Hi mechatronics fans!
Welcome to the latest post of the Sinadrives blog.
In today’s post we’d like to talk about how air skates work. This system is typically integrated in linear axes powered by electric motors. The concept of an air bearings system might sound like something related to aerospace technology, something that would be far too expensive and inaccessible for most industrial applications. Well, today we’re going to bust some myths and offer a simple explanation of how your company could use this technology to improve the performance of your machine.
1. What’s an air skate?
We’ve all seen air hockey machines in amusement arcades. They operate using simplified air skate technology. A plastic disc slides over a perforated surface through which compressed air comes out. This pressure makes the disc levitate, thus minimizing its friction with the surface.
WHAT DOES AN AIR SKATE LOOK LIKE?
An air skate, also sometimes called an air bearing or air dolly, is a rectangular or cylindrical element with small holes on one of its sides. The skate is supplied with a flow of compressed air at a pressure level of between 5 and 8 bars, generating an air cushion between the sliding surface and the skate. This air cushion can range from 2 or 3 microns to several millimetres high, depending on the application, the bearing technology used and the sliding surface.
EXAMPLE OF AN APPLICATION WITH AIR SKATES
If we take an industrial system that uses precise machinery to perform guided linear movements with electric linear motors or actuators, requiring surgical movements, the margin of error must be tiny.
Click on the link below to see an air skate in action:
2. What is friction?
From a friction efficiency perspective, the air skate is today’s undisputed winner. No other technology on the market offers such a small coefficient of friction. Since there is no element in contact, friction is restricted to the viscosity of the air itself and its friction.
HOW IS FRICTION CALCULATED?
FORMULA FOR ESTIMATING FRICTION
To calculate friction force in this type of application, the coefficient of friction that we’ll use in the formula is 0. That is, for a movement with constant speed (according to F = m x g x μ), the required force will always be 0. Acceleration and braking will require a force that depends on the mass and desired acceleration, but the force to be applied in a movement at constant speed is zero.
3. What is the air cushion? How can we know its height on air skates? How can air consumption be calculated?
As we’ve said above, depending on the sliding surface, the height of the air cushion can vary. In industrial applications in which the skate is moved over a granite table or a rectified steel plate, the height of the cushion tends to vary between 3 and 10 microns. Here are some technical concepts to bear in mind:
3.1 A small air cushion requires greater absolute accuracy on the sliding surface (greater flatness and straightness) and less roughness.
3.2 A large air cushion reduces the rigidity of the skate and has a negative impact on the behaviour of the system (see Graph 1 – Example of stiffness graph).
3.3 A large air cushion increases the compressed air consumption (see Graph 2 – Example of air consumption).
4. Mechanical configuration of air skates
The air skate restricts freedom of movement only by 1 degree of freedom. To complete the application at least 4 skates are needed to restrict the 2 directions on the Y axis and the 2 directions on the Z axis. This configuration is shown in the image below. The only degree of freedom left is the movement in the X direction.
There are a multitude of settings for restricting the degrees of freedom in a linear axis system. Some of them are shown below: 3+3+4, 4+4+4.
In a configuration where the skates are facing each other, a mechanical preload is obtained in order to be able to control the air cushion and the skate system. This must be constant to ensure the proper functioning of all the machinery.
5. What’s the load carrying capacity of an air skate?
Due to the large surface area of the skate, the load carrying capacity is very high, somewhere in the region of 150-250 N/mm² (with a compressed air pressure of 4 to 6 bar). This load carrying capacity allows compact applications to be deployed even with heavy loads.
Click on the link below to see a set of X-Z linear axes. The weight of the X axis is 150 kg. In the vertical direction there are just three 180 x 120 mm skates.
6. Accuracy of air bearings
If the air bearing preload is correctly executed and the base surface is highly accurate, the flatness values could be around ± 2-3 microns.
7. Wear and tear and maintenance of the bearing. Main advantage.
The main advantage of an air bearing is the absence of friction. There’s no contact between the fixed part and the moving part, which means there’s no wear and tear. The assembly can operate non-stop around the clock, without requiring adjustments and with no interruptions.
8. Conclusions on systems that integrate air skates.
A set of air skates combined with a linear motor constitute a 100% maintenance-free assembly. Due to the fact that there’s no wear and tear, one of the advantages is that the assembly can operate for millions of cycles without stopping once. In addition, its high efficiency means that energy expenditure is minimal, with few vibrations and great reliability. The use of air skates increases the availability of the machine and the safety of operations. They’re systems that can be seamlessly integrated in axis systems and linear guides with electric linear motors.
If you have an application in which you’re keen to improve the performance of your machine, be it in terms of speed, dynamics, precision or simply reducing maintenance needs, please contact us at email@example.com
Our specialists in Direct Drive technology and Linear Modules with linear motor technology will be happy to advise you free of charge.
Draw your own conclusions. Decide which innovation you want to implement in your machine to be competitive. We can help you.
Your SINADRIVES Team.