Principle of Speed Frame
Process Parameters for Speed Frame
Calculations of Speed Frame
Winding at Speed Frame
Principles of Drafting, Twisting & Bobbin Building in Roving Formation
1. Functions of the roving frame
1. Drafting
The basic function of the roving frame is attenuation or drafting so that the mass per unit length of the sliver may be reduced down to the extend which is which is suitable to be fed to the ring frame. The range of draft given at the roving frame is 5 to 20.
2. Twisting
After drafting the fine strand of fibres (roving) has little coherence and become unsuitable for further attenuation at the ring frame. So a protective twist must be imparted to give coherence to the fibres and to give strength to the roving. The amount of twist given at the roving frame is low and ranges from 0.7 to 2 TPI.
3. Winding
The drafted and twisted roving has to be properly wound on a package called as bobbin. This is done by the winding operation at the roving frame.The winding operation is a complex mechanical process which not only winds the roving on the bobbin but also maintains a special build of the package.
The drawn sliver is composed of clean and straightened fibres laying parallel to one another and to the axis of the sliver. These characteristics of a drawn sliver are ideal for creation of a yarn. However this is not possible because if the drawn sliver is to be directly converted in to a yarn it would require a mechanical draft of a range of 300 to 600. But even on most modern machines technologically it is not possible construct a ring frame that could give such high drafts in a single process. So an intermediate stage of drafting is carried out using the roving frame.The draft given at the roving frame reduces the linear density of the drawn sliver in to a less thick strand of fibres suitable as an input to the ring frame.
This roving which is fed to the ring frame can then be easily converted in to a yarn by giving a draft of 15 to 40. Another advantage of making roving is to have a better package as an input to the ringframe. The roving frame produces roving on compact small packages called as bobbins. The bobbins are much more convenient to transport and have less chances to damaged as compared to the can sliver mode of package.
2. Main parts of Roving Frame
1. Creel Section
The creel is the area designated for the drawn sliver cans which are positioned at the back of the machine.Just above the cans the no. of independently driven guide rollers are provided that help the sliver to move towards the drafting section. Since the fibres in the drawn sliver have less coherence so it is necessary to keep the surface speed of the guide rollers equal to the surface speed of the back drafting rollers so that any false drafting may be avoided that can damage the sliver. Just before the drafting section a photo electric stop motion is used which detects the presence of the sliver and as the sliver breaks it automatically stop the machine.
2. Drafting Section
A typical drafting section of the roving frame is composed of 3 over 3 roller arrangement. However some of the machines also make use of 3 over 4 roller drafting arrangement. Regardless of the type of roller arrangement the bottom roller used is always a steel fluted roller while the top roller is covered with some synthetic rubber covering.The top rollers are pressed down with sufficient force on to the bottom rollers to ensure proper grip of the fibres. A pressure of 100 to 250N per roller is provided on the top rollers by using mostly the spring pressure. However in addition to spring pressure, pneumatic and magnetic weighting is also used by some manufacturers. A simple 3 over 3 roller drafting arrangement is shown below:
In modern roving frames a double apron system is provided at the middle pair of drafting rollers. The top apron is short while the while the bottom apron is longer. Both of these aprons are made of either soft leather or synthetic rubber. Both these aprons in cooperation with each other guide and transport the fibres during drafting. The apron help to support the floating fibres and drastically reduce the drafting wave. It is important the apron should extend as close as possible to the nip line of the front rollers.The length of apron also called as the cradle length is kept approximately equal to the staple length of the fibres.
Use of condenser in the drafting system: Three special design trumpet guides or condensers are used in the drafting section of the machine.First trumpet guide also known as in-feed condenser is used just before the black pair of the drafting rollers and its purpose is to lead the sliver properly in to the drafting arrangement. The second trumpet guide is used near the nip of the middle pair of the rollers and third one is used just before the front pair of rollers. The main function of last two guide is to bring back the fibre mass in to a strand that tends to tear apart because of the drafting action.The trumpet guides are usually mounted on a reciprocating bar giving the sliver a traversing motion so that the wear of the roller may spread out gradually over the entire width of the roller.
Draft distribution-An important consideration of the drafting zone is the draft distribution i.e. how much draft should be given in the back and front zones. The draft at the back zone is called as break draft and it should be as low as feasible while majority of the draft is given at the front zone where there is an apron control over the fibres and is called as the main draft.The total draft is the product of break and main draft. Generally the break draft for cotton lies in the range of 1.05 to 1.15 and all the remaining draft is given at the main drafting zone. The break draft is just meant to straighten the fibre and to prepare them for a major draft at the front drafting zone. If the break draft is increased beyond an optimum value then the evenness of the spun yarn drastically reduces reduces due to the formation of thick and thin places.
3. Winding Section
The winding section comprises of a spindle and a flyer. A spindle is a long steel shaft that acts as a support and a driving element for the flyer. A spindle is a long steel shaft that acts as a support and driving element for the flyer. The flyer is a special component of the roving frame that help to insert twist in the roving. The spindle is mounted at its lower end in a bearing which gets its drive from the train of gears and transmits it to the layer. Just around the flyer a shaft is fixed around the spindle with a collar that gets its drive independently, from a separate set of gears. An empty hollow package made of wood or plastic is mounted on this shaft.The arrangement of a spindle and a flyer is shown below:
3. Objectives of Roving Frame
The winding portion of the machine with the help of the spindle and the flyer meets the following two main objectives of the roving frame:
a) Twisting
b) Package winding
1. Twisting
The sliver after being drafted out to form a roving combing from the front delivery rollers have little fibre cohesion among themselves and is weak to be wound on packages and also may not sustain further drafting at the next stage of the processing i.e. the ring frame .So in order to give cohesion and strength to the roving strand a small amount of protective twist is given. The twist is imparted by using a flyer method of twist insertion.The roving coming out of the front delivery roller is threaded through the top of the flyer ,passes through its hollow leg around the pressure arm on to the bobbin. The pressure arm maintains certain tension on the roving which is necessary for proper compact winding.
The flyer rotates with the spindle at a constant speed with each revolution of the flyer inserting one twist in the roving strand.
The relation is given by:
Twist per Inch = Flyer Rotary speed (rpm) / Delivery speed (Inches/min)
Since the rotary speed of the flyer is constant so the amount of twist inserted per unit length depend upon the delivery rate of the front delivery rollers.
2. Package Winding
In order for winding to take place on the surface of the bobbin, the surface speed of the roving coming from the flyer should be different from the surface speed of the bobbin .The bobbin therefore is driven independently of the flyer and it rotates with the collar around the spindle Winding at the roving frame is possible by one of the following two methods:
1)Bobbin lead method
2)Flyer lead method
In the bobbin lead method to facilitate winding,the surface speed of the bobbin is kept greater than the surface speed of the flyer.On the other hand in flyer lead method,the surface speed of the flyer is kept more than that of the bobbin.
In cotton roving frame, the bobbin lead method is used.
A. The package Built
The roving coming out of the hollow leg of the flyer is made to wind on a cylinder package is called as the bobbin. The bobbin is entire made of wood or plastic. The roving is wound on the bobbin in such a way so that tapered ends on both sides of the package are formed. The angle of taper of ends lies between 80 to 95 depending upon the adherence of the material. By increasing the angle of taper ,larger packages are possible to make however the adherence between the layers decreases. On the other hand low angle of taper gives better adherence to the material but only smaller packages are possible to make with it.
Bobbin is an ideal form of a supply package to be fed to the ring spinning frame because when full it can hold large length of roving owing to its compactness and when empty it is light and occupies small volume and is easy to handle and transport.
B. The Builder Motion
The above mentioned required package built is not easy to construct. There are lots of complex mechanical arrangement provided at the roving frame to achieve this, due to these special winding needs at the roving frame it makes it a very complicated machine. The builder motion is a device or series of mechanical arrangements necessary to obtain proper built of the roving bobbin. The builder motion performs the following important tasks related to the package built:
- Controlling Bobbin drive
- Controlling Lifter motion of Bobbin Rail
- Formation of tapered ends
1. Controlling Bobbin Drive
During the package winding with each new layer of roving wound on the bobbin, the diameter of the bobbin increases which causes the surface speed of the bobbin to increase as well. However for a uniform package winding, it is absolutely essential to keep the difference in the surface speed of the flyer and bobbin constant. Since the rotary speed of the flyer in a roving frame is kept constant so to keep this difference of the surface speed constant, the rotary speed speed of the bobbin is reduced proportionately with increase in its diameter.
The reduction in the rotary speed of the bobbin originates from a cone drum drive. With each new layer of roving wound on the bobbin, the builder motion shifts the cone belt enough to reduce the rotary speed of the bobbin so that its surface speed may remain constant. A simple arrangement of a cone drum drive is shown in the figure below:
2. Controlling Lifter Motion of Bobbin Rail
The roving package is created by placing layer upon layer of parallel coils of roving frame on the surface of the bobbin. The built of the package is carried out in a precision manner where each successive layer or coil of roving is laid in a precise spacing to the previous coil. This can be achieved by
a) Raising and lowering of the flyer
b) Raising and lowering of the bobbin
The flyer is never moved because by doing so the unsupported length of the roving coming from the front delivery roller to the top of the flyer will vary. Also the angle of the roving from the front delivery roller to the flyer top will also change. This can cause uneven winding tension and is not suitable. So the only way to achieve a proper built of the package is to move the bobbin up and down. For this purpose the bobbins are supported by a movable rail that lifts and lowers the bobbin as per requirement.
Since the package diameter is steadily increasing, the lift speed must also be reduced by a small amount after the completion of every layer. This is necessary because with a bigger diameter of a package more length of roving will be accommodated and the lifting and lowering of the bobbin should be slowed down. To obtain this,the drive to the lifter mechanism is also obtained from the same cone drum drive used for the bobbin. The lifter mechanism is also fitted with a reversing mechanism so that the bobbin rail is alternately raised and lowered so that the roving could be wound on the entire length of the bobbin.
3. Formation of Tapered Ends
In order to form special taper end on the bobbin the height of the lift through which the lifter motion is going to raise and lower the bobbin is gradually reduced after each layer of roving has been completed. For this the builder motion of the roving frame is fitted with special micro switches that perform this action.
Controlling the Roving Tension
The roving coming out of the lower end of the hollow leg of the flyer passes over a pressure arm. The pressure arm has to guide the roving form the lower exit of the flyer leg on to the package. Before the roving could be wound on the package it is wrapped two or three times on the pressure arm.
The number of turns determine the roving tension. More tension would means greater roving tension and would result in a harder and more compact packages may occur. The working principle of the flyer roving frame can be summarised as below:
- Roller drafting, delivers fibre strand at a constant speed Vd.
- Flyer rotate at nf (constant) to twist the strand
Twist level = nf/Vd
- bobbin rotates at nb (different to nf) to wind on the roving.
- Either bobbin lead or flyer lead, as long as there is a difference in rotational speed
- winding on speed vw=πDr(nf-nb) (Dr=roving diameter on bobbin)
- Dr varies as the long package builds up, change nb to match Vw with Vd.
Modern Developments in Roving Machine
1. Developments in Speed-Frame
The speed frame is the machine that has gone least change. This can be realised from the fact that a spindle speed of 1,500 rpm as on date as compared to 600 rpm in 1950s. Still there are certain changes that have been carried out for better production and quality like, increase in the roving bobbin diameter from 4″ to 7″ and lift from 8″ to 16″, use of straight cone drum instead of hyperbolic cone drum for better control over the roving tension etc. Now all closed (AC Type) flyers are used to overcome the problem of air drag on roving.
These flyers are aerodynamically balanced and are of light weight. The roving frames are equipped with auto doffing system that, apart from avoiding man handling, reduces doffing time. Even the Toyota claims to have auto doffing on FL100 roving frame in record time of 3½ minutes. Roving bobbins auto doffing and transportation, to the ring spinning through over head rails, becomes a standard feature of the roving frame. Apart from above, some of the developments those are really significant will be discussed in subsequent sections.
2. Equal Roving Geometry for Front and Back Row Roving
Deposition of roving spindles in two rows leads to variation in roving twist and count in front and back row due to different level of twist attained by roving in front and back row. This is due to change in roving path geometry. Modern speed frames have raised flyer top of the back row as compared to the front row to maintain the roving delivery angle.
3. Individual Motor Drive
In multi-motor drive system, drafting rollers, flyers, bobbins and bobbin rail are driven directly by individual servomotors and are synchronized throughout package build by the control system. The advantages of this system are like, no need of heavy counter weight for bobbin rail balancing and differential gear, reduced maintenance, lower energy consumption, etc. All machine manufacturers incorporate four-motor-drive system except Toyota that uses three servomotors on FL100 roving frame. One motor for drafting system and flyer while one motor each for bobbin drive and bobbin rail.
4. Roving Tension Sensors
Roving tension sensors measure and control the roving tension (constant) through out the bobbin build. These tension sensors do not actually contact the roving while measuring the tension. The tension is measured at periodic intervals and the required change in tension is actuated by changing the bobbin speed through servomotor. Rieter F15/F35 roving frame, Zinser 668 roving frame, Marzoli FTN roving frame, Lakshmi LFS 1660 speed frame and Toyota FL100 roving frame have incorporated roving tension sensor on their machines.