By Japh Humphries, UK Sales & Commercial Manager, Matara UK Ltd.
When it comes to linear motion systems, one question that gets repeatedly asked is ‘which should I use – ball screw or lead screw?’ They both perform the same task, converting rotational motion into linear movement. However, assuming they are interchangeable because of this would be a mistake, and a potentially costly one at that. Their design and subsequent operation differ, providing each with its own set of advantages and disadvantages; weighing these up against the application at hand is essential to ensure your linear motion system performs to the specified criteria.
Design & operation compared
Both lead and ball screws feature a threaded shaft which acts as the screw element of the unit.
A lead screw - the simpler of the two options when it comes to design – has a threaded mating nut which is in direct contact with the screw. Rotational motion turns the screw, causing the nut to move along in a linear motion, thereby converting the motion from rotary to linear. Lead screws come in different formats with one of the most popular for industrial applications being trapezoidal lead screws. The screws are manufactured from steel and the nuts are available in a variety of materials from steel through to bronze, brass and plastic.
In the case of ball screws, a threaded shaft is also used, but it features a helical path around which ball bearings move. The ball assembly behaves as the nut. Ball screws are available in different formats, with rolled and ground types mostly favoured by manufacturers.
Lead screws: pros & cons
As mentioned above, lead screws are the more basic design of the two and that is reflected in the cost. Lead screws have a lower capital outlay than ball screws; ball screws can be two to three times the price of a comparable lead screw. They also do not require a braking system as they are self-locking, which is a further plus point when it comes to total cost.
This price differential often makes lead screws the first port of call, but this could be a false economy as it may not operate well in your specific application; it’s horses for courses, as the saying goes!
Apart from a lower purchase cost, lead screws have the advantage of generally being self-lubricating and they tend to be quieter in operation than ball screws. They are compact with minimal parts, and are easy to design into a system.
Where they don’t score so well is efficiency, which runs between 20% - 25%, as lead screws require greater torque and a larger motor and drive. They also have higher friction and therefore run at a hotter temperature.
Ball screws: pros & cons
With ball screws relying on point contact to support a load, as opposed to line contact, less friction is produced during use. This means that ball screws typically operate at a minimum of 90% efficiency, plus they run at cooler temperatures, and they produce a considerably smoother movement.
Due to recirculating balls supporting the load, ball screws have a higher load capacity than lead screws of a similar size.
Ball screws are also more versatile than lead screws. They are available in higher accuracy grades for greater positional accuracy, and they can be adjusted to increase/decrease preload.
But there are some drawbacks. Obviously cost is one, as outlined above, which is further increased by the need for braking mechanisms to eliminate the risk of back driving. Back driving occurs when the motor shuts off and the ball screw free-falls as a result. Brakes must be fitted as a safety precaution in this case. This is particularly pertinent if the load is to be carried vertically.
With moving balls within the mechanism, it is likely to be noisier than a lead screw, and will also require regular lubrication to ensure the balls move freely. Furthermore, the overall size of ball screw device is larger than a comparable lead screw due to the need for the balls to be recirculated.
What to use where: the rule of thumb
So, which should it be – ball screw or lead screw?
Whilst a definitive answer isn’t practical – each application will have other factors and trade-offs that may need to be taken into account – generally speaking, lead screws are more suitable for transfer applications for which speed, accuracy, precision and rigidity are not as critical. That’s because most lead screws are not well suited to high throughput, high speed applications or those with continuous or long cycle times. Typically, you will find lead screws in use in fluid handling, data storage applications and life science equipment.
Ball screws, on the other hand, are mostly better for high precision applications where accuracy and control are critical. They are also a good option for high load applications and for where thrust force is required to move those loads across a distance in a smooth motion, whilst maintaining speed. Examples of applications include 3D printers and automobile and aeroplane manufacturing.
So, when deciding on whether a ball screw or lead screw is right for your machine, whilst they both perform the same task of converting rotational motion into linear movement, the way in which they realise this and type of end result you achieve are markedly different.
Matara designs, manufactures and supplies pneumatic and linear automation products, including lead screws and ball screws.