What factors lead to friction in linear motion systems?

Designers and engineers often try to avoid or mitigate friction in systems of linear motion.While friction isn't always a bad thing in some applications,it can provide damping effects and help improve servo adjustment-when it comes to linear motion systems,it can increase the force needed to move the load,generate heat,increase wear,and shorten life.

Static friction must be overcome before an object will move.Once motion begins,dynamic friction dominates.

Linear motion systems are subject to friction from a variety of sources,some of which can be mitigated by design and proper maintenance.Here,we will examine the factors that cause friction in linear moving systems and discuss ways to reduce friction through component selection and system design.

1.Sliding vs rolling contact

One of the main ways to reduce friction in linear motion systems is to use parts that have rolling rather than sliding contact.For example,because of the large contact area between load-bearing surfaces,the lead screw and plain bearing guide rail that rely on sliding motion will naturally suffer higher friction than the rolling body.

Bearings with sliding contact also experience a greater difference between static(starting)and dynamic(kinetic)friction,which leads to an effect called stick-slip or adhesion effect.Stick-slip causes the system to exceed its target position at the beginning of motion due to the transition from(higher)static friction to(lower)dynamic friction.

2. Raceway geometry

Although the friction of rolling bearings is much lower than that of plain bearings,they are not completely frictionless.Many factors,many of which are inherent in bearing design,can cause friction in rolling body bearings.One factor is the raceway geometry,or the type and area of contact between the rolling body and the raceway.

Rolling bearings usually use one of two raceway geometries:a two-point circular arc or a four-point Gothic arch(although some variations of these two designs do exist).For low friction applications,a two-point circular arc geometry is usually preferred because it experiences less differential slip than a four-point Gothic arch design and therefore has lower friction.

Bearings with Gothic arch raceway geometry have higher friction than those with circular arc geometry.

3.Recirculation

In cyclic ball and roller bearings,the number of load-bearing elements fluctuates as the rolling body moves in and out of the load zone.This can lead to changes in friction,which can be detrimental to highly sensitive applications such as micromachining and metrology.To reduce these frictional variations,manufacturers of circular linear guides(and ball screws)have invested significant research and development efforts to optimize recirculation components and processes.Generally,bearings with higher accuracy grades have smoother and more consistent friction curves.

4.Preload

The preload eliminates the clearance between the bearing and the guide rail(or nut and screw)by increasing the contact area between the components.This provides higher stiffness for the bearing and reduces deflection,but also leads to higher friction.This is why it is recommended to use the lowest preload level that will provide the required stiffness and accuracy.

Preload increases the stiffness of a bearing,but it also increases friction

5.Seals

Of all the design and operation characteristics of linear guides and screws,the one that usually creates the most friction is the use of seals.In most applications,linear bearings that rely on balls or rollers(whether recirculated or not)require seals to maintain lubrication and prevent contaminants from entering.In highly contaminated environments,side(side)seals and end seals are usually required.

Seals can be a significant source of friction in linear guides and ball screws.

While manufacturers offer a variety of seal materials and types - from seals with smaller clearances to those with double-sided full-contact profiles-the most effective seals are,of course,those that have the most contact with the guide or screw parts.But more contact means more friction.As with preloading,for sealing,use options that are appropriate for the application and environment,but don't overdo it.

6.Lubrication

One of the key functions of lubrication is to reduce friction between rolling or sliding elements.But using too much lubricant,or using a high viscosity lubricant,can actually increase friction.Therefore,it is important to follow the manufacturer's instructions and use the correct type and amount of lubricant.

7.Radial bearings

Radial bearings are found in almost all linear motion systems and support rotating parts such as ball or lead screw shafts or pulleys in belt drives.Although relatively small compared to linear guides or screws,these radial bearings also introduce friction that should be considered during system design and selection.

For applications requiring extremely low,consistent friction,consider piezoelectric ceramic actuators,voice coil motors,and air bearing stages driven by linear motors -- all of which use little or no components with sliding or rolling contacts,and thus avoid many sources of friction in typical linear motion systems.