Clutches and Brakes

ClutchesA clutch is a device which enables two inline shafts to be connected or disconnected whilst at rest or in relative motion. In most transmission units it is usually necessary to make changes to the conditions of the unit being driven without stopping the power source (electric motor) which may be used to drive one or more units

In order to control the units individually it must be possible to:

• Stop the unit• Select a new speed or direction• Re-engage the drive• Prevent over-run or run away of the driven unit whilst these changes are made

Clutches as a fail safe

Where expensive equipment is used in a transmission system to prevent damage and costly repairs a fail safe clutch may be employedThe clutch is adjusted on the on position and only slips when an overload occurs ( usually adjusted at rest)

Dog or Claw clutchThis device is really a form of sliding coupling, where a shaft is connected together by two flanges. Each of the flanges has teeth which mate into the opposing teeth on the other flange

Cone Clutch (friction)An old design still used today in variable speed drives and can be used in gearboxes to act as a synchronising device to provide easier gear changing. can usually be adjusted to slip when subjected to overload

Single Plate Friction ClutchThe most common type of friction clutch (used in the car) It has three major parts

Driving Plate

Driven Plate

Friction Plate

With this arrangement the clutch provides two friction surfaces and hence increased torque capacity. The friction plate is lined on both sides with a friction material which allows slip when engaging and disengaging. This also provides a smooth take up of drive

Multi-Plate Friction ClutchesUsed in many factory plant applications, where high torque levels are encountered. They may be operated by electro-magnetic, mechanical hydraulic or pneumatic means.

They may also operate in totally dry conditions or fully immersed in oil (wet) . Dry clutches rely on circulating air to remove the heat generated on engagement

The more friction plates fitted the higher the power output (smaller diameter plates can also be used)

Wet Clutches

Oil acts as a cushion and energy released by the heat is carried away by the oil, there is

a reduction in friction but as long as there is an oil film between the plates, the friction

characteristics and engagement torque remain low, as soon as the film breaks the

engagement torque rises rapidly and can lead to rapid acceleration

Electromagnetic friction clutchesA very popular clutch where light loads are in force, used in automatic control systems, switched byrelays or solid state devices in conjunction with electronic devices and plc’s

Single Plate Multi-Plate

Centrifugal ClutchThese are used when engagement of a prime load is to be achieved at apredetermined speed. A typical design has spring loaded weights located into slots in the driving element. The outer faces of the weights (which are faced with friction material) engage with the inner surface of the driven element usually the drum

Centrifugal clutches are often used in mopeds, lawnmowers, go-karts, chainsaws, and mini bikes to:• keep the internal combustion engine from stalling when the blade is stopped abruptly • disengage load during starting and idle

Freewheeling or over running clutch

This type permits the shaft to be driven in one direction only, it slips when the speed ofthe driven shaft exceeds that of the driver, there are two main types:

Roller Freewheeling Clutch - rollers are situated in a wedge shaped space and supported by springs. In one direction the rollers run up the wedge and lock to providethe drive. In the other direction the rollers lie at the bottom of the wedge and the clutchslips. If the driven outer unit speeds up or overruns then the rollers fall back to maintaina constant speed

Sprag ClutchThis operates in the same way as the freewheeling clutch and allows the driven elementto disengage in the reverse direction. Instead of rollers the clutch uses sprags. These are shaped so that in one direction they wedge the inner and outer races and providedrive and in the other direction they tilt and allow the clutch to slip

Expanding Ring and expanding band Friction Clutches

Normally used where high torque is required at low speeds. The centrifugal force assists the gripping power of the clutch

Fluid Clutches or couplingsDry Fluid Clutch

A centrifugal clutch that uses tiny metal shot which compacts into the outer cavity underthe turning action of the centrifugal force. A disc attached to the driver shaft helpsdistribute the shot evenly into the outer cavities. The unit allows the driven unit to attainspeed without overloading the driver unit. Once running it acts as a flywheel and stores rotational energy and dampens out vibration in the drive system

Fluid Flywheel couplingWhilst this device is called a coupling it acts in the same way as a clutch. Both the inputand output shafts have an impellers which have radial vanes filled with oil. The oilcirculates in the vanes when the coupling rotates, The input wheel acts as a pump andthe output wheel as a turbine so that power is transmitted. There is usually some loss ofspeed due to slip. The coupling when running acts as a flywheel, storing rotationalenergy, dampening vibration and providing smoother rotational movement

BrakesA brake is a device which – applies a resistive force to a moving body in orderto stop it. It may also be used to absorb and /or measure power, in the majority of cases the movement involved is a rotational one.

The pressure applied to brakes in order to operate them can come from a variety of sources ranging through: mechanical levers, cables, hydraulic, pneumatic and electrical

Brakes can be classified into three groups

• Band Brakes• Drum Brakes• Disc Brakes

Band Brakes (External Drum)A relatively simple device consisting of a simple band usually made from steel with afriction material lining

Drum Brakes (Internal and External drum)

The drum normally covers the brake shoes and a force pushes the shoes outward and onto the inside of the drum to provide the slowing and stopping action (in a car this force is hydraulic)Each shoe has a friction lining material either riveted or bonded to the outer face of theshoe. The lining will normally cover an arc of 90-110 deg

Disc BrakesThe brake consists of a finely ground plate (disc) which is either part or bolted to the main rotating member. The braking effect comes from segmental blocks (pads) whichnormally oppose one another and are forced against the plate in order to stop it, theseare situated in a calliper from which the actuating force is applied. The main advantageof the disc brake is its ability to sustain the braking force with minimal fading (fall inbraking efficiency over a prolonged time) which other systems have a tendency to do

Brake Blocks• Single Brake Block- a simple

device using a block from a frictional material, which is held against the rim of a rotating wheel in order to slow it down or stop it rotating

• Double Brake Block – uses two brake blocks usually situated either side of the wheel or drum, activated simultaneously.

Spring Set BrakeThis is a double brake block in which the blocks are normally held in contact with thedrum, thus preventing its rotation. A compressed spring applies the load and theoperating lever compresses the spring still further to release the drum. The lever may be operated by hand, electrical solenoid or hydraulic/pneumatic actuator. This type of brake is fail safe and is used on lifts and power presses etc

Friction LiningsBrake linings are composed of a relatively soft but tough and heat-resistant material witha high coefficient of dynamic friction typically mounted to a solid metal backing usinghigh-temperature adhesives or rivets.

Impregnated woven cotton

Sintered Metals

Asbestos based friction materials - Due to its suitability, asbestos was often a component in brake linings; as a result, brake dust should never be inhaled. Brake linings or pads have not used asbestos since 2003, having been replaced by Aramid. However there still remains a serious health risk especially when removing old worn linings and accumulations of lining dust

Mating SurfacesMating surface main requirements are:

• Requisite strength and low thermal expansion• Hardness sufficient to give long wear life and resist abrasion• Heat soak capacity sufficient to prevent heat spotting and crazing

Close Grained Pearlitic Grey Cast Iron meets these requirements usually with thefollowing additives:

• Carbon• Silicon• Manganese• Chromium• Sulphur• Phosphorus• Molybdenum• Copper• nickel

Methods of Actuation