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

荷物を運ぶ角ネジジャッキを考えてみましょう。モーメントを抜いてもネジの位置が保持されている場合、それはセルフロックと呼ばれます。
これは、摩擦力の方向が逆になり、反力がねじ山の法線の反対側に作用した場合に発生します。
ここで、静摩擦角はリード角より大きいか、それ以上にすることができます。
セルフロックネジを下向きに巻くには、ネジに反対方向にモーメントを加える必要があります。力の平衡方程式を適用することにより、モーメントの大きさを決定できます。
両方の角度が等しい場合、反作用は垂直に作用し、ねじが下向きに巻かれる寸前になるように荷重のバランスを取ります。
摩擦角がリード角よりも小さい場合、ネジはセルフロックしておらず、反対方向のモーメントが加えられてネジの下向きの巻き上げが停止します。
この瞬間により、傾斜面上の糸の滑りを防ぐ水平方向の力が生まれます。自由体図から、このモーメントの大きさを決定できます。

<ol>
	<li>ヒベラー、RC(2016)。エンジニアリングメカニクス:静力学。第14版、ニュージャージー州:ピアソン。432-433ページ。</li>
	<li>メリアム、JL、クレイジ、LGおよびボルテン、JN(2016)。エンジニアリングメカニクス:静力学とダイナミクス。第8版、シンガポール:John Wiley & Sons。355-356ページ。</li>
	<li>ビール、FP、ジョンストン、ER、マズレック、DF、コーンウェル、PJおよびセルフ、BP(2016)。エンジニアのためのベクトル力学。第11版、ニューヨーク:マグロウヒル教育。451ページ。</li>
</ol>

自己ロック式ねじ

正方形のスレッドねじのジャッキは、重い荷重を持ち上げたり相当な力を加えるために広く使われる機械装置です。自己ロック能力を持つことで、ねじジャッキの効果と信頼性を向上させることができるとされています。

荷物を持ち上げたり負荷をかけた状態の正方形スレッドねじジャッキは、そのねじが位置を維持する場合、自己ロックとみなされます。ねじにかかる力が取り除かれた後でも、ねじの位置がそのまま保たれるのです。

<img alt="Figure 1" src="/files/ftp_upload/14320/14320_PT_Figure_1v2.svg" style="text-align: center; height: 350px;" />

これは、摩擦力の方向が逆転し、反作用力がねじの軸線の反対側で働くときに生じます。この状況では、静止摩擦角がねじの山型の角度以上に大きいか等しいとなります。自己ロック式ねじを下方に巻き込むには、逆方向にねじに力矩を加える必要があります。

<img alt="Figure 2" src="/files/ftp_upload/14320/14320_PT_Figure_2v2.svg" style="text-align: center; height: 350px;" />

ねじにかかる力についての釣り合いの方程式を適用することで、下方に巻き込むために必要な力矩の大きさを求めることができます。

<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;" />

もし静止摩擦角とねじの山型の角度が等しい場合、反作用力は垂直方向に働き、負荷をバランスさせるためにねじが下方に巻き込まれる寸前の状態になります。この場合、ねじは追加の力矩なしでその位置を維持できる平衡状態にあります。

しかしながら、静止摩擦角がねじの山型の角度よりも小さい場合、ねじは自己ロックしていません。この場合、ねじの下方への巻き込みを止めるために逆方向の力矩が必要です。この逆方向の力矩は、ねじジャッキの傾斜面上でのねじの滑りを防止する水平方向の力を発生させます。

<img alt="Figure 3" src="/files/ftp_upload/14320/14320_PT_Figure_3v2.svg" style="text-align: center; height: 350px;" />

これらの条件下でねじジャッキの自由体図を分析することで、必要な逆方向の力矩の大きさを求めることができます。

<img alt="Equation 4" src="/files/ftp_upload/14320/14320_PT_Equation_4v2.svg" style="text-align: center; height: 13px;" />

この情報は、効果的なねじジャッキの設計と運用において重要であり、エンジニアが特定の応用に適切なパラメータを選択することを可能にします。

角ねじねじジャッキは、重い荷物を持ち上げたり、かなりの力を加えたりするために広く使用されている機械装置です。主な機能の1つ...

Education

JoVE Core

Mechanical Engineering

機械工学

Unit 1

ユニット1

Friction

摩擦

KEY TERMS AND CONCEPTS

主要用語と概念

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

Static friction is the force that keeps two surfaces from starting to slide past each other. It acts when there is a tendency for motion, but not yet actual slipping. This force helps explain everyday actions like walking on the ground and driving a car. A towing example shows how static friction changes as force is applied. A truck pulls a car with a rope, and the car stays still at first. The car is in static equilibrium, which means the forces on it are balanced. Gravity and the normal...

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

Static and Kinetic Dry Friction

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types of friction problems

Dry Friction in Crate, Ladder, and Frame Problems

Dry friction problems often focus on crates, ladders, and frames that are close to slipping. Dry friction is the force that resists relative motion, or the tendency for motion, between two surfaces in contact. In many physics and engineering problems, this force helps keep an object still or defines the point where motion is about to begin. One common case has no clear impending motion. A force may be applied to a crate, but the crate stays at rest. The friction force between the floor and the...

friction: problem solving

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wedges

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A wedge is a simple machine that can lift, support, and split heavy objects. It can also change the height of structural or mechanical parts. Because it amplifies force, a wedge makes it easier to move or hold large loads with less effort. One example is lifting a heavy slab. In that case, the wedge turns the applied force into a much larger force that acts almost perpendicular to the original push. That force change helps raise the load without requiring a large input force. To analyze wedge...

frictional forces on screws

Square-Thread Screw Mechanics and Friction

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upward impending motion

Screw Jack Force and Friction Analysis

A square-threaded screw jack lifts heavy loads by turning handle force into a twisting moment. Its upward motion depends on overcoming static friction between the screw threads and the jack. To study this motion, the thread can be treated as if it were a block on the groove of the jack after the thread is completely unraveled. A free-body diagram helps show the forces acting on this simplified system. The force at the handle creates a horizontal force that pushes the movable thread up the...

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

Screw Torque and Self-Locking

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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

Rolling resistance is the force that makes a rolling object harder to move across a surface. It is also called rolling friction. It comes from deformation in the object and the surface that touch each other. Internal friction, hysteresis, and other energy losses in the materials also play a role. Because of this force, extra energy is needed to keep a wheel, tire, or ball moving. The video applies this idea to a lawn roller. The roller has a mass of 100 kg, a radius of 25 cm, and a coefficient...

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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mechanical engineering

Screw Jack Self-Locking Behavior

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