Friction and Area of Contact

There is a lot of information on the 4 types of friction on the web.

Uniform wear is very rare and hard to design for. Some part will wear out first, the idea is to design for the part you want to wear out first to wear out first. Usually the cheaper, easier to replace part.

For example: when I design with a rack and pinion, I would normally want the pinion to wear out first because it is usually easier to source and replace than the rack BUT the pinion will see many more cycles of wear opportunities so it may be even harder than the rack but may still wear out first.

the basic idea of posing this question is that when a bicycle with full air tires needs smaller force to move than when it has flat tires or lesser air in the tires. Here all the parameters like weight of bicycle, normal resultant force, the materials in contact etc, are same but the frictional force differs. similarly two bicycles, one with narrow tires and the other with wide tires, do need different forces to move eventhough all other parameters are same. the info provided by the websites gives the variation of frictional resistance coefficient with respect to the radius but not the width, I feel this has also some bearing on the friction factor.

A wider tire has a larger radius at the contact point even though overall wheel diameters may be equal.

A tire with lower air pressure can be seen as a different material than a tire with higher pressure as the 'air' is seen as changing the density of the material making the fuller tire 'harder' than the underinflated tire.

So a skinny, fully inflated, hard tire will roll much easier than a wide, flat one. Even though it is only flat on the bottom....sorry couldn't resist that last part.

"A wider tire has a larger radius at the contact point even though overall wheel diameters may be equal."

if the radius at the contact point is larger, it takes lesser force to move the vehicle. Do you mean to say that the wider tires require lesser force than the slender ones? if so, why narrow tires are used by the cyclists in the races?

I think Massey was referring to the transverse radius (across the direction of travel, not parallel with travel.

Correct.

Is there a question here?

The trick is that the friction force per unit area is proportional to the normal force per unit area (i.e. pressure). So (FrictionForce/Area) = μ x (NormalForce/Area). Area cancels out, which is why friction force is independent of Area.

...true for sliding friction, not so much for rolling friction?

In the case of railway locomotion, a high level of wheel/rail friction is desirable, to transfer the wheel power tangentially. In most other machines, a low friction level is desired, to reduce power wastage and to contain frictional wear.

If it's low friction you seek to analyse, the effects of 'lubricants' and of material transfer between the opposing surfaces have been widely reported. Tribological studies - not just of sliding but also of rolling - often wisely focus on the physical aspects of pressure micro-welding between metals. I put the word 'lubricants' in quote marks because the real function of a 'lubricant' in rolling friction often is to prevent micro-welding.

Between rolling elements, micro-welding destroys the surface integrity of both elements, dramatically affecting perceived friction when the micro-welds are ripped apart.

The role of a "lubricant" is to separate the two surfaces. Its role in SLIDING and ROLLING relative movements is different.

In sliding the lubricant which has a smaller shear resistance than the two surfaces leads to a reduction of the resistance forces. His presence reduces as well the risk of direct contact between roughness peaks and the local high temperature flash or the high local pressure with micro weldings as result.

In rolling the behavior is different due to the high increase of viscosity under pressure. This leads to a elastohydrodynamic local zone which totally separates the surfaces and transmits the force without direct contact.

The pressure distribution in the contact zone is not uniform thus the resulting force is at a certain distance from the normal passing through the rotation center and leads to a resistant torque which gives the "friction" in rolling.

Although both resistances to the movement are called "friction" they are in fact totally different.

If sliding without lubricant is not acceptable due to many reasons which I will not mention here since the list is too long, in rolling there are applications without any lubricant as far as the load does not go over limits correlated to the materials in contact. This kind of application has as limit only the surface fatigue and can fail if for any reason a sliding even very small occurs.

In the tire example it should be thought this way:

-the load is transmitted to the ground by the contact pressure which is depending on the tire internal pressure and the tire stiffness in radial local deformation.

- the lower the pressure in the tire the bigger will be the contact area with the ground.

When a movement occurs the tire is deformed there where it goes to the contact zone and is released when it leaves this same zone. This deformation leads to the non uniform load distribution and to the distance between radial load and reaction force on the contact zone thus to a rolling "friction".

In fact the tire rolling "friction" depends on the tire own stiffness and hysteresis it is -on an horizontal ground- given by the work to deform the tire itself. The lower the internal pressure the more the tire will be deformed and "friction" will be higher. There is as well an other aspect the tire deformation leads also to local sliding there where the force parallel to the rolling ground is higher than the friction force under the normal load. As one sees the process in the tire contact is much more complex due to the low stiffness of one of the parts in contact.

Thank you Guru nicholas for your reply. Could you please suggest some books or web reference to have better understanding and perception of the behaviour of rolling friction. Thanks in advance.

Microwelding is mostly present in sliding movements, less in rolling. In rolling if the l loads are over some limit the under layer shear maximal value lead to a local fatigue with exfoliation of surface chips. This is the most present surface destruction in case of rolling if no sliding occurs.

As for the the adherence it is not only a problem of the railways in fact you can walk every day ONLY because the sliding friction between your shoe sole and the ground is high enough to transmit the force you apply to push your body. This can not be the case on for instance ice where under pressure a thin water film builds up and becomes a lubricant.

The same when the road if covered with water and your tires are not able to push it away and get a direct contact to the road. This is one of the reasons it is recommended to reduce speed on a humid road to give the tire profile the time to evacuate the water and get the "dry" contact. It is a problem of flow between the tire and the ground and the time needed to push it by the profile groves aside. It is also the reason not to brake on a humid road since the wheel can be stopped and the water layer is no more moved away thus the adherence is lost.

As you see it is not only limited to the railway.

I would think it depends if there is any thrusting involved. The hypoid gear that mates with a screw is usually brass with extreme thrust controlled by double spherical bearing and cup and cones back to back. A single helical gears thrust is controlled by the inboard bearing be locked. The pinion gears are usually made of a softer material than the bull gear. Spur gears are all made from the same material and don't thrust but are noisy. Your double helical or herring bone gears relatively uniform because of minimal thrusting. Root clearances and backlash clearances play an important role as well for proper wear on the gear

I love this blog, many thanks to all here.

To better understand the role of Friction, as it affects our daily lives in so many ways, some good, some not, is REALLY important, for me at least, but I am sure I am not alone either.

I like the links as well.

Especially when your or my comments general a third class of friction.

You just have to make a "questionable" comment to one that could not have upset Lucifer......WOW!!

You don't SEEK trouble, you MAKE it!!

Fourth type??????

Why?

I get that plate-plate or plate-point sliding friction is Type I, and that rolling friction is Type II. The third type, Type III, is verbal friction! I have not a clue what the fourth kind is, Andy?

I made a "questionable" comment? Would you care to obliviate on that topic?

Are we talking slip-stick friction as being the actual real Type III friction? At the macroscopic and probably the nanoscopic level, there is evidence that surface structure has a lot to do with stick and slip.

Here's one for you Andy: What kind of friction do you get when you put lipstick on a pig? You tell me...

"...obliviate on that topic..." - priceless good phrase .

Tribology considers the contact as being build of 3 parts the 2 "solid" and the stuff in between (which can be a fluid -liquid or gas-). Surface plays an important role as well from the roughness magnitude as from its elasticity point of view. In stick-slip the elasticity is THE parameter.

from Wikipedia the friction definition is:

"Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other."

It would be more correct to use " resistance to relative displacement " since friction is valid only for sliding.

http://www.tutorvista.com/physics/4-types-of-friction

4 Different Types of Friction

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There are four types of friction namely

1. Static friction
2. Kinetic friction
3. Rolling friction
4. Fluid friction

I think we are creating a lot of friction here.

OK, gents, what happens to friction when the "fluid" between the frictional surfaces intermeshed with each other (on a nanoscopic scale) consists of nothing but a pure vacuum? Does the cohesion of the two surfaces come into play? Can one weld alumium to steel under such conditions? Just a thought experiment...

Since you generate the question please write what YOU think.

Do not forget that aluminum can develop an oxide layer and in the case you describe it is welding between steel and aluminum oxide not between steel and aluminum.

I think it is fairly well known that spin welding takes place even in air. It is also known that unless metal surfaces are carefully prepared (and somehow lubricated with a non-voltatile) that metals will contact weld in outer space.

I suspect the answer to my question about friction (say spin welding) of aluminum and steel can take place under the correct conditions in a vacuum, but the question would remain about oxygen inclusions in the HAZ during the spin. I suspect the weld might form (if that is useful at all), but that the weld could be somewhat brittle with respect to bending tests. Other materials with aluminum could and would present the same results.

This gives rise to yet another question: How are certain refractory metals bonded to glass to form metal-glass transition seals in high vacuum equipment, or electrical isolators? Is the molten glass simply fused to the metal through surface interaction? Could it be possible to friction weld tantalum to glass (for example)? I think these sorts of material transitions have definite practical uses in the field.

Have you considered earthquakes, stick-slip, various othe stuff recently on CR4. We even got it to poly cups.

Many have pointed out the difference between static and sliding friction, but it's not all bull. It's academic versus common sense. Yeah, I did say poly cups, ans I defy anybody to state they had managed that one.

Your comment is very interesting could you please elaborate since I did not fully understand your point of view.

Thanks

Nickname

The 'poly cup' was a ref to a recent Challenge Question. In that case, elastic energy is involved. There is no smooth transfer of stress release, it's a bit like two saw-tooth edges made of rubber. On a minor scale, you could force a sweaty palm across a desk and hear the juddering noise it makes. The are of contact between palm and surface is not constant.

Not sure if that helps, but I was ineptly trying to show that dynamic friction is not some sort of smooth and constant process. Conventional thinking on dynamic friction is fine in the right context, but stored elastic energy and stick-slip can mess up that as a model.

Feel free to ask for more if that makes no sense - I'm only just convincing myself !

What you mention is the so called stick-slip sliding. There are different explanations for it.

The simplest is to consider the difference between static and kinematic friction. If one of the bodies (the hand for instance) is moved, due to the initial friction and its deformability the force at the contact region will grow but a sliding will occur ONLY when the force becomes > static friction. In this situation since kinematic friction is less, stored energy in the previous deformation will accelerate the surface of the body which makes a jump but due to friction energy is destroyed and speed decreases till surface stops and friction becomes again static thus higher and cycle repeats.

This is one of the problem which penalyse hydraulic or pneumatics servo-systems. It occurs as well when a heavy load is pulled by cables in sliding. In servo-systems the problem was solved by special compounds (ptfe based most of them) with a lower friction, lower differences between static and kinematic and also with stiffer seals design in axial direction. For the sliding of heavy loads instead of pulling with cables which are relatively compliant the technology (often applied for moving houses) is based on pushing via short stroke high pressure rams in order to obtain a high stiffness at the force generator level.

This happens at an other scale between the continental plates and is among other reasons an earthquake generator. Geologists think that this was the origin of the last earthquake in the Himalaya region.

Geologists just don't think it is stick-slip, they know it is. Trouble is, the thing can not be predicted.Whilst we discuss, have a chuckle......