Clearing the Air
By Tim Wheeler, Alkon Corp.
Introduction
Air-over-oil devices offer an economical solution to introduce precise control into simple compressed-air motion systems. While other technologies have attempted to replace these systems, air-over-oil controls remain cost-effective, easy to maintain, and simple to adjust. Despite some concerns about leaks, oil mist, and aesthetics, air-over-oil systems can be safely implemented, providing the benefits of savings and simplicity in a well-engineered design.
The Challenges with Pneumatic Motion Control
Since the late 1800s, fluid power has been used to drive motion in automation, and pneumatic devices (compressed air) continue to play a prominent role in industries today. Air cylinders and actuators are common due to their speed, ease of availability, and simple maintenance. Compressed air is clean, easy to generate, and readily available on most plant floors.
However, certain applications demand more control than a basic compressed-air device can provide. Pneumatic systems have inherent challenges due to the variability in pressure, volume, and force required for motion. This variability can result in inconsistent, jerky movement, particularly when encountering variable loads or friction.
An easy way to understand compressed air characteristics is through the equation:
PV = K,
where Pressure (P) times Volume (V) equals a Constant (K).
For example, if a 1-unit volume of compressed air at 100 pressure is involved, P * V = 100. When the volume changes, the pressure adjusts to maintain the constant, which can cause drops in pressure and inconsistent motion.
Optimizing Compressed Air Motion Control
One solution to address this variability is converting the motion system to hydraulic power. Oil is incompressible, offering greater control and smoother, more consistent motion. However, hydraulic systems can be expensive, especially in environments where compressed air is already used extensively.
Alternatively, a motor/ball-screw drive can be used, but this adds complexity to what is otherwise a simple operation. To maintain the advantages of compressed air while minimizing its drawbacks, an air-over-oil system can be introduced. This system regulates the speed of linear movement, reducing the variability caused by compressed air.
There are three main methods for integrating oil control into a pneumatic system:
- Air-Oil Tank Systems
- Tandem Air-Oil Devices
- Self-Contained Oil-Control Units
Air-Oil Tank Systems
An air-oil tank system uses compressed air to drive hydraulic oil from two separate tanks into opposite ends of a cylinder, creating the force needed for motion. The incompressibility of the oil allows for better flow rate control. However, these systems can be complex to build, with potential for oil leaks, frothing, and oil mist in exhausts.
Tandem Air-Oil Devices
Tandem air-oil units combine pneumatic and hydraulic cylinders. The air cylinder and oil cylinder work in tandem to move the load. While these devices provide the benefits of air-over-oil motion control, they can result in oil mist in the system exhaust and require careful design to avoid leaks and frothing. They are more specialized devices and can be difficult to source and maintain.
Self-Contained Oil-Control Units
The most straightforward and cleanest solution is the self-contained oil-control unit, which incorporates a standard pneumatic actuator along with a separate oil-control unit. This design ensures no oil mist or oil migration occurs, as the systems remain fully separated. Self-contained units are simple to produce and offer high configurability with shorter lead times compared to other options.
System Options: Versatile, Self-Contained Oil-Control Units
Self-contained oil-control units offer various configuration options for different needs, such as:
- Single Direction Control: Allows free flow in one direction.
- Dual Direction Control: Provides control in both directions.
- Dual Speed Control: Allows different control levels at different travel points.
- Skip Check: Enables rapid movement between control regions.
- Stop Check: Stops travel completely, such as in emergency stop situations.
Methods of Connecting Oil-Control Devices
For effective motion control, oil-control devices need robust and secure mechanical connections. These can include:
- Parallel Connection through Cylinder Rod: Connecting the oil-control unit to the cylinder rod for direct integration.
- Tandem Mounted Cylinder: Adding a tandem mounting option to extend the footprint for specialized configurations.
- Work-Load Attachments: Securing the oil-control device directly to the load for better control.
Conclusion
Self-contained oil-control units offer a simplified solution to smooth motion control by maintaining the benefits of pneumatic motion while providing precise, regulated oil control. They are an ideal choice for improving motion control in new and existing processes, eliminating the traditional issues associated with air-over-oil systems.