Web Guiding System: The Hydraulic Drive Behind Accurate Edge Guiding
A web guiding system is a control loop that keeps a moving web at a fixed sideways position: a sensor reads the edge, a hydraulic power pack converts the reading into oil flow, and a cylinder moves a stand or frame to correct the path. Krishna Engineering Works builds web guiding systems in Ahmedabad for flexible printing, packaging and converting machinery.
What is a web guiding system?
A web guiding system is the equipment that stops a web from wandering across the machine. Every web drifts to some degree, because parent reels are wound with small edge errors, rollers are never perfectly parallel and tension is never perfectly even. Left alone, the drift reaches the knives, the print units or the rewind and becomes scrap.
The system corrects automatically and continuously. It is used to guide flexible materials of many kinds, including film, paper, foil, laminates and fabric, at the unwind, in the middle of the machine and at the rewind. Krishna Engineering Works offers the system with edge guiding, aligner units and tracking roller assemblies.
This page looks at the system from the drive side. The sensor decides what to correct, but the hydraulic power pack and cylinder decide how fast, how hard and how smoothly the correction is made.
The four parts of the loop
A web guiding system has four parts, and the loop is only as good as the weakest of them. Each is described below with the question it answers.
| Part | Question it answers | What limits it |
| Sensor | Where is the edge now? | Web type, dirt, flutter, position on the machine |
| Power pack and servo valve | How much oil, and in which direction? | Pump flow, line pressure, oil cleanliness |
| Cylinder | How far and how hard to move? | Bore, stroke, mounting stiffness |
| Guide frame or stand | What moves the web? | Friction, play, entry and exit spans |
Together they form a control system with feedback: the sensor sees the effect of each correction and the valve keeps adjusting until the error is gone. The power pack is covered part by part on the hydraulic power pack page.
Unwind, steering and rewind guiding
There are three places to guide a web, and each uses the cylinder in a different way. Choosing the right one matters more than any other decision.
Unwind guiding
At the unwind, the cylinder shifts the whole reel stand sideways so that the web leaves it on the correct line. The sensor is fixed to the machine frame, just after the first idler roller that moves with the stand. This is the most common position, because it corrects reel errors before they enter the process. The cylinder must move the full weight of the reel, so this is where power pack force matters most.
Steering or intermediate guiding
In mid machine neither end can be moved, so a pivoting frame with idler rollers turns the web back onto its line. The frame is the tracking roller assembly, built with centre pivot or end pivot guiding. The load on the cylinder is small, but the web needs enough free span before and after the frame to twist without creasing.
Rewind guiding
At the rewind the logic is reversed: the web cannot be moved, so the reel is moved to meet it. The sensor is mounted on the moving rewind stand and travels with it, and the cylinder shifts the stand until the sensor finds the edge. The result is a reel with straight, flat sides, which is what the next customer sees first.
| Guiding position | What moves | Sensor mounted on | Load on the cylinder | Main benefit |
| Unwind | Unwind stand and reel | Fixed machine frame | High, grows with reel weight | Web enters the process on line |
| Steering | Pivoting roller frame | Fixed frame after the guide | Low | Corrects drift after long sections |
| Rewind | Rewind stand and reel | Moving rewind stand | High, grows as the reel builds | Straight sided finished reel |
Matching the power pack to the guide
Match the power pack to the mass that has to move and the speed at which the error appears. The two standard models give the following published figures.
| Item | KEW-17 | KEW-30 |
| Motor rating (3 phase, 50 Hz) | 0.5 HP | 1 HP |
| Pump flow rate | 3.4 l/min | 5.8 l/min |
| Tank capacity | 17 litres | 30 litres |
| Line pressure | 14 bar | 21 bar |
| Maximum operating force (50.8 mm diameter cylinder) | 260 kg | 389.5 kg |
| Follow-up speed | 30 mm/sec | 40 mm/sec |
| Weight including oil | 40 kg | 60 kg |
Both use Servo 32, Enklo 44 or Hydro 37 oil, and the air sensor works at 5 to 28 mbar, infinitely adjustable. The points below turn the figures into a choice.
- For steering frames and light unwinds the smaller model is normally enough, since the cylinder only has to overcome the friction of a light frame.
- For heavy reels, or stands that run on plain slides, compare the push needed with the maximum operating force and move to the larger model if the margin is thin.
- For fast lines and badly wound reels, look at follow-up speed. The guide must be able to move faster than the edge can drift.
- For machines with two or three guides, a double or triple power unit carries two or three servo valves on the same reservoir.
The force a guide needs is set by friction, not by weight alone. A stand on clean linear bearings needs a small fraction of the push that the same stand needs on dry, worn slides. Freeing the mechanics is often the cheapest way to improve a sluggish guide.
Why is hydraulic correction used for guiding?
Hydraulic correction is used because it gives high force, smooth motion and firm holding from a compact cylinder. A reel stand with a full reel is heavy, and the correction has to be gentle enough not to shake the web and stiff enough not to drift between corrections.
Oil is nearly incompressible, so a hydraulic cylinder holds its position rigidly when the valve is closed. The servo valve is proportional, so small errors produce slow movements and large errors produce fast ones. The cylinder is small for the force it gives, which makes it easy to fit under a stand. The power pack can stand a few metres away, joined only by pipes.
The cost is that the oil must be kept clean and the pipes tight. Those duties are light when they are done routinely.
Choosing the sensor
Choose the sensor by the web and the surroundings. Two sensing methods are offered with the hydraulic drive.
| Sensing method | How it reads the edge | Suits | Take care with |
| Air sensor, hydro-pneumatic unit | The edge partly blocks a low pressure air jet | Clear and printed films, paper, foil, dusty or solvent areas | Open or porous webs, strong draughts |
| Photocell sensor, electro-hydro unit | The edge partly blocks a light beam | Opaque webs, positions where an electrical panel is preferred | Clear films, dust on the lens |
The air sensing version is described on the hydro-pneumo web aligner page. The electro-hydro web aligner (power pack) unit uses a photoelectric sensor for the edge, with an electrical panel board to operate the complete unit, a hydraulic cylinder, an adjuster and an oil pipe.
Sensor technology is a subject in its own right. The client’s web guide system site compares edge, line and ultrasonic sensing and the actuator types that go with them.
How to plan a web guiding system
Plan from the process backwards. The steps below give a sound order.
- Mark the points on the machine where web position really matters: knives, print units, coating head, lamination nip, rewind.
- Place a guide immediately before each of those points, or as close as the layout allows.
- Decide the guiding position for each: unwind, steering or rewind.
- Decide what is guided: one edge, the centre line or a printed line.
- Check the spans. A steering frame needs a free entry span and a free exit span, each generous in relation to the web width.
- Work out the moving load at each guide and select the power pack model, or a multiple unit for several guides.
- Fix the position of each sensor and each power pack, and check the pipe runs.
A guide corrects only what happens before it. Drift that arises after the guide, in a long dryer for example, needs a second guide at the far end.
Which machines use a web guiding system?
Machines that print, coat, laminate, slit or rewind a web all benefit. Krishna Engineering Works supplies guiding for rotogravure printing, flexo, lamination, coating, slitting rewinding, doctoring rewinding and inspection rewinding machines, among others. A slitter rewinder with a web guiding system holds its slit widths and trim.
Lamination is a good example of several guides working together, because two webs must arrive at one nip with their edges matched. That case is set out on the page for the power pack on a lamination machine. The other machine pages are listed under power packs by machine.
Common web guiding faults and the hydraulic causes
When a guide misbehaves, check the mechanics and the oil before the sensor. The table links each symptom to its usual cause on the drive side.
| Symptom | Likely cause on the drive side | Action |
| Guide swings slowly from side to side | Sensor too far from the guide, or play in the cylinder mounting | Move the sensor closer, tighten pins and brackets |
| Guide chatters in small quick movements | Air in the oil or sensitivity set too high | Bleed the cylinder, reduce sensitivity |
| Guide follows too slowly | Low flow from a blocked filter, cold oil, or a stiff stand | Clean the filter, check the oil grade, free the slides |
| Guide stops short under a full reel | Force too low for the load | Check line pressure, reduce friction, consider the larger model |
| Guide drifts when the line stops | Internal leakage in the valve or cylinder seals | Have the unit serviced |
| Guide runs to the end of its stroke | Reel loaded off centre, or sensor not at the edge | Centre the reel, reposition the sensor |
Units of Indian and imported makes, including E+L and BST web guiding systems, can be sent for repairs and servicing.
From single unit to complete system
A single guide is usually bought as a web aligner unit, a kit with power pack, sensor, adjuster, filters, pipes and cylinder. A complete system is a set of such units, each placed where the process needs it, sometimes sharing one reservoir. The main brand site shows the product under web guiding system with the rest of the converting range.
Krishna Engineering Works has the capacity to supply 100 to 150 units per month and invites enquiries from machinery manufacturers who buy in bulk.
What to send for a web guiding system quotation
Send a simple sketch of the web path with the data below, and the guides can be proposed position by position.
- Machine type, and the points where web position matters
- Web materials, width range and line speed
- For each guide: unwind, steering or rewind
- Heaviest reel at each unwind and rewind, and how each stand is carried
- Free web spans available where a steering frame is planned
- What is to be guided: one edge, the centre or a printed line
- Conditions at each sensor: dust, solvent vapour, heat, draughts
- Existing guiding equipment, with make and model
Send the enquiry through the contact page, or see the whole range on the hydraulic power pack products page.
Frequently Asked Questions
What are the main types of web guiding?
There are three main types: unwind guiding, steering guiding and rewind guiding. Unwind guiding shifts the reel stand so the web enters the machine on line. Steering guiding uses a pivoting roller frame to correct the web in mid machine. Rewind guiding shifts the rewind stand so the reel builds with straight sides.
How fast does a web guide need to correct?
A web guide needs to correct faster than the web edge can drift at full line speed. Reels with a gradual offset need little speed, while reels with stepped or wandering edges on a fast line need much more. The KEW-17 power pack gives a follow-up speed of 30 mm/sec and the KEW-30 gives 40 mm/sec on the published cylinder size.
Should I guide to the edge or to the centre of the web?
Guide to one edge when the web width is constant and the process refers to that edge, as slitting and most printing do. Guide to the centre when the width varies from reel to reel and the web must stay central on the machine. Centre guiding needs two sensors, one at each edge, working together.
Why does my web guide hunt from side to side?
A guide hunts when it corrects too late or too strongly. The usual causes are a sensor mounted too far from the guiding roller, sensitivity set too high, air in the hydraulic oil, or loose pins and brackets on the cylinder. Move the sensor closer, bleed the cylinder, tighten the mounting and then reduce sensitivity.
How much free span does a steering guide need?
A steering guide needs a free span of web before and after the frame, so the web can twist without creasing. As general practice each span is at least a substantial fraction of the web width, and stiffer webs such as paper and foil need more than extensible films. If the space is not available, guide at the unwind instead.
Can a web guiding system remove wrinkles or baggy edges?
No. A web guiding system controls sideways position only. Wrinkles are treated with spreading rollers, baggy edges come from the parent reel or from uneven tension, and neither is changed by moving the web sideways. Correct those faults at their source, then let the guide hold the web on its line.