Sometimes in a large flow of information (especially new) it is very difficult to find some important little things, to single out the "seeds of truth." In this short article I will talk about the gear ratios of gears and the drive in general. This topic is very close to the topics covered in ...
A drive is a motor and everything that is and works between the motor shaft and the shaft of the working body (couplings, gearboxes, various gears). What is "engine shaft" is understandable, I think, to almost everyone. What is the "shaft of the working body" is probably not clear to many. The shaft of the working body is the shaft on which the element of the machine is fixed, which is set in rotary motion by the entire drive with the required given torque and speed. This can be: wheel of a cart (car), drum of a belt conveyor, sprocket of a chain conveyor, winch drum, pump shaft, compressor shaft, and so on.
U Is the ratio of the engine speed nдв to the frequency of rotation of the shaft of the working body of the machine nro.
U = nдв / nro
General ratio drive U often, in practice, a sufficiently large number is obtained from the calculations (more than ten, or even more than fifty), and it is not always possible to perform it in one gear due to various restrictions, including power, strength and overall dimensions. Therefore, the drive is made consisting of several gears connected in series with their optimal gear ratios Ui... In this case, the total gear ratio U is found as the product of all gear ratios Ui included in the drive.
U = U1 * U2 * U3 *… Ui *… Un
Gear ratio Ui Is the ratio of the frequency of rotation of the input shaft of the transmission nin to the speed of the output shaft of this gear nout.
Ui = nin i / nouti
When choosing, it is desirable to give preference to values close to the beginning of the range, that is, to the minimum values.
The proposed table is just a recommendation and not a dogma! For example, if you assign a chain drive U= 1.5, then this will not be an error! Of course, everything has to be justified. And, perhaps, to reduce the cost of the entire drive, it is better to U= 1.5 "hide" inside the gear ratios of other gears, increasing them accordingly.
Various scientists have paid a lot of attention to optimization issues in the design of gear reducers. Dunaev P.F., Snesarev G.A., Kudryavtsev V.N., Niberg N.Ya., Niemann G., Wolf V. and other famous authors tried to achieve equal strength at the same time gear wheels, compactness of the gearbox as a whole, good conditions lubrication, reduction of losses due to oil splashing, uniform and high durability of all bearings, good stiffness of the shafts. Each of the authors, having proposed their own algorithm for dividing the gear ratio into gear stages, has not completely and unambiguously solved this controversial problem. Very interesting and detailed about this is written in the article at: http://www.prikladmeh.ru/lect19.htm.
I will add a little more ambiguity to the solution of this issue ... Let's look at one more table in Excel.
We set in the combined cell C4-7 the value of the total gear ratio of the gearbox U and read the calculation results in cells D4 ... D7 - Ub and in cells E4 ... E7 - UT performed for four variants of different conditions.
The values given in the table are calculated using the formulas:
1. In cell D4: = H4 * $ C $ 4 ^ 2 + I4 * $ C $ 4 + J4 =4,02 Ub = a * U ^ 2 + b * U + c
in cell E 4: = $ C $ 4 / D4 =3.91 UT = U / Ub
in cell H 4: a =-0,0016111374
in cell I 4: b =0,24831562
in cell J 4: c =0,51606736
2. In cell D5: = H5 * $ C $ 4 ^ 2 + I5 * $ C $ 4 + J5 =5.31 Ub = a * U ^ 2 + b * U + c
in cell E 5: = $ C $ 4 / D5 =2.96 UT = U / Ub
in cell H 5: a =-0,0018801488
in cell I 5: b =0,26847174
in cell J 5: c =1,5527345
3. In cell D6: = H6 * $ C $ 4 ^ 2 + I6 * $ C $ 4 + J6 =5.89 Ub = a * U ^ 2 + b * U + c
in cell E 6: = $ C $ 4 / D6 =2.67 UT = U / Ub
in cell H 6: a =-0,0018801488
in cell I 6: b =0,26847174
in cell J6: c =1,5527345
4. In cell D 7: = C4 / E7 =4.50 Ub = U / UT
in cell E 7: = 0.88 * C4 ^ 0.5 =3.49 UT =0,88* U ^0,5
In conclusion, I dare to recommend: do not design a single-stage spur gearbox with a gear ratio U> 6 ... 7, two-stage - with U> 35 ... 40, three-stage - with U>140…150.
On this, a short excursion into the topics "How to optimally" split "the gear ratio of the drive by stages?" and "How to choose a gear ratio?" completed.
Dear readers, subscribe to receive announcements of my blog articles. A window with a button is at the top of the page. If you don't like it, you can always unsubscribe.
There are three important characteristics camshaft design, they control the engine power curve: valve timing, valve opening time and valve lift. Further in the article we will tell you what the design of the camshafts and their drive is.
Valve lift is usually measured in millimeters and is the distance the valve will move farthest from the seat. The duration of the valve opening is a period of time, which is measured in degrees of crankshaft rotation.
The duration can be measured in various ways, but due to the maximum flow with a small valve lift, the duration is usually measured after the valve has already risen from the seat by some amount, often 0.6 or 1.3 mm. For example, a particular camshaft might have an opening time of 2,000 turns at a stroke of 1.33 mm. As a result, if you use the 1.33mm tappet lift as the stop and start point for valve lift, the camshaft will hold the valve open for 2,000 crankshaft rotations. If the duration of the valve opening is measured at zero lift (when it is just moving away from the seat or in it), then the duration of the crankshaft position will be 3100 or even more. The moment when a particular valve closes or opens is often referred to as the camshaft timing.
For example, the camshaft may act to open the intake valve at 350 before top dead center and close at 750 after bottom dead center.
Increasing valve lift distance can be beneficial in increasing motor power, as power can be added without significantly interfering with engine performance, especially at low revs... If you delve into the theory, then the answer to this question will be quite simple: such a camshaft design with a short valve opening time is needed in order to increase the maximum engine power. It will work theoretically. But, the actuator mechanisms in the valves are not so simple. In this case, the high speed of the valves, which are caused by these profiles, will significantly reduce the reliability of the engine.
When the opening speed of the valve increases, there is less time left for the valve to move from the closed position to full lift and return from the point of departure. If the travel time becomes even shorter, more force valve springs are needed. This often becomes mechanically impossible, let alone driving the valves at fairly low RPM.
As a result, what is a reliable and practical value for maximum valve lift?
Camshafts with a lift greater than 12.8 mm (minimum for a motor in which the drive is carried out using hoses) are in an impractical area for conventional motors. Camshafts with an intake stroke of less than 2900, combined with a valve lift of more than 12.8 mm, provide very high closing and opening speeds. This, of course, will create an additional load on the valve drive mechanism, which significantly reduces the reliability of: camshaft cams, valve guides, valve stems, valve springs. However, a shaft with a high valve lift rate can work very well in the beginning, but the service life of the valve guides and bushings will most likely not exceed 22,000 km. It is good that most camshaft manufacturers design their parts in such a way that they provide a compromise between valve opening times and lift values, while ensuring reliability and long service life.
The timing of the intake and the valve lift discussed are not the only design elements of the camshaft that affect the final power of the engine. The moments, closing and opening of valves in relation to the position of the camshaft, are also such important parameters for optimizing the performance of the engine. You can find these camshaft timing in the datasheet that comes with any quality camshaft. This datasheet graphically and numerically illustrates the angular positions of the camshaft when the exhaust and intake valves are closed and opened.
They will be accurately measured in degrees of crankshaft rotation before TDC or TDC.
The cam angle is the offset angle between the cam center line of the exhaust valve (called the exhaust cam) and the cam center line of the intake valve (called the intake cam).
Cylinder angle is often measured in "camshaft angles" because we are discussing the offset of the cams relative to each other, this is one of the few times when the characteristic of the camshaft is indicated in degrees of rotation of the shaft, and not in degrees of rotation of the crankshaft. The exception is those engines where, two camshafts in the cylinder head (cylinder head).
The angle chosen in the design of the camshafts and their drive will directly affect the valve overlap, that is, the period when the exhaust and intake valves are simultaneously open. Valve overlap is often measured by SB crankshaft angles. When the angle between the centers of the cams decreases, the intake valve opens and the exhaust valve closes. It should always be remembered that valve overlap is also influenced by a change in the opening time: if the opening time is increased, the overlap of the valves will also become large, while ensuring that there are no angle changes to compensate for these increases.
Good day, dear motorists! Let us together try to put on the shelves, in the literal sense of the word, the device of one of the important components of the gas distribution mechanism (timing) of the engine - the camshaft.
Camshaft device
The camshaft performs far from the last function in the operation of a car engine - it synchronizes the intake and exhaust of engine strokes.
Depending on the type of engine, the timing can be with a lower valve arrangement () or an upper valve arrangement (c).
In modern engine building, preference is given to the upper timing belt. This allows you to simplify the process of maintenance, adjustment and, thanks to ease of access to the timing parts.
Structurally, the camshaft is connected to the engine crankshaft. This connection is made by means of a belt or chain. The camshaft belt or chain is fitted to the camshaft pulley and crankshaft sprocket. The camshaft is driven by crankshaft.
The most effective is the camshaft pulley, which is used to increase the power characteristics of the engine.
On the cylinder head there are bearings in which the camshaft journal journals rotate. In the case of repairs, camshaft repair liners are used to secure the bearing journals.
Camshaft end play is prevented by camshaft clips. A through hole is made along the camshaft axis. Through it, the rubbing surfaces of the parts are lubricated. On the rear side, this hole is covered by the camshaft plug.
Camshaft cams- the most important component... Their number corresponds to the number of intake and exhaust valves in the engine. It is the cams that perform the main purpose of the camshaft - adjusting the valve timing of the engine and.
Each valve has its own, individual cam, which opens it, "running" on the pusher. When the cam leaves the tappet, a powerful return spring closes the valve.
The camshaft cams are located between the bearing journals. Two cams: intake and exhaust for each cylinder. In addition, a gear is attached to the shaft to drive the breaker-distributor and the oil pump. Plus an eccentric to drive the fuel pump.
The gas distribution phase of the camshaft is selected empirically, and depends on the design of the intake and exhaust valves and the engine speed. Manufacturers for each engine model indicate the camshaft phases in the form of diagrams or tables.
A camshaft cover is installed on the camshaft supports. Front camshaft cover - common. It has thrust flanges that enter the grooves in the camshaft journals.
The main parts of the timing
- Valves: inlet and outlet. The valve consists of a stem and a poppet. The valve seats are plug-in for easy replacement. The intake valve head is larger in diameter than the exhaust valve head.
- Rocker serves to transfer force to the valve from the rod. There is a screw in the short arm of the rocker for adjusting the thermal gap.
- Barbell designed to transfer force from the pusher to the rocker arm. One end of the bar rests against the pusher, and the other against the rocker arm adjusting bolt.
How the camshaft works
The camshaft is located in the camber of the cylinder block. The camshaft is driven by a gear or chain drive from the crankshaft.
The rotation of the camshaft causes the cams to act on the operation of the intake and exhaust valves. This happens in strict accordance with the valve timing and the order of the engine cylinders.
For correct installation valve timing there are timing marks located on the timing gears or on the drive pulley. For the same purpose, the crankshaft cranks and camshaft cams must be in a strictly defined position relative to each other.
Thanks to the installation made according to the marks, the sequence of the alternation of strokes is observed - the order of operation of the engine cylinders. The order of operation of the cylinders depends on their location and design features crankshaft and camshaft.
Engine duty cycle
The period when the intake and exhaust valves in each cylinder must open once is the engine's duty cycle. It takes 2 crankshaft revolutions. At this time, the camshaft should make one revolution. It is for this that the camshaft gear has twice as many teeth.
Number of camshafts in the engine
This value usually depends on. Engines with in-line configuration and one pair of valves per cylinder have one camshaft. If there are 4 valves per cylinder, then two camshafts.
Boxer and V-shaped engines have one camshaft in the camber, or two, one camshaft in each cylinder head. There are also exceptions due to the design features of the engine model. (for example, an in-line arrangement of four cylinders - one camshaft with 4 valves per cylinder, like the Mitsubishi Lancer 4G18).
Automotive expert. Graduated from ISTU named after M.T. Kalashnikov with a degree in Operation of Transport and Technological Machines and Complexes. More than 10 years of professional car repair experience.
Modern engines rarely have one camshaft, most often there are two, which ensures quieter engine operation, increased efficiency and increased power due to more valves (the intake-exhaust cycle is accelerated). One camshaft is responsible for the intake and the other exhaust valves. For more powerful cars with V-shaped engines, four camshafts are used due to design features power plant... The single camshaft timing gear is called Single OverHead Camshaft (SOCH), the double shaft system is called Double OverHead Camshaft (DOCH). At correct operation camshafts rarely fail, their main malfunction is natural wear of rubbing parts or deformation of the unit due to cracks. Wear is significantly accelerated in the following cases:
- low oil pressure (insufficient level);
- getting antifreeze or fuel into the oil;
- burnout valves or malfunctioning hydraulic lifters;
- violation of the valve timing.
Good luck in learning how your car's engine works.
The valve timing mechanism, abbreviated as timing, is something without which a four-stroke engine, in principle, cannot exist. It opens the intake valves, allowing air or a combustible mixture into the cylinders on the intake stroke, opens the exhaust valves on the exhaust stroke, and reliably locks the mixture burning in the cylinder during the working stroke. The power and environmental friendliness of the engine depend on how well it provides the "breathing" of the engine - air supply and exhaust gas release.
The valves open and close the camshafts with their cams, and the torque is transmitted to them from the crankshaft, which, in fact, is the task of the timing drive. Today, a chain or belt is used for this. But it was not always so…
Good old lower camshaft
At the beginning of the twentieth century, there were no problems with the camshaft drives - it was spun by ordinary gears, and pusher rods went to the valves from it. The valves were then located on the side, in the "pocket" of the combustion chamber, directly above the camshaft, and were opened and closed by rods. Then the valves began to be placed one opposite the other in order to reduce the volume and surface area of this "pocket" - as a result of the non-optimal shape of the combustion chamber, the motors had an increased tendency to detonation and poor thermal efficiency: a lot of heat went into the walls of the cylinder head. Finally, the valves were moved to the area just above the piston, and the combustion chamber became quite small and almost regular in shape.
The arrangement of the valves on top of the combustion chamber and the valve drive with longer tappets (the so-called OHV scheme), proposed at the beginning of the twentieth century by David Buick, turned out to be the most convenient. This design supplanted side-valve variants in racing designs as early as 1920. For example, it is she who is used in the famous Chrysler Hemi engines and Corvette engines in our time. And motors with side valves can be remembered by the drivers of GAZ-52 or GAZ-M-20 "Pobeda", where this scheme was used in engines.
And it was all so convenient! The construction is very simple. The camshaft, remaining at the bottom, is located in the cylinder block, where it is perfectly lubricated by splashing oil! Even rocker rods and cams with shims can be left outside if needed. But progress did not stand still.
Why did you abandon barbells?
The problem is being overweight. In the 30s, the speed of rotation of racing motors on the ground and aircraft motors on airplanes reached values at which it became necessary to facilitate the gas distribution mechanism. After all, each gram of valve mass forces to increase both the force of the springs that close it, and the strength of the pushers through which the camshaft presses on the valve, as a result of the loss on the timing drive, it quickly increases with increasing engine speed.
The solution was found in moving the camshaft up to the cylinder head, which made it possible to abandon a simple but heavy system with pushers and significantly reduce inertial losses. The operating speed of the engine has risen, which means that the power has also increased. For example, Robert Peugeot created a racing engine with four valves per cylinder and two overhead camshafts in 1912. With the transfer of the camshafts up to the head of the block, the problem of their drive arose.
The first solution was to introduce intermediate gears. There was, say, a variant with a drive with an additional shaft with bevel gears, as, for example, the familiar B2 engine and its derivatives on all tankers. This scheme was also used on the already mentioned Peugeot engine, the Curtiss K12 aircraft engines of the 1916 model and the Hispano-Suiza of 1915.
Another option was the installation of several cylindrical gears, for example, in the engines of Formula 1 cars from the period of the 60s. Surprisingly, the "multi-gear" technology has been used quite recently. For example, on several versions of diesel 2.5-liter Volkswagen motors on the Transporter T5 and Touareg - AXD, AX and BLJ.
Why did the chain come?
The gear drive had many "inherent" problems, the main one being noise. In addition, the gears required precise shaft alignment, calculation of clearances and mutual hardness of materials, as well as torsional vibration damping couplings. In general, the design, despite its apparent simplicity, was tricky, and the gears were by no means "eternal". Something else was needed.
It is not known exactly when the timing chain was first used. But one of the first mass-produced designs was the chain-driven AJS 350 motorcycle engine in 1927. The design turned out to be successful: the chain was not only quieter and simpler in design than the shaft system, but also reduced the transmission of harmful torsional vibrations due to the operation of its tension system.
Oddly enough, the chain did not find application in aircraft motors, and appeared in automobiles much later. At first, it appeared in the drive of the lower camshaft instead of bulky gears, but gradually began to gain popularity in drives with overhead camshafts, but it became especially relevant when motors with two camshafts appeared. For example, the timing chain was used in the 1948 Ferrari 166 and later versions of the Ferrari 250, although earlier versions were driven by bevel gears.
In mass motors, the need for a chain drive did not arise for a long time - until the 80s. Low-power engines were produced with a lower camshaft, and this is not only the Volga, but also the Skoda Felicia, Ford Escort 1.3 and many American cars - the pusher rods were on V-shaped motors to the last. But on highly accelerated motors from European manufacturers, chains appeared already in the 50s and until the end of the 80s remained the predominant type of timing drive.
How did the belt come about?
Around the same time, the chain had a dangerous competitor. It was in the 60s that the development of technology made it possible to create fairly reliable timing belts. Although the belt drive is actually one of the oldest, it has been used to drive mechanisms since antiquity. Development of a machine park with a group drive of mechanisms from steam engine or the water wheel provided the development of belt technology. From leather, they became textile and metal cord, with the use of nylon and other synthetic materials.
The first use of a timing belt goes back to 1954, when Bill Devin's Devin Sports Car was beaten in SCCA races. His motor, according to the description, had overhead camshaft and a toothed belt drive. The first production car with a timing belt is considered to be the 1962 Glas 1004 of a small German company, later taken over by BMW.
In 1966, Opel / Vauxhall began production of the Slant Four series with a timing belt. In the same year, a little later, the Pontiac OHC Six and Fiat Twincam motors appeared, also with a belt. Technology has become truly mainstream.
And the engine from Fiat almost hit our Zhiguli! The option of installing it instead of the lower shaft Fiat-124 engine on the future VAZ 2101 was considered. But, as you know, old motor just altered for the overhead valves, and put a chain as a drive.
As you can see, at first the belt was used exclusively on inexpensive motors. After all, its main advantages were low price and low noise of the drive, which is important for small cars that are not burdened with noise insulation. But it had to be regularly changed and monitored so that aggressive liquids and oil did not get on it, and the replacement interval was already rather big and amounted to 50 thousand kilometers.
And yet he managed to get the glory of a not too reliable method of timing drive. After all, it was enough to bend one pin or fail one roller, as its resource decreased significantly.
Seriously reduced the resource and oiling - even a sealed casing did not always help here, because the engines of those years had a very primitive ventilation system for crankcase gases and oil still got on the belt.
However, all the nuances of using low-quality timing belts are familiar to the owners of front-wheel drive VAZs. The 2108 motor was developed just in the 80s, at the peak of the hobby for belts. Then they began to be installed even on large motors like the Nissan RB26, and the reliability of the best samples was at the level. Since then, the debate about which is better - a chain or a belt does not subside for a minute. Be sure, right now, while you are reading these lines, on some forum or in a smoking room, two apologists of different drives are arguing to the point of exhaustion.
In the next post, I will analyze in detail all the pros and cons of chain and belt drives. Stay in touch!
The D0HC valve timing mechanism of the four-stroke engine is an improvement on the SOHC scheme and is designed to eliminate the only remaining reciprocating mass of the rocker arms (although this would require the return of the tappets). Instead of a single center camshaft, steam is used, mixed directly above the valve stems (see Fig. 1 (see below) 
1.Typical double overhead camshaft timing design
This design uses two camshafts, one above each valve or row of valves. The valve is opened by a "cup" type pusher, and the clearance is adjusted using washers. In this design, only the most necessary parts of the drive of the gas distribution mechanism remained.
To drive the timing mechanism, a chain drive is used - the most traditional and cheapest to manufacture, although a design is known (but not yet widespread), following trends in the automotive industry, in which a pulley and a toothed belt are used instead of a chain drive. Examples of the use of this design include the Honda JGoldwing, Pan European, Moto Guzzi Daytona, Centauro and a number of Ducati motorcycles. The advantages of a belt drive include the following: they are quieter, do not stretch like chains, and pulleys do not wear out like sprockets, although belt replacement should be done more often.
Another way to drive the camshafts is used on Honda VFR models and is a crankshaft-driven gear train (see figure 2). This design eliminates the need for a tensioner and is also quieter than a chain, although the gears of the gear train are subject to wear.
2.Gear-driven camshaft mechanism .
Bowl-shaped camshaft pushers. work in the bore of the cylinder head. When using "cup" tappets, the valve clearance is adjusted using small round shims called shims. Since the washers themselves are unadjustable, they must be replaced with washers of different thicknesses before reconditioning. correct clearance... On some engines, the washer practically coincides with the diameter of the pusher and is installed in a socket located in the upper part of the pusher; this is referred to as a “pusher with shims on top” (see Fig. 3). The washer can be replaced by holding the pusher in the down position using a special tool so that there is enough clearance between the pusher and the camshaft to remove and install the washer.
3 Typical DOHC Timing Actuator Sectional View showing cup-shaped tappets with shims on top
On other engines, the washer is much smaller and is located under the tappet in the center of the valve spring retainer. In this case, it rests directly on the end face of the valve stem: this design is called a "pusher with shims from below" (see Fig. 4).
4 Typical DOHC Timing Actuator Sectional View showing cup-shaped tappets with shims underneath
Thus, the mass of parts moving back and forth is reduced even more when using small gaskets, but it becomes necessary to dismantle the camshaft with each procedure for adjusting the valve clearance, which increases the cost and laboriousness of maintenance. To avoid the hassle of having to use special tools or dismantling the camshaft, some DOHC engines use small, lightweight rocker arms instead of "cup tappets" (see Fig. 5).
5. The DOHC type timing drive mechanism demonstrating an indirect effect on the valve using short rocker arms or rockers, which simplify the adjustment of clearances in the valve mechanism
On some engines with this arrangement, the rocker arms are equipped with a traditional adjusting screw and locknut. In others, the rocker arms rest on a small washer located in the center of the valve spring holder, and the rocker arms themselves are mounted on shafts that are longer than the width of the rocker arm. A spring is located on the shaft to hold the rocker arm over the valve. To replace the adjusting washer, the rocker arms are shifted towards the spring so that the washer can be removed …….
…… continued in the next article
