Friction in the Machine Elements

FRICTIONIN THE MACHINE ELEMENTS

1 Surface contacts:

·        Basic laws of friction

·        Pivot and collar, introducti on and types.

·        Problem on flat pivot, Problems on conical pivot.

Friction

Friction is a measure of how hard it is to slide one object over another. Take a look at the figure below. Both of  the blocks are made from the same material, but one is heavier. I think we all know which one will be harder for the bulldozer to push.

Friction force versus weight

To understand why this  is, let's take a close look at one of the blocks and  the table:

Because friction exists at the microscopic level, the amount of force it takes to move a given block is proportional to that block's weight.

Even though the blocks look smooth to the naked eye, they are actually quite rough at the microscopic level. When you set the block down on the table, the little peaks and valleys get squished together, and some of them may actually weld together. The weight of the heavier block causes it to squish together more, so it is even harder to slide. Different materials have different microscopic structures; for instance, it is harder to slide rubber against rubber than it is to slide steel against steel. The type of material determines the coefficient of friction, the ratio of the force required to slide the block to the block's weight. If the coefficient were 1.0 in our example, then it would take 100 pounds of force to slide the 100-pound (45 kg) block, or 400 pounds (180 kg) of force to slide the 400-pound block. If the coefficient were 0.1, then it would take 10 pounds of force to slide to the 100-pound block or 40 pounds of force to slide the 400-pound block.

So the amount of force it takes to move a given block is proportional to that block's weight. The more weight, the more force required. This concept applies for devices like brakes and clutches, where a pad is pressed against a spinning disc. The more force that presses on the pad, the greater the stopping force.

FRICTION

Dry friction – Friction in screw jack – Pivot and collar friction - Plate clutches - Belt and rope drives - Block brakes, band brakes.

Friction:

The opposing force which acts in the opposite direction of the movement of the upper block is called the force of friction or friction.

Types or friction:

1.     Static friction: It is experienced by a body, when at rest.

2.     Dynamic friction: It is friction experienced by a body when in motion.

a.     Sliding friction: It is friction experienced by a body when it slides over another body.

b.     Rolling friction: It is friction experienced between the surfaces which the balls or rollers interposed between them.

c.      Pivot friction: It is the friction experienced by a body due to motion of rotation.

Further classified

1. Friction between unlubricated surfaces

2. Friction between lubricated surfaces. Laws of dry or solid friction:

^Ø   The force of friction directly proportional to the normal load between the surfaces.

^Ø   The force of friction is independent of the area of the contact surface for a given normal load.

^Ø   The force of friction depends upon material which the contact surfaces or made.

^Ø   The force of friction is independent of the velocity of sliding of one body relative to other body.

Coefficient of friction (µ):

It is as the ratio the limiting friction (F) to the normal reaction (RN) between the two bodies.

µ= F/ RN

Angle of friction:

It may be defined as the angle which the resultant reaction R makes with normal reactions

tan ϕ= F/ RN

2 Friction drives:

3 Friction in screw threads:

·                    Friction in screw and nut

·                    Friction in screw jack

·                    Problems in screw jack

Screwjack:

The screw jack is a device lifting loads. For lifting heavy loads by applying a comparatively smaller effort at its handle. The principle on which a screw jack works in a smaller to that of an inclined plane.

1. Torque required lifting the load by a screw jack

Let     p=pitch of the screw

d =mean diameter of the screw

α= helix angle

P= effort applied at the circumference of the screw to lift the load

W=load to be lifted

µ=coefficient of the friction

tan α=p/Πd

Torque required overcoming friction between the screw and nut

The torque required to overcome friction at the collar

Total Torque required overcoming friction

If an P1 is applied at the end of a lever of arm l, then the total torque required to overcome friction must be equal to the torque applied at the end of the lever

2. Torque required lower the load by a screw jack

Let     p=pitch of the screw

d =mean diameter of the screw

α= helix angle

P= effort applied at the circumference of the screw to lift the load

W=load to be lifted

µ=coefficient of the friction

tan α=p/Πd

Torque required overcoming friction between the screw and nut

Efficiency of the screw jack:

The efficiency of the screw jack may b defined as the ratio between the ideal efforts to actual effort.

Self locking and over hauling of screws

Torque required to lower the load

In the above expressions ,if ϕ< α ,then the torque required to lower the load will be negative ,the load will start moving downward without application of force, such a condition is known as overhauling of screw.

If ϕ> α ,then the torque required to lower the load will be positive ,indicating that an efforts applied lower the load , such a condition is known as self locking of screw

4 Clutch

For other uses, see Clutch (disambiguation).

Clutch for a drive shaft: The clutch disc (center) spins with the flywheel (left). To disengage, the lever is pulled (black arrow), causing a white pressure plate (right) to disengage the green clutch disc from turning the drive shaft, which turns within the thrust-bearing ring of the lever. Never will all 3 rings connect, with no gaps.

Rear side of a Ford V6 engine, looking at the clutch housing on the flywheel

Single, dry, clutch friction disc. The splined hub is attached to the disc with springs to damp chatter.

A clutch is a mechanical device, by convention understood to be rotating, which provides driving force to another mechanism when required, typically by connecting the driven mechanism to the driving mechanism. Clutches and brakes are similar; if the driven member of a clutch is fixed to the mechanism frame, it serves as a brake.

Clutches are useful in devices that have two rotating shafts. In these devices, one shaft is typically attached to a motor or other power unit (the driving member), and the other shaft (the driven member) provides output power for work to be done. In a drill, for instance, one shaft is driven by a motor, and the other drives a drill chuck. The clutch connects the two shafts so that they can either be locked together and spin at the same speed (engaged), or be decoupled and spin at different speeds (disengaged).

Multiple plate clutch

This type of clutch has several driving members interleaved with several driven members. It is used in race cars including F1, Indy car, World rally and even most club racing, motorcycles, automatic transmissions and in some diesel locomotives with mechanical transmissions. It is also used in some electronically controlled all-wheel drive systems.

Vehicular

There are different designs of vehicle clutch but most are based on one or more friction discs pressed tightly together or against a flywheel using springs. The friction material varies in composition depending on many considerations such as whether the clutch is "dry" or "wet". Friction discs once contained asbestos but this has been largely eliminated. Clutches found in heavy duty applications such as trucks and competition cars use ceramic clutches that have a greatly increased friction coefficient. However, these have a "grabby" action generally considered unsuitable for passenger cars. The spring pressure is released when the clutch pedal is depressed thus either pushing or pulling the diaphragm of the pressure plate, depending on type. However, raising the engine speed too high while engaging the clutch causes excessive clutch plate wear. Engaging the clutch abruptly when the engine is turning at high speed causes a harsh, jerky start. This kind of start is necessary and desirable in drag racing and other competitions, where speed is more important than comfort.

5.4.1 Friction clutches:

Single plate clutches and Multi-plate clutche Uniform wear theory and Uniform pressure theory Problems in clutches

5 BELT

The belt or ropes are used to transmit power from one shaft to another shaft by means of pulleys which rotate at the same speed or at different speed.

Types of belt drives.

1.     Light drives: belt speed upto10m/s

2.     Medium drives: speed 10m/s to22m/s

3.     Heavy drives :Speed ove r22m/s

1Types of belt

1.Flat belt

2.V- belt

3.Circular belt or rope

Type of flat belt drives

Open belt drive

Cross belt drive

Quarter turn belt drive

Belt drive with idler pulley

Velocity of belt drive

It is the ratio between the velocies of the driver and follower or driven.

d₁      =       diameter of the driver

d₂      =       diameter of the follower

N₁      =       Speed of the driver r.p.m

N₂      =       Speed of the driven r.p.m

Let r1 and r1  =    Radii of the larger or smaller pulleys

x         =Distance between the centres of the two pulleys

L   =       Total length of the belt.

Length of an open belt drive

Length of the cross belt drive.

Power transmitted by a belt

T₁ and T₂    =       Tensions in the tight and slack side of the belt respectively N

r1 and r1     =       Radii of the larger or smaller pulleys

v        =Velocity of the belt m/s

Power         =       (T1-T2)v W

Belt and rope drives:

6 Friction aspects in Brakes:

BRAKES