Posts Tagged ‘hardness’

Mohs Scale of Hardness, Ceramic vs. Concrete

Tuesday, March 15th, 2016

    Last time we watched as the kinetic energy of our falling coffee mug was transformed into the work of creating a crater in a pan of soft kitty litter.   Shock absorbing materials are often placed strategically to cushion valuable objects should they fall, and as an engineering expert I’ve sometimes had to implement break-its-fall solutions.  Today we’ll place our mug into a less kind scenario, one in which it makes impact with the unforgiving hardness of a concrete floor.   In so doing we’ll compare the mug’s ceramic to the floor’s concrete, and we’ll familiarize ourselves with the Mohs Scale of Hardness.

The Mohs Scale of Hardness, Ceramic vs. Concrete

The Mohs Scale of Hardness, Ceramic vs. Concrete


    Material hardness is commonly measured by the Mohs Scale of Hardness, which ranks the relative hardness of a material by observing how resistant it is to scratching by other materials harder than itself.   This standard was developed by German mineralogist Friedrich Moh in 1812, and it rates objects’ hardness on a scale from 1.0, very soft, to 10.0, very hard.   A fingernail, for example, ranks 2.5 on the scale, while a diamond ranks 10.0.

    Now let’s take a look at the materials in our scenario, a ceramic mug and concrete floor, and see how they compare.   The mug’s ceramic was created by mixing together clay, water, and other materials and then heating them in a kiln, a process known as firing.   This firing causes a chemical reaction that bonds the individual materials tightly together, and when it cools it becomes the product we know as ceramic, a hard, brittle solid which registers at about 7.5 on the Mohs Scale.

    The floor the mug falls to is poured-in-place cement, a compound consisting of primarily limestone, clay, pebbles and sand.   When these materials are combined with water a chemical bonding takes place and forms the hard, stone-like matter we know as concrete, which comes in at about 8.0 on the Mohs Scale.

    Although the mug’s ceramic is comparably hard to the floor’s concrete, its inherent brittleness, along with certain design features, most notably its handle, causes it to be fragile.   Anyone broken a coffee mug lately?

    As for the concrete floor the mug falls onto, it won’t yield to the mug’s freefall kinetic energy and form a crater like the litter did.   So where does the mug’s energy go?

    According to the Work-Energy Theorem, most of the mug’s kinetic energy is still converted into work, just as it was when it met up with the litter, but because the concrete floor is harder and thicker than the mug’s thin ceramic, the mug’s kinetic energy at impact falls back on itself rather than transferring externally into the concrete.   The result is a shattered mug and a mess to clean up.

    But we haven’t yet accounted for all the mug’s energy.   We’ll find out what happens to the rest of it next time.

Copyright 2016 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog



Mechanical Power Transmission – The Centrifugal Clutch Feels The Heat

Sunday, May 27th, 2012

     Ever get out of bed on a cold winter morning and feel as stiff as a ladder?  Summer’s heat doesn’t have the same effect on aging joints as winter’s chill, and many retirees have been motivated to move into warmer climates because of it.

     Heat can change the properties of metals like steel, too.  By properties, I mean qualities such as hardness and stiffness–where hardness relates to steel’s ability to resist wear and denting, while stiffness relates to its ability to resist a force that is trying to bend it.

     Obviously, if things get hot enough, say in the thousands of degrees Fahrenheit, steel will soften and eventually melt into a blob of glowing liquid.  At lower temperatures the change will be less dramatic, but its atomic structure will be undergoing change nonetheless.  Varying temperatures cause atoms to become energized, causing them to move around within their atomic structure.  Depending on how quickly things cool back down, the iron and carbon atoms that make up the steel can end up in different locations, causing a permanent change.  The steel could end up softer or harder.  For example, slow cooling hot steel in air makes it softer, while rapid cooling, such as when you submerge hot steel quickly into cold oil, makes it harder.

     How does heat play a part in the ongoing discussion of the centrifugal clutch in a grass trimmer?  Well, friction between the shoes and housing generates heat as a result of centrifugal force.  Clutch springs are made of steel, which is hard and resistant to bending.  But during operation they may heat up to hundreds of degrees, then slowly cool down again when the grass trimmer is shut off.  Without getting into a complex explanation of metallurgy, this slow cooling makes the steel in the springs softer, and with time they will lose their stiffness and weaken.

     Over time the springs become so weak they are unable to overcome the centrifugal force acting on the clutch shoes, causing the clutch to fail at its task of disengaging the cutter head from the engine at idle speed.  In other words, as soon as the engine is started, the cutter head will rapidly begin to spin.   With these conditions in place, the cutter head poses a threat to anything or anyone making contact with it. 

     Next time we’ll look at another cause of centrifugal clutch failure, that is, component wear due to friction between the clutch shoes and clutch housing. 


centrifugal clutch springs damaged by overheating