Friction resists relative sliding.
At a contact between surfaces, friction opposes sliding or the tendency to slide. It can act even when the surfaces are not slipping.
Static friction adjusts to the demand up to a limit. Sliding friction acts once surfaces slide relative to each other. The conditions at the contact affect both.
The same principle has different jobs.
- TractionTransfers force without gross slipping.
- BrakingResists movement or holds a load.
- LossesSliding and other resistance turn energy into heat.
A simple friction model.
If the normal force pressing surfaces together is 1,000 N and the static friction coefficient is 0.3, the model gives a maximum static friction force of 300 N.
That is a limit, not a force that always acts. If only 100 N is needed to prevent sliding, static friction can be 100 N. This simple flat-contact example is not a lift traction calculation.
A traction sheave transfers force to ropes.
In a traction lift, frictional grip between the ropes and drive sheave allows force to be transmitted. Wrap angle, groove geometry, rope condition and the tension relationship all matter.
Traction is not the same as teeth positively engaging a gear. A sheave’s ability to transmit force without slipping has limits.
Lubrication belongs to a specific contact.
An appropriate lubricant helps separate surfaces and manage wear and heat in many bearings and gears. Type, quantity and condition must suit the component.
Adding lubricant indiscriminately is not an improvement: friction surfaces and traction arrangements have their own requirements. Maintenance follows the equipment and rope manufacturer’s instructions.
A closer look
Lubricated contacts can operate with full fluid films or with some surface contact, depending on speed, load, viscosity and geometry. A single friction coefficient cannot describe every operating condition.
Check your understanding
Is friction always something an engineer tries to eliminate?
Reveal the explanation
No. Traction and friction brakes depend on it. The aim is to manage friction appropriately at each contact.