GN 248 Leveling Feet Steel, with Vibration Damping

Interference frequency [Hz]:
is the frequency emanating from a machine,
e.g. from the machine main shaft speed [rpm].

Static load F [N]:
is the load acting on each vibration-dampening element (levelling foot).

Degree of insulation [%]:
measure for absorbing the interference frequency (dampening)

Compression s [mm]:
is the change in the height of the dampening element (spring excursion).

Stiffness R [N/mm]:
is the load causing the dampening element to be compressed by 1 mm (spring rate).

The first step is to determine the static load F for each levelling foot. With favourably arranged levelling feet and therefore an evenly distributed load F, this value is calculated using the following equation:

Force due to weight of the machine [N] / Number of levelling feet

= static load F [N] for each levelling foot

Use the calculated static load F to select a levelling foot from the table, making sure that the static load F lies as close as possible to the static load capacity without exceeding it. The associated stiffness R of the selected levelling foot is also taken from the table.
The actual compression is then calculated using the equation below:

Static load F [N] per levelling foot /
Stiffness R [N/mm]

= actual compression s [mm]

Starting from the calculated actual compression s, the achievable degree of insulation as factor of the interference frequency can now be taken from the graph shown above.
To optimise the achievable degree of insulation, the number of levelling feet may be changed such that the static load F for each foot is as close as possible below a load capacity value given in the table.
This will increase the compression s which, in turn, results in a higher degree of insulation.
In general it can be said that medium to high frequencies can be very well insulated with sufficient compression.

Static load F (machine weight) = 48,000 N, number of feet = 4, ergo: static load per foot = 12,000 N
Selected foot: d₁ = 160, static load capacity 20,000 N, R = 9,000 N/mm
Resulting in an actual compression s of:

12,000 N (static load/foot) /
9,000 N / mm (stiffness R)

= 1.3 mm

With an interference frequency of 20 Hz (1,200 rpm), the above graph delivers a degree of insulation of only about 20 %.

To optimize, the number of feet may be increased to 5, resulting in a static load per foot of 9,600 N. A levelling foot whose static load capacity is closer to the new result may now be selected.
Newly selected foot: d₁ = 120, static load capacity 10,000 N, R = 4,000 N/mm
This results in an actual compression s of:

9,600 (static load/foot) /
4,000 N/mm (stiffness R)

= 2.4 mm

With the same interference frequency of 20 Hz (1,200 rpm), the above graph now delivers a degree of insulation of approx. 75 %.