Stick-slip is a phenomenon that occurs in mechanical systems where resistance to motion is due mostly to coulomb type friction. Coulomb friction is characterized by resistance that is one value when contacting surfaces are stationary relative to one another and lower when they are sliding. The threshold between static and dynamic friction forms a discontinuity which leads to a sharp change in resistance across a narrow change in relative speed. This effect can lead to unsteady motion in some cases.
Stick-slip refers to relative motion between two parts that is characterized by cyclic periods of static non-motion followed by rapid sliding motion. Stick-slip motion occurs in many places, both in natural and manmade situations. Two commonly known occurrences are squealing breaks and earth quakes at fault lines. These examples represent two vastly different time frames as well as resulting effects.
Imagine a long shaft string that is resting horizontally in a dry trough and is driven from one end. Friction between the shaft OD and the trough resists the rotation. During the stick phase the driven end rotates while the free end sits still. During this period the shaft twist increases causing strain energy and torque to build up in the shaft. It is worth noting that during this phase some slippage does occur, mostly on the driven end of the shaft.
When twist in the shaft becomes built up enough to generate the torque required to overcome the static frictional resistance then sliding occurs along the entire length of the shaft. The friction force flips to dynamic along with a sudden drop of resistance. This allows a rapid release of trapped strain energy, (slip phase). During slip, untwisting motion drains strain energy along with a decline of available torque. At some point, the twisting toque in the shaft drops below the dynamic frictional resistance torque, allowing the shaft to stop. Then the cycle starts over.
Stick-slip in shaft strings causes a number of undesirable effects including:
- Fatigue damage of shaft components
- Unsteady motion which could affect the quality of the process
- Reverse rotation at the shaft end leading to unwanted loading of shaft end components and damage
Various equipment that includes long shaft strings or the dynamic equivalent are:
- Conveyer systems with long spans or with many interconnected short spans
- Drill pipe
- Cable driven devices such as string trimmers and speedometer cables
One method of overcoming stick-slip from occurring in long shaft strings is to operate the shaft at a speed high enough to prevent the return transition to stick.
Another possible solution would be to change the friction in the system from primary coulomb to viscous by adding or changing fluids in the system.
These solutions may not always be possible because of various factors which may be related to the process of stability of the equipment. In such cases one can add a viscous resistance to the design that is locate at the shaft end farthest from the driver. This technique will smooth out the transition between the stick and the slip motion yielding a smoother operation. Riverhawk supplies resistive devices to help correct stick-slip.