What is an aerostatic tool motor spindle?
An aerostatic motor spindle is a shaft supported by an radial and axial bearing system and driven by an electric motor.
For an aerostatic bearing function pressurized air or gas is fed into a small gap between the shaft (rotor) and the bearing (stator) and forms a thin layer between both. A groove structure or jets (orifices) spread the pressure within the bearing gap in such way that a restoring force is created in the opposite direction of a disturbance or a shift. The higher the shift of the shaft the higher the restoring force. Thus, an aerostatic bearing has a stiffness and, considering dynamic and squeeze film damping, high damping values.
The properties of the thin gas or air layer between the rotor and the stator also include a significant averaging effect on shape errors of either the shaft or the bearing. This means that the resulting movement or rotation of the rotor in the stator is significantly more accurate than the sum of the shape errors of both would actually allow. Thus, this type of motor spindles is used in ultra-precision applications where low spindle errors (DIN ISO230-7) are crucial to achieve a certain surface finish level.
Although the shear-friction within the bearing gap increases non-linear with the rotation speed of the rotor, an aerostatic bearing spindle can reach approx. double the speed of a similar spindle with ball bearings.
The shaft iself can feature a clamping system for tools or a work-holding to hold and position parts during a machining. For static applications where no roation is required, shaft and bearing can be of any (not rotationally symmetric)
shape. Up until now such motor spindles with aerostatic bearings are used for high-speed applications like PCB drilling, micro machining or for ultra-precision applications.
Bottom line: Aerostatic bearing spindles offer higher speeds and significantly lower spindle errors and are thus the perfect solution for high-speed applications and/or to machine optical components.
User benefits
Considering a static application with no relative movement between the rotor and the stator, air bearings offer an extremely low friction level. This allows a very precise positioning and gives an excellent position control
with no backlash or stick-n-slip effects.
Although wear-free, because of shear losses being a quadatic funcion with speed, air bearing spindles can create a significant amount of heat in the bearing gap at high relative speeds between the rotor and the stator. Still,
the achievable rotation speed is approx. double the rotation speed of ball bearings.
Unlike it is with ball bearings, no part except the rotor is moving in a gas bearing. Where the cage of ball bearings limit the relative speed between the rotor and the stator, gas bearings can be designed at the stress limits
of the rotor allowed, but considering instabilities like the half-speed whirl. Thus, gas bearings can accelerate and spin faster than ball bearings. Often, and if no instabilities are in the speed range, gas bearing can use
the maximum motor torque/current for acceleration. Changes in speed of up to 75,000 rpm/s are achieved with Levicron motor spindles (
U/ASD-H20A).
The gap between the rotor and the stator can be only a few microns across. This thin pressurized layer includes a significant averaging effect of shape errors of the rotor and the stator and allows errors in motion far smaller
than the sum of all errors would actually allow. Thus, gas bearings allow spindle errors (DIN ISO230-7) that can be 100 times smaller than spindle errors of a comparable spindle with ball/roller bearings.
Same applied to spindle vibrations. Roller elements and cages rotate with the shaft, but at a different frequencies. As these part also have shape errors, the vibration level of spindles with gas bearings are signicantly lower compared to those with ball bearings. They also allow a more controllable balancing and improved shaft dynamics your machining results will greatly benefit from.
Although there is shear-friction in the gap between the rotor and the stator, there is no wear.
This means, once warmed through, there is no change in properties we all know from roller bearings.
You high-speed application and machining results thus benefit from non-changing properties at high speeds, even if user over weeks.
Because they are wear-free, the total cost of ownership of spindles with gas bearings is lower compared to concentional machining spindles.
Because there is no grase or oil, but only gasto lubricate the bearings, gas bearins are suitable for clean room applications or can be used in the food industry. They are also suitable in fuel cells, where oxygen itself is used instead of air.
Why motor spindles from Levicron?
Because most manufacturers of air bearings spindles use designs and manufacturing technologies from the mid nineteenth or prefer to just modify existing designs for a specific applications, these spindles lack of innovation, performance
and robustness. Same applies to the electric motors.
Levicron uses innovative design and manufacturing technologies like laser drilling, diamond CNC turning or. Instead of modifying existing technologies, a spindle design from Levicron targets on and is perfectly optimized for the application.
This also includes components other spindle manufaturers buy to save costs, but end up with compromises in design. Thus we took the trouble to develop, manufacture and integrate bespoke low-vibration electric motors, spring-less
HSK clamping or even rotary encoder systems. Over the years these technologies were more and more interlinked. With our internally used and very comprehensive
ShakesBear System we could also make a step forward in quality consistency and part/spindle traceability. Here our
ShakesBear System not only is an analyzer system, but can be used on ervey computer within the network (
ShakesBear Server), for static and dynamic test stands and is basically a technical ERP system linked with our standard ERP system for a seamless quality and process control.
Such an “approach without any compromise” not only lead to unique spindle solutions with lowest spindle errors, best dynamic and thermal stability and robustness to machine ultra-precision parts in a highly automated CNC environment, it gives us an outsanding quality and production control to serve you with ultra-precision spindle solution at very short lead and response times.
…..”Ultra-Precision meets CNC Performance”
Your benefits at a glance
Because of the averaging effect of the thin gas layer between the rotor and the stator and that is simply based on flow physics (see section “User benefits”), aerostatic bearing spindles have a very low spindle error level in general.
However, this averaging effect still depends on shape errors, and the lower these, the lowet also the spindle errors. Thus, Levicron is doing everhing to minimize and control shape errors, no matter if in part production or during the design stage to optimize their thermal and dynamic stability. Where shaft and bearings faces can be machined within 0.1 micron shape accuracy even for larger spindles, the patented bearing design with small capillaries and no pockets greatly redude spindle errors while giving higher stiffness and load capacity values at the same time.
Based on a great deal of experience designing high-speed drilling spindles, Levicron strictly optimize their spindle solitions for dynamic stability over their entire speed range. This includes the rigid-mode and bending criticals. Along with methods that allow to simulate and predict the bearing properties over speed and a unique and very efficient thin-film spindle liquid cooling speeds are possible beyond the speed of any other comparable spindle solution on the market.
Surface velocities of up to 200 m/s in journal bearings and up to 450 m/s in thrust bearings are feasable while securely clamping a tool. With its simple and robust design, even our spring-less HSK clamping system
SLH-x is ready for these speeds. 100,000 rpm with HSK-E25 or 150,000 rm with HSK-E20 you can get from us as a standard.
S1/100% describes the highest continuous power an electric motor can deliver. For motor spindles this is generally at top speed. Because of changes in ball/roller bearings, the bearings in conventional motor spindles define the time you can work at high speeds. After a specified time, the spindle has to ramp down to regrease or cool down the bearings.
Not for aerostatic bearing spindles. Once warmed through and because there is no wear within the bearing system, they can be used at top speed for an unlimited amount of time.
Along with the very efficient thin-film liquid cooling and a design that compensates changes in shaft length due to centrifugal expansion with changes in shaft length due to temperatur, Levicron manages to provide
spindle solutions with extremely small shaft growth at shortest soak times. Our
ASD-H20A
, for example, offers a shaft growth of less than 1 micron at a soak time under 3 minutes and still covers a speed range from standstill to 120,000 rpm.
As explained above (section: “Higher Speeds”), optimization of the shaft dynamic of all spindle products is a cucial part of the design. Here not only instabilities like the well-known half-speed whirl is considered,
but also rigid-mode and bending criticals that are optimized in a Campbell diagramm. Because of no instabilities and resonances in the speed range and because of a controlled bearing gap with speed, ramping up a
spindle to top speed can use the max. power/torque of the motor. Thus we manage to accelerate our
ASD-H20, for example, from standstill to 120,000 rpm within 1.6 seconds what again represent a change in speed of 75,000 rpm/s. Thus, chip-to-chip times with Levicron spindle solutions can significantly reduced.
Where ball/roller bearings need a pre-lubrication and warm-up cycle, spindles from Levicron are designed to work even at top speed from standstill and cold conditions right away. This reduces the machining auxiliary
time in your process.
Due to their design limitations, ball/roller bearing spindles show a significant change in axial tool position dur to centrifugal and thermal changes within the spindle and bearings. Most machine centers thus use compensation system which again are known error-sources.
Not with spindle solution from Levicron. Their designs target on compensating the change in shaft length due to centrifugal effects with the change in shaft length due to temperature. Thus and along with the high-efficient thin-film spindle liquid cooling our work-holding spindle ASD-Px, for example, has an axial shaft growth (dilatation) of under 0.7 micron within a spindle soak time of under 9 minutes.
Although all single parts are manufactures with sub-micron tolerances, Levicron designs are modular and simple. Not only it is possible to convert our
ASD-H25/A from a 60,000 rpm to an 80,000 rpm version by just swapping the bearing cartridge, the modular design greatly reduces the time required to service or repair a spindle.
As gas (air) is used to lubricate the bearing system, no oil or grease is required fo those. Along with our maintenance-free and spring-less HSK clamping system SLH-x our spindles can be used in environments where oil or grease are prohibited.
Ball/roller bearing spindle that use ceramic material for the roller elements require anti-static measures like brushes to prevent spark discharges. As the area of an aerostatic bearing is huge compared to the tiny Hert’sche contact of the roller elements to the bearing race and the thin bearing gap, the capacitor between the rotor and stator is that large that no anti-static measure are necessary.
Should you still need an electric contact to detect a contact between the tool and the work piece e.g., please contact us. We also have a solution for this task.
Ball/roller bearing spindles with, for example, a conventional internal HSK claming system require a system that holds the shaft when performing a tool change cycle. This is because the actuation system has to overcome
the pre-load of the spring system first before getting to the point to apply the load required to eject the tool.
Axial or thrust bearings in aerostatic bearing spindles are large in area. During a tool change and even if the load to change the tool is larger than the bearing load capacity, the rotor just touches down onto the
stator with no damage to either of these. Of course, no rotation is allowed in this case. Thus aerostatic bearing spindles fonr’t require a shaft retention system to protect the bearing from getting damaged during
a tool change.
Because of our in-house developed spring-less HSK clamping system
SLH-x doesn’t require springs to permanently clamp a tool, the actuation system does not have to overcome the pre-load of the spring before it starts to eject the tool. This again reduces the eject load onto
the bearings and further reduces the need of such a bearing protection system.
