Lead Screws and Backlash
What is backlash?
Backlash can be described as the amount of play between moving parts of a mechanism, and has the potential to appear in almost any mechanical system, particularly gears and lead screws. Backlash occurs when one or more moving parts within a mechanism is not engaged with a sufficiently tight fit to the next part of the machine, allowing for some degree of free-movement within the mechanism, which results in lost motion. Backlash can affect both the accuracy and responsiveness of a mechanism, and can also increase wear as loose-fitting moving parts collide together. In systems where a mechanism calculates its own position, backlash can also add up over numerous cycles, resulting in increasingly inaccurate positioning throughout a machine's service life. The issue of backlash is particularly problematic with lead screws, which are generally used in high-accuracy precision actuation - for this reason, components such as anti-backlash lead screw nuts are frequently employed as a preventative measure. While zero-backlash systems are, in theory, the ultimate goal in preventing backlash within a machine, this is not actually strictly achievable in most applications. Backlash is necessary to allow moving parts to move freely – this is because, with no free space at all, systems would be tightly locked in place, and would jam when operated. Backlash is also an unavoidable by-product of the gaps which must be left for factors such as moisture, dust, lubrication, and thermal expansion.
How to prevent backlash in lead screws
Use two nuts
Arguably the simplest way to mitigate the effects of backlash is to use two nuts, spaced apart to press against opposing flanks of the lead screw thread. This method is effective, but can increase costs significantly if using expensive nuts. This method is a static solution, meaning that it can also be difficult to adjust, and can be particularly problematic if backlash changes after installation due to wear or thermal expansion.
Use an anti-backlash nut
Anti-backlash lead screw nuts come in a selection of shapes and sizes. Accu offers an anti-backlash nut solution which utilises an inbuilt compressions spring to ensure that the teeth of a lead screw mechanism remain consistently engaged and preloaded. This method of compensating for backlash is reactive, and can remain effective even if the level of backlash is altered over time – this is because the elasticity of the spring will compensate for any changes throughout the service-life of a mechanism. The downside to this option is the reduced load-bearing ability of the mechanism when relying on the strength of a compression spring rather than a solid construction.
Use backlash compensation software
In many automated applications, it is possible to predict and compensate for backlash with the use of software. Software can counteract backlash by calculating the amount of backlash in play, and remembering the direction in which the led screw last moved. Using this data, a device such as a CNC router can calculate which side of the nut is not in contact with the thread, and how much further a lead screw must rotate to subtract backlash from the final adjustment.
Obviously, applications requiring ultra-precise actuation such as 3D printers, CNC lathes, and robotics cannot simply ignore backlash, but for systems where pinpoint accuracy is not a necessity, such as vices, presses or jacks, backlash is not always a significant issue. Backlash is one of many factors which can affect the accuracy of a mechanism, and is not necessarily catastrophic to the operations of a machine. Using high-quality, high-precision components will keep backlash to a minimum, and if the accuracy of a machine stays within tolerance, then the positioning inaccuracies caused by backlash can potentially be written off as an acceptable margin of error within an imperfect system. Anyone who has ever used a pillar drill or a milling machine will be familiar with a certain degree of acceptable play between moving parts, which is often simply accounted for during operation. Mechanisms will generally still function with even a large degree of backlash, but when effectiveness and quality is desired, or in absolutely precision-critical applications, worthwhile solutions are available.