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Friction

We developed the idea of making AutoSock out of a high-technology textile fabric with high friction properties according to new theories on the effect of electrostatic charging and pressures on friction.

 

 

Tribology/ Dry vs. wet friction

Tribology is defined as the science and technology of interacting surfaces in relative motion and of the practices related thereto.

 

A tribological system consists of three parts:

  • Upper surface
  • Lubricant
  • Lower surface

 

 

In the case of friction on icy or snowy roads the upper surface is the tyre or AutoSock, the lubricant is water film created by frictional melting and liquid layer on the ice/snow, and the lower surface is the icy or snowy road.

 

 

When the water film thickness is insignificant, we have dry friction; this is the case when braking a car at -20ºC. When the water film separates the two surfaces, we have wet friction; this is the case when a car water planes.

 

 

AutoSock wants to have as large an area as possible under the AutoSock with dry friction, since dry friction gives the highest friction coefficient.

 

Snow vs. Ice friction

AutoSock has:

 

  • A surface pattern that makes the total contact area exposed to friction sufficiently large under both soft and hard snow or ice conditions
  • A combination of surface pattern, strength and elasticity that make the contact points sufficiently sticky under hard snow or ice conditions

 

To some extent we have a trade off between good friction properties on snow relative to ice. It is favourable to open up the structure in order to increase the total contact area exposed to friction on snow. At the same time the contact points need to be sufficiently sticky on hard ice.

The contact configuration of a tyre or AutoSock can be quite different on snow relative to ice.

 

The upper figures show the contact configurations of a tyre on soft snow.

The figure to the left simulates compaction resistance, while the figure to the right simulates impact resistance.

 

 

 

The lower figure shows the contact configuration of a tyre on ice where the

macroscopic impact and compaction resistances are negligible.

 

 

 

 

 

 

 

 

Electrical parameters

The AutoSock is made of a special high friction textile fabric.

In a frictional AutoSock sliding situation, electrostatic pressures can be defined in:

 

  • The air gaps between: a) the sock surface and the icy or snowy road, b) the sock surface and the water film, c) the water film and the icy or snowy road surface.
  • The interfacial contacts between: a) the sock surface and the icy or snowy road surface, b) the sock surface and the water film, c) the water film and the icy or snowy road surface.
  • The water film.

 

The dry friction process is dominated and characterised by accumulation of electrostatic charges in the slider contact points. The frictional water film initiates discharge of potential differences between the slider and the sliding surface due to the much higher electrical conductivity of water relative to snow/ice.

The topography of the slider and the sliding surface is decisive for the electrical contact configuration between the slider and the sliding surface.

The electrolytic conductivity of a melted snow/ice sample may indicate the rate of ions introduced to the interface between snow/ice and  the slider by frictional melting and thereby the rate and ease of discharge between the slider and snow/ice through the frictional water film during braking. Larger frictional electrification should take place on snow/ice with low electrical conductivity compared to snow/ice with high electrical conductivity.

 

Friction properties on ice and snow

 

AutoSock has been developed with good friction properties both on ice and snow.

            The AutoSock is characterised by:

 

  • A specially designed surface pattern that makes the total contact area exposed to dry friction as large as possible both under dry and wet snow or ice conditions.
  • Hairs between and at the contact areas in order to increase the total contact area exposed to friction.
  • A surface pattern that makes the total contact area exposed to friction sufficiently large under both soft and hard snow or ice conditions.
  • A combination of surface pattern, strength and elasticity that make the contact points sufficiently sticky under hard snow or ice conditions.

 

Development of friction materials

The story

 

When Henry Ford had problems getting his car going under ice/snowy conditions, his wife used to put a carpet in front of the tyre, and Henry got going. The idea behind the AutoSock has been to use textile in a similar, but more efficient way. One day Bård Løtveit found out he should friction test his own cotton shirt, and the friction properties turned out to be surprisingly good.

 

Was it possible to learn something from the cotton shirt?

 

We put Bård’s cotton shirt under our laser microscope.

 

The image shows the cotton surface of Bård’s shirt viewed through our microscope. We can see that the contact areas of the cotton surface are small and dense and that the total contact area is relatively large. Hairs on the cotton surface increase the totally exposed contact area to friction.

 

 

 

 

 

 

 

This image shows a rubber summer tyre
The rubber summer tyre has a large total contact area and small flexible points.

 

 

 

 

 

 

 

 

 

 

 

The image shows a sock with poor friction properties on ice. The totally exposed contact area to friction is too small under hard snow or ice conditions.

 

 

 

 

 

 

 

 

From testing these, we developed the idea of making AutoSock out of a high-technology textile fabric with high friction properties according to new theories on the effect of electrostatic charging and pressures on friction.

 

 

 

 

First Generation AutoSock 

 

The contact area of the first generation AutoSock in the image is larger and stickier than for the previous sock. The grooves and roughness given by the surface pattern exposes the sock to more snow under soft snow or ice conditions, and increases the totally exposed contact area  to dry friction under wet snow or ice conditions.
At the same time the totally exposed contact area to friction is larger under hard snow or ice conditions than the previous sock. The hairs between the contact points increase the totally exposed contact area under all snow and ice conditions.

 

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