Hydraulic Power Pack for Doctoring Rewinding Machine: Guiding Badly Wound Reels
A hydraulic power pack for doctoring rewinding machine guiding moves the unwind stand so that a badly wound reel is rewound with a straight edge. Krishna Engineering Works builds the power pack and web aligner unit in Ahmedabad, and the web aligner is used on doctoring rewinding, winding rewinding and rewinder unwinder machines to guide the material.
What does the guide do on a doctoring rewinding machine?
The guide takes the sideways error out of a faulty reel while the operator takes out the other faults. A doctoring rewinder exists to rescue reels: the web is run from one shaft to another so that bad splices, printing faults, torn sections and loose winding can be cut out or corrected. A reel that arrives with a ragged or telescoped side has to leave with a flat one.
That makes the duty unlike any production machine. On a press the guide trims small errors from good reels. On a doctor machine the reel is on the machine because it is bad, so the guide works near the limit of its stroke and speed as a matter of routine, and it does so through constant stops and starts.
Which reel faults can the guide correct?
The guide can correct any fault in which the edge is displaced sideways but the web itself is sound. The table sets the common faults against what they ask of the cylinder and the power pack.
| Reel fault | What the edge does during unwinding | What it asks of the guide |
| Telescoped reel | Moves steadily to one side from outside to core | Stroke: the total offset must fit within the cylinder travel |
| Stepped reel | Jumps sideways at each step, then runs straight | Speed for a moment, then holding |
| Dished or conical reel | Slow drift that reverses part way | Little speed, a centred starting position |
| Wavy edge from poor winding | Wanders irregularly | Steady following, moderate sensitivity |
| Reel loose on its core | Shifts when tension changes | Fix the core first, the guide cannot hold a slipping reel |
| Damaged or torn edge | No clear edge to read | Guide from the opposite edge |
Stroke is the limit that surprises people. The hydro-pneumatic web aligner unit is supplied with one hydraulic cylinder of 6 inch stroke. With the stand central that leaves about half of the travel to each side, so a reel telescoped further than that must be loaded off centre in the opposite direction before the run starts. The operator who knows this never sees the cylinder reach its end stop.
A guide moves the web, it does not repair it. Slack edges, baggy lanes and creases wound into the reel come from the earlier process, and the method of removing them is covered on the client’s site about the doctoring process.
How does the guide behave through stops and restarts?
A hydraulic guide holds its position when the web stops, which is exactly what a doctoring machine needs. The operator stops the machine many times in one reel: to cut out a fault, to make a splice, to tighten a loose section. Each time, the edge is sitting still in the sensor.
With the web stationary the sensor still reads the edge. If the edge is where it should be, the servo valve stays closed and the oil trapped in the cylinder locks the stand. Oil is nearly incompressible, so the stand does not sag or creep while the operator leans on the reel or pulls the web to make a splice. When the machine restarts the guide simply carries on.
Three situations need care.
- Splicing. While the web is cut, there is no edge in the sensor, or the wrong one. A guide left in automatic will drive the stand to the end of its stroke looking for the edge. Take the edge of the new tail to the sensor before releasing the web, or shut the guide off during the splice.
- Tension pulled by hand. Pulling the web sideways at the splicing table moves the edge in the sensor and the guide will answer. This is normal and it settles as soon as the web is released.
- Restart after a long stop. If the pack was switched off, run it for a short time before restarting the web, so the cylinder is under pressure before the first correction is called for.
What happens when the machine runs in reverse?
When the web runs backwards the unwind guide no longer has the right geometry, so reverse running needs a plan. In forward running the sensor sits just after the guided stand and sees the result of each correction at once. In reverse, the same stand is now receiving web, and the sensor is in front of it instead of after it.
On most doctor machines reversing is brief: a few metres back to find a fault that has passed the table. For that, the usual practice is to leave the stand where it is and let the short length wind back as it came. Where long reverse runs are part of the work, the guiding has to be arranged for both directions, with the sensor placed and the stand chosen to suit. State in the enquiry how far and how often the machine runs backwards, so the arrangement can be proposed accordingly.
Response at low and changing speed
At low speed the guide has speed to spare, and the thing to watch is smoothness. A fault in the reel arrives slowly when the web is crawling, so the 30 mm/sec follow-up speed of the KEW-17 power pack is far more than the web demands. What matters instead is how the stand starts to move.
A stand that sticks on its slides needs a certain push before it breaks free. At crawl speed the error grows slowly, the valve opens slowly, pressure builds, and then the stand lets go and jumps past the target. The edge of the rewound reel shows a fine sawtooth. The cure is mechanical: clean, lubricated slides or wheels, and a cylinder whose pins and brackets have no play.
The speed also changes all the time, from crawl to full speed and back within one reel. A proportional servo valve copes with this naturally, since it meters oil according to the size of the error and not according to line speed. Set the sensitivity at the fastest speed the operator uses, check that the guide does not hunt, and it will be stable at every slower speed.
Air in the oil is the other enemy of slow, exact movement. A cylinder with air in it is springy: it moves late and then too far. Bleed it by stroking end to end after any work on the pipes.
The unit supplied for a doctor rewinder
The hydro-pneumatic web aligner unit is supplied complete with a 0.5 HP motor, air pipe of 5 metres, oil pipe of 5 metres, one sensor, one adjuster, one air filter, one oil filter and one hydraulic cylinder of 6 inch stroke. The adjuster carries the sensor and lets the operator slide it to the edge of each new reel, which on a doctor machine is done many times a shift.
Air sensing is a practical choice here, because the reels that come for doctoring are of every kind: clear film, printed film, metallised film, paper and laminate. The air jet reads them all by the physical edge, with no setting to change between a clear reel and a printed one. Details are on the hydro-pneumo web aligner page. An electro-hydro unit with a photocell sensor for the edge and an electrical panel board is the alternative where the webs are opaque.
The kit is itemised on the web aligner unit page and the pack is explained part by part on the hydraulic power pack page. Where the machine handles heavy reels, compare the push needed with the maximum operating force of the two standard models: 260 kg for the KEW-17 and 389.5 kg for the KEW-30, both on a 50.8 mm diameter cylinder.
Operator routine for each reel
A fixed routine gets a straight reel from a bad one with the least waste. The steps below suit a single edge guide at the unwind.
- Look at the faulty reel before loading it, and judge which way and how far the edge is displaced.
- Centre the unwind stand, or offset it against a telescoped reel so the cylinder has travel where it will be needed.
- Load the reel with its best edge towards the sensor.
- Thread the web, set a light tension and slide the sensor on its adjuster until the edge half covers the sensing gap.
- Switch the guide to automatic and start at crawl speed. Watch the cylinder position, not only the web.
- Bring the speed up once the stand has settled near the middle of its travel.
- At each stop for a cut or splice, keep an edge in the sensor or shut the guide off until the web is joined.
- Near the core, slow down. Core faults are the sharpest and the reel is turning fastest.
Oil care on a machine that starts and stops all day
Short, broken running is easy on the oil but hard on habits. A doctor machine may stand idle for hours and then work a full shift, so the checks tend to be forgotten.
| Check | Reason on a doctoring machine | When |
| Oil level | Slow weeps go unnoticed on a machine that is often idle | Weekly |
| First movement after standing | Cold oil is thick and the first corrections are slow | Each start, run the pack a little first |
| Sensor gap and ports | Trimmed slivers and paper dust collect at the splicing table | Every shift |
| Air pipe | Pipes get trapped or kinked when the sensor is moved often | When the sensor is repositioned |
| Stand slides or wheels | Low speed guiding depends on free movement | Monthly |
| Oil colour and filter | Dark or cloudy oil means a change is due | Every few months |
Use Servo 32, Enklo 44 or Hydro 37 oil. Spares are kept readily available, and a unit of any Indian or imported make that drifts or hunts can be sent for repairs and servicing.
Related machines
A doctor machine is close in layout to a slitter, where the same unwind guide protects slit width, as set out on the page for the power pack on a slitting rewinding machine. Final checking of printed reels is covered on the page for the power pack on an inspection rewinding machine. Krishna Engineering Works also builds the doctoring rewinding machine itself, and all applications are listed under power packs by machine.
What to send for a doctoring rewinder guiding quotation
Send the details below, so the stroke, model and sensor can be matched to the reels you receive.
- Materials handled, with thickness range and whether any are clear
- Web width range and the heaviest reel
- The worst edge displacement you expect in a faulty reel
- Working speed range, from crawl to maximum
- How often and how far the machine runs in reverse
- How the unwind stand is carried: slides, wheels or linear bearings
- Whether the rewind is to be guided as well
- Make and model of any guide fitted now
Send the enquiry through the contact page.
Frequently Asked Questions
Can a web guide straighten a telescoped reel on a doctoring rewinding machine?
Yes, within the stroke of the cylinder. The guide shifts the unwind stand against the telescoping, so the web leaves on a constant line and rewinds with a flat side. If the reel is displaced further than the travel available to one side, load it off centre in the opposite direction before starting.
What happens to the web guide when the doctoring machine stops?
The guide holds its position. With the web still, the sensor continues to read the edge, the servo valve stays closed and the oil trapped in the cylinder locks the stand. The stand does not creep while the operator cuts or splices, and guiding continues as soon as the web moves again.
Why does the guide run to one end during a splice on a doctor rewinder?
It runs to one end because the sensor has lost the edge. With the web cut, the sensor sees either no web or full cover, reads that as a large error and drives the cylinder to its limit. Keep an edge in the sensor during the splice, or shut the guide off until the web is joined.
Does a hydraulic web guide work at crawl speed on a doctoring rewinder?
Yes. Hydraulic correction does not depend on line speed, because the valve responds to the position of the edge. At crawl speed the demand on flow is very small. Smooth movement then depends on a free running stand, tight cylinder mountings and oil that has been bled of air.
Which edge should the sensor follow on a damaged reel?
The sensor should follow the better edge. A torn, folded or ragged edge gives a confused signal and the guide will copy every tear into the rewound reel. Load the reel so that its cleaner edge faces the sensor, or move the sensor to the other side of the machine if the adjuster arrangement allows.
Is one power pack enough to guide both unwind and rewind on a doctor machine?
One reservoir can serve both when a double power unit is used. Krishna Engineering Works offers double and triple power units with two and three servo valves on the same tank. Each guide then has its own valve, sensor and cylinder, and the two work independently from a shared oil supply.