Pneumatic motors suited for winding

23rd January 2015

  

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Pneumatic motors are ideal for winding applications, owing to their ability to automatically adapt speed as load changes, Germany-based air tools company Deprag says.

Winding drives can be found in many production processes, including the continuous winding of materials before or after their handling on spirals, rolls, reels or balls.

Different materials place different requirements on the winding drive, owing to its specific features in terms of surface properties, hardness, tensile strength, cross-section profile or thickness.

A pneumatic-vane motor consists of a rotor which turns inside an offset chamber in the rotor cylinder. The vanes in the rotor are pressed against the wall of the rotor by centrifugal force, creating working chambers. Inside these chambers, the sealed compressed air expands and pressure energy is changed into kinetic energy, which turns the rotor.

A pneumatic motor runs idle when fully loaded, but when a reduced load is applied, with a lower torque on the motor spindle, the working speed slows to an idle. The working speed is reduced as soon as the torque increases.

At 50% of the idle speed, the pneumatic motor reaches maximum power. This is also the optimal working range of the pneumatic motor. In the range between 40% and 50% of the idle speed, the pneumatic motor is particularly energy efficient. Deprag pays particular attention to this criterion when it designs pneumatic motors.

Additionally, when compared with an electric motor, a pneumatic motor can be loaded until the unit being driven comes to a standstill without problems. Electric motors cannot be loaded until standstill and may suffer from overheating. After reducing the load on a pneumatic motor, it will immediately run again.

In a characteristic curve, another advantage of pneumatic motors is that their energy requirement decreases with increased torque.

Using pressurised air provides an unproblematic energy source, with hazards such as electric cables or other electric connections being eliminated, and short circuits not being an issue.

Pneumatic Vane Motors
Two factors are decisive for pneumatic motor design: the required winding speed and the maximum torque.

To calculate the maximum torque, the largest possible roll diameter is taken, for example, the size when the roll is fully wound.

The winding speed should also be determined when fully wound.

When the roll is carrying less material, the roll diameter will be smaller and the motor will automatically wind the material more quickly, as the working speed adjusts correspondingly to the load (the lower torque).

If the motor rotates too quickly, the speed can be steplessly adjusted by altering the air supply, the operating pressure, or a combination of both.

Regulation Speed with Air Supply
By regulating the air supply, the speed can be simply and flexibly reduced.

There are two options, depending on the application situation, including throttling supply air or exhaust air.

By throttling the exhaust air, the speed of the motor is reduced without noticeably reducing the power or the torque of the pneumatic motor. A throttle valve keeps the exhaust air back and creates so-called back pressure or counterpressure, resulting in the speed being reduced.

Throttling of the supply air is recommended to reduce the speed of the air motor and also the power or the torque.

Another specification for winding applications could be to keep the continuous material taut. In this case, the pneumatic air must be present at all times to keep it taut. To reduce the air consumption, the motor’s supply air is throttled and operated with reduced operating pressure.

Speed Regulation through Operating Pressure
Apart from regulating the air supply, the speed of a pneumatic motor can also be adjusted using operating pressure.

The technical data of Deprag pneumatic drives is based on an operating pressure of 6 bar, with every Depdrag pneumatic motor being able to operate between 4 bar and 6.3 bar, as required to adjust the speed and torque.

A reduction of operating pressure is useful if the motor used for the winding material is too powerful. By reducing operating pressure by 1 bar, torque is reduced by 17%; therefore, if the pneumatic motor is operated at 4 bar, the torque is reduced by 33%.

A pneumatic motor operated at 4 bar can still be too strong in a winding application, such as empty running rolls.

To use the torque range of the air motor further, Deprag suggests equipping the motor with spring-loaded vanes, also known as forced-started vanes. Using these vanes with a torsion spring makes it possible to run pneumatic motors with an operating pressure of less than 1 bar.

This also makes it possible to almost fully exploit the power control range of the motor (from 0 watt to maximum power) – ideal conditions for a winding drive.

A pneumatic motor, therefore, offers several advantages for a winding drive application. The power density of the motor is high, and an air motor requires only two-thirds of the size and a third of the mass of a comparable electric motor.

Edited by Megan van Wyngaardt
Creamer Media Contributing Editor Online

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