First step in thrust spiral groove bearing design may be to minimize the
rotational speed at which full film lubrication begins. For
this the optimal non dimensional load is aimed at minimum film thickness
h_{0}, where h_{0} is based on machining accuracy. The
optimum value for the non dimensional load F' is a function of α,
k, λ and δ (Figure 9.22). A combination of these parameters which are much used in
practice is calculated with eq.9.54 and is filled in as default.
S.g.b.'s operate with air lubrication as well with oil
or grease. The grooves are usually etched in the smooth finished surface
(photo etchmask on the right). The grooves have a shape such that the
angle between the tangent to the groove and the local velocity vector (wxr)
has at all times the value a
(construction).
From the numerical example above it follows that if a grooved disc of 150 mm diameter is placed on a smooth
one and rotated in the right direction, a rotational speed of 1 revolution
per second will be enough to separate the discs by a layer of air about 12
micron thick, which will then support the weight of 1 kg. The friction
losses then are 0.0025 W. Next step in spiral groove bearing design, after that the bearing
geometry is optimized, is to compute the bearing parameters at nominal
operating conditions. For this step compute the angular velocity with
trial value for h_{0}.
