Electromagnetic attraction

13 July 2010



Merobel web tension control equipment is an essential component in Daco’s converting machines.


UK based manufacturer of finishing machinery for the narrow web, label printing and converting industries Daco Solutions builds slitter rewinders, inspection rewinders and rotary diecutters ranging from entry level machines to high speed, high volume equipment, as well as providing customised systems. Daco has been using Merobel electromagnetic brakes for the past 10 years, supplied by Andantex, the French company’s UK subsidiary.

“Analysing the different options on the market, I would say that it is the best solution,” explains Dave Beynon, owner and Managing Director of Daco. “Pneumatic brakes may be an option, but are less linear with a lot more wearing parts. Furthermore, there is the additional risk of leakage.”

Pneumatic brakes feature a torque ratio from 1-30 max, compared with an electromagnetic brake offering a braking range from 1-100. For this reason, several pneumatic braking disks may be necessary.

“Other suppliers of electromagnetic brakes are not acceptable to me,” Mr Beynon continues. “Either their reliability is not good enough or they are too complex to implement, or overly expensive. Of the 200 machines we have delivered over the past 10 years, we have had fewer than five problems. The best thing about Merobel brakes is that you install them and forget about them.”

Merobel EMP brakes apply a controlled torque to the web shaft to maintain constant tension during winding or unwinding. The EMP technology is said to be both extremely reliable and often simpler to integrate than friction systems, as well as being more economical and much easier for users to control than motor and drive based systems.

This reliability results from a number of technological improvements: innovative surface treatment especially formulated to limit wear; special steels chosen to incorporate optimised magnetic properties; and ball bearings selected to resist high temperatures. The residual torque has been reduced considerably and magnetic circuits optimised to increase the linear operating range.

An example of the Daco/Merobel combination in action is a vinyl diecutting machine delivered to a customer in May 2010.

“We had to consider a closed loop electronic control system, because the backing that the vinyl is on is much more flexible than paper and the conventional system wouldn’t have been accurate enough as the product is also sprocket punched for use in sign making equipment,” recalls Dave Beynon. “Tension control was paramount to the success of the project, as the pitch of the sprocket hole can change if tension is not constant throughout the production run.

“Merobel’s DGT300 digital control really suited our needs: the software is extremely user-friendly and the complete system with loadcell was provided by Andantex. It was just a couple of hours to run the first tests.”

This Daco machine was an adaptation of the company’s DTD 250 bench-top rotary diecutter - a ‘cost effective’ system with a production speed of 150m/min. It is available with either one or two rotary die stations, rotary slitting and independent tension control for the unwind and rewind. The diecutter can also be used as a slitter rewinder for finishing of pre-printed labels, by removing the rotary die.

The Merobel DGT300 digital control regulates web tension whatever the constraints: product elasticity, coil inertia, cyclic operation, large variations in diameter or roundness. Its multi-purpose capabilities are said to ensure control in both open loop and closed loop regulation applications.

With a user-friendly Windows PC interface on the front panel keyboard, the unit allows the definition of all application parameters required to start a web tension control system. These include emergency stop proportional to the set point, smooth start-up, freewheel and blocking modes, taper tension and flying splice web replacement.

Magnetic particle brake to deliver 30% more torque

Higher torque values are now claimed for Magpowr’s recently released Model F magnetic particle brake. Originally rated to deliver up to 250lb/ft of torque, the company says it can now be placed in demanding applications requiring up to 325lb/ft.

“The benefits of magnetic particle brakes - no dust, smooth stops and starts, and little or no maintenance - make them preferred for most applications,” says Darren Irons, Magpowr’s Product Manager. “These new models give us flexibility to place the technology in applications where we couldn’t before.”

Brake discs to be manufactured from recycled material

The Rebrake project is a UK Government backed consortium of companies developing new techniques that will allow carbon brake discs to be made from manufacturing scrap, providing a lower cost, more environmentally friendly solution to very high braking performance for applications such as paper converting machinery.

“The programme has recently passed a major gateway by successfully manufacturing small disc samples that prove the concept, so we are already moving on to the manufacture and testing of full-scale prototypes,” says David Holme, Technical Manager of Federal-Mogul, one of the project partners that is developing compatible pad materials. “Following further refinement, the new system will be benchmarked against current bestin- class products.”

Tension control systems for difficult applications

A difficult application for tension control systems involves intermittent web movement due to sudden changes in web speed and direction of dancer roll movement as the intermittent motion begins and ends.

UK agent Jarshire says the Nexen RSD100 and RSD200 dancer tension controller systems have proved capable of handling these variations in a variety of applications, including a medical products operation where an RSD100 is fitted on an intermittent feed machine that has an unwind stand operating at a constant web speed. Here the dancer system needs to absorb the excess material coming off the rewind while the process is momentarily stopped, then pay the excess into the machine as the process begins again. All this must happen at a constant web tension, due to the nature of the product.

A second example is an OEM machine builder who tested an RSD200P system on an in-house slitter rewinder test machine being prepared for sale. This has two unwind stands placed one behind the other for manual splicing as each roll depletes. The machine accelerates the web from 0 to 1,500ft/min in 10sec – however, a production version will do it in 7-8sec.

The total cycle time on the production machine is 30sec: 10sec to accelerate, 10sec dwell, and 10sec to decelerate. To add further complexity, the tension can vary from 0.5-3pli and the web width can vary between 24-72 inch.

As the controllers closely monitor the response of the system each time the output is changed, the need for expensive extra sensors to measure roll diameter, web speed and other parameters is overcome. PID gains of the control loop are optimised to get the fastest response and keep the output stable under all circumstances, states Jarshire.

Always putting safety first

Mayr power transmission says it always follows proven safety principles during the development and manufacture of its Robastop safety brakes. All components are safely dimensioned and manufactured from high quality, tried and tested materials. For example, the screw thrust springs for producing the braking force are made of stainless steel and are tested for a service life of 10 million load changes.

Additional safety has resulted from the application of the tried and tested safety principle ‘multiplication of parts’. Generally, more than five springs are put into one brake. This lowers the influence caused by an error. In addition, mayr applies proven design principles. It uses sufficiently guided springs with a wire diameter which is larger than the distance between two coils. Should a spring actually break, the broken ends cannot coil into each other. The original spring length and the spring force are maintained.

Every brake is tested in detail before being sent to the customer. Among other things, the spring force is tested, the tightening voltage, the coil capacity, the air gap and the safety of the electrical insulation.

Further characteristics which are checked depending on the customer or the product are, for example, the attraction and drop-out times when the brake is switched, the braking torque or switching noises. For use in vertical axes, for example, mayr has designed redundant brake systems from the Roba –topstop construction series.

As an independent module, the Roba-topstop is also capable of holding the vertical axis in any position, even without a servomotor. This produces substantial cost and time savings, for example if the drive motor is replaced, and reduces downtime during repairs.


Daco machines run smoothly thanks to Merobel. Daco More torque from Magpowr. Magpowr Nexen's RSD200 system. Nexen The Robastop-topstop. Robastop Testing at Federal-Mogul. Federal-Mogul

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Andantex
Daco
Magpowr
Federal-Mogul
Jarshire
Mayr

Magpowr Magpowr
Nexen Nexen
Robastop Robastop
Daco Daco
Federal-Mogul Federal-Mogul


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