Hydraulic Power Pack for Offset Printing Machine Infeed Guiding
A hydraulic power pack for offset printing machine guiding moves the paper reel stand, or a steering frame at the infeed, so the web enters the first printing unit of a web offset press on the same line throughout every reel. Krishna Engineering Works builds the power pack in Ahmedabad and supplies it with an air sensor or a photocell edge sensor.
Which offset presses need a web guide?
Web offset presses need a web guide, because they print on a continuous web drawn from a reel. Sheet-fed offset presses do not: each sheet is registered against front and side lays before it is printed, so there is no running web to steer.
On a reel-fed press nothing in the printing units holds the paper sideways. The blanket cylinders grip the web and carry it through, wherever it happens to be. The sideways position is therefore decided before the first unit, and that is where the power pack does its work. The types of press concerned are listed below.
- Commercial heatset web offset presses with a dryer and chill rolls
- Coldset presses for newspapers and books
- Narrow web offset presses for business forms, labels and security printing
- Reel-to-sheet presses that print from the reel and deliver sheets
What does infeed guiding protect on a web offset press?
Infeed guiding protects the position of the print on the paper and, through it, the position of every fold and cut that follows. The table shows what depends on the sideways position of the web.
| Part of the press | What it needs from the web | Result when the web wanders |
| First printing unit | Web entering on a fixed line | Whole image shifted sideways on the paper |
| Later units | Web arriving where the first unit left it | Sidelay register system has to work harder |
| Former and folder | Print centred on the former nose | Fold runs off the centre of the page, margins unequal |
| Slitters and ribbon section | Print lanes under fixed slitters | Ribbons of unequal width, pages out of position in the signature |
| Sheeter or rewind | Constant edge margin | Sheets or reels with a varying margin, trouble in finishing |
Colour to colour register is the job of the register system of the press. The guide gives that system a steady web to work on, and it fixes the one thing register control cannot: where the paper itself is.
Where are the guides placed?
Guides on a web offset press are placed at the reel stand and, on longer presses, again after the infeed and before the folder. Each uses the cylinder of the power pack in its own way.
At the reel stand
The reel stand is the first and most important position. On a simple stand, the whole stand sits on slides and the cylinder shifts it. On a shaftless stand, the arms that hold the reel between two chucks are carried on a cross slide, and the cylinder shifts the arms together with the reel. In both cases the edge sensor is fixed to the press frame, just after the first roller that follows the reel.
A stand with two or three reel positions for automatic splicing is a heavy moving mass. The force margin of the power pack should be checked against the full weight of everything that shifts, not against one reel.
After the infeed
Between the reel stand and the first unit the web passes the splicer, a festoon or dancer and the infeed nip. Where that path is long, the web can settle to a new line of its own. A pivoting roller frame placed just ahead of the first unit makes the last correction close to where it matters. The frame is the tracking roller assembly, driven by a servo cylinder from the same kind of power pack.
Before the folder, sheeter or rewind
After a heatset dryer and chill rolls the web has travelled a long way and has lost moisture, and it may have moved. A second steering frame ahead of the former or the sheeter brings it back, so the fold or cut falls where the print is. Reel-to-reel forms presses guide at the rewind instead, with the sensor carried on the moving rewind stand.
How does paper change width through the press?
Paper grows wider as it takes up fountain solution, and this decides which edge to guide from. Offset printing puts water on the web at every unit. The fibres swell, and the web leaving the last unit is slightly wider than the web that entered the first. Printers call the effect fan-out.
For guiding, the consequence is simple. An edge guide holds one edge still, so all of the width change appears at the other edge. Three habits keep that harmless.
- Choose one reference edge and keep it. If the reel stand guides to the operator side edge, any steering frame further down the press and the finishing line after it should refer to the same side.
- Do not chase the far edge. A varying margin on the unguided side during a run is the paper changing width, not the guide failing.
- Tell the supplier if centre guiding is needed. A press that must hold the centre line of the web, as some folder layouts require, needs sensing on both edges, which is a different arrangement from a single edge sensor.
Reel width also varies a little from reel to reel. Guiding from one edge places that variation on the other side every time, where the trim or the margin can absorb it.
Which sensor suits paper dust at the infeed?
An air sensor suits a dusty infeed, and a photocell sensor suits it too if the lens is kept clean. The choice is practical, since paper is opaque and solid and both sensing methods can read it.
The infeed of a web offset press is one of the dustiest places in a press hall. Loose fibre and filler come off the reel edge and the reel face, the splicer cuts paper at every reel change, and coldset inks add a fine mist. The table compares the two sensors in those conditions.
| Condition at the infeed | Air sensor | Photocell edge sensor |
| Paper dust and lint | Outflowing air keeps the sensing gap clear | Lens collects dust and needs regular wiping |
| Ink mist | Little effect while the air is flowing | Film on the lens weakens the signal slowly |
| White, coloured and preprinted paper | Same signal for all, the sheet blocks the air | Reads all opaque paper, setting may need checking for preprint |
| Very light, open grades | Some air passes through the sheet, so test first | Reads the edge by light |
| Maintenance skill | Mechanical fitter, clean air and a clean air filter | Electrician for the sensor and the panel |
| Signal path | Air pipe to the servo valve | Cable to the electrical panel |
The air sensor works at 5 to 28 mbar, infinitely adjustable, and is the sensing part of the hydro-pneumo web aligner. The photocell version is the electro-hydro web aligner (power pack) unit, which consists of a photocell sensor for the edge, an AC motor, an electrical panel board to operate the complete unit, a hydraulic cylinder, an adjuster and an oil pipe. How a photoelectric sensor detects an edge is described in general references, and the unit has its own page under electro-hydro web aligner.
Whichever sensor is chosen, mount it where the splicer does not shower it with offcuts, and where the operator can reach it to move it for a new reel width.
Which power pack model suits a web offset press?
The model follows the weight that the cylinder has to shift. Paper reels are dense, so the reel stand of an offset press is usually a force question.
| Press and position | Moving mass | Model usually considered |
| Narrow web forms or label press, shifting reel stand | Small reel and light stand | KEW-17 |
| Commercial press, single reel stand on rolling bearings | Medium to heavy reel | KEW-17 or KEW-30, by reel weight |
| Commercial or newspaper press, shaftless arms on a cross slide | Heavy reel, arms and chucks | KEW-30 |
| Multi position reel stand for automatic splicing | Two or more reels with the frame | KEW-30, or a larger model on enquiry |
| Steering frame at the infeed or before the folder | Light roller frame | KEW-17 |
The KEW-17 has a 0.5 HP motor, a line pressure of 14 bar and a maximum operating force of 260 kg. The KEW-30 has a 1 HP motor, 21 bar and 389.5 kg. Both force figures are for a cylinder of 50.8 mm diameter, and the follow-up speeds are 30 mm/sec and 40 mm/sec. Power pack units are available in different models from 0.25 HP to 3 HP for stands outside this range. The full table and the sizing steps are on the hydraulic power pack page.
Follow-up speed is used at the splice. When a new reel is joined to the running web, its edge may sit a few millimetres to one side of the old one. The sensor sees the step, the servo valve opens fully, and the stand moves at its follow-up speed until the edge is back. Loading each new reel in line with the running one keeps that step, and the waste copies it causes, small.
Where should the power pack stand beside the press?
The power pack should stand on the floor beside the reel stand, off the press frame and clear of the reel loading path. Four points decide the place.
- Vibration. A running press shakes its frame. A tank bolted to the frame passes that vibration to its pipes and fittings, so a separate footing on the floor is better.
- Reel handling. Reels arrive on trolleys or by hoist. Keep the pack and its pipes out of that path, with the pipes run in a protected channel if they cross the floor.
- Wash-up solvent. Blanket wash and roller wash are solvents. Do not store or pour them over the tank, and wipe any splash from the cylinder rod, since solvent attacks rod seals.
- Access. The filler, the oil filter and the air filter should be reachable without climbing over the reel stand.
The standard kit, described on the web aligner unit page, carries an air pipe and an oil pipe of 5 metres each, which is enough to reach from the floor beside the stand to the cylinder and the sensor.
Setting the guide at make-ready
The guide is set once for each reel width and then left alone. The order below fits into a normal make-ready.
- Centre the reel stand or the arms at mid travel before the first reel of the job is loaded.
- Load the reel centrally between the chucks or on the shaft.
- Web up and run the press at crawl speed with the guide switched off.
- Move the sensor to the reference edge until the web half covers the sensing gap.
- Switch on the guide and raise the sensitivity until the stand follows the edge without overshoot.
- Set the sidelay of the printing units to the guided web, not the other way round.
- Run a splice and watch the stand take up the step. Reduce the sensitivity slightly if it overshoots.
- Note the sensor position against the scale on its bracket for the next job of that width.
Keep the oil clean with the same care as the press itself. Use only Servo 32, Enklo 44 or Hydro 37, clean the oil filter and the air filter at planned stops, and send any unit that creeps or hunts after long service for repairs and servicing.
Other printing machines and related pages
Offset is one of several print processes served. Packaging presses in general are covered on the power pack for a flexible printing machine page, and presses for textiles and other rotary processes on the power pack for a rotary machine page. Heavy reels and dust away from the press, in paper converting, are treated on the power pack for a paper machine page. The full list is under power packs by machine.
Sensor types for web guides are compared in more detail on the client’s web guide system site, and the same maker’s web guiding system is shown on the main Krishna Engineering Works site.
What to send for a web offset guiding quotation
The unit is matched to the press and its reel stand. Send the details below.
- Make and type of press: heatset, coldset, narrow web or reel-to-sheet
- Type of reel stand: shifting stand, shaftless arms, or multi position stand with splicer
- Maximum reel weight, diameter and width, and the minimum width
- Paper grades and the range of basis weight
- Maximum press speed
- Positions to be guided: reel stand, infeed, before the folder or sheeter, rewind
- Reference edge, or centre guiding if the folder requires it
- Sensing preferred: air sensor or photocell
- For a replacement: make and model of the existing guide
Send the enquiry through the contact page. Press builders who buy units in bulk are invited to enquire.
Frequently Asked Questions
Does a sheet-fed offset press need a hydraulic web guide?
No. A sheet-fed offset press registers every sheet against front lays and a side lay before printing, so there is no continuous web to steer. A web guide and its power pack are used on reel-fed presses: heatset and coldset web offset, narrow web forms presses and reel-to-sheet presses, where the paper runs as one long web.
Where is the edge sensor mounted on a web offset reel stand?
The edge sensor is mounted on the fixed press frame, immediately after the first roller that follows the reel and before the splicer and the infeed nip. It reads the reference edge of the paper. Keeping it close to that roller lets it see each movement of the stand at once, which keeps the guide stable.
Is an air sensor or a photocell better for paper on an offset press?
Both can read paper, since it is opaque and solid. An air sensor is the easier one to live with at a dusty infeed, because its outflowing air keeps lint and ink mist out of the sensing gap. A photocell works well when its lens is wiped as a routine. Very light, open grades should be tested with air sensing first.
Why does the margin on one side change during a run?
The usual cause is the paper itself. The web takes up fountain solution in each unit and becomes slightly wider, and reel width varies a little from reel to reel. An edge guide holds the reference edge still, so the change appears at the other edge. Check the guided edge before adjusting anything.
Can the guide correct a fold that runs off centre?
It can when the cause is web position. If the web arrives at the former to one side, the fold runs off the centre of the print, and a steering frame ahead of the former brings the web back. If the web is on line and the fold is still off, the fault lies in the former or folder setting.
Which model moves a shaftless reel stand with heavy paper reels?
The KEW-30 is the model usually considered. It gives a maximum operating force of 389.5 kg on a 50.8 mm diameter cylinder, with a 1 HP motor and a line pressure of 21 bar. Add the weight of the reel, the arms and the chucks, apply the friction of the cross slide, and keep a clear margin.