The first consideration is that the spindle (unless specifically designed with aerodynamic bearing capability) should never be run without an air supply connected. Similarly, if an ATC (automatic tool change) system is incorporated, the spindle must not be run without a tool fitted.
Operation outside of the specification parameters, even for a fraction of a second, could cause damage that may render the spindle unfit for the intended application.
Correct balance of any associated tooling is critical, as is the method of connecting the tool to the spindle. It should be remembered that tool holder geometry and location might alter with increasing rotational speed, due to centrifugal, thermal or windage effects.
If installing a high speed spindle, especially as a retrofit or upgrade of an existing machine, care should be taken that the resultant vibrational frequencies, induced by cutting material (number of cutting edges multiplied by speed) or by the drive mechanism for the spindle (e.g. six step inverter output multiplied by number of pairs of motor poles multiplied by speed; number of turbine buckets multiplied by speed, etc.) does not correspond with a modal frequency for any part of the machine construction or operation.
Vibrational levels produced by a rotating, unloaded, spindle are very small and, unless very high positional accuracy is required for the specific application e.g. optical, etc., these vibrations should not induce noticeable problems in other machine components.
Individual components and gas films within the spindle will however have their own resonant frequencies and forced vibrations coinciding with these frequencies (or sometimes Harmonics of these frequencies) can conceivably cause ‘softened’ bearing performance.
It should be remembered that the spindle shaft is ‘floating’ on a gas film and can therefore be considered as being suspended, axially and radially, on a compound spring system.
This means that the action of the shaft, in space, will be affected by the ‘system’ stiffness (kS) in the plane considered, where kS is calculated from an equation.
1/ kS = 1/ kB + 1/ k1 +………1/ kN
- kB being the bearing stiffness, itself variable with speed and temperature.
- k1… kN being the stiffness’s of all individual components, joints, mechanisms, etc. between the bearing a rigid ground.
The result of this may be seen as an effect on system dynamic performance (whirl speeds) or on resonance frequencies or harmonics.
The shaft will also experience machine induced spindle movements (intentional or otherwise) through the bearing ‘spring’ system; this further complicates the calculation of dynamic effects.
In an application where it is known that the spindle will be subject to controlled, externally produced, movements (e.g. drilling, spraying, etc.), it is necessary to estimate the bearing loading produced by shaft gyroscopic or inertia effects, resulting from this action.
Most spindle outline specifications as well as theoretical and test performances, consider the spindle in isolation; i.e. unaffected by external stimuli. It is therefore necessary to establish the angular and linear accelerations acting on the system before calculating the forces involved.
As the distances, between shaft and bearing, are so small and the time requirements to move these distances are very short, it is necessary to consider all changes in acceleration (jerk) experienced by the spindle. It is best to measure the effects of such actions at the shaft with the spindle mounted in the machine to be used, as calculation can be very difficult.
Typical control movement accelerations are in the order of 1g to 5g but system clearances, backlash, bounce and resonance can create two to ten times this value at the spindle.
A major problem associated with excessive acceleration, or applied vibration, is that it is one of the very few mechanisms that can cause minor damage to a bearing and hence cause a degradation of performance over a period of time.
Although of very small frictional resistance, gas film shearing will produce some heat at high rotational speeds. This will be dissipated by the cooling system but some minor performance changes may occur in the first few seconds after start-up (all theoretical design and practical testing is based on stable dynamic conditions).