Are you madabout kit cars

   
 "We've Got Kit Cars Covered" Information about Madabout-Kitcars.com Contact Madabout-Kitcars.com         Home of UK kit cars - madabout-kitcars.com Various kit car write ups All the latest kit car news Kit car related and general discussion
Search Madabout
Manufacturers
Kit Cars
Kit Car Data sheets
Picture Gallery
SVA Knowledgebase
Clubs & Communities
Build cost estimator
Kit cars for sale
Knowledge Base

KitcarUSA.com
Classic-Kitcars.com
 

CATEGORIES (articles) > Engine, Gearbox > Technical > Gearbox synchromesh, a detailed look

Gearbox synchromesh, a detailed look


Transmission types
Manual

Automatic

  • Tiptronic

Semi-automatic

  • Direct-Shift Gearbox
  • Twin-clutch Gearbox

Continuously-variable
Derailleur gears
Hub gears

A manual transmission (also known as a stick shift or standard transmission) is a type of transmission used in automotive applications. Manual transmissions often feature a driver-operated clutch and a movable gear selector, although some do not. Most automobile manual transmissions allow the driver to select any gear at any time, but some, such as those commonly mounted on motorcycles and some types of race cars, only allow the driver to select the next-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential manual transmission.

Manual transmissions are characterized by gear ratios which are selectable by engaging pairs of gears inside the transmission. Conversely, automatic transmissions feature clutch packs to select gear ratio. Transmissions which employ clutch packs but allow the driver to manually select the current gear are called semi-automatic transmissions.

Contemporary automotive manual transmissions are generally available with between 4 and 6 forward gears and one reverse gear, although manual transmissions have been built with as few as 2 and as many as 7 gears. Some manuals are referred to by the number of forward gears they offer (e.g., 5-speed) as a way of distinguishing between automatic or other available manual transmissions. In contrast, a 5-speed automatic transmission is referred to as a 5-speed automatic.

Other types of transmission in mainstream automotive use are the automatic transmission, semi-automatic transmission, and the continuously variable transmission.

Manual transmissions come in two basic types: simple unsynchronized systems where gears are spinning freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging "clashing" and "grinding" when trying to mesh the rotating teeth, and synchronized systems that eliminate this necessity while changing gears.

Unsynchronized transmission

The earliest automotive transmissions were entirely mechanical unsynchronized gearing systems. They could be shifted, with multiple gear ratios available to the operator, and even had reverse. But the gears were engaged by sliding mechanisms or simple clutches, which required skills of timing and careful throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged; otherwise the teeth would refuse to mesh.

When upshifting, the speed of the gear driven by the engine had to drop to match the speed of the next gear; as this happened naturally when the clutch was depressed, it was just a matter of skill and experience to hear and feel when the gears managed to mesh. However, when downshifting, the gear driven by the engine had to be sped up to mesh with the output gear, requiring engagement of the clutch for the engine to speed up the gears. Double declutching, that is, shifting once to neutral to speed up the gears and again to the lower gear, is sometimes needed. In fact, such transmissions are often easier to shift from without using the clutch at all. The clutch, in these cases, is only used for starting from a standstill. This procedure is common in racing vehicles and most production motorcycles.

Even though automotive transmissions are now almost universally synchronised, heavy trucks and machinery as well as dedicated racing transmissions are still usually nonsynchromesh transmissions, known colloquially as "crashboxes", for several reasons. Being made of brass, synchronizers are prone to wear and breakage more than the actual gears, which are cast iron, and the rotation of all the sets of gears at once results in higher frictional losses. In addition, the process of shifting a synchromesh transmission is slower than that of shifting a nonsynchromesh transmission. For racing of production based transmissions, sometimes half the dogs on the synchros are removed to speed the shifting process, at the expense of much more wear.

Similarly, most modern motorcycles still utilise Unsynchronised transmissions. Synchronisers are generally not necessary or desirable in motorcycle transmissions. The low gear inertias and higher strengths mean that 'forcing' the gears to alter speed is not damaging, and the selector method on modern motorcycles (pedal operated) is not conducive to having the long shift time of a synchronised gearbox. Because of this, it is still necessary to synchronise gear speeds by 'blipping-the-throttle' when shifting into a lower gear on a motorcycle.

Synchronized transmission

A modern gearbox is of the constant mesh type, in which all gears are always in mesh but only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, the others being allowed to rotate freely; thus greatly reducing the skill required to shift gears.

Most modern cars are fitted with a synchronised gear box, although it is entirely possible to construct a constant mesh gearbox without synchromesh, as found in motorcycle for example. In a synchromesh gearbox, the teeth of the gears of all the transmission speeds are always in mesh and rotating, but the gears are not directly rotationally connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual gear consists of a collar on the shaft which is able to slide sideways so that teeth or "dogs" on its inner surface bridge two circular rings with teeth on their outer circumference; one attached to the gear, one to the shaft. (One collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other) When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and determines the output speed of the transmission. To correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped brass clutch which is attached to the gear, which brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or balk rings, the latter being spelled "baulk" in the UK). The gearshift lever manipulates the collars using a set of linkages, so arranged so that only one collar may be permitted to lock only one gear at any one time; when "shifting gears", the locking collar from one gear is disengaged and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.

The first synchronized transmission system was introduced by Cadillac in 1929. The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone synchronizers were called "Porsche-type" for many years after this. In the early 1950s only the second-third shift was synchromesh in most cars, requiring only a single synchro and a simple linkage; drivers' manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again. With continuing sophistication of mechanical development, however, fully synchromesh transmissions with three speeds, then four speeds, five speeds, six speeds and so on became universal by the 1960s. Reverse gear, however, is not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears in reverse is not required.

Internals

Like other transmissions, a manual transmission has both input and output shafts. Pairs of gears are attached to these shafts such that, when selected, will cause the output shaft to rotate at a given ratio of the input shaft speed. When a driver selects a gear, he is simply selecting a pair of these gears to be used; mechanical connections translate the driver's selection into an appropriate connection of gears and prevent more than one set of gears being engaged at any given time (as that would cause the transmission to lock). The teeth on gears of mass market automobiles are not straight-cut, but are helically cut, in order to reduce gear whine. Reverse gear often is straight-cut, however, leading to a characteristic whine from many cars when reversing.

In racing vehicles (most commonly those involved in drag racing), sometimes a trans-brake is incorporated, allowing the driver to lock the transmission into both first gear and reverse gear at the same time. This serves the purpose of allowing the driver to increase the engine speed without changing the vehicle's speed (much as one would do while in neutral, or while the clutch is disengaged), but being able to transfer as much of the resultant power to the tires in a shorter period of time.

The input shaft of a manual transmission comes from the clutch, and is connected to a layshaft. The lay shaft has one gear on its input end and several on the output end, usually one per selectable gear. The output gears of the layshaft connect to the drive gears. These are fixed in place on the output shaft, which leads to the differential and tires.

Manual transmissions are often equipped with 4, 5, or 6 forward gears. Nearly all have exactly one reverse gear. In three or four speed transmissions, in most cases, the topmost gear is "direct", i.e. a 1:1 ratio. For five speed or higher transmissions, the highest gear is usually an overdrive gear, with a ratio of less than 1:1. Older cars were generally equipped with 3-speed transmissions, or 4-speed transmissions for high performance models and 5-speeds for the most sophisticated of automobiles; in the 1970s, 5-speed transmissions began to appear in low priced mass market automobiles and even compact pickup trucks, pioneered by Toyota (who advertised the fact by giving each model the suffix SR5 as it acquired the fifth speed). Today, mass market automotive manual transmissions are essentially all 5-speeds, with 6-speed transmissions beginning to emerge in high performance vehicles in the early 1990s, and recently beginning to be offered on some high-efficiency and conventional passenger cars.

On earlier models with three or four forward speeds, the lack of an overdrive ratio for relaxed and fuel efficient highway cruising was often filled by incorporation of a separate overdrive unit in the rear housing of the transmission, separately actuated by a knob or button, often incorporated into the gearshift knob.

Clutch

In all vehicles utilizing a transmission (virtually all modern vehicles), a coupling device is utilized to be able to separate the engine and transmission when necessary. The clutch is what accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and anytime the vehicle is at a stop, so would be the engine. In a car the clutch is usually used by a pedal; on a motorcycle, a lever on the left handlebar serves the purpose.

  • When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque is transferred from the engine to the transmission, and by extension to the drive wheels. This allows for the transmission's gears to be independent of the engine (spinning purely through momentum or, for any engaged gear, the motion of the vehicle). This allows for shifting without gear grinding.
  • When the clutch pedal is fully released, the clutch is fully engaged, and essentially all of the engine's torque is transferred.
  • In between these extremes, the clutch "slips" to varying degrees. Clutch slippage is useful, because the entire purpose the transmission serves is gear reduction. Because the engine and tires are designed to be linked in order to drive, one must dictate the speed of the other. If there was no slippage, the tires would dictate engine speed, and as such, getting a vehicle to move from rest would be extremely difficult. This slippage allows for the slow introduction of power, with less resistance introduced to the engine until enough momentum is built that the engine can operate normally without output reduction from the clutch.
  • Note: Automatic transmissions also utilize a coupling device, however, a clutch is not present. In these kinds of vehicles, the torque converter is used to separate the engine and transmission.

Gear selection

Floor-mounted shifter

In most modern cars, gears are selected through a lever attached to the floor of the automobile—this selector is often called a gearstick, gear lever, gear selector, or simply shifter. Moving this lever forward, backward, left, and right allows the driver to select any given gear. In this configuration, the gear lever must be pushed laterally before it is pushed longitudinally.

A common layout for a 5-speed transmission is shown below. N marks neutral, or the position where no gears are engaged. In reality, the entire horizontal line is a neutral position, although the shifter is usually equipped with springs so that it will return to the N position if not left in another gear. The R denotes reverse, which is technically a sixth gear on this transmission. There is usually a mechanism that only allows selection of reverse from the neutral position, so reverse will be less likely to be accidentally chosen when downshifting from 5th to 4th (or by someone used to a 6-speed transmission and trying to shift from 5th to the non-existent 6th).

Image:Manual Layout.PNG

This layout is called the shift pattern. The shift pattern for a specific transmission is usually printed on the shifter knob.

Another common five-speed shift pattern is:

Image:Manual Layout 2.PNG

Transmissions equipped with this shift pattern usually feature a lockout mechanism that requires the driver to depress a switch or the entire gear lever when entering reverse, so that he does not accidentally select it when trying to find first gear.

Most front-engined, rear-wheel drive cars have a transmission that sits between the driver and the front passenger seat. Floor-mounted shifters are often connected directly to the transmission. Front-wheel drive and rear-engined cars often require a mechanical linkage to connect the shifter to the transmission.

A 4-speed floor shifter is sometimes referred to as "Four on the Floor".

Column-mounted shifter

Some older cars feature a gear lever which is mounted on the steering column of the car. Many automatic transmissions still use this placement, but manual column shifters are no longer common.

Column shifters are mechanically similar to floor shifters, although shifting occurs in a vertical plane instead of a horizontal one. Column shifters also generally involve additional linkages to connect the shifter with the transmission.

Image:Manual Layout 3.PNG

The 3-speed shift pattern is typical of American cars, trucks, and vans produced with manual transmissions until the 1950s and 1960s. This pattern is not "intuitive", as the shifter has to be moved forward (into R) to make the car go backward (and vice-versa).

First gear in a 3-speed is often called "low," while third is usually called "high." There is, of course, no overdrive.

A 3-speed column shifter is sometimes referred to as "Three on a Tree".

Note that reverse in a car with a column shift is in nearly the same position as park (P) is on a car with a column-mounted gear selector with an automatic transmission.

Sequential manual

Some transmissions do not allow the driver to arbitrarily select any gear. Instead, the driver may only ever select the next-lowest or next-highest gear ratio. These transmissions often provide clutch control, but the clutch is only necessary when selecting first or reverse gear from neutral. Most gear changes can be performed without the clutch.

Sequential transmissions are generally controlled by a forward-backward lever, foot pedal, or set of paddles mounted behind the steering wheel. In some cases, these are connected mechanically to the transmission. In many modern examples, these controls are attached to sensors which instruct a transmission computer to perform a shift—many of these systems can be switched into an automatic mode, where the computer controls the timing of shifts, much like an automatic transmission.

Motorcycles typically employ sequential transmissions, although the shift pattern is modified slightly for safety reasons. In a motorcycle the gears are usually shifted with the left foot pedal, the layout being this:

   5
  4┘
 3┘
2┘
N
1

The pedal goes one step - both up and down - from the center, before it reaches its limit and has to be allowed to move back to the center position. Thus, changing multiple gears into one direction is accomplished by repeatedly pumping the pedal, either up, or down. Although neutral is listed as being between first and second gears for this type of transmission, it "feels" more like first and second gear are just "further away" from each other than any other two sequential gears. For inexperienced riders, this can lead to difficulty in finding neutral. The reason neutral does not actually have its own spot in the sequence is to make it quicker to shift from first to second when moving. You will not accidentally shift into neutral. The reason for having neutral between the first and second gears instead of at the bottom is that when stopped, the rider can just click down repeatedly and know that they will end up in first and not neutral.

Semi-manual

Some very new transmissions (BMW's Sequential Manual Gearbox (SMG) and Audi's Direct-Shift Gearbox (DSG), for example) are conventional manual transmissions with a computerized control mechanism. These transmissions feature independently selectable gears but do not have a clutch pedal. Instead, the transmission computer controls a servo which disengages the clutch when necessary.

These transmissions vary from sequential transmissions in that they still allow nonsequential shifts: BMWs SMG system, for example, can shift from 6th gear directly to 4th gear when decelerating from high speeds.

Comparison with automatic transmissions

Manual transmissions are typically compared to automatic transmissions, as the two represent the majority of options available to the typical consumer. These comparisons are general guidelines and may not apply in certain circumstances. Additionally, the recent popularity of semi-manual and semi-automatic transmissions renders many of these points obsolete. It should be kept in mind that many some of these points are true of "conventional" automatic transmissions which shift gears and are coupled to the engine with a torque converter but are not a true comparison or do not apply to other kinds of automatic transmissions, like the continuously-variable transmission.

Advantages

  • Manual transmissions are typically more efficient than automatic transmissions. This is because manuals generally involve a clutch instead of a torque converter, which can cause significant power losses and because an automatic transmission introduces parasitic losses through the high pressure hydraulic pumps it requires. This results in both better acceleration and fuel economy.
  • It is generally easier to build a very strong manual transmission than a very strong automatic transmission. Manual transmissions usually have only one clutch, whereas automatics have many clutch packs.
  • Manual transmissions are generally significantly lighter than torque-converter automatics.
  • Manual transmissions are typically cheaper to build than automatic transmissions.
  • Manual transmissions generally require less maintenance than automatic transmissions.
  • Manual transmissions normally do not require active cooling, because not much power is dissipated as heat through the transmission.
    • The heat issue can be important in certain situations, like climbing long hills in hot weather, particularly if pulling a load. Unless the automatic's torque converter is locked up (which typically only happens in an overdrive gear that would not be engaged when going up a hill) the transmission can overheat. A manual transmission's clutch only generates heat when it slips, which does not happen unless the driver is riding the clutch pedal.
  • A driver has more direct control over the state of the transmission with a manual than an automatic. This control is important to an experienced, knowledgeable driver who knows the correct procedure for executing a driving manoeuver, and wants the machine to obey his or her instructions exactly and instantly.
    • An example: the driver, anticipating a turn, can downshift to the appropriate gear while the steering is still straight, and stay in gear through the turn. This is the correct, safe way to execute a turn.
    • Another example: when starting, the driver can control how much torque goes to the tires, which is useful for starting on slippery surfaces such as ice, snow or mud. This can be done with clutch finesse, or possibly by starting in second gear instead of first. The driver of an automatic can only put the car into drive, and play with the throttle. The torque converter can easily dump too much torque into the wheels, because when it slips, it acts as an extra low gear, passing through the engine power, reducing the rotations while multiplying torque.
    • Yet another example: passing. When the driver is attempting to pass a slower moving vehicle by making use of a lane with opposite traffic, he can select a lower gear for more power at exactly the right moment when conditions are right to begin the manoeuver. Automatics have a delayed reaction time, because the driver can only indicate his intent by pressing the throttle. The skilled manual transmission driver has an advantage of superior finesse and confidence in such situations.
  • Many people prefer driving a car with a manual transmission because it provides a more direct interaction in the driving experience
  • Many people feel that driving a manual forces the driver to pay more attention to the road and to other cars, making it more difficult to become distracted.
    • A related point is that the driver of a manual transmission car can develop an accurate intuition for how fast the car is traveling, from the sound of the motor and the gear selection. It's easier to observe the lower speed limits like 30 km/h and 50 km/h without glancing at the instrumentation.
  • Cars with manual transmissions can be started when the battery is dead by pushing the car into motion (or allowing it to roll down a hill) and then engaging the clutch in third or second gear.
    • Caveat 1: if the battery is too drained, there is the risk that the spark plugs won't fire during the first few strokes of the engine, and consequently that unburned fuel-air mixture will escape into the exhaust system, where it can later ignite, possibly damaging or destroying the catalytic converter. Consequently, push-starting should be regarded as an extreme emergency measure. However, the choice is there, and so this can be regarded as an advantage of a manual transmission automobile in spite of the inherent risks.
    • Caveat 2: it can't be assumed that every car with a manual transmission can be started this way. Some modern cards have engine-management electronics which must be active in order for the engine to start. These components might not obtain sufficient power from the push.
  • Manual transmissions work regardless of the orientation angle of the car with respect to gravity. Automatic transmissions have a fluid reservoir (pan) at the bottom; if the car is tilted too much, the fluid pump can be starved, causing a failure in the hydraulics. This could matter in some extreme off roading circumstances.

Disadvantages

  • Manual transmissions require more driver interaction than automatic transmissions.
  • A driver may inadvertently shift into the wrong gear with a manual transmission, potentially causing damage to the engine and transmission as well as compromising safety.
  • Manual transmissions are more difficult to learn to drive as one needs to develop a feel for properly engaging the clutch.
  • The smooth and quick shifts of an automatic transmission are not guaranteed when operating a manual transmission.
  • Manual transmissions are slightly harder to start when stopped upward on a hill, but this is easily overcome with a little experience.

Applications and popularity

Many types of automobiles are equipped with manual transmissions. Small economy cars predominantly feature manual transmissions because they are relatively cheap and efficient, although many are optionally equipped with automatics. Economy cars are also often powered by very small engines, and automatic transmissions can make them comparatively very slow.

Sports cars are also often equipped with manual transmissions because they offer more direct driver involvement and better performance. Off-road vehicles and trucks often feature manual transmissions because they allow direct gear selection and are often more rugged than their automatic counterparts.

Very heavy trucks also feature manual transmissions because they are efficient and, more importantly, can withstand the severe stress encountered in hauling heavy loads.

Conversely, manual transmissions are no longer popular in many classes of cars sold in North America, although they remain dominant in Europe. Nearly all cars are available with an automatic transmission option, and family cars and large trucks sold in the US are predominantly fitted with automatics. In Europe and Asia most cars are sold with manual transmissions. Most luxury cars are only available with an automatic transmission. In situations where automatics and manual transmissions are sold side-by-side, the manual transmission is the base equipment, and the automatic is optional—although the automatic is sometimes available at no extra cost. Some cars, such as rental cars and taxis, are nearly universally equipped with automatic transmissions in the US.

Driving technique

See Manual transmission driving technique.

Maintenance

Because clutches use changes in friction to modulate the transfer of torque between engine and transmission, they are subject to wear in everyday use. A very good clutch, when used by an expert driver, can last hundreds of thousands of kilometres. Weak clutches, downshifting, inexperienced drivers, and aggressive driving can lead to more frequent repair or replacement.

Manual transmissions are lubricated with gear oil, which must be changed periodically in some cars, although not as frequently as the automatic transmission fluid in a vehicle so equipped. (Some manufacturers specify that changing the gear oil is never necessary except after transmission work or to rectify a leak.)

Gear oil has a characteristic aroma, due to the addition of molybdenum disulfide compounds, to lubricate the large degree of sliding friction seen by the teeth due to their helical cut, which in turn is done to eliminate the characteristic whine of straight cut gears. Some manufacturers, however, such as Honda, do not use this additive in their gear lube, specifying regular motor oil until recently, and now their own brand of gear lube which seems to be an enhanced version of motor oil. On motorcyles with "wet" clutches (clutch is bathed in engine oil), there is usually nothing separating the lower part of the engine from the transmission, so the same oil lubricates both the engine and transmission.




Related Articles

CATEGORIES (articles) > Engine, Gearbox > Technical > Gearbox synchromesh, a detailed look

 
Search for keyword     



This content from Wikipedia is licensed under the GNU Free Documentation License.


copyright madabout-kitcars.com 2000-2017
terms and conditions | privacy policy