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Left-Hand vs Right-Hand Threads, Which Is Best?

When working with precision engineering components, it's crucial to understand the differences between left-hand and right-hand threads. It's not just about knowing which way to loosen left-hand threads or tighten right-hand screws; it's also about understanding where you might choose to use what are often referred to as reverse head screws in practice and where they can bring advantages to your projects.

In this guide, we'll explain the meanings, benefits and drawbacks of left hand vs right hand thread screws and bolts, explore the concept of 'handedness', and show where you could choose to use left-hand threads in your projects.

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How Do You Know if You Have a Left or Right-Hand Thread?

To begin with, if you're looking to answer the question "Do you screw left or right?" you first need to determine the thread's direction or to be more specific, are your threads left-handed or right-handed. The best way to discover which way the threads on your screw turn is by observing which way the threads point.

An easy way to see this is to place the screw between your thumb and index finger. With the screw head at the bottom (resting on your thumb) and the tip touching your index finger, if the threads point diagonally upwards and to the right, then the screw has a right-hand thread. For left-hand threads, the direction of the threads will point diagonally upwards and to the left.

A Diagram Showing the Orientation of Left and Right Hand Threads.

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What Is Screw Handedness?

Handedness refers to the preference or dominance of one hand over the other for performing various tasks. In the context of engineering, the term is often associated with the direction of rotation or orientation of mechanical components like screws, nuts, and bolts.

The term "screw handedness" in relation to a singular screw is related to the orientation that the thread continuously coils around the screw's shaft. The question then is whether this thread progresses in a direction that is either clockwise or counterclockwise as you look down the screw shaft. Therefore, which way to tighten a screw depends on its handedness.

So What Does Right-Hand Thread Actually Mean?

All right-hand threads run in a clockwise direction. That is to say that when the component is rotated in a clockwise direction (viewed from above), it moves away from the observer, typically tightening into the material it's being threaded into. In practical terms, if you take any point along a right-hand thread and mark it, as you tighten it into a workpiece, the point you have marked will turn clockwiseBy contrast, to unfasten a right-hand threaded screw, it must be turned anticlockwise. This is consistent with the well-known mnemonic "righty-tighty, lefty-loosey."

And What Does Left-Hand Thread Mean?

Left-hand threads are the opposite of right-hand threads, so they are often also referred to as ‘reverse thread screws’ - their threads run in the anti-clockwise direction and must be turned anticlockwise to fasten them or clockwise to unfasten them.

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Right-Hand Thread and Left-Hand Thread Origins

Most engineering screws and components in use today feature a standardised, right-hand thread - finding left-handed threads is quite rare in general applications. Originally the reason for this was that the majority of the human race was (and currently still is) right-handed and the hand naturally twists (or ‘pronates’) in the clockwise direction. This decision was made at a time when screws were installed by hand in every application, so it made the most practical sense to manufacture components with this right-hand thread.

To this day, it is unusual to find left-handed screws in general applications, but some are used in more specialist instances when a specific characteristic is needed in the component, especially regarding locking which we'll cover next.

A Black and White Image of the Ducati Motorbike Assembly Line.

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Why Do Left-Handed Threads Exist?

Reverse thread screws have many practical benefits over the standard right-hand screw. They are often used to counteract the movement that causes a right-hand screw to loosen. So, left-hand screws are typically used in applications where the screw must not loosen, even when moving anti-clockwise.

One example is on the right-hand side of exotic and Formula 1 racing cars that have a singular wheel nut. This nut is threaded onto a left-hand tapped axle so that when the wheels spin clockwise, that nut doesn't come loose. Another example that may be a bit more relatable is on the left pedal of a bike, where the pedal must remain tight, even when turning in an anti-clockwise motion.

Left-hand threads are also used to deliberately distinguish between different pipes and parts. For example, gas pipes are often tapped with left-hand thread screws to distinguish them from water pipes.

Despite turning in a different direction, there is no such thing as a left-hand screwdriver as most tools are ambidextrous so they can be used with all standard components.

A Zoomed in Image of a Racing Cars Wheels With Left Hand Threaded Nuts To Keep the Wheels Secure.

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What Is the Difference Between Right and Left-Hand Threads?

Understanding the difference between a left hand vs right hand thread is crucial for anyone working with engineering components. Essentially, the distinction lies in the direction of the helix or the "twist" of the threads. Remember:

  • Right-hand threads twist clockwise.
  • Left-hand threads twist anti-clockwise.

Now let's take a look at some of the benefits, drawbacks and when you should choose a right or left-handed screw.

  Right-Hand Screws Left-Hand Screws
Orientation An Icon Showing a Right-Hand Threaded Screw. An Icon Showing a Left-Hand Threaded Screw.
Which Way To Tighten a Screw?

Clockwise

Clockwise Icon

Anti-Clockwise

Anti-Clockwise Icon

Benefits
  • Universality and cross-compatibility.
  • Higher availability and lower cost.
  • Standard component for many applications making repairs simple.
  • More ergonomically inclined to the majority of the human race.
  • Does not loosen when subject to unwanted anti-clockwise forces.
  • Can be used to distinguish between components.
Drawbacks
  • In some applications, anticlockwise movements can cause them to unfasten. For example on the left wheels of a car, where the torque can cause right-hand threaded fasteners to loosen.
  • Not a standardised component used in most general applications - lower availability from suppliers.
Applications
  • All types of general and specialist engineering.
  • Most applications where a threaded component is required.
  • Used to counteract anti-clockwise rotational forces that would typically loosen a right-handed threaded fastener.
  • Create deliberate incompatibility as a safety indicator. E.g. gas and water lines.

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Can You Combine Right and Left-Hand Threads?

Can you use a left-hand nut on a right-hand thread? Or vice versa? Unfortunately, because of the opposing thread directions, you can only use right-hand nuts with right-hand screws as their threads interlock and form a strong bond when tightened. The same can be said with left-handed fasteners. By attempting to combine right-hand and left-hand threaded components, the threads will cross so they cannot interlock and tighten together.

However, it is important to note that you can use both right and left-hand threads in the same product or component, such as on a turnbuckle to create tension.

In the field of precision engineering, understanding the nuances of left-hand and right-hand threads is paramount. From ensuring the safety and functionality of machinery to making informed decisions in component selection, mastering this knowledge is invaluable. At Accu, we're not just suppliers; we're educators, ensuring that every professional has the insights they need for success. Whether you're working with a traditional right-hand thread or navigating the unique challenges of left-hand threads, we're here to guide and support you.

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FAQs

What is the point of a left-handed thread?

A left-handed thread is designed to counteract movements that might cause a standard right-hand screw to loosen, ensuring components remain securely fastened even under anti-clockwise forces. Additionally, it can be used to deliberately differentiate components, such as distinguishing gas pipes from water pipes for safety reasons.

How Do You Identify Left-Hand Thread on a Drawing?

On drawings, it is very common to mark left-hand threads on components by using the letters ‘LH’, making it very obvious what thread is used. It is good practice for designers to include this as it saves time later in the manufacturing process.

What is the difference between a right-hand tap and a left-hand tap?

A right-hand tap is designed to cut threads that twist in a clockwise direction, aligning with the common right-hand thread orientation for most screws and bolts, whereas a left-hand tap cuts threads that twist in an anti-clockwise direction, used for creating left-handed threads in specific applications where counter-rotational forces or differentiation is desired.

Does left-hand thread mean reverse thread?

Yes, a left-hand thread is often referred to as a "reverse thread" because it twists in the opposite (anti-clockwise) direction compared to the more common right-hand thread (clockwise).

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