Hydraulic Power Pack for Rotary Machine Fabric and Web Guiding
A hydraulic power pack for rotary machine guiding moves the fabric batch or a steering frame at the entry, so the cloth is laid straight on the machine before printing or processing begins. Krishna Engineering Works builds the power pack in Ahmedabad and supplies it as a complete guiding unit for rotary printing and rotary textile machines.
What is guided on a rotary machine?
On a rotary machine the incoming fabric or web is guided, at the point where it enters. Rotary machines process a continuous length with rotating tools: engraved or perforated cylinders, screens and rollers that turn at the speed of the material. The term covers several machines in a textile or printing plant.
- Rotary screen printing machines for textiles, where fabric is carried on a printing blanket under a row of rotating screens
- Rotary printing machines for paper, transfer paper, wall coverings and other webs
- Rotary coating and finishing machines that apply paste or finish through a screen or roller
- Entry and batching sections that feed and collect fabric for these machines
In all of them the tools are fixed across the machine. The design is printed where the screens are, and the fabric has to be there to receive it. The control task is a simple closed loop: a sensor watches the selvedge, a servo valve meters oil, and a cylinder moves the fabric until the selvedge is back at the sensor.
Why must fabric be guided at the entry?
Fabric must be guided at the entry because on a rotary screen printing machine there is no second chance. The cloth is pressed onto an adhesive coated printing blanket at the feed end and stays stuck to it under every screen. From that moment it cannot be shifted. If it is laid down off line, or laid down on a slant, the whole design is printed off line or on a slant.
The blanket has its own guide to keep it on its rollers, and that is part of the machine. The guide described here controls the fabric before it touches the blanket.
| Fault at the entry | What appears on the printed fabric | What entry guiding does |
| Fabric drifts to one side | Unprinted strip on one selvedge, print running off the other | Holds one selvedge on a fixed line |
| Fabric enters on a slant | Design drifts steadily across the width along the piece | Corrects the batch before the slant builds up |
| Selvedge wanders after each seam | Print position jumps at every joined piece | Follows each new piece back to the line |
| Operator steers by hand | Uneven result that depends on attention | Corrects continuously without the operator |
| Fabric width set wider than needed for safety | Extra unprinted selvedge and wasted cloth | Lets the margin be reduced to what the fabric needs |
On other rotary machines, where the web runs free between rollers, guiding at the entry still does the most good. A correction made there is carried through the whole machine.
How is fabric guided from a batch or from a trolley?
Fabric is guided by shifting the batch when it comes from a roll, and by a steering frame when it comes plaited from a trolley. The supply decides the method.
Fabric from a batch roll
Prepared fabric often arrives wound on a large batch roll carried on an A-frame. The batch station is mounted on rails across the machine, and the cylinder of the power pack shifts the station with the roll on it. The sensor is fixed to the machine frame after the first roller and reads the selvedge. This is the same arrangement as the shifting unwind of any converting machine, with a heavier and wider load.
Fabric from a trolley or a scray
Fabric that is plaited in a trolley, or that comes from a scray or J-box, has no roll to shift. It is drawn up over tension bars and rollers, and it is steered on the way by a pivoting roller frame. The tracking roller assembly is built for this: a movable frame with idler rollers, swung by a servo cylinder that answers the hydraulic signals of the power unit. The fabric must grip the rollers of the frame, so it needs steady tension and a reasonable wrap around them.
After the dryer
Printed fabric leaves the blanket, passes through a dryer and is batched or plaited at the exit. Where it is batched on a roll, a second guide at the batcher winds a roll with level selvedges for the next process. Both guides can share one tank. Double and triple units, described on the double and triple hydraulic power units page, carry two and three servo valves on the same reservoir.
What makes a fabric selvedge harder to sense than a film edge?
A fabric selvedge is thicker, rougher and less regular than the cut edge of a film or paper web, and the sensor has to cope with that. Four properties matter.
- Selvedges curl. Knitted fabric and some light wovens roll at the edge under tension. A curled edge is not where the cloth ends, so the edge should be opened by an uncurling device before the sensor.
- Selvedges are hairy. Loose threads and fringe stand out from the edge. A sensor that reads a broad band averages them out better than one that reads a fine line.
- Fabric is porous. Open weaves and light knits let air pass. An air sensor reads a dense woven fabric well, but an open fabric blocks less of the air jet and gives a weaker signal.
- Seams pass through. Every joined piece brings a seam and often a small step in width. The guide should follow the step smoothly, without a sharp jerk that would wrinkle the cloth.
The choice of unit follows from the fabric.
| Fabric or web | Sensing that suits | Reason |
| Dense woven fabric, canvas, coated fabric | Air sensor | The cloth blocks the air jet, and the sensor has no electrical parts at the wet end |
| Open weaves, light knits, nets | Photocell edge sensor | Air passes through the cloth, while the photocell reads the edge by light |
| Paper and transfer paper on rotary printing machines | Air sensor | Solid, opaque web, often with dust |
| Fabric with a strong curl at the edge | Either, after an uncurler | The edge must be opened flat before it is read |
The air sensing unit is described on the hydro-pneumo web aligner page. Its air sensor works at 5 to 28 mbar, infinitely adjustable, so the jet can be set softly for light cloth. The photocell version is described on the electro-hydro web aligner page.
A web guide moves the whole fabric sideways. It does not straighten a bowed or skewed weft, which is the work of a weft straightener, and it does not set the width, which is done on the stenter. For light selvedge guiding at stenter entries and similar points, the client’s cloth guider site shows the devices made for that duty.
How do water and humidity affect the power pack?
Water and humidity affect the power pack by getting into the oil and by corroding exposed steel. A print room is wet. Screens and blankets are washed, floors are hosed, the dryer and any steamer release vapour, and the air stays humid all day.
Water enters a tank in two ways. It is splashed or hosed in through the filler and the breather, and it condenses inside the tank when warm, moist air is drawn in and the tank cools after a shift. Water does not lubricate. It rusts the pump and the valve, and it helps the oil to foam.
| Sign | What it means | Action |
| Oil looks milky or cloudy | Water is mixed into the oil | Change the oil and the filter, then find where the water enters |
| Free water at the bottom of the tank | Condensation has collected over time | Drain it from the lowest point at planned intervals |
| Foam on the oil surface | Water or air in the oil | Check the oil level and the suction side, change contaminated oil |
| Rust spots on the cylinder rod | Rod is being wetted and left to dry | Wipe and oil the rod, shield the cylinder from spray |
| Guide hunts or sticks after a wash-down | Water has reached the valve or the sensor air | Dry and clean the air filter, check the oil |
| Air sensor signal drifts | Moisture in the sensor air | Clean and drain the air filter |
Prevention costs little. The points below cover most of it.
- Stand the power pack on a raised base, above the level that floor water reaches.
- Place it outside the wash-down area. The 5 metre pipes of the standard kit give room to do so.
- Fit a simple cover over the top of the tank if spray can reach the filler and the breather.
- Never hose the unit. Wipe it clean.
- Keep the filler closed and top up from a closed, dry can kept for this oil only.
- Check the level and the colour of the oil every week, and drain any water at the same time.
- Wipe print paste from the cylinder rod before it dries, since dried paste cuts the rod seal.
Use only Servo 32, Enklo 44 or Hydro 37 oil. The part played by the oil filter in holding back rust particles and dirt is the same as in any oil system, and in a wet room it should be cleaned more often. Filter elements, seals and pipes are listed on the web aligner spare parts page.
The motor of the power pack is an electrical item, which is one more reason to keep the unit out of the wet zone. With air sensing, the only parts at the fabric are the air sensor and the cylinder, and neither carries current.
Which power pack model suits a rotary machine?
The model is set by the weight of the batch station or by the frame to be moved. Fabric batches are wide and can be heavy, so the batch station is the demanding case.
| Guiding position | What moves | Model usually considered |
| Batch station at the entry, large rolls | A-frame, roll and station | KEW-30 |
| Batch station, small or light rolls | Roll and station | KEW-17 |
| Steering frame for fabric from a trolley | Light roller frame | KEW-17 |
| Batcher at the exit | Roll and batcher carriage | KEW-30 |
| Paper or transfer paper unwind on a rotary printing machine | Reel and stand | By reel weight |
The KEW-17 gives a maximum operating force of 260 kg at a line pressure of 14 bar, and the KEW-30 gives 389.5 kg at 21 bar, both on a cylinder of 50.8 mm diameter. Their follow-up speeds are 30 mm/sec and 40 mm/sec. Fabric drift is slow, so speed is seldom the limit on a rotary machine. Force is, and the rails of a batch station in a wet room corrode and stiffen if they are not looked after.
A wide station pushed off its centre line twists and jams. Fix the cylinder on the centre line, low down, between two parallel rails. Power pack units are available in different models from 0.25 HP to 3 HP, and the components of each are explained on the hydraulic power pack page.
Related machines and industries
Rotary machines sit beside other printing and coating machines in this range. Web presses for paper are covered on the power pack for an offset printing machine page, and packaging presses on the power pack for a flexible printing machine page. Fabric that is coated after printing meets the heat and margin questions set out on the power pack for a coating machine page.
Textile, technical textile, jute and carpet applications are grouped on the power packs by industry page. Units that have suffered from water in the oil can usually be recovered through repairs and servicing.
What to send for a rotary machine guiding quotation
The unit is matched to the fabric and to the way it is supplied to the machine. Send the details below.
- Type of rotary machine and its working width
- Fabric or web: woven, knitted, coated, paper, and how open or dense it is
- Maximum and minimum fabric width
- Supply at the entry: batch roll on an A-frame, plaited trolley, scray or reel
- Maximum weight and diameter of the batch or reel
- Whether the exit batcher is to be guided as well
- Machine speed
- How wet the area is, and where a dry place for the power pack is available
- Sensing preferred: air sensor or photocell
- For a replacement: make and model of the existing guide
Use the contact page to send the fabric and batch data. Printing, coating and rewinding machines outside the textile field are listed under power packs by machine.
Frequently Asked Questions
Why is the fabric guided before the printing blanket and not after?
The fabric is guided before the blanket because it is stuck to the blanket for the whole printing length. Once it has been pressed onto the adhesive it cannot move sideways, so any error at that point is printed in. The guide therefore has to place the selvedge correctly in the entry section, ahead of the feed roller.
Can an air sensor read the selvedge of a woven fabric?
Yes, when the fabric is dense enough to block the air jet. Shirting, sheeting, canvas and coated fabrics are read well. Open weaves, light knits and nets let much of the air through and give a weak signal, so a photocell edge sensor is the better choice for them. State the fabric construction in the enquiry.
What should be done if the oil in the tank turns milky?
Milky oil contains water and should be changed. Drain the tank, clean it, clean or renew the oil filter and refill with fresh oil of the stated grade. Then find the way in, which is usually the filler, the breather or wash water aimed at the unit, and correct it so the new oil stays dry.
How is fabric from a plaited trolley guided when there is no roll to shift?
It is steered by a tracking roller assembly placed in the entry section. The frame carries idler rollers and is swung by a servo cylinder from the power pack, which turns the fabric back towards the sensor. The cloth needs steady tension and enough wrap on the rollers of the frame to follow them without slipping.
Does the web guide correct bowed or skewed fabric?
No. The guide shifts the whole fabric sideways so that one selvedge runs on a fixed line. Bow and skew are distortions of the weft across the width, and they are corrected by a weft straightener. Width is set on the stenter. The guide works alongside those machines and does not replace them.
Where should the power pack stand in a wet print room?
Stand it on a raised base outside the wash-down area, within reach of the 5 metre pipes. Keep the top of the tank covered if spray can reach it, and never hose the unit. Only the cylinder and the sensor need to be at the fabric, and with air sensing neither of them carries electrical current.