What is a Centrifugal Clutch?
A centrifugal clutch is an automatic clutch that uses centrifugal force to operate. The output shaft is disengaged at low rotational speed and engages more as speed increases.
It is often used in mopeds, under bones, lawnmowers, go-karts, chainsaws, mini bikes, and some paramotors and boats to keep the engine from stalling when the output shaft is slowed or stopped abruptly and to remove load when starting and idling. It has been superseded for automotive applications by fluid coupling and automated manual transmissions.
As the engine speed is increased to or above the set engagement speed of the centrifugal clutch the mechanical drive will become engaged. This enables the operator to run the engine at a designated idling speed without driving the equipment, thus allowing the engine to reach its optimal torque before experiencing load.
More Resources: What is a Clutch?
How Does A Centrifugal Clutch Work?
A centrifugal clutch works, as the name suggests, through centrifugal force. The rotation of the hub forces the shoes or flyweights outwards until they come into contact with the clutch drum, the friction material transmits the torque from the flyweights to the drum. The drive is then connected.
As we have experienced since childhood in roundabouts on playgrounds, when a mass is rotating/spinning a force is generated that points away from the axis of rotation and is known as centrifugal force. The faster the rotation gets, the stronger that force gets and that is why it is so difficult to stay in roundabouts that are spinning quickly.
The operation of the centrifugal clutch is based precisely on the generation of this centrifugal force. In particular, the rotating shaft is connected to the crankshaft of the engine and arranged in the middle of the clutch.
The outer housing is positioned concentrically and connected to the driven shaft. As the engine speed increases, the speed of the drive shaft increases, forcing the shoes installed on the inside of the assembly to expand and reach the inside wall of the drum.
Both the inside of the drum and the outside of the shoe are made of materials with high friction so that a reliable engagement is possible, similar to braking. However, this also causes high temperatures if the engagement/disengagement action occurs more frequently than intended.
As soon as the engine speed drops again, the clutch is disengaged immediately so that the driven shaft no longer receives any rotational movement energy. This helps prevent the engine from stalling when the load is overwhelming and the engine has trouble handling it and makes overall operation safe for both the systems and the user.
Centrifugal clutches can have two, three, four, five, or even six shoes that follow a symmetrical positioning on the drive shaft. In order for them to expand in a controlled manner and in relation to engine speed, manufacturers use expansion springs to compensate for this expansion.
Manufacturers can use different spring strengths and lengths to achieve specific engagement and disengagement points for their centrifugal clutch, whereby the engagement should generally be maintained in the optimum engine torque range.
This means that not every centrifugal clutch is ideal for every type of engine and that both must be considered and optimized together in order to achieve the best possible results.
Parts of Centrifugal Clutch
1. Shoes
These are sliding shoes that slide in the guideways. It consists of friction lining at the end and this friction lining contacts the drum during the engagement.
2. Spring
Spring is a controlling element here, it is used to release the clutch when the engine is turning more slowly.
3. Spider or Guides
The spiders are mounted on the drive shaft (motor shaft) or the motor shaft. The spiders are evenly distributed. Equally spaced means, if there are four guides, each guide is 90 degrees apart. The shoes are held between these guides and each guide holds a spring.
4. Friction Lining
The outer surface of the sliding shoes is provided with friction lining. It helps hold the inside surface of the drum in place.
5. Drum
The clutch drum serves as a housing that encloses all parts of the clutch, including sliding shoes, guides, springs, etc. It is connected to the driven shaft of the transmission system or to chains or belts.
Advantages of Centrifugal Clutch
There are several reasons to choose the centrifugal clutch over alternative types:
- Automatic. Centrifugal clutches automatically engage and disengage. You don’t need a control mechanism. The centrifugal clutch is based on the laws of physics.
- Lower costs. Centrifugal clutches are relatively inexpensive compared to other types of clutch. In part, because fewer parts are involved, no battery or additional components are required.
- Low maintenance requirements. In terms of maintenance and servicing, centrifugal clutches are more cost-effective. As long as the original design and construction are solid, there is little chance the system will break or wear out.
- Less chance of engine stalling and damage. Heavy loads and abrupt starts can put stress on the engines. Instead, progressive intervention in the load is safer. Centrifugal clutches specialize in smooth and shock-free acceleration.
- Greater engagement speed control. The centrifugal clutch gradually engages to increase the torque from the engine to the load. This ensures a much longer lifespan and saves more energy at startup.
Disadvantages of Centrifugal Clutch
There are also several disadvantages of centrifugal clutch:
- Power transmission is limited due to slippage.
- The problem of overheating is due to friction between drum and shoes. If they go too far due to poor driving/usage habits, permanent damage can completely destroy the clutch.
- The centrifugal clutch assembly must be oiled frequently to keep the temperature at a safe level.
- It cannot be used to transmit high torque.
- The power transmission depends entirely on speed control.
- There is always a loss of performance due to friction and slippage.
Applications of Centrifugal Clutch
Centrifugal clutches that correctly transmit a movement with centrifugal force are mainly used in industrial machines such as sweepers, mowers, chain saws, generator sets, motor pumps, lawnmowers, or fans. They are also used in cooling systems and mini karts.
While friction clutches offer numerous advantages, they are not always suitable for every application. Here are some considerations to keep in mind when planning to use a centrifugal clutch:
- Low-speed Engines can cause slippage. In the case of devices with a relatively low speed, there would be greater friction values, which would heat up the clutch drum and lead to a loss of power.
- The maximum speed of a centrifugal clutch depends on the clutch size. The smaller the clutch, the faster the speed. This is due to the centrifugal force required to lock it into place.
- Ideal for engines with a wide speed range. When your engine reaches a certain speed, the clutch is activated. If the load exceeds its rated capacity, the clutch will automatically “slip”, protecting the motor and driven equipment.