PICKING MECHANISM

A.MURALIKRISHNAN

LECTURER,

P.A.C.RAMASAMY RAJA POLYTECHNIC COLLEGE

RAJAPALAYAM, TN

Introduction:

Picking is the 2nd primary motion in weaving.

The action of inserting weft yarn through the warp yarns is

called picking.

The objectives of picking mechanism are:

* To propel the weft carrying element (shuttle, projectile or

rapier) along the correct trajectory .

* To throw the weft carrying element at a predetermined

speed.

Types of weft insertion in weaving

Weft Insertion

Conventional

Shuttle

Over pickUnder pick

Un conventional

Projectile Rapier Air jet Water jet

Mechanism of picking

1st method:

• Mechanisms will generate the energy for picking when it is

required

• Done by means of cam to displace the picker.

– Example : shuttle loom

2nd method:

• Mechanisms will generate the energy required for picking

and stored in springs or torsions and release it suddenly

when it required

– Example : Projectile loom

Ideal picking• Should have minimum consumption of power

• Should have minimum momentum of a shuttle on entering into shuttle box (i.e. at checking)

• Kinetic energy in the picking mechanism should remain constant irrespective of fluctuation in the speed and variation in shuttle weight

• Shuttle should have slow speed at start, high at middle and again slow at other end

• Shuttle should be under complete control

• Picking and checking mechanism should be integrated so that kinetic energy of shuttle as it enters a box can be utilized for subsequent picking instead of dissipating as a heat and noise. This will reduce the power required for picking

Complexities of shuttle propulsion

• A shuttle weighting about half Kg is used to insert a weft yarn

weighting only a few grams

• During shuttle movement, it follows a complex path in all the

three axis with the motion of sley

• If there is a variation in loom speed resulting in a variation in

picking force

• If there is a variation in the checking and leads to variation in

shuttle speed

• Shuttle is accelerated from rest to a speed of 10 – 15 m/s in a

distance of only 15 – 20 cm.

Mechanism in shuttle loom

• The energy generated by the cam is used to displace

the picker against the inertial resistance offered by

the shuttle

• The shuttle speed varies with the loom speed

• The force acting on the shuttle tends to increase to

a maximum about the half way through the period

of acceleration.

• Then to decrease to zero at the instant at which the

shuttle loses contact with the picker.

Timing diagram

Picking occupies

1/3 of loom timing in a cycle

Picking and Checking

• 80°-110° : Picking mechanism operates

• 105°-110° : Shuttle enters in the shed

• 240°-250° : Shuttle leaves the shed

• 270° : Shuttle strikes the swell in the shuttle box

• 300° : Shuttle comes to rest

Cone over Pick

•

• A picking cam attached to bottom shaft displaces the picking cone (picking cone) which is attached with the picking shaft.

• This causes rotation of the picking shaft. As a result, the picking stick attached to the picking shaft swing in a horizontal plane over the loom .

• This motion is further transferred to the picker by the picking strap.

• The picker is constrained by the spindle to move in a straight line which otherwise would have followed a path of arc.

• Finally, picker hits the shuttle and passed through

the warp shed.

• It is robust and easy to adjust and maintain.

Mechanism of Over Picking

How to Increase PPM:

- By increasing motor speed.

- By setting the cone stud nearer to the picking tappet.

- By decreasing the picking strap.

- By altering the position of picking arm towards the centre

of the loom.

- By decreasing the length of the stroke of picking tappet.

Under picking mechanism

Phases in shuttle movement

• There are 3 phases in weft carrier motion

– Acceleration

– Free flight

– Checking ( Retardation)

Initial speed of shuttle

• It is a speed at the instant, shuttle leaves the picker and starts

its free flight.

• Initial speed of shuttle should be almost equal to the average

shuttle speed.

• Unless the shuttle is seriously impeded by the top shed.

Relation between Shuttle Velocity and Loom Speed

Let us assume the following notations:

P = Loom speed (picks/min.) or number of revolution of crank shaft/min.

R = Width of the reed (m)

v = Average shuttle velocity (m/s)

L = Effective length of shuttle (m)

θ = Degree of crank shaft rotation available for the passage of the shuttle through the shed.

t = time required for the shuttle passage through the shed (s)

– Now the distance covered by the shuttle in t sec = (R+L) m

Power Required for Picking• The energy used to accelerate the shuttle is equal to its

kinetic energy when it leaves the picker

Cost of power required for picking

• From the previous equation 7, for a given width of loom,

the power required for picking is proportional to the cube of loom speed (picks/min).

• if the loom speed is increased by 20%,

– the power requirement for picking will increase by 73% • (1.23 times = 1.728 times).

– the productivity of fabric (m/min) will also increase by 20% due to increase loom speed.

– So, power cost per unit length (m) of fabric will increase by 44% (1.728/1.2 times).

Power of picking is also influenced as follows.

• Increases linearly with the mass of the shuttle.

• Increases proportionately with the square of loom width

• Decreases proportionately with the square of degree of crank shaft rotation available for the passage of the shuttle through the shed

Nominal Vs Actual Displacement of Shuttle during Picking

• If the loom is turned over slowly by hand, the profile of cam gives the nominal movement to shuttle when all the flexible parts become rigid. This movement is called nominal displacement.

• But, in actual running, the displacement differs from nominal because all the parts become flexible. Under actual running condition of loom is known as actual displacement of shuttle.

• This study is more useful for the machine designer to get better understanding of movement of shuttle.

Projectile picking

Mechanism used in projectile loom• The generated energy is stored in a spring or torsion rod and

release it suddenly when required.

• Because the energy is released suddenly, the weft carrier speed is independent of the loom speed.

• The force exerted by the torsion rod is greatest at the instant of release and decreases steadily to zero.

• In this mechanism, we can obtain more uniform acceleration is achieved.

Features of projectile picking• Picking unit and projectile receiving unit is separated from

moving sley.

• The weft is drawn directly from a large, stationary cross

wound package. No pirn winding

• Projectile is picked across the warp at very high speed.

• Small amount of shed opening is sufficient to pass the small

projectile

• Projectile runs in a steel guides, so that warp threads are not

touched neither by projectile nor by weft thread

Torsion mechanism – Before picking

Torsion bar mechanism – After picking

Velocity and acceleration of projectile

• The velocity is continuously

varying through out the

projectile movement.

• Acceleration of projectile

occupies only 0.007 sec.

• Retardation of projectile

takes longer time and

projectile comes to rest in

gentle manner.