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Ball Screw vs. Lead Screw: What’s the Difference?

Ball Screw vs Lead Screw

Whether you’re a newbie to engineering or a seasoned engineer, you’ll have come across the terms “ball screw” and “lead screw.” But do you know their differences?

Ball screws and lead screws are two screw mechanisms in industrial equipment. They are used in machines to transmit power by converting rotary motion to linear motion. These two devices are similar, but they have several differences.

Both have different types of threads. A ball screw uses ball bearings on the nut to turn the threaded rod. Lead screws have angular threads, and they use a nut to turn the inner thread on the rod.  Besides, lead screws have a threaded nut, while ball screws have recirculating balls.

In this article, we provide an in-depth look at ball screw versus lead screw to help you make an informed decision whenever you should need one or the other.

tool dividerOverview of Ball Screw

The ball screw is a precision linear actuator used in automated systems. It uses ball bearings to transfer torque and rotate the nut. In other words, a ball screw moves objects in a straight line by rotating the screw.

How Does It Work?

A ball screw transmits power from one part of a machine to another. It has a rod with a groove along its length. The groove of the ball screw holds balls in place. The balls roll along the groove as the rod turns.

The balls in the screw will move forward with each rotation until they reach their maximum distance. At this point, they stop moving forwards and start rolling back toward their starting point.

When using ball screws, it’s crucial to consider proper lubrication to withstand the temperatures at which the ball screw will operate and the pressures on the balls.

There are three types of ball screws:

  • Return-Pipe Method Ball Screws: Return-pipe ball screws are a type of ball screw that uses a return pipe to guide the nut. The screw has two fittings on it. One is in the middle (the return fitting). Another is at the end of the nut (the adjustment nut). The purpose of these fittings is to provide for backlash-free motion.
  • Deflector Method Ball Screws: Deflector Method Ball Screws have a deflector plate attached to the back of the nut and a roller on the other end. The deflector method is for high-speed applications where accuracy is crucial. The deflectors on the nuts guide the balls into contact with the mating surface of the nut. This way, they’re less likely to jump out of contact under load.
  • End-Cap Method Ball Screws: End-cap ball screws transmit torque from the motor to a driven load. They are used for high-speed conveyance. The shaft of an end-cap ball screw is a hollow tube with balls on its ends. A nut inside the tube holds the balls in place. It also allows them to rotate around the axis of the shaft.

Application

Ball screws are used in robotic arms and other industrial machinery such as building medical equipment and surgical robots to assist doctors and surgeons with challenging procedures.

They are also helpful when positioning objects during manufacturing processes. It’s especially true when the position must be accurate. Usually, it’s the case when working with circuit boards and other electronic devices.

Load Capacity

This is the maximum allowable thrust that a screw can transfer without causing permanent deformation of the screw. The load capacity depends on the diameter, pitch, and length of the screw. A larger diameter ball screw can carry more load than a smaller ball because it has more surface area to distribute the load across its surface area.

A ball screw has a higher load capacity compared to a lead screw.

Rotational Speed

The rotational speed of a ball screw is measured in revolutions per minute (rpm). It’s crucial as it determines the distance the screw will travel when it makes one complete revolution.

The balls’ diameter and their number determine the rotational speed. The rotational speed increases as the diameter of the ball increases and the pitch decreases.

The rotational speed of ball screws is between 4,500 rpm for 3-millimeter ball bearings and 1,050 rpm for 125-millimeter ball bearings.

Pros
  • More efficient and need less torque
  • It doesn’t need frequent replacement
  • You can adjust ball screws to lessen or increase preload
  • Less friction
  • Highly accurate
  • Available in ground and rolled types
  • High speed
Cons
  • Requires a braking system
  • More costly
  • Needs lubrication

divider 4Overview of Lead Screw

A lead screw transmits power between two points. It has a shaft threaded along its length and a nut that threads onto the end of this shaft.

Usually, the thread on the shaft has a helical groove cut into it. The nut has teeth that mesh with these grooves. They allow it to be turned by the shaft.

How Does It Work?

Lead screws convert rotary motion into linear motion or vice versa. In other words, they’re nuts on a screw. You can use them to drive other devices such as cams, cranks, and linkages.

The screw thread touches the nut thread. The lead screw works by applying force at an angle to a nut threaded onto the shaft of the screw.

As the nut moves up or down along the shaft, it causes a corresponding linear movement of whatever is attached to it on one end. The force needed depends on how much friction there is between the nut and shaft.

There are several types of lead screws, all grouped depending on the thread type used.

  • Buttress Thread: You can use a buttress thread if you’re transmitting force in one direction. It has a triangular shape. The load flank has a 7-degree inclination, while the trailing flank inclines by 45 degrees. Also, you can use a buttress thread to create small precise movements in one direction.
  • Acme Thread: The most common type of lead screw is the acme thread. It has a constant lead per unit length. Its pitch varies linearly with the diameter of the screw. Your pitch gets smaller as you go up in size, so you can have higher precision parts. Acme thread has a thread angle of 29 degrees.
  • Square Thread: A square thread has a thread angle of 0 degrees. The angle stops any radial pressure on parts and lessens friction between them. Also, it has square-shaped threads. The square design reduces the contact surface between the screw and the nut. So, square threads reduce friction and have high efficiency.

Application

Lead screws have many applications. They’re used in medical equipment, industrial machinery, robotics, and automation systems.

They have been used for centuries to power anti-friction bearings in windmills. Their work can also be seen in engraving, paper processing machines, and bottle labeling.

Load Capacity

The load capacity of a lead screw is determined by the type of lead screw, its material, and size. A lead screw has a high load capacity but not more than a ball screw.

The load capacity also depends on how smooth and efficient the lead screw is. Smoothness is critical for accurate positioning and high-speed applications.

Rotational Speed

The rotational speed of a lead screw depends on the pitch diameter and the number of threads per unit length. It also depends on whether you’re using external or internal threads.

The rpm of lead screws is equivalent to the linear speed (inches per minute) divided by the lead of the screw (inches per revolution). Lead screws can deliver speed variations of up to 70 inches every second.

Pros
  • It doesn’t need a braking system
  • Suitable for vertical applications
  • Affordable
  • Less noise than ball screws
  • Comes in screw diameters as small as 6mm
  • It’s self-lubricating
Cons
  • Have more friction
  • Not cost-effective in the long run because of maintenance costs
  • Not suitable for high-speed applications
  • Needs greater torque

divider 4

Differences Between a Ball Screw and a Lead Screw

Ball screws and lead screws are two types of linear motion systems you can use. While they both perform the same job, they have some significant differences as seen below.

  • Mechanism: The primary difference between ball screws and lead screws is that ball screws have rounded balls, whereas lead screws do not. Rounded balls allow for smoother movement and less friction than a lead screw. Ball screws are also more accurate than lead screws because ball bearings inside the ball screw allow for precise movement at high speeds.
  • Cost: Ball screws are usually more expensive than lead screws. It’s because they need special manufacturing equipment and greater precision. This means that you will pay more for ball screws up front, but you’ll save money over time due to their superior performance compared to lead screws.
  • Design: Ball screws have spherical balls, while lead screws have cylindrical threads. Also, ball screws have a complex design, while lead screws have a simple design and are easily customizable for applications.
  • Applications: Ball screws are used in applications where precision and accuracy are required. Besides, they’re used in large, industrial applications. On the other hand, lead screws are used in smaller applications.

How Do You Choose a Suitable One for You?

You can choose the screw to use depending on the application you want to use it for. Ball screws are used in applications requiring high precision such as fine adjustments or when moving heavy loads.

Lead screws are less precise but can handle higher loads than ball screws. Usually, they’re used where traveling is needed over short distances, such as an elevator. Ball screws are used for applications where there are small changes in position over long distances.

Key factors that will impact your decision include:

1. Load Capacity

Ball screws have a higher load capacity than lead screws. Their design allows them to handle higher loads than lead screws.

The recirculating balls also help support the load. Lead screws that have bronze nuts can carry heavy loads. But, as the load increases, so does the friction. So, it reduces their duty cycle.

This means that if you need a screw with a high load capacity, ball screws may be the better option for your application.


2. Vertical Operations

If most of your applications involve vertical operations, choose a lead screw. Lead screws have a self-lock mechanism, so a load will not fall or back drive even if there’s no power on the motor. It’s helpful if the load must remain on top of strokes during the process.

Lead screws don’t have a braking device to work. On the other hand, ball screws need a braking device.


3. Duty Cycle

Ball screws have a higher duty cycle than lead screws. This means they can run for long periods without overheating or damaging themselves. The balls in ball screws reduce friction.

Lead screw motors overheat more often than ball screw motors. The friction in lead screws causes overheating. Choose ball screws for automotive applications since they’re durable and resist wear for a long time.

gloves and screwdrivers
Image Credit: Bru-nO, Pixabay

4. Material

The material of a screw will determine how long it will last, how much weight it can support, and how much torque it can generate.

Most people choose lead screws for applications in harsh environments and those that ball screws cannot endure. It’s because lead screws don’t have recirculating elements, which are prone to contamination.

Ball screws are made of carbon steel and stainless steel. Carbon steel screws are more affordable than stainless steel screws, but they’re susceptible to corrosion.

On the other hand, lead screws are made from various materials including steel, stainless steel, brass, aluminum, and plastics. The material used to make lead screws impacts their strength and durability.

When to Choose a Ball screw
  • When you have high loads with high speeds
  • When you want high precision
  • Applications with regular cycling
When to Choose a Lead Screw
  • When on a budget
  • If you have positioning applications
  • In applications where lubricating can cause damage

tool dividerConclusion

Ball screws and lead screws are similar, and both types work on the same mechanical principle. But if you’re looking to use a ball screw or lead screw, you’ll want one engineered for your application.

Screws come in many sizes and have different capabilities depending on the intended use. While each has its unique perks and drawbacks, both have proven to be effective precision devices. With the in-depth info in this guide, users can now make informed decisions, especially at the point of purchase and use.


Featured Image Credit: (L) Aumm Graphixphoto, Shutterstock | (R) Asharkyu, Shutterstock

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