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Ever wondered if a running horse lifts all four of its feet off the ground at the same time? Leland Stanford, an industrialist and horseman of the late 19th Century did, and he hired photographer Eadweard Muybridge to find out. Muybridge’s series of 24 photographs of Stanford’s horse, Sallie Gardner, came to be known as Sallie Gardner at a Gallop and is regarded to be an early example of silent film.
The Muybridge photos were viewed at increased speed on a zoopraxiscope, a device he invented in 1879. A precursor to modern movie projectors, it projected a series of independent photographs as a moving image through the use of multiple cameras shooting the subject at different points in time. In this way it was disclosed that yes, indeed, there were moments when all four of a galloping horse’s feet hover in mid air. Today’s moving images are displayed at between 24 and 300 fps, depending on the application. Muybridge’s experiment proved that not only are moving images more engaging than static ones, they are also more explicit, able to convey information still images are not. Take for example this series of stills of a centrifugal clutch assembly.
Are you able to tell by looking at these two-dimensional images how a clutch works? How as the engine speeds up the spinning shoes move out and make contact with the clutch housing, this pressure causing the entire assembly to spin? Unless you’re familiar with clutches, probably not. Now here’s the same clutch brought to life through animation:
In today’s fast paced, internet-laden society, people’s attention spans are shorter than ever, and their demands to be visually engaged are high. It’s been proven that holding a modern day viewer’s attention for more than three seconds is a difficult task. This truth is evident in the courtroom as well, where trial attorneys are obliged to increase the production value of evidence presented in order to win over juries, and animation is becoming their tool of choice. What held true more than 100 years ago still holds true today: Nothing tells a story like a moving image. Next time we’ll switch gears, quite literally, to understand how a series of gears work together to power machinery. ________________________________________ Note: If you are viewing this blog article in an email and the animation video does not appear, then click on this link to view the article with your web browser. ________________________________________ |
Posts Tagged ‘clutch’
See How a Centrifugal Clutch Works With Animation
Monday, December 16th, 2013
Mechanical Power Transmission – Centrifugal Clutch Shoe Wear
Sunday, June 3rd, 2012| My first car was a used 1963 Dodge 880. It was reliable for the most part, but one day when I stepped on the brake in a supermarket parking lot, nothing happened. I began to roll down an incline, and I struggled to steer around the maze of parked cars in the lot. After what seemed to be an eternity I managed to navigate my way out of the lot into an adjacent cornfield. The soft ground and corn stalks finally brought me to a stop. I later discovered that the reason my brakes failed is because their linings had completely worn away.
Like the brakes in cars, centrifugal clutch shoes also have linings as shown in Figure 1. Brake linings are typically made of a rough, high friction materials, such as ceramic compounds. These materials are bonded to the brake shoes, or in the case of clutches, to the clutch shoes. When centrifugal force comes into play, pressing the clutch shoes against the inside wall of the clutch housing, the roughness of the linings provides a good grip, preventing slippage between the shoes and the housing.
Figure 1
As we learned in previous articles, slip between the clutch shoes and clutch housings can create problems. In our grass trimmer for example, we learned that slippage reduces the amount of power the engine can effectively transmit to the cutter head. It also tends to produce a lot of heat. This heat can adversely effect the clutch springs and cause clutch failure. Although the high friction lining of the clutch shoes prevents most slippage, it can still occur, as when the throttle is depressed and engine speed increases beyond idle. There is some slipping as the clutch shoes first engage with the clutch housing, and it will continue until the engine speed increases to the point where centrifugal force causes the clutch shoes to firmly press into the clutch housing. Slippage also occurs when gasoline powered tools are subjected to operating stress. Figure 2 shows two views of a chainsaw. The first view is complete, the second shows the chain and clutch housing in isolation.
Figure 2
With the engine housing removed, we see that the saw chain is connected to a sprocket located on the centrifugal clutch housing. This sprocket is similar to those that engage the chains on bicycle wheels. Now suppose someone decides to use the chainsaw to cut a green, sap-filled log. To make matters worse, let’s suppose the chainsaw has a dull saw chain. If you’ve ever tried doing this, you know that the sticky, sappy wood will eventually gum up the chain and stop it from moving. Since the chain is connected to the clutch housing, it stops as well. However the clutch shoes, which are driven by the engine, keep trying to move the gummed-up clutch housing, because the engine’s power is enough to overcome some of the friction. The result is that the shoes slip uselessly inside the housing. Over time, continued slippage will cause the clutch shoes’ high friction lining to wear away. Once the lining is gone the clutch shoes will slip excessively, even when the gasoline powered tool is being employed to perform the lightest task. That’s because slipping prevents a good portion of the engine’s power from being transmitted to the cutting head. That’s it for our series on centrifugal clutches. Next we’ll be discussing transistors, how they’re used in electronic controls to switch things on and off and perform other functions. ____________________________________________ |
Mechanical Power Transmission – Putting the Centrifugal Clutch Together
Sunday, April 29th, 2012| I’ve never been one to enjoy table top puzzles, yet I love to examine the way mechanical things fit together. Manipulating parts to see how they interrelate to form an operational system is a pastime I very much enjoy. In fact, I spend many evenings at my work bench doing just this. I often become so engrossed in the activity I forget what time it is. The result is yet another night without TV. So sad…
Last week we looked at how a centrifugal clutch mechanism operates when it’s coupled to a gasoline engine shaft spinning at idle speed, and then we depressed the engine throttle trigger to speed things up. Let’s now introduce a new component called the clutch housing to see how it interfaces with the clutch mechanism to drive the cutter head in a grass trimmer.
Figure 1
The clutch housing shown in Figure 1 resembles a rather short cup. One end is open, the other closed. Figure 2 shows the closed end of the clutch housing connected to the cutter shaft’s coupling. On the cutter shaft coupling resides a ball bearing which enables the clutch housing to both spin freely and act as a support for the clutch housing. The open end of the clutch housing allows the clutch mechanism to fit neatly inside.
Figure 2
Next time we’ll put the assembly shown in Figure 2 into operation. First we’ll examine how the centrifugal clutch mechanism and clutch housing operate with the engine at idle speed, then compare that to the engine operating at actual cutting speed. ____________________________________________
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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.
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.
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 – 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. 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.
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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Mechanical Power Transmission – Centrifugal Clutches
Monday, April 2nd, 2012| I remember the days when trimming grass around trees, fences, and flower beds involved the use of hand operated clippers. You know, those scissor-like things that require you to squeeze the handles together to move the blades. Cutting seemed to take forever and there was a lot of bending, stooping, and kneeling which would kill your back and turn your knees green from grass stains. Worst of all, the repetitive motion of squeezing the handles dozens of times would cramp your hands. It was a great day when gasoline powered grass trimmers came along. Just pull the recoil starter cord and you’re ready to go. It’s fast, easy, and the final result looks better too.
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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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.
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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