April 30th, 2021
Knowing which linear motion system to choose can be a difficult task and choosing the right one is important so that you are not left disappointed with the performance of the system. Each type of linear motion system has its pros and cons, so read this helpful guide to find out which one you should choose.
Generally, there are two different systems – roller and plain bearing linear motion systems. Let’s have a look at the important things to consider when choosing the right one: load and adjustment.
Getting the right adjustment force for the end user is important. Have you ever tried to adjust the seat in your car and the seat either moves too quickly, without control, sending you flying backwards, or it’s very difficult to move, causing stick slip issues. This is a result of getting the adjustment force wrong.
The adjustment force is all about physics! The friction that the roller bearing or linear slides generate and the load applied, give the resulting adjustment force. Roller bearings typically have 10 times less friction than sliding plain bearings and therefore have minimum adjustment force. The actual values you can achieve vary a lot depending on the materials you use, the grease and operating temperature. When it comes to grease, the amount and type of grease that is applied can alter the adjustment force. The viscosity of the grease changes with temperature and can affect the sliding force of any such application.
It’s important when designing a linear motion system to get this balance of load, friction and adjustment effort right.
As an example, if the system must handle heavy loads and the adjustment should be as low as possible, linear roller systems are the perfect choice (100kg load will result in adjustment force of roughly 10N). Although there is still potential for stick slip; grease will be required to manage this, yet grease viscosity varies depending on temperature and dirt ingression.
In applications without high loads, linear slide bushings may be a better option.
The ball bearing design is a three-way assembly which includes the ball carriers, the inner rail and the outer rail. To ensure everything runs smoothly, the system needs to be greased. The adjustment length of linear roller systems depends on both the length of the inner and outer rail. Linear plain bearing slider systems have an advantage here as the adjustment length only depends on the length of the outer rail, that’s because the outer rail is fixed to the inner rail.
Linear slider systems are self-lubricating (therefore requiring no grease). During operation, small particles of PTFE are worn off the linear slider and stick to the rails. This, in combination with the low coefficient of friction of PTFE, leads to a self-lubrication therefore no grease is needed. PTFE elements work like a seal which keeps the sliding surface clean. Linear sliders work perfectly in harsh environments. All Saint-Gobain sliders are manufactured with the same thick laminated layer of PTFE as NORGLIDE® Bearings. Making them grease-free and maintenance-free, delivering consistent performance from the first cycle to the last.
Linear rollers require grease as a metal to metal contact comes with several challenges. Grease avoids corrosion, reduces noise and reduces the coefficient of rolling but collects dust and particles from the environment. Dependent on the application linear ball bearings come with enough grease for the whole lifecycle. But this is a potential topic during maintenance.
When you are specifying different systems, it’s often easy to overlook the hidden aspects of producing those parts.
Roller bearing type sliding systems are ubiquitous in the industry, but they also come with their fair share of manufacturing challenges. The fit between each of the tracks and ball bearings is critical to get the right effort – too tight and the adjustment can be too high, too loose and it could lead to rattle and noise. To avoid noise and vibration the balls and carriers need to be under load at all time, this can limit the system design and packaging options.
Because of this, special attention needs to be paid to the tolerances of the tracks and balls. In most cases there is some matching process done (where all the components are measured one by one, then matched to each other to give the perfect fit).
In a linear slider, firstly, there are fewer sliding elements than in a roller type one, making the assembly of them easier. Secondly, due to the inherent properties of the PTFE and spring elements, they automatically compensate for the tolerances of the rails. This means that the tolerance of the rails doesn’t need to be so tight, saving some cost. Steel balls work better with point contact which leads to Brinelling, whereas slider requires line- or surface contact which avoids Brinelling. Brinelling occurs where the balls can dig into or mark the mating sliding surfaces that can cause stick slip and running noise.
In most cases linear roller bearings come in standard sizes that are predetermined, while linear bearings, in most cases, can be made to your own specific dimensions giving more freedom for design.
Our engineers will work in close collaboration with you, making sure that your system is robust and reliable. Contact us today to discuss your custom-made linear sliders. For further information on our sliding bearing technology visit the NORGLIDE® Bearings product page or email: email@example.com