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Table of contents
Introduction to linear guide preload
Linear guide preload eliminates the clearance between the linear guide and the linear rail. Manufacturers achieve this by using ball bearings or roller bearings. By eliminating the clearance, it increases rigidity and reduces deflection when external loads are applied. To correctly specify a linear guidance system one key thing to consider is preload.
The ball bearings or roller bearings used to achieve preload are slightly larger than the distance between the raceways of the linear guide and the rail. Common preload amounts are between 2 and 8 percent. The preload of a linear guide is given as a percentage of the dynamic load capacity.
For roller bearing linear guides, most commonly used in applications requiring extremely high loads and minimal deflection (machine tool), linear guide preloads up to 13 percent are common. An example, a linear guide with a dynamic load capacity of 20,000N with a preload of 5% would encounter a preload force of 1000N.
CPC offers market leading smoothness for linear guides with all types of preload available. ‘Custom’ linear guide preloads for particularly demanding applications are also available.
In most applications it may seem logical then to choose a high linear guide preload; less deflection, the more accurate the assembly will be. However, when specifying a linear guidance system, it is best to consider the application first rather than choosing the highest preload possible. Let’s look at the main criteria to consider when selecting profiled linear guide preload.
More is not always better when considering linear guide preload
As we have just looked at the highest linear guide preload possible is not always best. The higher the preload the more force is required move the linear guide which may mean having to use a larger motor and other related components. A higher preload also causes more heat to be generated within the linear guide which can increase wear and reduce the lifetime of the linear guide.
Finally, preload creates an internal force on the guide which acts a static force, and this must be taken account of when calculating lifetime in addition to the applied load. This however is counteracted when an external load is applied and is greater than 2.8 times the preload force.
Positioning accuracy
Most applications, such as pick and place, dispensing, inspection, will dictate a certain requirement for positional accuracy and as such a linear guide with light preload (2%) is generally recommended by manufacturers.
A higher linear guide preload, as we’ve looked at, doesn’t necessarily provide improved positioning accuracy, unless the application (machine tool for example) has deflection and vibration, then this can be considered. A linear guide with clearance – without preload – can negatively affect positioning accuracy as the clearance between linear guide and rail allows some play in motion.
As such manufacturers will generally suggest a light preload. Deflection Now we know that the higher the preload the more rigid the linear guide will be, and it will see less deflection. Any deflection, no matter how small will affect the overall repeatability of the application. This will be exaggerated if the load or working area is located far from the linear guide system. If the application will also experience shock loads and vibration, yet high positional accuracy is a necessity, for example machine tools, a heavy preload of 6-8% will be required.
Deflection
Now we know that the higher the preload the more rigid the linear guide will be, and it will see less deflection. Any deflection, no matter how small will affect the overall repeatability of the application. This will be exaggerated if the load or working area is located far from the linear guide system. If the application will also experience shock loads and vibration, yet high positional accuracy is a necessity, for example machine tools, a heavy preload of 6-8% will be required.
Linear guide arrangement
A common configuration for profiled linear guides is the setup of two rails in parallel with two linear guides per rail, creating a square type layout. This setup provides the best support for momentary loads in roll, pitch, and yaw directions.
Alignment is always key when using multiple linear guides and rails. If the guides are preloaded as they will mostly always be, alignment becomes critical. A working example: Two rails are used in parallel with two linear guides per rail creating the square type layout. The linear guides are preloaded.
The permissible offset in the vertical direction (typically denoted as ‘e1’) is smaller than if the guides were not preloaded. So, a precision machined mounting surface will be critical to maintain positioning accuracy and alignment. If the maximum vertical offset is exceeded, the linear guides will then see a roll moment load which will in turn reduce the lifetime of the linear guide.
CPC supply linear systems for all applications ranging from machine tool to microscopy equipment, from XY tables to pick and place semiconductor machinery.
Kiwi Motion are partners and distributors for the whole CPC range. They are experts in the linear field and can advise on the most suited linear system for your application.
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