The device and principle of operation of the overrunning clutch
Auto Terms,  Car transmission,  Car device

The device and principle of operation of the overrunning clutch

The device of some mechanisms of the car includes an overrunning clutch. In particular, it is an integral part of the generator. Now we will focus on what kind of mechanism it is, on what principle it will work, what kind of breakdowns it has, and also how to choose a new clutch.

What is a freewheel alternator

Before you figure out why this spare part is in the generator, you need to delve into the terminology a little. As the well-known service Wikipedia explains, an overrunning clutch is a mechanism that allows you to transfer torque from one shaft to another. But if the driven shaft starts to rotate faster than the drive, the force does not flow in the opposite direction.

The simplest modification of such mechanisms is used in bicycles (a five-piece installed in the rear wheel structure or a ratchet in sports models). When the pedals are depressed, the roller element is triggered and the sprocket begins to spin the wheel. When freewheeling is performed, for example when going downhill, the overrunning mechanism is triggered and the torque from the wheel is not applied to the pedals.

The device and principle of operation of the overrunning clutch

A similar mechanism is used in generators. It is worth mentioning that in many older cars this element is not provided. With an increase in the power of the internal combustion engine, the load on the car generator began to increase. Installing a freewheel clutch provides an increase in the working life of the timing belt (this detail is described in detail in another article) or the drive of the power supply itself.

The presence of a roller element in the generator drive device provides a balance between the revolutions of the crankshaft (from it, the torque is transmitted through the timing belt to all attachments, and through a separate belt to the generator) and the driven shaft of the power source. When the engine in the car is running, it is the generator that becomes the main source of electricity, although the electrical circuit of the car is looped through with the battery. While the power unit is running, the battery is recharged by generating electricity from the generator.

Let's figure out what is the purpose of the freewheel clutch.

Why do you need an overrunning clutch

As most motorists know, electricity in the car during the operation of the internal combustion engine is generated by transferring torque from the crankshaft to the generator drive. We will not go into the intricacies of its device - in detail about why the machine needs a generator and what its work is, it is told in another review.

Modern power units differ from older versions by high torsional vibrations generated on the crankshaft. This is especially pronounced in diesel engines, starting with those that comply with the Euro4 environmental standard and higher, since even at low speeds such engines have high torque. Because of this, the drive pulley does not rotate as evenly as it does when the starter spins the motor at the moment of starting.

Excessive vibration of attachments leads to the fact that the timing belt develops its resource after about 30 thousand kilometers. Also, these forces negatively affect the serviceability of the crank mechanism. To do this, a dual-mass flywheel is installed on many cars (for details on how this part differs from the standard analogue, read here), as well as a damper pulley.

The essence of the clutch is to ensure that the motor does not experience additional loads when switching to another mode. This happens when the driver changes gear. At this moment, the gas pedal is released and the clutch is depressed. The engine slows down for a split second. Due to the inertial force, the generator shaft continues to rotate at the same speed. Because of this, it becomes necessary to eliminate the difference between the rotation of the driving and driven shafts.

The device and principle of operation of the overrunning clutch

While the internal combustion engine picks up speed suitable for driving the generator, the shaft of the energy source can rotate freely at its own speed. The synchronization of the rotation of these elements occurs at the moment when the crankshaft spins up to the required speed and the generator shaft drive mechanism is again blocked.

The presence of this freewheel damper mechanism ensures the safety of the belt (in the process of changing the operating modes of the motor, torque surges are not formed). Thanks to this, in modern machines, the operating resource of the belt can already reach 100 thousand kilometers.

In addition to the generator, the overrunning clutch can also be installed in some modifications of the starter (for details about their device and what their principle of operation is, read separately). This mechanism is also installed in classic automatic transmissions with a torque converter. In all these cases, the torque must be transmitted in only one direction, and in the opposite direction, the connection must be interrupted. This is necessary so that the devices do not collapse and do not suffer from vibrations that are formed during the operation of the engine.

The advantages of these mechanisms include:

  1. There is no need for additional actuators to decouple the drive from the follower (no drive, no electronic interlocks, etc. are needed). The device self-locks and disconnects without the need to control this process.
  2. Due to the simplicity of the design, the mechanisms in which the product is used are not complicated by different actuators. This makes the repair of the units a little easier, as if they were equipped with additional electronics, which may malfunction.

How the clutch works

Despite the fact that there are several types of overrunning clutches, they all have the same operating principle. Roller type devices are widely used in the automotive industry. Let us discuss the principle of operation of the mechanism using this modification as an example.

The device and principle of operation of the overrunning clutch

This type of construction has two parts. One coupling half is installed on the drive shaft, and the other on the driven shaft. When the drive half of the coupling rotates clockwise, the frictional force moves the rollers (located in the cavities between the clips of the half of the couplings) into the narrow part of the mechanism. Due to this, a wedge of the mechanism is formed, and the driven part begins to rotate with the drive.

As soon as the rotation of the drive shaft slows down, overtaking of the driven shaft is formed (starts to rotate at a higher frequency than the driving part). At this moment, the rollers move into the wider part of the clips, and the force does not come in the opposite direction, since the half-couplings are separated.

As you can see, this part has a very simple principle of operation. It transmits rotational movements only in one direction, and only scrolls in the opposite direction. Therefore, the product is also called a freewheel.

Device and main components

Consider the roller clutch device. This modification consists of:

  • Outer cage (inside there may be special grooves on the wall);
  • Inner cage with projections;
  • Several springs attached to the outer cage (their availability depends on the design features). They push the rollers out to make the device work faster;
  • Rollers (friction element of the device), which, when moved into a narrow part of the structure, clamp both parts, and the clutch rotates.

The photo below shows a drawing of one of the modifications of the freewheel clutches.

The device and principle of operation of the overrunning clutch

This part replaces the standard alternator pulley. The power supply itself does not visually differ from the classic type. The only difference is that a thread will be made on the shaft of such a model. With its help, the coupling is firmly attached to the generator drive. The pulley is connected to the power unit in the same way as in the classic generator model - through the timing belt.

When the motor switches to a lower speed, the accelerating effect of the weighty generator shaft does not create a runout in the belt, which increases its working life, and makes the work of the power source more uniform.

Varieties of overrunning alternator couplings

So, the universal type of freewheel mechanisms allows the generator rotor to rotate freely due to the transfer of force from the crankshaft. In this case, an important condition is a higher rotation speed of the drive shaft - only in this case the mechanism will be blocked, and the shaft of the power source will be able to unwind.

The disadvantages of the roller modification are:

  1. Non-demountable construction;
  2. The axes of the driving and driven shafts must match perfectly;
  3. Due to the use of rolling elements (as in a bearing), the product requires greater precision in the manufacturing process, therefore, a high-precision lathe is used in production. Only in this case is it possible to achieve the ideal geometry of all components of the device;
  4. They cannot be repaired or adjusted.

The ratchet version has a similar design. The only difference is that teeth are made inside the outer cage, and the friction element is represented by pawls, which are fixed on one side to the inner cage, and on the other hand are spring-loaded. When the driving half of the coupling rotates, the pawls rest against the teeth of the cage, and the coupling is blocked. As soon as there is a difference in the speed of rotation of the shafts, the pawls slip by the ratchet principle.

The device and principle of operation of the overrunning clutch

Naturally, the second modification has several advantages over the roller type. The main thing is that such a modification provides a more rigid fixation of the two half-couplings. Another plus of the ratchet type is that it can be repaired, but the roller type cannot.

Despite the higher reliability, ratchet clutches are not without drawbacks. These include:

  • Impact effect at the moment when the clutch is blocked. This is due to the fact that the dogs abruptly abut against the teeth of the outer coupling half. For this reason, ratchets are not effective in units with high drive shaft speeds.
  • In the process of overtaking, the clutch emits characteristic clicks (the dogs slip on the teeth). If the device often overtakes the driven shaft, the pawls or teeth in the mechanism (depending on the metal used) will quickly wear out. True, today there are already modifications of ratchet overrunning clutches that work much quieter due to the fact that when overtaking the dogs do not touch the teeth.
  • At high speeds and frequent locking / unlocking, the elements of this mechanism wear out faster.

To independently determine which pulley is installed on the generator of a particular car, just look at its mount. The overrunning clutch is not secured with a lock nut on the machine shaft. But in modern cars there is not much free space under the hood, so it is not always possible to consider what kind of fastening the generator pulley has (the option with a freewheel clutch in most cases will simply screw onto the shaft). Generators equipped with the mechanism under consideration are closed with a dark protective cover (housing casing), so many craftsmen determine the type of generator drive specifically for this cover.

Symptoms of a malfunctioning overrunning clutch

Since this device is in constant motion, its breakdowns are not uncommon. The most common causes of failure include contamination of the mechanism (attempts to overcome a deep, dirty ford) or natural wear of parts. These factors lead to the fact that the overrunning clutch can be completely blocked or the fixation of the coupling halves may not occur.

It is possible to determine the malfunction of the overrunning clutch by malfunctions in the generator. So, with sharp jumps in the crankshaft revolutions (the driver abruptly presses the gas pedal, and the revolutions jump), a rupture of the half-couplings may occur. In this case, even if the rollers move to a narrower part of the device, due to serious damage, they simply slip. As a result, the crankshaft rotates, and the generator stops working (torque stops flowing to its shaft).

The device and principle of operation of the overrunning clutch

With such a breakdown (the half-couplings do not engage), the power source stops generating electricity or does not recharge the battery, and the entire on-board electrical system is powered by the battery. Depending on the parameters of the battery in this mode, the machine can work up to two hours. When doing this, take into account the battery charge level. More details on how to check the generator are described here.

If a breakdown occurs, as a result of which the coupling halves are jammed, then in this case the mechanism will work like a conventional drive pulley of a generator until, due to wear, the rollers stop resting on the cage. Not a single overrunning clutch malfunction can be ignored, because this will adversely affect the operation of the power source, up to deformation of its shaft.

Also, a malfunction of the mechanism may be accompanied by a crash at the time of starting or stopping the power unit. During the operation of the motor, a constant noise is heard from the generator side (this is also a symptom of a failed power source bearing).

How to determine that the clutch is out of order

With the introduction of a freewheel in the design of modern generators, according to many experts, the resource of the energy source has increased by 5-6 times. As we have already figured out, this element is necessary to eliminate torsional vibrations on the generator shaft. Thanks to this, the mechanism works more evenly, without premature wear of the bearing, and its operation is not accompanied by noise.

The device and principle of operation of the overrunning clutch

But there are no parts in the car that do not need to be replaced. The same can be said about the overrunning clutch. Its key malfunction is common to all bearings - it is subject to wear and often its wedge occurs. The approximate resource of the generator clutch is in the region of 100 thousand kilometers.

If the clutch jams, it will stop absorbing inertia, and will work like a normal bearing. Because of this, the load on the alternator belt will increase. If it is already old, then it can break. The belt tensioner will also wear out faster.

You can identify the freewheel wedge by the following features:

  1. The smooth operation of the generator disappeared - vibrations appeared in it. As a rule, during engine operation, this malfunction will be accompanied by a bouncing of the alternator belt.
  2. In the morning, when starting the engine and until it runs a little, the belt whistles a lot.
  3. The belt tensioner began to work with clicks.

Much less often, the clutch does not wedge, but stops rotating the generator shaft. Such a breakdown is much more difficult to determine visually without dismantling the mechanism. The main symptom of such a malfunction is the lack of battery charge or its undercharging (of course, this malfunction has other reasons).

Overrunning clutch diagnostics

Checking the overrunning clutch is necessary in the following situations:

  1. The battery indicator (yellow or red) on the tidy came on. This happens when the battery is not being charged or is not receiving enough power.
  2. When changing gears (the clutch is squeezed out and the gas is released), small vibrations are felt, as if the engine is forcibly slowed down by some mechanism. This effect occurs in the event of a jammed clutch. In this case, when the motor switches to low speed, the generator shaft creates a short-term resistance to the motor due to inertial forces. This effect increases the load on the belt, causing it to wear out faster.
  3. Scheduled vehicle maintenance. At this stage, the automatic transmission, the driveshaft is checked (if it is present in the transmission, then its malfunctions also cause vibrations when changing the operating modes of the internal combustion engine), the starter, the clutch (insufficient opening of the basket also provokes jerks of the engine at idle speed).
The device and principle of operation of the overrunning clutch

To check the serviceability of the freewheel clutch, it is necessary to contact a specialist, since this work is accompanied by the dismantling of the mechanism. If the standard pulley is removed by unscrewing the clamping nut, then the freewheel is removed with a special tool. Improvised means in this situation can seriously damage the generator shaft.

How do I know if the alternator freewheel needs to be replaced?

To accurately determine whether the overrunning clutch has failed, the generator will need to be dismantled. But there are other ways that will help determine the malfunction of the clutch by indirect signs.

Consider the option of checking with the dismantling of the coupling and without removing the generator.

Dismantled test

After removing the coupling from the generator shaft, the inner race is clamped with two fingers so that the outer race can rotate freely. The principle of operation of the overrunning clutch is that the scrolling of the clips in one direction must be independent, and in the other direction - synchronous.

With the inner race locked, try turning the outer race in the direction of belt rotation. In this direction, the clips should rotate together. If the outer race can be turned even slightly, it means that the clutch does not work, and with great effort the shaft will not rotate, which will lead to undercharging of the battery. In this case, the clutch must be replaced.

The device and principle of operation of the overrunning clutch

A similar procedure is carried out to determine if the clutch is jammed. With the inner ring clamped, an attempt is made to turn the outer race in the direction opposite to the rotation of the alternator belt. A good clutch should rotate freely in this direction. If it works with noticeable jerks or does not spin in any direction at all, then it is jammed and the part must be replaced.

Inspection without dismantling

Here are some indirect signs that indicate wear or problematic freewheel operation:

  1. The motor is running at idle. The alternator belt tensioner should rotate evenly, without twitching;
  2. The motor is brought to a speed of 2-2.5 thousand per minute. ICE stops. At this point, you need to listen to the sounds coming from the generator. If after stopping the motor a short buzz is heard (1-5 seconds), then this is a sign of wear on the pulley bearing;
  3. During the start of the engine or its stop, clicks coming from the generator are clearly audible. This happens when an inertial load is applied to the clutch, and it is blocked and slips under heavy load;
  4. Belt whistling may be a sign of a jammed clutch.

Special Checks for Alternator Freewheels

The remaining types of checking the performance of the overrunning clutch (if a special type of inertial disconnecting mechanism is installed) are carried out in the conditions of specialized car services.

A routine test allows you to determine whether the mechanism is working or it has already broken. With a detailed check on special stands, experts can approximately say how soon the part will fail.

Choosing a new mechanism

Choosing a new overrunning clutch is no different from choosing another auto part. The safest thing to do is to seek advice from an auto parts store. It is enough for the seller to name the model of the car and the year of manufacture. You can also search for overrunning clutches for specific generators by catalog number or markings on the product itself (if any).

If the motorist is sure that the car fully complies with the factory configuration, then the selection of a new mechanism can be performed using the VIN code (read about where to look for this code and what information about the car it contains) separately).

Many motorists prefer original auto parts, but in many cases this does not always mean that the part will be of the best quality, but the price will always be high. The same applies to overrunning clutches. There are not so many companies producing original options for the factory configuration. Many of them supply their products to the secondary market as well. Noteworthy budget analogs of the originals of overrunning clutches are offered by brands such as:

  • French Valeo;
  • German INA and LUK;
  • American Gates.
The device and principle of operation of the overrunning clutch

Even cheaper, but less quality products are offered by the following companies:

  • Brazilian ZEN;
  • Japanese Lynxauto, although this brand sells products made in other countries;
  • American WAI;
  • Dutch Nipparts;
  • Italian ERA.

When purchasing a part, it is important to take a close look at the product. Any mechanical damage or visual defects are unacceptable, since this spare part must have perfect geometry.

Installing a new overrunning alternator clutch

Usually, the overrunning clutch is replaced in a specialized service station, since many modern cars have a complex engine compartment, which makes access to the part difficult. Also, for this procedure, a tool is used that is rarely used anywhere else, so an ordinary motorist often does not have such keys.

To dismantle and replace the mechanism from the generator shaft, you will need:

  • A special puller for the coupling (he needs a multi-faceted nozzle with a double-sided bit);
  • Open-end wrench of the appropriate section or suitable head;
  • Torque wrench;
  • Vorotok Torks.
The device and principle of operation of the overrunning clutch

It is best to carry out the work after dismantling the generator, since some cars do not have enough space in the engine compartment to replace the clutch. Depending on how the engine compartment is organized, the work is carried out in the following sequence;

  • The terminals are removed from the battery (how to do this correctly is described here);
  • The alternator belt is weakened;
  • The power supply is dismantled;
  • Using a puller, the coupling is unscrewed from the shaft (while the shaft must be held so that it does not turn);
  • A new mechanism is screwed up instead of the old;
  • The device is tightened on the shaft using a torque wrench with a force of about 80 Nm;
  • The structure is installed in its place;
  • The battery terminals are connected.

One small feature of overrunning clutch replacement. It must be closed with a plastic casing (protects against dust and foreign objects from entering the mechanism). If this item was not included, you must purchase it separately.

How to change - repair with your own hands

To replace/repair a failed clutch, it is necessary to dismantle it from the generator. To do this, loosen the belt tension, dismantle the generator itself, and then unscrew the nut that fixes the coupling on the shaft.

Installation of a new clutch is carried out in reverse order. The only difficulty is that manufacturers use a special bolt that requires a special key. Usually such a nozzle is present in professional tool kits for motorists. Therefore, when choosing a new set of tools for the machine, you should pay attention to the presence of a nozzle for the TREX bolt.

If we talk about the repair of the overrunning clutch, then this mechanism is not repairable, although there are craftsmen who are trying to restore the broken mechanism. But in the case of a clutch, the reason for repair is the same as in the case of a seized or badly worn bearing. Such elements should always be replaced with new counterparts.

Related videos

Here is a short video about the device and the purpose of the freewheels of the generator:

Overrunning clutch purpose and device

Hack and predictor Aviator

So, while it is not mandatory for older vehicles to install an overrunning clutch on the alternator, this mechanism ensures smoother operation of the power supply, and also prevents premature wear of the drive belt. If such machines can easily do without this element, then in modern models its presence is mandatory, since the power unit creates large torsional vibrations, and with abrupt transitions from high speeds to the XX mode, the inertial effect is much higher than in low-power engines.

These mechanisms have a simple design, due to which they have a long working life. But if there is a need to repair or replace the device, it is better to seek help from specialists.

In conclusion, we offer a short video on how you can check the overrunning clutch without removing it from the generator:

Questions and answers:

What does an overrunning alternator clutch do? It is part of the pulley in many modern car models. This device provides a smooth shaft movement and independent rotation of the pulley with the unidirectional movement of these parts.

What happens if the generator clutch gets stuck? Vibration of the alternator belt will appear, the noise from it will increase. The tensioner will make a clicking sound and the belt will whistle. Over time, the belt and its tensioner wears out and breaks down.

How to remove the clutch from the generator? The battery is disconnected, interfering parts are dismantled. The alternator belt is loosened and removed. Retains the pulley shaft (using a torque wrench). The pulley fastening nut is unscrewed.

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