Archive for the ‘Professional Malpractice’ Category

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. 

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centrifugal clutch springs damaged by overheating

Mechanical Power Transmission – The Centrifugal Clutch and Metal Fatigue

Sunday, May 13th, 2012

     When I’m under a lot of stress I sometimes have the nervous habit of grabbing a paper clip, straightening out the bends, then repetitively bending it back and forth.  Eventually the wire reaches a point where it just breaks apart.

     My paper clip broke due to metal fatigue.  Metal parts are said to become fatigued when they’re subjected to forces of a repetitive nature such as occur due to twisting and bending.  The metal cracks, then eventually breaks due to the stress.

     So what’s happening when metal becomes fatigued?  Figure 1 shows the simplified atomic structure of a sample metal.

Figure 1

 

     When the metal is deformed, such as during bending, its rows of atoms are forced to move with respect to each other as shown in Figure 2.

 

Figure 2

 

     The movement of rows of atoms leads to an alteration in structure, breaking bonds between atoms.  This results in small cracks forming along the metal’s surface, cracks which eventually migrate deeper inside the metal with each subsequent bend.  With time the metal will become so compromised by the cracks that breakage occurs.

     Metal fatigue can occur in centrifugal clutch mechanisms as well.  Power tools such as grass trimmers typically operate between idle and working speeds many times during a day’s usage.  As we learned in previous articles, when the engine runs at idle speed, the springs in the centrifugal clutch mechanism stay retracted.  As the engine speeds up, the centrifugal force acting on the clutch shoes extends the springs.  Successive extensions and retractions cause the metal in the springs to bend, and over time they, like my paper clip, will become fatigued and metal springs will break.  

     Next time we’ll continue talking about centrifugal clutch failures and learn how the springs of a clutch mechanism can fail without its metal being brought to the breaking point.

 

 

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Mechanical Power Transmission – The Centrifugal Clutch in Operation

Sunday, April 22nd, 2012
     Just the other day I unexpectedly experienced the effects of centrifugal force while  driving home from the grocery store.  The checker had packed my entire order into one bag, making it top heavy.  Then en route someone cut me off at an intersection, and I had to make a sharp turn to avoid a crash.  During this maneuver centrifugal force came into play, forcing my grocery bag out of its centered position on the front seat next to me.  It lurched into the passenger’s door, fell over, and spilled its contents onto the floor.  Fortunately the eggs didn’t get smashed.

     In previous articles we identified the component parts of a centrifugal clutch mechanism and learned how centrifugal force makes objects spinning in a circular path about a fixed point move outward.  We can now explore what happens when we couple a centrifugal clutch mechanism to the engine of a grass trimmer.

     Figure 1 depicts the spinning clutch mechanism of a gas engine when it’s just been started and is operating at a slow idle speed.

centrifugal clutch mechanism

Figure 1

 

     Like the red ball in my previous article on centrifugal force, the blue centrifugal clutch shoes each have a mass m.  They spin around a fixed point P, situated at the center of the yellow engine shaft coupling.  Point P is located a distance r from the center of each shoe.  The shoes in motion have a tangential velocity V, and in accordance with Sir Isaac Newton’s Law of Centrifugal Force, the force Fc acts upon each shoe, causing them to want to pull out from the center of the mechanism, away from the fixed point.  Since idle speed is rather slow, however, the centrifugal force exerted upon the shoes isn’t strong enough to overcome the tension of the two springs and the coils connecting them remain coiled, holding the shoes tightly in position on the green boss.

     So what happens when we press the throttle trigger on the gas engine and cause the engine to speed up?  See Figure 2.

clutch shoes

Figure 2

 

     Figure 2 shows the clutch mechanism spinning at an increased velocity.  The tangential velocity V increases, and according to Newton’s law, the centrifugal force Fc acting on the clutch shoes increases as well.  The force is so strong that it overcomes the tension in the springs and they extend.  The clutch shoes are caused to move out and away from fixed point P, as well as from each other, traveling along the ends of the boss.

     When we remove our finger from the throttle trigger, the engine will slow down and return to idle speed.  The centrifugal force will decrease and the springs will pull the shoes back towards fixed point P.  The mechanism will return to its previous state, as shown in Figure 1.

     Next time we’ll insert the centrifugal clutch mechanism into the clutch housing to see how mechanical power is transmitted from the engine to the cutter head in our grass trimmer.

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Mechanical Power Transmission – The Centrifugal Clutch Mechanism

Sunday, April 15th, 2012
     My journey through engineering school was marked by a cast of colorful characters from around the world.  I remember one Russian professor in particular, fond of extolling the virtues of Russian engineering by the statement, “In Soviet Union steel ingots roll in one door, military tanks roll out other door.”  During that period of history in his homeland, it was not uncommon for all components down to the smallest screw to be manufactured within the same factory.

     That professor taught me all about clutch mechanisms, and whether they’re present in Soviet tanks or grass trimmers they perform the same basic function.  Let’s take a look at one now.

Centrifugal Clutch Mechanism

Figure 1

 

    Figure l shows my color-enhanced clutch illustration, which makes it easy to identify the different components of a centrifugal clutch.  The main part of the clutch is colored green and it’s respectfully referred to as the “boss.”  I assume it’s earned the title due to its role in keeping all component parts of the clutch assembly together.

     The blue portion shows two clutch shoes.  The boss fits loosely into notches within the shoes.  The curved surfaces on the shoes are composed of a high friction material, and we’ll see why later.  Two springs attached to the shoes cause them to pull towards each other and keep them from falling off the ends of the boss.

     The yellow portion shows the engine shaft coupling.  It’s permanently affixed to the center of the boss.  This coupling has a hole in it that enables the clutch mechanism to be attached onto an engine shaft with a threaded nut or some other type of mechanical fastener.

     Now that we’re familiar with a centrifugal clutch’s parts we can see how they come into play in a real world application, that of an engine shaft.   We’ll explore that next week.

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Mechanical Power Transmission – Centrifugal Force and Centrifugal Clutches

Monday, April 9th, 2012
     I’m not a big fan of amusement parks.  The first time I rode on a Tilt-A-Whirl I was caught off guard and flung onto my side by the centrifugal force acting upon my body, the lower half of which was constrained by a seat belt so I wouldn’t be catapulted out during the ride.  To make matters worse, the centrifugal force started to force the lunch I’d made the mistake of eating just before back up my throat.  It was a very unpleasant experience to say the least.

     Centrifugal force is an interesting phenomenon, and its principles are involved in the operation of a centrifugal clutch, which we’ll see later.  For now, let’s get a basic understanding of what it’s all about, thanks to the discoveries of Sir Isaac Newton in the late 17th Century.

 Centrifugal Force

Figure 1

 

     Figure 1 shows a red ball, whose mass we’ll notate m, attached to a string, the other end of which is attached to a fixed point, such as if you held it taught between your fingers.  If you’re in a playful mood, you might enjoy twirling the ball above your head on its string.  The distance between the center of the ball and the fixed point is labeled r, which stands for the radius of the circular path traveled by the ball as it twirls around the fixed point.   The speed at which the ball travels through the air is called its straight line velocity, or tangential velocity in scientific-speak, and it is generally notated as a V.  The centrifugal force, or Fc, that is exerted upon the ball as it whirls around your head is, Sir Isaac tells us, measured by the equation:

Fc = mV2/r

     Centrifugal force in the simplest of terms is an outward-pushing force that pulls objects in motion away from the point about which they’re rotating.  Let’s hold as fact that if m and r don’t change, then Newton’s equation tells us that the centrifugal force exerted upon the object in motion increases by the square of the velocity, or speed, of the ball.  In other words, the faster the ball moves as you spin it around your head on the string, the harder the centrifugal force that acts upon it.  As you spin the ball faster and faster, it will pull outward more and more strenuously, exerting ever greater resistance upon the string you hold between your fingers.

     Now suppose we replace the string in this example with a spring as shown in Figure 2. 

centrifugal force clutch spring

Figure 2

 

     Why a spring?  Because that’s what’s used within a centrifugal clutch.  Just as with the string, the ball’s velocity increases as you increase rotation speed around the fixed point, and the centrifugal force acting upon its mass by the spinning action increases as well.  The spring expands, extending further and further out from its beginning position of attachment to the fixed point, your fingers.  As velocity decreases, the spring will retract, eventually returning to its original coil size.  This extending and retracting action is the major mechanism at play within a centrifugal clutch.

     Next time we’ll explore a centrifugal clutch mechanism in more depth to observe its behavior relative to its spring under the influence of centrifugal force.

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Industrial Control Basics – Disconnect Switches

Sunday, March 25th, 2012
     Last week our kitchen ceiling fan and light combo decided to stop working.  We don’t like eating in the dark, so I was compelled to do some immediate troubleshooting.  As an engineer with training in the workings of electricity I have a great respect for it.  I’m well aware of potential hazards, and I took a necessary precaution before taking things apart and disconnecting wires.  I made the long haul down the stairs to the basement, opened the circuit breaker in the electrical panel, and disabled the flow of electricity to the kitchen.  My fears of potential electrocution having been eliminated, my only remaining fear was of tumbling off the ladder while servicing the fan.

     Just as I took the precaution to disconnect the power supply before performing electrical maintenance in my home, workers in industrial settings must do the same, and a chief player in those scenarios is the motor overload relay discussed last week.  It automatically shuts down electric motors when they become overheated.  Let’s revisit that example now.

Industrial Control System

Figure 1

 

     Our diagram in Figure 1 shows electric current flowing through the circuit by way of the red path.  Even if this line were shut down, current would continue to flow along the path, because there is no means to disconnect the entire control system from the hot and neutral lines supplying power to it, that is, it is missing disconnect switches.  Electric current will continue to pose a threat to workers were they to attempt a repair to the system.  Now let’s see how we can eliminate potential hazards on the line.

Disconnect Switches

Figure 2

 

     In Figure 2 there is an obvious absence of the color red, indicating the lack of current within the system.  We accomplished this with the addition of disconnect switches capable of isolating the motor control circuitry, thereby cutting off the hot and neutral lines of the electrical power supply and along with it the unencumbered flow of electricity.

    These switches are basically the same as those seen in earlier diagrams in our series on industrial controls, the difference here is that the two switches are tied together by an insulated mechanical link.  This link causes them to open and close at the same time.  The switches are opened and closed manually via a handle.  When the disconnect switches are both open electricity can’t flow and nothing can operate.  Under these conditions there is no risk of a worker coming along and accidentally starting the conveyor motor.

     To add yet another level of safety, disconnect switches are often tagged and locked once de-energized.  This prevents workers from mistakenly closing them and starting the conveyor while maintenance is being performed.  Brightly colored tags alert everyone that maintenance is taking place and the switches must not be closed.  The lock that performs this safety function is actually a padlock.  It’s inserted through a hole in the switch handle, making it impossible for anyone to flip the switch.  Tags and locks are usually placed on switches by maintenance personnel before repairs begin and are removed when work is completed.

     Now let’s see how our example control system looks in ladder diagram format.

Control System Ladder Diagram

Figure 3

 

     Figure 3 shows a ladder diagram that includes disconnect switches, an emergency stop button, and the motor overload relay contacts.  The insulated mechanical link between the two switches is represented by a dashed line.  Oddly enough, engineering convention has it that the motor overload relay heater is typically not shown in a ladder diagram, therefore it is not represented here.

     This wraps up our series on industrial control.  Next time we’ll begin a discussion on mechanical clutches and how they’re used to transmit power from gasoline engines to tools like chainsaws and grass trimmers.

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Industrial Control Basics – Motor Overload Relay In Action

Sunday, March 18th, 2012

    Last week we explored the topic of thermal expansion, and we learned how the bimetal contacts in a motor overload relay distort when heated.  We also discussed how the overload relay comes into play to prevent overheating in electric motor circuits.  Now let’s see what happens when an overload situation occurs.

motor overload relay

Figure 1

 

     Figure 1 shows a motor becoming overloaded, as it draws in abnormally high amounts of electric current.  Since this current also flows through the electric heater in the overload relay, the heater starts producing more heat than it would if the motor were running normally.  This abnormally high heat is directed towards the bimetal switch contacts, causing them to curl up tightly until they no longer touch each other and open up.  They will only close again when the overload condition is cleared up and the heater cools back down to normal operating temperature.

     Let’s now take a look at Figure 2 to see how the motor overload relay fits into our example of a conveyor belt motor control circuit.  Once again, the path of electric current flow is denoted by red lines.

motor overload relay

Figure 2

 

     The circuit in Figure 2 represents what happens after Button 1 is depressed.  That is, the electric relay has become latched and current flows between hot and neutral sides through one of the N.O. contacts along the path of the green indicator bulb, the motor overload relay heater, and the conveyor belt motor.  The current also flows through the other N.O. contact, the Emergency Stop button, Button 2, the electric relay’s wire coil, and the motor overload relay bimetal contacts.  The motor becomes overloaded, causing the overload relay heater to produce abnormally high heat.  This heat is directed towards the bimetal contacts, also causing them to heat up.

industrial control

Figure 3

 

     In Figure 3 the bimetal contacts have heated to the point that they have curled away from each other until they no longer touch.  With the bimetal contacts open, electric current is unable to flow through to the electric relay’s wire coil.  This in turn ends the magnetic attraction which formerly held the relay armatures against the N.O. contacts.  The spring in the electric relay has pulled the armatures up, causing the N.O. contacts to open, simultaneously closing the N.C. contact. 

     These actions have resulted in a loss of current to the green indicator bulb and electric motor.  The red indicator bulb is now activated, and the conveyor motor is caused to automatically shut down to prevent damage and possible fire due to overheating.  This means that even if the conveyor operator were to immediately press Button 1 in an attempt to restart the line, he would be prevented from doing so.  Under these conditions the electric relay is prevented from latching, and the motor remains shut down because the bimetal contacts have been separated, preventing current from flowing through to the wire coil. 

     The bimetal contacts will remain open until they once again cool to normal operating temperature.  Once cooled, they will once again close, and the motor can be restarted.  If the cause of the motor overload is not diagnosed and its ability to recur eliminated, the automatic shutdown process will repeat this cycle. 

     Next time we’ll see how the overload relay is represented in a ladder diagram.  We’ll also see how switches can be added to the circuit to allow maintenance staff to safely work.

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Industrial Control Basics – Thermal Expansion Effect on Overload Relays

Sunday, March 11th, 2012
     Imagine driving on steel tires, not rubber.  Don’t think it would work too well?  On asphalt highways maybe not, but on the steel rails that steam locomotives travel upon, steel wheels work surprisingly well and it’s due in large part to the principles of thermal expansion and the different rates at which metal alloys expand and contract.  Allow me to explain by analyzing how a locomotive“tire” is changed.

     As you can imagine changing locomotive tires isn’t easy.  Firstly, locomotive shop mechanics have to actually build a fire around the steel tire to heat it up.  The intense heat causes its steel tire to thermally expand, meaning its steel molecules become energized by the heat and begin to vibrate.  This causes the molecules to move away from each other, and this results in the tire actually growing slightly in size.  This enlargement is just enough to enable mechanics to slip the tire back onto the locomotive’s wheel.  Now in place, the tire is allowed to cool back down to ambient air temperatures.  Cooling results in the tire’s steel molecules relaxing and moving closer to each other.  The tire shrinks back to its original preheated size and tightly wraps itself around the wheel. 

     Thermal expansion properties of metals comes into play in many other instances, including the workings of motor overload relays.  Please refer to Figure 1.

motor overload relay

Figure 1

 

     Here overload relay components are shown in the foreground box.  We see that the relay includes an electric heater and a set of two peculiar looking curved objects.  These are bimetal switch contacts, so named because each is made of two, that’s the “bi” part, metal strips with different thermal properties.  These strips are positioned back to back, then bonded together and curved into a shape resembling a question mark.

     Each of the two metals has different properties, namely, one expands at a faster rate and to a greater extent than the other when heated.  This differing rate of expansion is indicative of the two metals’ diverse thermal properties.  When the bimetal contact is exposed to heat, one metal strip wants to expand a lot, but it is bonded to the other metal strip which only wants to expand a little.  The end result is that their point of contact distorts and changes shape.  When allowed to cool back down, the metal strips contract and the contact point returns to its original shape.  In our next blog we’ll see how the contact shape changes and why this shape change is important.

     In Figure 1 the motor is running normally and there is no overload situation.  Under these conditions the motor draws electric current within the normal limits of its design.  That current also flows through the heater in the overload relay causing it to generate heat, but in this situation the heat change is small enough that it doesn’t affect the bimetal switch contacts and cause them to change shape.  The temperature at which the switch contacts will warp depends on the overall design of the overload relay as well as its application.

     Next time we’ll see what part a motor overload plays in conjunction with the overload relay’s heater and bimetal contacts.

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Industrial Control Basics – Motor Overload

Sunday, March 4th, 2012

     Last summer my wife and I did a lot of work in the garden.  Many holes were dug, bags of garden soil lifted, and plants planted.  It’s a new garden, and my wife has very big plans for it, so needless to say there was a lot of work to be done.  On more than one occasion we would end the day moaning about our body aches and how we had overdone it.  The next day we would hurt even worse, and we’d end up taking time off to recuperate.  Pain is your body’s way of telling you that it needs attention, and you’d better listen to it or you may have an even heavier price to pay down the road. 

      Electric motors can get overworked, just like our bodies.  Motors are often placed into situations where they are expected to perform tasks beyond their capability.  Sometimes this happens through poor planning, sometimes due to wishful thinking on the user’s part.  Motors can sustain damage when stressed in this way, but they don’t have a pain system to tell them to stop.  Instead, motors benefit by a specific type of electric relay known as an overload relay.  But before we get into how an overload relay works, let’s get a better understanding of how overloads happen.

     Suppose we’re back in the telephone factory discussed in previous blogs, watching a conveyor belt move phones through the manufacturing process.  An electric motor drives the conveyor belt by converting electrical energy into mechanical energy.  Everything is moving along normally when all of a sudden a machine malfunctions.  Telephones start piling up on a belt, and the pile up gets so bad the belt eventually gets jammed and its motor overloaded.  If the electricity flow to the motor isn’t shut down promptly by means of a nearby emergency stop button or an astute operator sitting in central control, then an even bigger problem is in the making, that of a potential fire. 

     When electricity is applied to motors they begin to operate, and their natural tendency is to want to keep operating.  They do so by continuously drawing energy from the electric current being supplied to them.  The greater the workload demand on the motor, the more current it requires to operate. 

     When motors become overloaded as in the scenario presented above, they continue to draw energy unless forced to a stop.  The result is an overabundance of current flowing through the motor and no outlet for its task of converting electrical energy into mechanical energy.  And where is all that pent up energy to go?  It becomes heat energy trapped inside the motor itself, and this heat can build up to the point where the motor becomes damaged or even bursts into flames.

     Next time we’ll look at how overload relays work to keep electric motors from overheating, just as our body’s pain sensors protect us from overdoing it. 

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Industrial Control Basics – Emergency Stops

Sunday, February 26th, 2012
     Ever been in the basement when you heard a loud thud followed by a scream by a family member upstairs?  You run up the stairs to see what manner of calamity has happened, the climb seeming to take an eternity.  Imagine a similar scenario taking place in an industrial setting, where distances to be covered are potentially far greater and the dangerous scenarios numerous.

     Suppose an employee working near a conveyor system notices that a coworker’s gotten caught in the mechanism.  The conveyor has to be shut down fast, but the button to stop the line is located far away in the central control room.  This is when emergency stop buttons come to the rescue, like the colorful example shown in Figure 1.

emergency stop pushbutton

Figure 1

 

     Emergency stop buttons are mounted near potentially dangerous equipment in industrial settings, allowing workers in the area to quickly de-energize equipment should a dangerous situation arise.  These buttons are typically much larger than your standard operational button, and they tend to be very brightly colored, making them stick out like a sore thumb.  This type of notoriety is desirable when a high stress situation requiring immediate attention takes place.  They’re easy to spot, and their shape makes them easy to activate with the smack of a nearby hand, broom, or whatever else is convenient. 

     Figure 2 shows how an emergency stop button can be incorporated into a typical motor control circuit such as the one we’ve been working with in previous articles.

emergency stop button in motor control circuit

Figure 2

 

     An emergency stop button has been incorporated into the circuit in Figure 2.  It depicts what happens when someone depresses Button 1 on the conveyor control panel.  The N.C. contact opens, and the two N.O. contacts close.  The motor starts, and the lit green bulb indicates it is running.  The electric relay is latched because its wire coil remains energized through one N.O. contact.  It will only become unlatched when the flow of current is interrupted to the wire coil, as is outlined in the following paragraph.  The red lines denote areas with current flowing through them.

     Both Button 2 and the emergency stop button typically reside in normally closed positions.  As such electricity will flow through them on a continuous basis, so long as neither one of them is re-engaged.  If either of them becomes engaged, the same outcome will result, an interruption in current on the line.  The relay wire coil will then become de-energized and the N.O. contacts will stay open, preventing the wire coil from becoming energized again after Button 2 or the emergency stop are disengaged.  Under these conditions the conveyor motor stops, the green indicator bulb goes dark, the N.C. contact closes, and the red light comes on, indicating that the motor is not running.  This sequence, as it results from hitting the emergency stop button, is illustrated in Figure 3.

emergency stop button unlatches electric relay

Figure 3

 

     We now have the means to manually control the conveyor from a convenient, at-the-site-of-occurrence location, which allows for a quick shut down of operations should the need arise.

     So what if something else happens, like the conveyor motor overheats and catches on fire and no one is around to notice and hit the emergency stop?  Unfortunately, in our circuit as illustrated thus far the line will continue to operate and the motor will continue to run unless we incorporate an additional safeguard, the motor overload relay.  We’ll see how that’s done next time. 

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