locking Screw

A square-threaded screw jack is a mechanical device widely used for lifting heavy loads or applying considerable force. One of the key features that can ...

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screw jack self-locking behavior

A square-threaded screw jack can hold a heavy load because of its self-locking behavior. This feature makes the device more reliable when it is used to lift large weights or apply a strong force. If the screw keeps its position after the turning moment is removed, the jack is self-locking.

Self-locking happens because friction changes the way the forces act on the thread. The friction force reverses direction, and the reaction force moves to the other side of the thread’s normal. In this case, the static friction angle is larger than or equal to the lead angle of the screw.

To wind a self-locking screw downward, an opposite moment must be applied. The required moment can be found by using the equilibrium equations for the forces on the screw. If the static friction angle and the lead angle are equal, the reaction force acts vertically and the screw is on the verge of winding down.

When the static friction angle is smaller than the lead angle, the screw is not self-locking. Then an opposite moment is needed to stop the screw from winding downward. That moment creates a horizontal force that prevents the thread from sliding on the screw jack’s inclined plane. Engineers can determine the size of this opposing moment from a free-body diagram, which helps them choose the right screw jack design for the job.

Screw Jack Self-Locking Behavior

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key terms and concepts

Learn how a square-threaded screw jack stays in place, when it is self-locking, and how friction and lead angle affect the moment needed to move it.

Screw Jack Self-Locking Behavior | JoVE K12

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Screw Jack Self-Locking Behavior

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