| 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.
Posts Tagged ‘heat’
I got my first 10-speed bike when I was in high school. It was nice, except for one nasty hangup, the brakes were always going out of adjustment. Once it did this at the worst of times, when I was going down a steep hill. I squeezed hard on the brake handles, and nothing happened. The bike started to go out of control in its ascent down the hill, and in desperation I took my feet off the pedals and pressed the soles of my shoes as hard as I could into the road surface. To my relief my emergency measure was effective. The harder I pressed into the pavement, the less my shoes slipped, and the more the bike slowed down. I had good rubber treads on the sneakers I was wearing that day, and the friction between the soles of my shoes and the surface of the pavement was strong enough to stop my runaway descent. Something very similar occurs during the operation of a centrifugal clutch.
If you recall from previous articles in this series, when the clutch mechanism spins faster than engine idle speed, the centrifugal force acting upon the clutch shoes overcomes the tension in the springs. This causes the clutch shoes to make contact with the clutch housing. But although there is contact, the clutch shoes will initially slip somewhat. That is, the clutch housing and cutter head won’t spin at exactly the same speed when a faster spin is first employed, although the slip between the clutch shoes and housing decreases as engine speed increases.
Faster speed means there’s more centrifugal force at play, forcing the shoes harder against the drum of the clutch housing. The increase in centrifugal force makes the shoes move tighter and tighter against the housing, and this causes an increase in friction. Eventually the engine speed will increase to full throttle, the point where the shoes are pressed into the housing so hard there is no more slip. The cutter head will then turn at the same rate as the engine, and the engine’s power will be fully transmitted to the cutter head, allowing you to trim grass effectively.
Friction is a double edged sword. On the one hand it reduces slip between the clutch shoes and clutch housing. On the other, the friction between the slipping shoes and clutch housing generates a lot of heat, particularly if the grass trimmer is cutting thick grass. We’ll see how that heat impacts the clutch mechanism components next week.
| Ever take a peek inside the toaster while you’re waiting for the toast to pop up? If so, you would have noticed a bright orange glow. That glow is produced when the toasting wires heat up, which in turn creates a nice crusty surface on your bread or waffle. It’s the same phenomenon as when the filament inside an incandescent bulb glows. The light and heat produced in both these cases are the result of the Joule, pronounced “jewel,” effect at work.
To understand Joule heating, let’s first refresh our memories as to electrical current resistance. We learned previously that wire is not a perfect conductor, and as such resistance to flow is encountered. This resistance causes power to be lost along the length of wire, in accordance with this equation:
Power Loss = I2 × R
Where I is the electric current flowing through a wire, and R is the total electrical resistance of the wire. The power loss is measured in units of Joules per second, otherwise known as watts, “watt” denoting a metric unit of power. It is named after the famed Scottish mechanical engineer, James Watt, who is responsible for inventing the modern steam engine. A Joule is a metric unit of heat energy, named after the English scientist James Prescott Joule. He was a pioneer in the field of thermodynamics, a branch of physics concerned with the relationships between different forms of energy.
Anyway, to see how the equation works, let’s look at an example. Suppose we have 12 feet of 12 AWG copper wire. We are using it to feed power to an appliance that draws 10 amperes of electric current. Going to our handy engineering reference book, we find that the 12 AWG wire has an electrical resistance of 0.001588 ohms per foot, “ohm” being a unit of electrical resistance. Plugging in the numbers, our equation for total electrical resistance becomes:
R = (0.001588 ohms per foot) × 12 feet = 0.01905 ohms
And we can now calculate power loss as follows:
Power = I2 × R = (10 amperes)2 × (0.01905 ohms) = 1.905 watts
Instead of using a 12 AWG wire, let’s use a smaller diameter wire, say, 26 AWG. Our engineering reference book says that 26 AWG wire has an electrical resistance of 0.0418 ohms per foot. So let’s see how this changes the power loss:
R = (0.0418 ohms per foot) × 12 feet = 0.5016 ohms
Power = I2 × R = (10 amperes)2 × (0.5016 ohms) = 50.16 watts
This explains why appliances like space heaters and window unit air conditioners have short, thick power cords. They draw a lot of current when they operate, and a short power cord, precisely because it is short, poses less electrical resistance than a long cord. A thicker cord also helps reduce resistance to power flow. The result is a large amount of current flowing through a superhighway of wire, the wide berth reducing both the amount of power loss and the probability of dangerous Joule heating effect from taking place.
Our example shows that the electric current flowing through the 12 AWG wire loses 1.905 watts of power due to the inconsistencies within the wire, and this in turn causes the wire to heat up. This is Joule heating at work. Joule heating of 50.16 watts in the thinner 26 AWG wire can lead to serious trouble.
When using a power cord, heat moves from the copper wire within it, whose job it is to conduct electricity, and beyond, on to the electrical insulation that surrounds it. There the heat is not trapped, but escapes into the environment surrounding the cord. If the wire has low internal resistance and the amount of current flowing through it is within limits which are deemed to be acceptable, then Joule heating can be safely dissipated and the wire remains cool. But if the current goes beyond the safe limit, as specified in the American Wire Gauge (AWG) table for that type of wire, then overheating can be the result. The electrical insulation may start to melt and burn, and the local fire department may then become involved.
That’s it for wire sizing and electric current. Next time we’ll slip back into the mechanical world and explore a new topic: the principles of ventilation.
Last week we talked about an area of thermodynamics that concerns refrigeration cycles as presented through the example of an air conditioner. This week, we’ll learn about stoichiometry, which is concerned with the math behind chemical reactions, like those that take place during the burning of fuels.
During the combustion process, heat energy is released from a fuel when the combustible elements in the fuel combine with oxygen. This is known as oxidation, or in common everyday language as burning. The most important thing to remember about oxidation is that it obeys the first law of thermodynamics. That is, mass cannot be created or destroyed. In a chemical reaction like combustion, particles of fuel and air are rearranged in space and then combine to form different substances. However, despite the rearranging, the mass that goes into the reaction must equal the mass that comes out. This conservation of mass is the basis of stoichiometry.
For example, if pure carbon (represented by the chemical symbol “C”) is burned in pure oxygen (O2), you can represent the combustion process as:
C + O2 → CO2
This is chemistry shorthand for representing how carbon and oxygen combine during burning to form carbon dioxide (CO2). The elements to the left of the arrow are known as “reactants” and the elements to the right are known as “products.” In stoichiometry, the mass of the reactants must equal the mass of the products. But, how do we quantify the mass of reactants and products? Now this is where it gets a little weird.
To make use of our chemistry shorthand above, we have to consider something called moles. No, these aren’t the little furry creatures that tunnel under your lawn and eat your tulip bulbs. In stoichiometry a mole is considered to be 6.02×1023 molecules of a substance. That is 602,000,000,000,000,000,000,000 molecules!
Okay, so we have one heck of a lot of molecules in a mole. So, what does that have to do with figuring out how much mass we are dealing with in the combustion process? Well, in order to make moles work for us, we have to take into consideration the differing molecular weights of substances. Molecular weight is the number of grams (g) of mass that are contained within one mole of a substance, like the element carbon in our example above. To help make stoichiometry more workable, scientists created a table that provides the molecular weight of all known chemical elements. This table is known as the Periodic Table of Elements, or the “Periodic Table” for short.
Now going back to our example above, if we know from the Periodic Table that carbon has a molecular weight of 12 g per mole and oxygen has a molecular weight of 16 g per mole, then how many grams of carbon dioxide do we get by burning carbon in pure oxygen? The combustion process can be represented by this equation:
C + O2 → CO2
(12 g/mole) × (1mole of carbon) + (16 g/mole) × (2 moles of oxygen)
= 44 g of carbon dioxide
This is a fairly straightforward example of how stoichiometry works. In reality, things can get far more complicated. In a power plant for example, fuels like coal contain substances in addition to carbon, such as hydrogen and sulfur, and they, too, must be factored into the stoichiometric accounting system.
To further complicate things, fuels are usually burned in air, rather than pure oxygen. Air, too, contains substances other than oxygen, including nitrogen, argon, and molecules of water. These other substances’ presence in fuel and air make the combustion process more challenging to account for, because they all get mixed together, and they can combine into all sorts of other substances. Despite these complicating factors, the first law of thermodynamics must be obeyed, so the balancing act is still the same: mass of the reactants must equal the mass of the products.
Once mechanical engineers use stoichiometry to figure out what’s going in and coming out of the combustion process, they can then use the data provided by chemical analysis of the fuel to calculate the heat energy that is released. They can also calculate the air required for proper combustion. This helps them to design things capable of delivering enough fuel and air to meet the heat input requirements for a diversity of power cycles, from the engine in your car to the coal fired power plant supplying electricity for your home.
Next week, we’ll talk about psychrometric analysis. No, this has nothing to do with psychiatry. Psychrometrics involves the analysis of gas and vapor mixtures like air and water.