Stick-slip, also referred to as “stiction,” is the bane of engineers and machine builders who need smooth, precise motion. Simply put, it’s the jerkiness that can plague both plain and recirculating linear bearings when they transition from standstill to motion. Here’s why stick-slip happens…
When two objects slide against each other, friction works against their motion. But there are two types of friction: static (also referred to as breakaway) and kinetic (also referred to as dynamic). Friction is a sticky (pun intended) phenomenon to define and measure, but at a fundamental level, static friction is caused by the molecular bonding that occurs when two surfaces are in contact. Kinetic friction is primarily caused by surface roughness, which impedes the motion of the two bodies relative to each other.
Static friction increases with time of contact. In other words, the longer two surfaces have been in contact at rest (not moving), the higher their static friction will be. Kinetic friction has been found to be mostly constant regardless of velocity, although some variation can occur at very slow speeds. (The velocity of the surfaces determines how long any two areas on the surfaces are in contact with each other.)
The relationship between friction and velocity for lubricated surfaces (as with recirculating bearings) is shown on a Stribeck curve. The thickness of the lubrication film is an important parameter in friction, and there are three regions of lubrication in which bearings can operate:
- Boundary Lubrication—where friction is dominated by surface properties
- Mixed Lubrication—where friction is affected by both surface properties and the properties of the lubricant, and is also dependent on speed
- Hydrodynamic Lubrication—where friction is dominated by the viscosity of the lubrication film
Because linear motion is not continuous—unlike radial bearings, linear bearings must stop at the end of the guideway and return in the opposite direction—linear recirculating bearings spend more time in the mixed-lubrication zone, which makes them susceptible to stick-slip.
With recirculating bearings, the initiation of motion draws lubrication into the contact area between the two surfaces, reducing surface-to-surface contact and causing friction forces to drop. As the speed increases, the lubricant film increases and friction is further reduced. But beyond a certain point, friction actually increases and generates viscous drag. This is why using both the proper type and amount of lubrication for recirculating bearings is important.
In plain bearings, stick-slip can also be an issue, due to the higher coefficient of friction of sliding (rather than rolling) contact. To minimize stick-slip when using plain linear bearings, it’s important to ensure that the bearings are properly spaced, according to the 2:1 rule or 2:1 ratio.
The transition that occurs when motion is initiated, from higher static friction to lower dynamic friction, can cause a system to overshoot its target position and negatively affect positioning accuracy. But the effects of stick-slip can be minimized through proper design of plain bearing systems, and through proper lubrication of recirculating bearings.
Feature image credit: Dr. James Hedberg, CUNY