How to recognize left hand vs right hand threads in lead screws

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How to recognize left hand vs right hand threads in lead screws


A lead screw (or power screw) is a mechanical component that converts rotational motion into linear movement by sliding threads, commonly used for precise positioning and lifting heavy loads in machines.

Key examples include CNC machine axes, 3D printers, robotic arms, laboratory equipment, and automated lab fluid handling.

Identifying the direction of a lead screw thread is a useful skill, especially when working on mechanical assemblies or DIY projects. Here is how you can distinguish between Right-Hand (RH) and Left-Hand (LH) threads.

​1. The Visual "Slope" Test

​The easiest way to tell is to hold the screw vertically and look at the angle (the helix) of the threads.

​Right-Hand Thread: The threads slope upward to the right.

​Left-Hand Thread: The threads slope upward to the left.

​2. The Rotation Test

​If you have a nut that fits the screw, or if you are trying to drive the screw into a threaded hole:
​Right-Hand: Follows the "Right-Hand Rule." You turn it clockwise (tighten) to move it away from you and counter-clockwise (loosen) to bring it toward you. This is the standard for 95% of screws.

​Left-Hand: It works the opposite way. You must turn it counter-clockwise to tighten it. These are often used on rotating parts (like bicycle pedals or fans) to prevent the rotation from accidentally loosening the screw.

​3. The "Thumb" Rule

​Hold the screw in your hand and align your thumb with the axis of the screw, pointing in the direction you want the screw to travel.

​Right-Hand: If the threads follow the natural curve of your right hand’s fingers as you close them, it is a right-hand thread.

​Left-Hand: If the threads follow the curve of your left hand’s fingers, it is a left-hand thread.




​한글 번역 (Korean Translation)

​리드 스크루(Lead Screw)에서 오른나사와 왼나사를 구분하는 방법은 다음과 같습니다.

​1. 시각적 경사 확인 (기울기 방향)

​스크루를 수직으로 세우고 나사산의 기울기를 확인합니다.

​오른나사 (Right-Hand): 나사산이 오른쪽 위 방향으로 기울어져 있습니다.

​왼나사 (Left-Hand): 나사산이 왼쪽 위 방향으로 기울어져 있습니다.

​2. 회전 방향 테스트

​오른나사: 시계 방향(오른쪽)으로 돌릴 때 조여집니다. 우리가 사용하는 대부분의 일반적인 나사입니다.
​왼나사: 시계 반대 방향(왼쪽)으로 돌릴 때 조여집니다. 주로 회전하는 부품(자전거 페달, 선풍기 날개 등)에서 회전 진동으로 인해 나사가 풀리는 것을 방지하기 위해 사용됩니다.

​3. 엄지손가락 규칙

​엄지손가락을 나사가 나아가야 할 방향으로 향하게 하고 나사를 감싸 쥐어 봅니다.

​나사산의 방향이 오른손 손가락이 감기는 방향과 일치하면 오른나사, 왼손 손가락 방향과 일치하면 왼나사입니다.





What is the difference between left-hand and right-hand threads and how can I recognize the thread direction?


A screw’s handedness indicates which direction the thread of the screw faces as it moves upward along the screw. In a right handed screw, the thread will move to the right as it slopes upward, and a left handed screw will move to the left as it slopes upward.

How to drive left-handed screws vs right-handed screws


The thread direction of a screw will determine which way it needs to be turned to be driven into a material. For right handed screws, turning them clockwise will drive them downward. For left handed screws, turning them counterclockwise drives them downward.

Thread direction in lead screws
Left-handed and right-handed screws are not just limited to standard fasteners — lead screws can also be manufactured in either thread direction. Thread direction plays a much more crucial role, however, in lead screw applications than in other types of screws.

Right-hand lead screw threads
For lead screws with right-hand thread, the thread flanks rise from bottom left to top right. In production drawings, right-hand threads are often marked with the letters RH. For example: TR20x4-RH.

Right-handed lead screws (and fasteners in general) are significantly more popular than their left-handed counterparts. However, this is often due to ergonomics, rather than technical differences. The majority of people are right-handed and can therefore apply more torque when turning a lead screw clockwise than counterclockwise, and the motion itself is more intuitive.



Left-hand lead screw threads


A left-hand thread rises from bottom right to top left when viewed from the front. Production drawings will typically denote a left-handed thread with the letters LH. For example: TR20x4-LH.

Left-handed lead screws are popular in rotary applications where self-loosening is a concern. In these applications, the rotational movement can potentially cause a right-handed screw to loosen over time. By instead using a left-handed screw, this same motion will instead keep the screw tightened and secure.

Other applications like turnbuckles or actuators that need to move two screws together and apart in sync also rely on left-handed screws, or a double-ended (reverse) screw with a left-handed thread on one side and a right-handed thread on the other that meet in the middle of the screw.



Thread directions other than left and right, are of course, impossible. But what about a combination of both cutting directions?

The dryspin product range of lead screws include left-handed, right-handed, and reverse lead screws in each of the major lead screw thread profiles, as well as a proprietary high-helix thread perfect for achieving a long service life and smooth, efficient motion.

Diagram of a reverse lead screw, demonstrating how the left hand and right hand threads meet
If you'd like advice on how to determine the best lead screw for your application, contact us. You can also visit this webpage to learn more about dryspin® lead screw assemblies or to try out our lead screw product finder and lead screw configurator to calculate service life, build lead screws in 20 minutes or less, and more.

Frequently Asked Questions

What is the core difference between left- and right-handed threads?


Which thread direction is the standard choice?


Why would I use a left-handed lead screw?


Is there a performance difference?


Can I use a left-hand nut on a right-hand screw?


How are left-hand threads used for dual-action?


Are left-hand threads used for safety?


How can I visually identify the thread direction of a lead screw?