When high friction is beneficial

Friction, along with wear and lubrication, is one of the fundamental concepts of tribology. Much of the time, I find myself thinking about friction only as an unwanted property of a tribological interface. This is because much of my work has been aimed at increasing the efficiency and power density of machines, which drives effort to  reduce friction coefficients as a means of reducing the internal power losses.



However there are significant tribological systems for which a minimum friction level is essential. In these cases, achieving a high-enough friction coefficient, often in spite of the presence of a lubricant, and in a stable manner (so NOT significantly reducing with run-in effects) can be a valid and essential goal.



Quite simply, there are some tribological interfaces that simply MUST operate as far to the left of the Stribeck curve as possible, whether that is viewed at the macro or micro scale.



Here are some examples. Can you add any more?

1. 
   Brakes (pads - discs, or shoes - drums, etc)

• 
  Airport runways (tyre  - runway surface)

• 
  Rolling bearings (roller - cup or cone)

• 
  Threaded fasteners (inner thread - outer thread)

• 
  Walking/running (shoes - the ground)

• 
  Railway traction (wheel - rail)

• 
  Traction drives, e.g. Torotrak

• 
  Clutch plates

It's interesting to think in each case how engineers have tried to maximise the friction coefficient, for example by carefully selecting materials, or including surface features to channel the lubricant film away from the contact patch.