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.

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 F_c 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.

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 F_c 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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     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.

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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     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 17^th Century.

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 F_c, that is exerted upon the ball as it whirls around your head is, Sir Isaac tells us, measured by the equation:

F_c = 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. 

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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     If you’ve ever operated a gasoline powered tool like a grass trimmer, you probably noticed that the cutter action isn’t immediate once the engine is started.   Instead, the engine enters into a much slower initial speed mode, the idle speed.  The cutter moves only after the throttle trigger is depressed.  This introduces more gas to the engine, causing it to speed up, and this action is due to a device called the centrifugal clutch.

     A centrifugal clutch, or any type of clutch for that matter, serves one basic function, to physically disconnect, then reconnect a gasoline engine from whatever it is powering.  For example, if the engine in a weed trimmer stayed permanently connected to the cutter when the engine was started, it would pose a definite safety hazard.  Even at idle speed, the cutter would immediately kick into high speed operational mode, and if someone wasn’t prepared for this instant response there would be a good probability of injury.

     When a centrifugal clutch is placed between the engine and the cutter, it automatically disconnects the engine from the cutter during starting and at idle speed.  We’ll see how it does that in a later blog.  For now, let’s consider the fact that the idle function serves as a “get ready.”  The user is able to both psychologically and physically prepare themselves to use their tool.  Pressing the trigger revs the engine up and causes the centrifugal clutch to connect the engine to the cutting action.  When the operator takes their finger off the throttle trigger the engine returns to idle speed, and the clutch automatically disconnects the engine from the cutter.  The cutter becomes idle.

Figure 1

     Figure 1 shows a gas trimmer and its centrifugal clutch.  The engine is on one end of the trimmer and the cutter at the other.  A hollow metal tube runs between them.  This tube  contains the cutter drive shaft.  The centrifugal clutch and its clutch housing are located in a cone shaped compartment between the engine and the metal tube.  The clutch is connected to the engine drive shaft and the clutch housing is connected at the other end of the cutter drive shaft.  When they’re assembled into the grass trimmer, the clutch fits within the clutch housing.

     Next time we’ll see how the centrifugal clutch on a grass trimmer uses centrifugal force and friction to automatically transmit mechanical power from the gas engine to the cutter.

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     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.

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.

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.

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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    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.

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.

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.

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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     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.

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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