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

考虑一个承受载荷的方牙螺纹千斤顶。如果在去除转矩后螺纹仍能保持其位置不变，则称为自锁。
当摩擦力方向反转且反作用力作用于螺纹法线的另一侧时，就会发生这种情况。
此处，静摩擦角可以大于或等于导程角。
要使自锁螺钉向下旋转，必须对螺钉施加一个相反方向的力矩。通过应用力的平衡方程，可以确定该力矩的大小。
如果两个角度相等，反作用力将垂直作用，使载荷达到平衡，此时螺钉处于即将向下旋动的临界状态。
如果摩擦角小于导程角，则螺杆不具备自锁性，需施加反向力矩以阻止螺杆向下旋动。
这一力矩产生一个水平方向的力，可防止螺纹在斜面上滑动。根据受力分析图，可确定该力矩的大小。

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	<li>希伯勒，RC（2016 年）。工程力学:静力学。第十四版，新泽西州:皮尔逊。第 432-433 页。</li>
	<li>Meriam， JL， Kraige， LG 和 Bolten， JN（2016 年）。工程力学:静力学和动力学。第八版，新加坡:John Wiley & Sons。第 355-356 页。</li>
	<li>比尔，FP，约翰斯顿，ER，Mazurek，DF，康威尔，PJ 和塞尔夫，BP（2016 年）。工程师的矢量力学。第十一版，纽约:麦格劳-希尔教育。第 451 页。</li>
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自锁螺丝

方螺纹螺旋千斤顶是一种广泛用于提升重物或施加相当大力的机械设备。使螺旋千斤顶更有效和可靠的关键特征之一是其自锁能力。 如果施加在其上的力矩消除后螺钉仍保持其位置，则承载负载的方螺纹螺旋千斤顶被认为是自锁的。 <img alt="Figure 1" src="/files/ftp_upload/14320/14320_PT_Figure_1v2.svg" style="text-align: center; height: 350px;"> 当摩擦力的方向发生反转时，便会出现这种情况，并且反作用力作用在螺纹法线的另一侧。在这种情况下，静摩擦角大于或等于螺杆的导程角。要向下拧紧自锁螺钉，那么必须向螺钉施加相反方向的力矩。 <img alt="Figure 2" src="/files/ftp_upload/14320/14320_PT_Figure_2v2.svg" style="text-align: center; height: 350px;"> 通过应用施加在螺旋上力的平衡方程，可以确定其向下进行拧紧所需扭矩的大小。 <img alt="Equation 1" src="/files/ftp_upload/14320/14320_PT_Equation_1v2.svg" style="text-align: center; height: 29px;"> <img alt="Equation 2" src="/files/ftp_upload/14320/14320_PT_Equation_2v2.svg" style="text-align: center; height: 13px;"> <img alt="Equation 3" src="/files/ftp_upload/14320/14320_PT_Equation_3v2.svg" style="text-align: center; height: 13px;"> 如果静摩擦角和导程角都相等，则反作用力则会在垂直方向产生作用，以此来平衡负载，使螺旋处于即将向下拧紧的状态。在这种情况下，螺旋处于平衡状态，同时能够保持其位置而无需施加额外的扭矩。 但是，如果静摩擦角小于导程角，则螺钉不能自锁。在这种情况下，必须施加相反方向的力矩来阻止螺钉向下缠绕。这个反向力矩将会产生水平方向的力，以防止螺纹在螺旋千斤顶的斜面上发生滑动。 <img alt="Figure 3" src="/files/ftp_upload/14320/14320_PT_Figure_3v2.svg" style="text-align: center; height: 350px;"> 通过分析在这些条件下螺旋千斤顶的受力分析图，以此来确定所需反向扭矩的大小。 <img alt="Equation 4" src="/files/ftp_upload/14320/14320_PT_Equation_4v2.svg" style="text-align: center; height: 13px;"> 这些信息对于有效的设计和操作螺旋千斤顶至关重要，因为它允许工程师为特定的应用选择适当的参数。

方牙螺杆千斤顶是一种广泛用于提升重物或施加较大力的机械装置。其关键特性之一是……

Education

教育

JoVE Core

Mechanical Engineering

机械工程

Unit 1

第一单元

Friction

摩擦

KEY TERMS AND CONCEPTS

关键术语与概念

Self-Locking Screw

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

dry friction

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static friction

Static Friction at the Point of Slip

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kinetic friction

Sliding Friction Between Moving Surfaces

Kinetic friction is the sliding friction that acts when two surfaces move against each other. It opposes relative motion and acts parallel to the contact surface, in the opposite direction of travel. In the truck and car example, the car first stays still because static friction holds it in place. When the truck’s force becomes greater than the limiting static frictional force, the friction at the contact surface drops to a lower value. The car then begins to slide, and its speed increases. At...

characteristics of dry friction

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types of friction problems

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friction: problem solving

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frictional forces on screws

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upward impending motion

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self-locking screw

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,...

screw: problem solving

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frictional forces on flat belts

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flat belts: problem solving

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pivot bearings

Axial Load Support in Pivot Bearings

Pivot bearings support axial loads on a rotating shaft. A bearing helps two parts move relative to each other while reducing friction and wear. In this design, the pivot is placed at the end of the shaft to carry the axial thrust. The pivot surface can be flat, conical, or truncated, depending on the application. In a flat pivot bearing, the axial load is spread over the bearing area. The pressure is uniform and can be found by dividing the load by the area. To study the forces more closely,...

collar bearings

Friction Torque in Collar Bearings

Collar bearings support axial loads on rotating shafts, and their friction helps determine the torque needed to start motion. They are used in many machines and may have a single collar or multiple collars, depending on the application and load needs. For a single collar bearing under an axial load, the contact area is the ring-shaped region between the external radius and the internal radius of the collar. If the bearing is assumed to support the load evenly, the uniform normal pressure is...

journal bearings

Friction Moment in Journal Bearings

Journal bearings support rotating shafts and axles while helping keep them stable. They are used in machines such as engines, turbines, and pumps. Their job is to reduce friction, wear, and vibration. A journal bearing works by keeping a thin film of lubricant between the bearing surface and the rotating shaft. This film limits direct contact and lowers frictional force. The video uses a rope winch with dry or only partly lubricated journal bearings to show the idea. As the spool turns...

bearings: problem solving

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rolling resistance

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rolling resistance: problem solving

Lawn Roller Force Calculation

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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

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

mechanical engineering

Screw Jack Self-Locking Behavior

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