Putting the brake on accelerates film processing

Contact-free hysteresis brakes and ERM controllers provide RTI with the perfect solution for managing the increased output from its new 35mm film processing machine systems.

ZF contact-free hysteresis brakes and ERM controllers provide specialist RTI with the perfect solution for managing the increased output from its new 35mm film processing machine systems.

RTI's latest systems have been designed to process a 1200m reel of film every 20 minutes and use larger reels than normal to allow for this increase in processing capacity.

However, the previous braking system could not ensure a smooth transition between reels, overheating at the end of the on-wind process due to the higher reel speeds and greater stopping loads.

A ZF hysteresis brake solution proved ideal as the systems are easy to fit and commission and are now the tension control system of choice for new RTI machines and upgrading existing equipment.

To meet demand for faster turnaround times from the worldwide film industry, RTI have developed new machines and system upgrades that enable 35mm movie film to processed at a rate of up to 3500m/h.

"Due to the introduction of larger reels required by faster processing machines the speed and weight of the reels has increased, causing problems in many processing machines as the higher moment of inertia makes controlling the film tension on the feed-in panels problematic", explains David Addison, Head of Engineering at RTI.

"We were initially asked to redesign the tension control system for a repeat order of a high speed processing machine because the customer's existing processing machine was suffering from film looping during reel changeovers, incurring high material and downtime costs".

Developing movie film is a continuous operation, which involves taking unprocessed film from one reel and keeping it at a constant tension while it is being subjected to processing chemicals and wound onto a new reel.

Both ends of the machines are fitted with film elevators that enable the operator to load up to three new reels containing 1200m of film per hour, and remove developed film without stopping the process.

The old friction brakes used to control the reels suffered from overheating if set for a full reel and failed to accurately control the film speed.

If the machine was set for a smaller reel the problem would cause the "film break warning" to activate as film tension was lost when loops of film came off the reel.

After some enquiries David Levett, from the Bedfordshire based tension and motion control specialists DJ Automation, suggested a ZF contact free hysteresis braking system as a possible solution to the problem.

"The ZF system allows RTI to accurately control the speed and tension of the film, varying the speed of the spindles from 33rev/min for a full reel of film to 230rev/min at the end of a reel", explains Levett.

"The weight also varies from 9.3kg to 35g at the same time, so parameters had to be carefully calculated to specify a brake unit capable of controlling the spindle speed and stopping it as it reaches the end of a reel".

"To compound the problem, when the unprocessed film end comes away from the core, a clamp is automatically activated to prevent the film end from entering the elevator".

"A new reel of film is then loaded and the clamp released, during which time the elevator will rise as the machine continues to process film".

"When the ZF brake is released the new film starts to feed in at about 7000m/h to replenish the elevator, and when the elevator is full the film speed will return to the standard machine process speed of 3500m/h".

"With such a wide variation in speed and torque to control, we first tried a pulse signal from the spindle and the input shaft to a ZF ERM controller, but still looking for the ideal solution we then tried the ZF ERM with a 'closed loop' control system using a small dancer arm and it worked with a small adjustment to the standard ZF controller parameters".

The chosen solution involved fitting a ZF EBU 5000/3 unit to each spindle in the machine and linking the ZF ERM programmable controller unit to the existing rotary pulse encoders on the rewind system at the top of the machine.

During commissioning Levett used a portable ZF programmer unit to set up the controller and account for the changes in speed and weight of the reels, and very quickly had the new system working with seamless changeovers.

The results were so successful that Addison ordered a further set of brakes and controller for another machine soon afterwards.

Because the parameters had already been saved in the ERM programmer, DJ Automation could program the controller in seconds and package it up with brake units, reducing set up time to virtually nothing.

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