BMW X5 for 2004
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BMW X5 for 2004
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New xDrive all-wheel drive system, Valvetronic V-8 engine and freshened design highlight extensive enhancements for BMW's Sports Activity Vehicle Woodcliff Lake, New Jersey - Four years after its introduction, the BMW X5 has long since firmly established itself as a unique and coveted offering in the luxury sport-utility field. If the very combination of "BMW" and "SUV" seemed unlikely back then, well, it was; for this reason, BMW decided to give the X5 a more apt name -Sports Activity Vehicle - to distinguish it from the ordinary run of SUVs. In the meantime, the SAV concept has become so established and respected that the X5 Series is BMW's third-biggest seller in the U.S. In fact, so successful is the X5 that it is soon to be joined by a second SAV line, the X3. |
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For 2004, the X5 Series gets an extensive round of product enhancements that freshen its steady, strong appeal to customers who value the attributes of an SUV - robust construction, all-weather/all-road and trailer-towing capabilities, commanding driving position, ease of loading cargo - but also seek the attributes of a BMW: precision handling, brilliant performance, elegant design, tasteful luxury and premium quality.
6-speed automatic transmission
Photos: BMW
The changes include:
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A new all-wheel drive system, xDrive, that adjusts front/rear torque proportioning according to road and operating conditions - with lightning-fast, ultra-sensitive response that optimizes both traction and handling.
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Adoption of the acclaimed Valvetronic V-8 engine in the X5 4.4i model, resulting in greater power (+25 hp) and significantly improved fuel economy.
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New 6-speed transmissions - manual in the 6-cylinder X5 3.0i model, automatic in the X5 4.4i.
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Freshened exterior design, with an entirely (yet subtly) new front end, new taillights and several fresh colors.
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Standard rain-sensing windshield wipers and automatic headlight control.
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New wheel designs for four of the five available wheel/tire configurations.
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Enhanced interior equipment, including 8-way power seats in both models, and new optional wood treatment. Here too, the color selection is expanded.
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Availability of the BMW Assist services and telematics system.
There are two X5 models for 2004: the X5 3.0i at $40,995 base price including destination charge; and the X5 4.4i, at $52,195. A successor to the ultra-high-performance X5 4.6is will be announced later in the '04 model year.
HOW A MECHANICAL TRANSMISSION WORK
If you have ever driven a car with an automatic transmission, then you know that there are two big differences between an automatic transmission and manual transmission:
- There is no clutch pedal in an automatic transmission car
- There is no gear shift in an automatic transmission car. Once you put the transmission into Drive, everything else is automatic!
In this edition of HowStuffWorks we'll work our way through an automatic transmission. We'll start with the key to the whole system: planetary gearsets. Then we'll see how the transmission is put together, learn how the controls work and discuss some of the intricacies involved in controlling a transmission.
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Some Basics
Just like that of a manual transmission, the automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds.
Without a transmission, cars would be limited to one gear ratio, and that ratio would have to be selected to allow the car to travel at the desired top speed. If you wanted a top speed of 80 mph, then the gear ratio would be similar to third gear in most manual transmission cars.
You've probably never tried driving a manual transmission car using only third gear. If you did, you'd quickly find out that you had almost no acceleration when starting out, and, at high speeds, the engine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly undriveable.
So the transmission uses gears to make more effective use of the engine's torque, and to keep the engine operating at an appropriate speed.
The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission...
Planetary Gearsets
When you take apart and look inside an automatic transmission, you find an amazing assortment of parts in a fairly small space. Among other things you see:
- An extremely ingenious planetary gearset
- A set of bands that lock parts of a gearset
- A set of 3 wet-plate clutches to lock other parts of the gearset
- An incredibly odd hydraulic control system that controls the clutches and bands
- A large gear pump to move transmission fluid around
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From left to right: the ring gear, planet carrier, and two sun gears. |
Any planetary gearset has three main components:
- the sun gear
- the planet gears and the planet gears' carrier
- the ring gear.
One of the planetary gearsets from our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset.
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Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. Notice that the first gear ratio listed above is a reduction -- the output speed is slower than the input speed. The second is an overdrive -- the output speed is faster than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automatic transmission. You can try these out in the animation below:
Figure 2
Animation of the different gear ratios related to automatic transmissions
Click on the buttons on the left in the table above.
So this one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. With two of these gearsets in a row, we can get the four forward gears and one reverse gear our transmission needs. We'll put the two sets of gears together in the next section.
Gears
This automatic transmission uses a set of gears, called a compound planetary gearset, that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets.
Let's look at some of the parts.
Figure 3 shows how the gears in the transmission are put together.
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Figure 3 From left to right: the ring gear, planet carrier, and two sun gears. |
Figure 4 shows the planets in the planet carrier. Notice how the planet on the right sits lower than the planet on the left. The planet on the right does not engage the ring gear -- it engages the other planet. Only the planet on the left engages the ring gear.
 Figure 4 Planet carrier: Note the two sets of planets. |
Figure 5 shows the inside of the planet carrier. The shorter gears are engaged only by the smaller sun gear. The longer planets are engaged by the bigger sun gear and by the smaller planets.
 Figure 5 Inside of the planet carrier: Note the two sets of planets. |
Figure 6 shows all of the parts are hooked up in a transmission.
Figure 6
Move the shift lever to see how power is transmitted through the transmission.
First Gear
In first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. The planet carrier tries to spin counterclockwise, but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output. The small gear has 30 teeth and the ring gear has 72, so referring to the chart on the previous page, the gear ratio is:
So the rotation is negative 2.4:1, which means that the output direction would be opposite the input direction. But the output direction is really the same as the input direction -- this is where the trick with the two sets of planets comes in. The first set of planets engages the second set, and the second set turns the ring gear; this combination reverses the direction. You can see that this would also cause the bigger sun gear to spin; but because that clutch is released, the bigger sun gear is free to spin in the opposite direction of the turbine (counterclockwise).
Second Gear
This transmission does something really neat in order to get the ratio needed for second gear. It acts like two planetary gearsets connected to each other with a common planet carrier.
The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of the sun (the smaller sun gear), the planet carrier, and the ring (the larger sun gear).
The input is the small sun gear; the ring gear (large sun gear) is held stationary by the band, and the output is the planet carrier. For this stage, with the sun as input, planet carrier as output, and the ring gear fixed, the formula is:
The planet carrier turns 2.2 times for each rotation of the sun gear. At the second stage, the planet carrier acts as the input for the second planetary gear set, the larger sun gear (which is held stationary) acts as the sun, and the ring gear acts as the output, so the gear ratio is:
To get the overall reduction for second gear, we multiply the first stage by the second, 2.2 x 0.67, to get a 1.47:1 reduction. This may sound wacky, but it works.
Third Gear
Most automatic transmissions have a 1:1 ratio in third gear. You'll remember from the previous section that all we have to do to get a 1:1 output is lock together any two of the three parts of the planetary gear. With the arrangement in this gearset it is even easier -- all we have to do is engage the clutches that lock each of the sun gears to the turbine.
If both sun gears turn in the same direction, the planet gears lockup because they can only spin in opposite directions. This locks the ring gear to the planets and causes everything to spin as a unit, producing a 1:1 ratio.
Overdrive
By definition, an overdrive has a faster output speed than input speed. It's a speed increase -- the opposite of a reduction. In this transmission, engaging the overdrive accomplishes two things at once. If you read How Torque Converters Work, you learned about lockup torque converters. In order to improve efficiency, some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission.
In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output.
So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow.
Reverse
Reverse is very similar to first gear, except that instead of the small sun gear being driven by the torque converter turbine, the bigger sun gear is driven, and the small one freewheels in the opposite direction. The planet carrier is held by the reverse band to the housing. So, according to our equations from the last page, we have:
So the ratio in reverse is a little less than first gear in this transmission.
Gear Ratios
This transmission has four forward gears and one reverse gear. Let's summarize the gear ratios, inputs and outputs:
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After reading this section, you are probably wondering how the different inputs get connected and disconnected. This is done by a series of clutches and bands inside the transmission. In the next section, we'll see how these work.
The Clutches and Bands
In the last section, we discussed how each of the gear ratios is created by the transmission. For instance, when we discussed overdrive, we said:
In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing.
To get the transmission into overdrive, lots of things have to be connected and disconnected by clutches and bands. The planet carrier gets connected to the torque converter housing by a clutch. The small sun gets disconnected from the turbine by a clutch so that it can freewheel. The big sun gear is held to the housing by a band so that it could not rotate. Each gear shift triggers a series of events like these, with different clutches and bands engaging and disengaging. Let's take a look at a band.
Bands
In this transmission there are two bands. The bands in a transmission are, literally, steel bands that wrap around sections of the gear train and connect to the housing. They are actuated by hydraulic cylinders inside the case of the transmission.
 Figure 7 One of the bands |
In Figure 7 you can see one of the bands in the housing of the transmission. The geartrain is removed. The metal rod is connected to the piston, which actuates the band.
 Figure 8 The pistons that actuate the bands are visible above. |
In Figure 8 you can see the two pistons that actuate the bands. Hydraulic pressure, routed into the cylinder by a set of valves, causes the pistons to push on the bands, locking that part of the gear train to the housing.
Clutches
The clutches in the transmission are a little more complex. In this transmission there are four clutches. Each clutch is actuated by pressurized hydraulic fluid that enters a piston inside the clutch. Springs make sure that the clutch releases when the pressure is reduced. Figure 9 shows the piston and the clutch drum. Notice the rubber seal on the piston -- this is one of the components that is replaced when your transmission gets rebuilt.
 Figure 9 One of the clutches in a transmission |
Figure 10 shows the alternating layers of clutch friction material and steel plates. The friction material is splined on the inside, where it locks to one of the gears. The steel plate is splined on the outside, where it locks to the clutch housing. These clutch plates are also replaced when the transmission is rebuilt.
 Figure 10 The clutch plates |
The pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches and bands are energized at any given moment.
When You Put the Car in Park
It may seem like a simple thing to lock the transmission and keep it from spinning; but there are actually some complex requirements for this mechanism:
- You have to be able to disengage it when the car is on a hill (the weight of the car is resting on the mechanism).
- You have to be able to engage the mechanism even if the lever does not line up with the gear.
- Once engaged, something has to prevent the lever from popping up and disengaging.
The mechanism that does all this is pretty neat. Let's look at some of the parts first.
 Figure 11 The output of the transmission The square notches are engaged by the parking-brake mechanism to hold the car still. |
Figure 11 shows the output section of the transmission. The parking-brake mechanism engages the teeth on the output to hold the car still. This is the section of the transmission that hooks up to the driveshaft -- so if this part can't spin, the car can't move.
 Figure 12 The empty housing of the transmission with the parking brake mechanism poking through, as it does when the car is in park |
Figure 12 shows the park mechanism protruding into the housing where the gears are located. Notice that it has tapered sides. This helps to disengage the parking brake when you are parked on a hill -- the force from the weight of the car helps to push the parking mechanism out of place because of the angle of the taper.
 Figure 13 This rod actuates the park mechanism. |
Figure 13 shows the rod that actuates the park mechanism. This rod is connected to a cable that is operated by the shift lever in your car.
 Figure 14 The park mechanism |
Figure 14 shows the park mechanism from the top. When the shift lever is placed in park, the rod pushes the spring against the small tapered bushing. If the park mechanism is lined up so that it can drop into one of the notches in the output gear section, the tapered bushing will push the mechanism down. If the mechanism is lined up on one of the high spots on the output, then the spring will push on the tapered bushing, but the lever will not lock into place until the car rolls a little and the teeth line up properly. This is why sometimes your car moves a little bit after you put it in park and release the brake pedal -- it has to roll a little for the teeth to line up to where the parking mechanism can drop into place.
Once the car is safely in park, the bushing holds down the lever so that the car will not pop out of park if it is on a hill.
The Hydraulic System
The automatic transmission in your car has to do numerous tasks. You may not realize how many different ways it operates. For instance, here are some of the features of an automatic transmission:
- If the car is in overdrive (on a four-speed transmission), the transmission will automatically select the gear based on vehicle speed and throttle pedal position.
- If you accelerate gently, shifts will occur at lower speeds than if you accelerate at full throttle.
- If you floor the gas pedal, the transmission will downshift to the next lower gear.
- If you move the shift selector to a lower gear, the transmission will downshift unless the car is going too fast for that gear. If the car is going too fast, it will wait until the car slows down and then downshift.
- If you put the transmission in second gear, it will never downshift or upshift out of second, even from a complete stop, unless you move the shift lever.
 Figure 15 |
You've probably seen something that looks like Figure 15 before. It is really the brain of the automatic transmission, managing all of these functions and more. The passageways you can see route fluid to all the different components in the transmission. Passageways molded into the metal are an efficient way to route fluid; without them, many hoses would be needed to connect the various parts of the transmission. First, we'll discuss the key components of the hydraulic system; then we'll see how they work together.
The Pump
Automatic transmissions have a neat pump, called a gear pump. The pump is usually located in the cover of the transmission. It draws fluid from a sump in the bottom of the transmission and feeds it to the hydraulic system. It also feeds the transmission cooler and the torque converter.
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The inner gear of the pump hooks up to the housing of the torque converter, so it spins at the same speed as the engine. The outer gear is turned by the inner gear, and as the gears rotate, fluid is drawn up from the sump on one side of the crescent and forced out into the hydraulic system on the other side.
The Governor
The governor is a clever valve that tells the transmission how fast the car is going. It is connected to the output, so the faster the car moves, the faster the governor spins. Inside the governor is a springloaded valve that opens in proportion to how fast the governor is spinning -- the faster the governor spins, the more the valve opens. Fluid from the pump is fed to the governor through the output shaft.
The faster the car goes, the more the governor valve opens and the higher the pressure of the fluid it lets through.
 Figure 17 The governor |
Throttle Valve or Modulator
To shift properly, the automatic transmission has to know how hard the engine is working. There are two different ways that this is done. Some cars have a simple cable linkage connected to a throttle valve in the transmission. The further the gas pedal is pressed, the more pressure is put on the throttle valve. Other cars use a vacuum modulator to apply pressure to the throttle valve. The modulator senses the manifold pressure, which drops when the engine is under a greater load.
Manual Valve
The manual valve is what the shift lever hooks up to. Depending on which gear is selected, the manual valve feeds hydraulic circuits that inhibit certain gears. For instance, if the shift lever is in third gear, it feeds a circuit that prevents overdrive from engaging.
Shift Valves
Shift valves supply hydraulic pressure to the clutches and bands to engage each gear. The valve body of the transmission contains several shift valves. The shift valve determines when to shift from one gear to the next. For instance, the 1 to 2 shift valve determines when to shift from first to second gear. The shift valve is pressurized with fluid from the governor on one side, and the throttle valve on the other. They are supplied with fluid by the pump, and they route that fluid to one of two circuits to control which gear the car runs in.
 Figure 18 The shift circuit |
The shift valve will delay a shift if the car is accelerating quickly. If the car accelerates gently, the shift will occur at a lower speed. Let's discuss what happens when the car accelerates gently.
As car speed increases, the pressure from the governor builds. This forces the shift valve over until the first gear circuit is closed, and the second gear circuit opens. Since the car is accelerating at light throttle, the throttle valve does not apply much pressure against the shift valve.
When the car accelerates quickly, the throttle valve applies more pressure against the shift valve. This means that the pressure from the governor has to be higher (and therefore the vehicle speed has to be faster) before the shift valve moves over far enough to engage second gear.
Each shift valve responds to a particular pressure range; so when the car is going faster, the 2 to 3 shift valve will take over, because the pressure from the governor is high enough to trigger that valve.
Electronic Controls
Electronically controlled transmissions, which appear on some newer cars, still use hydraulics to actuate the clutches and bands, but each hydraulic circuit is controlled by an electric solenoid. This simplifies the plumbing on the transmission and allows for more advanced control schemes.
In the last section we saw some of the control strategies that mechanically controlled transmissions use. Electronically controlled transmissions have even more elaborate control schemes. In addition to monitoring vehicle speed and throttle position, the transmission controller can monitor the engine speed, if the brake pedal is being pressed, and even the anti-lock braking system.
Using this information and an advanced control strategy based on fuzzy logic -- a method of programming control systems using human-type reasoning -- electronically controlled transmissions can do things like:
- Downshift automatically when going downhill, to control speed and reduce wear on the brakes.
- Upshift when braking on a slippery surface to reduce the braking torque applied by the engine.
- Inhibit the upshift when going into a turn on a winding road.
Let's talk about that last feature -- inhibiting the upshift when going into a turn on a winding road. Let's say you're driving on an uphill, winding mountain road. When you are driving on the straight sections of the road, the transmission shifts into second gear to give you enough acceleration and hill-climbing power. When you come to a curve you slow down, taking your foot off the gas pedal and possibly applying the brake. Most transmissions will upshift to third gear, or even overdrive, when you take your foot off the gas. Then when you accelerate out of the curve, they will downshift again. But if you were driving a manual transmission car, you would probably leave the car in the same gear the whole time. Some automatic transmissions with advanced control systems can detect this situation after you have gone around a couple of the curves, and "learn" not to upshift again.
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مکانیزم کلی خودروها
مکانیزم کلی خودروها:
گروه تولید قدرت:
در این واحد انرژی شیمیایی بنزین به انرژی مکانیکی تبدیل می شود؛حرارت ناشی از سوختن هیدروکربورهابا بالا تر از ۷۰۰درجه میرسد.بازده مفیدسیستم حدود یک چهارم حرارت تولید شده است.
در یک موتور حدود ۱۲۰تا۱۵۰ قطعه ی متحرک موجود می باشد که نیاز به روغنکاری دارد.
با این وجود عمر مفید یک خودرو به طور متوسط ۸ سال و یا پیمودن۱۵۰۰۰۰ کیلومتر مسافت است.
گروه انتقال قدرت:
این مجموعا وظیفه دارد قدرت تولید شده از موتور را با چرخها انتقال دهدکه شامل قسمتهای زیر است:
۱.جعبه دنده یا مبدل گشتاور و سرعت؛که با این وسیله میتوان نیروی وارده به چرخها را تنطیم نمود.
۲.کلاچ:که عامل قطع و وصل کردن قدرت است وامکان تعویض دنده را فراهم می کند.
باید دانست که برای حرکت در شهرهای بزرگ به ازای پیمودن هر ۱۰۰ کیلومتر بیش از ۷۰۰بار عمل تعویض دنده یا گرفتن کلاچ انجام می شود.
۳.گروه چرخ بندی و ترمزها:
به طور متوسط در هر ۹۰۰۰۰ کیلومتر مسافت پیموده شده با هر شش سال کار خودروها هر چرخ حدود ۹۵۰۰۰۰۰۰بار چرخش میکند.
هر بار که سیستم ترمز نیرو صرف میکند تا یک اتومبیل با سرعت ۹۰ کیلومتر در ساعت را متوقف کند بر اثر اصطکاک؛گرمایی تلف میشود که میتواند یک لیتر آب را به جوش آورد.
۴.گروه بدنه و اطاق و شاسی:
بدنه ی اتومبیل ها طوری طراحی میشود که بتواند کلیه ی قطعات دیگر را نگهداری کند.
در بدنه اتومبیل ها ی متوسط حدود ۴۰ متر مربع ورق فولادی با ضخامتهای ۴/۰ تا ۲/۱ میلیمتر به کار میرودتا بتواند تنش های مختلف را به خوبی تحمل کند.
۵.گروه هدایت و فرمان:
نیروی متوسطی که لازم است تا بتواند اتومبیلی را در یک پیچ معمولی هدایت کند حبین ۵ تا ۱۰ کیلوگرم است ولی در سیستم های جدید فرما این نیرو به حدود ۳۰ گرم تقلیل می دهند.
۶.گروه مدارات الکتریکی:
از باطری های ۶ ۱۲ و ۲۴ ولتی برای راه اندازی و روشن کردن موتور استفاده میشود.سیستم جرقه زنی ولتاژ باطری را تا ۳۰۰۰۰ ولت افزایش داده و برای جرقه زنی موتور آماده میکند
هدف از به کار گیری کلاچ
سیستم انتقال توان خودرو توان موتور را به چرخهای محرک میرساند.در خودرو هایی که جعبه دنده ی دستی یا جهبه دنده های دیفرانسیل سر خود دستی دارند توان از طریق کلاچ منتقل میشود این وسیلهاتصال بین موتور و جعبه دنده ی دستی یا جعبه دنده دیفرانسیل سر خود دستی را بر قرار می کند.
کلاچ معمولا با نیرو ی پای راننده کار می افتد بعضی از کلاچها به وسیله ای هیدرولیکی مجهزندتا راننده نا گزیر نشود نیروی زیادی به کلاچ وارد کند.برای خودکار کردن کلاچ میتوان از وسایل الکترونیکی مختلفی استفاده کرد
کلاچ بین فلایویل وجعبه دنده یا جعبه دنده ی دیفرانسیل سر خود نصب میشوند.کلاچ به حرکت پدال کلاچ به کار می افتد .وقتی راننده پدال کلاچ را فشار میدهد کلاچ از فلایویل جدا و یا به اصطلاح خلاص می شود. در این حالت توان از موتور به جعبه دنده دیفرانسیل سر خود منتقل نمی شود.وقتی راننده پدال کلاچ را رها میکندکلاچ درگیر میشود و توان از موتور به جعبه دنده منتقل میشود
Get into Gear: A Practical Guide to Gearbox Selection
.Industrial gearboxes are not all the same. Without the right one, performance, flexibility and reliability will inevitably suffer. But a bewildering choice of gearboxes faces the industrial specifier.
To meet the expanding demands of industrial applications, gearbox manufacturers are producing an increasingly wide range of gearbox designs. To further match market needs, these different designs produce an extraordinarily broad range of outputs, ranging from a gentle 2-3 Nm to a leviathan 500,000 Nm and more.
Their functions can vary enormously: from the smoothly silent lifting of the curtain on an operatic performance in Paris, to the continuous powerful driving of conveyors carrying millions of tonnes of products ranging from delicate foods and wine, to bulk grain, coal and steel in Australia.
The machine that mixes your child’s ice cream; which powers the lifts that take us to work; and which drive the baggage escalators which (sometimes) produce our travel luggage: all require very rugged, very reliable industrial gearboxes.
But whether almost imperceptibly turning an observatory high in the Andes Mountains, driving agitators in waste water plants, or outloading mountains of cargo with slewing spouts in Esperance, industrial gearbox performance must be tailored to an endless diversity of specifier demands.
It isn’t just as simple as using a gearbox to multiply an engine’s torque to produce the output required. Specifier needs may encompass continuous operation, stop-start operation, compact installation, continuously consistent load and sharply changing load. Operating environments may vary from extremely dirty to hygienically clean, from aggressive marine, to food grade pure.
So perhaps it is not surprising that the choices, at first glance, can puzzle even professional engineers, and bewilder the plant operator who wants to select the best for his or her installation.
Naturally, anyone contemplating ideal gearbox selection will consult their gearbox specifying and engineering specialist. But even relative laymen can profit from an understanding of the basic gearbox types available. The range of such gearboxes is expanding considerably, and what was appropriate a few years ago, may not be the optimum now.
Classifications
Gearboxes, or speed reducers, are often classified by the relative position of input and output shafts. This is the concept behind such terms as in-line, parallel shaft or right-angle gear units.
Within the right-angle drive group are worm gear units and bevel helical units. While traditionally identified by the type of gearing rather than by shaft arrangement, both are right-angle drives.
Different again are gearboxes of planetary construction, which achieve exceptionally compact construction by departing from the traditional arrangement of a pinion driving one large gear on a parallel shaft. Instead, the planetary gearbox surrounds the pinion (called a “sun gear”) with three or more smaller planetary gears mounted in a planet carrier.
For each specific application, a design engineer may identify two or more product families that meet calculated torque and speed requirements. As a result, a design engineer will need to evaluate different factors in order to establish which among compatible configurations best suits the specific application and which proves best value for money.
Besides a variety of technical considerations, a significant issue can be space availability. The growing demand for more and more compact machines is placing increasing emphasis on space efficiency of motion control systems. This places increased emphasis on the importance of proper specification, installation, ventilation/cooling and maintenance.
Where no limiting factors over-rule technical ideals, design engineers are free to make the most of the features and advantages offered by each product line. A brief outline of such features follows.
Worm gear units
The worm design allows for very high transmission ratios (of up to i = 100 per single stage). This translates into greater cost-effectiveness. Also, worm reducers typically ensure quiet, vibration-free operation. They are inherently a right-angle drive.
Their typically low efficiency (from, say, 90 per cent down to even 35 per cent, depending on ratio, but typically 65-80 percent) suggests their use for low or middle-to-low power demand applications and/or those featuring intermittent duty. They can be ideal for applications that need to resist reversing, such as an inclined conveyor or hoist, but only for ratios of about 70-80 or higher.
In-line helical gear units
This style derives from the traditional form of pinion and gear drive and is characterised by high torque density (ie, transmitted torque per unit of volume) and high efficiency: 97-98 per cent per machine stage.
These efficient drives offer a natural extension of the electric motor. The load can be driven directly by the parallel output shaft or through an ancillary transmission (belt, chain, or gear type). Advantages generally include wide-span bearing support for the output shaft, which ensures good overhung load capacity and longer-term operational reliability.
Typically available in a wide range of speeds, these reducers generally offer reduction ratios in the range of 3 to 500, with the higher ratios being achieved by use of multiple stages in the gearbox. Ratios outside this range are possible, but less common in ordinary applications.
Major manufacturers offer various options for ease of mounting and/or enhanced space efficiency, including foot or flange mounting configurations, as well as combinations with compact or integrated motors.
Right-angle helical gear units
In this configuration, input and output shafts are arranged at right angles via a gear set with either intersecting (bevel helical) or non-intersecting (hypoid) axes. The right-angle helical design ensures great space efficiency in terms of width, and provides the primary alternative to worm reducers in applications involving right angle drives.
They once again are characterised by high efficiency and can extend to extremely high reduction ratios (even to 1700:1). The bevel set or hypoid set provide a significant ratio reduction in themselves. The hypoid arrangement used in some designs has an added advantage of being quieter and smoother running. Typically, these drives are a preferred choice where a right-angle drive requires high efficiency. This may be specified for applications involving continuous duty or large kW demand.
Right angle helical gear reducers come in a wide range of versions. Of particular interest in recent times is the shaft mounting type with hollow output shaft, with or without shrink disc or with tapering lock. In this configuration, the gearmotor is fitted directly onto the shaft of the driven machine, resulting in enhanced space efficiency, ease of mounting and avoidance of alignment issues.
Shaft-mounted gear units
These can be parallel shaft, helical or right-angle helical, planetary or worm gear units. They are frequently used in conveyor belts. Advantages of the shaft-mount style that make it the ideal selection for many applications are features such as:
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Simple, neat configuration
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Ease of installation
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Avoidance of the complexities of shaft alignment and of costly machining of mating surfaces
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Space saving
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Reduced angular backlash where shrink disc versions are used (because the keyway and its consequent basklash contribution are omitted)
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Torque arms that can be combined with torque-limiting devices, such as load cells
In the configuration featuring a solid input shaft, typically driven by a primary belt-and-pulley transmission, final speed can be adjusted (within limits) simply by changing either of the pulleys so as to modify the transmission ratio.
Parallel shaft gear units
In all manufacturers’ ranges, these reducers represent generally the heavy duty option, to cater for installed power ranging from a few kW to hundreds of kW and more.
Comprising helical gear sets, they offer high efficiency. These reducers can feature sturdy bearings, frequently the straight or taper roller type. Such bearings are suitable to withstand the high radial and thrust loading and impact loading typically encountered in many heavy industrial uses. Applications include:
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Wood, stone and ore crushers
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Extruders for plastic materials
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Bucket elevators, conveyor belts
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Dies and winding machines
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Fans and compressors
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Mixers, stirrers
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Many mining applications
Parallel shaft units can have a high number of reduction stages and can range in weight from fewer than 10kg to many tonnes. They generally have a male output shaft. The very large units are fixed in place and form a significant part of the whole machine.
Small-to-medium parallel shaft units with hollow output shaft are particularly favoured for shaft-mounted screw conveyor drives. Their geometry fits well with the geometry of the screw conveyor, providing a neat and compact drive arrangement.
Parallel shaft units are also frequently used in the shaft-mounted arrangement on belt conveyors. Once again, they provide a neat, compact and efficient arrangement.
Planetary gearboxes
The compact nature of planetary gearboxes is making them increasingly popular in the industry. Their arrangement of several smaller planetary gears around the input pinion (instead of one larger gear running to one side) offers distinct advantages for certain applications:
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High efficiency. The spur gears used in planetary gearboxes are inherently of high efficiency (97-98 percent per stage)
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Space saving. The planetary arrangement facilitates multiple reduction stages in a very compact space. These can be achieved because each reduction stage adds only a small increase in dimension. Compared with a parallel shaft arrangement, a planetary gearbox can often achieve the same ratio with one fewer reduction stage, with cost and dimension savings.
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High torque capacity. Because the torque being transmitted at any time is shared between multiple sets of teeth on the primary drive pinion, torque capability is greatly increased.
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Increased reliability - and higher radial loads permissible on the gearbox’s output shaft. Reliability and radial load capacity benefit because the shaft itself is not carrying any radial loads induced by the gears themselves (unlike parallel shaft systems). Benefits of reduced radial loads include extended bearing life, a critical component of machinery reliability. Reduced radial loads within the gearbox also permit it to tolerate higher radial loads from the equipment it is driving.
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Suitability for shaft mounting. Benefits of shaft mounting, as discussed previously, include elimination of the cost and complexity of couplings and elimination of the time and labour involved in ensuring correct alignment of the gearbox and the plant it is driving. Direct drive through shaft mounting also avoids radial loads imposed by chaindrives.
Planetary gearboxes are available in a wide range of sizes and can be modular in construction. Heavy duty industrial uses include water treatment agitators, crane slew drives, winches, conveyors, and equipment for the mining, quarrying and steel industries.
Planetary drives are particularly well suited to applications involving high torque and high reduction ratios. On the other hand, their compact size gives rise to reduced surface area. This can result in limitations due to the need for heat dissipation for applications involving high kW continuous operation. Such applications may, for example, require auxiliary cooling arrangements.
They are an excellent option for slew or winch drives, where intermittent high torque is required, or for large agitators, where low kW, high torque is required.
And now, what size?
Once the gearbox configuration that best suits the application has been identified, the design engineer will move on to selection of the proper size of gear unit and motor.
This is not as simple as it might sound. For example, it is important to select a properly rated gear box with adequate allowance for service factors, to take account of the number of starts, impact loading and the like. Then the specifier must select an electric motor that is powerful enough for the job, but not too powerful. Excessively large motors place unnecessary stress on all drive components (not to mention a large motor’s additional cost) and will require torque limiting, while an under-specified motor is obviously likely to fail prematurely.
Siting, installation, orientation, shaft loading, lubrication, ventilation, commissioning and maintenance are all engineering categories in their own right, which deserve the same attention as gearbox selection to achieve the optimum result.
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