Planning and Design Tips

Creep

Creep

Creep is a time-dependent increase in spring deflection for a spring under constant load. The phenomenon is a consequence of relaxation, i. e. it results from a gradual loss in the resilience of a spring subjected to constant deformation over time. It occurs with such a high degree of inertness that it does not take part in the vibrational movements of the rubber-metal mounts.

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Creep is influenced by

  • material
  • Spring rate in the load point
  • Shape of the characteristic curve

In the case of a linear spring characteristic at constant temperature, creep is linearly proportional to the time logarithm. It is independent of the type of stress (compression, shear).

The creep K of an elastomer material is defined as follows:

Spring deflection over time

K = Δ S S 6 × n

The following reference values apply for the elastomer materials used in SCHWINGMETALL mounts:

Elastomer hardness Creep Creep Creep
  NR CR NBR
40 Shore A 0,02 0,04 0,04
55 Shore A 0,03 0,05 0,04
65 Shore A 0,04 0,05 0,05
NR – Natural rubber; CR – Chloroprene rubber; NBR – Nitrile rubber

The values shown in the table were determined for compression/shear mounts with a diameter of 50 mm, 45 mm high and are subject to a tolerance range of ±15 %. Creep values for large-volume parts may be approx. 20 % greater.

The slight increase in spring deflection due to creep can generally be ignored. The increase in spring deflection over time must be taken into account only in the case of assemblies whose shafts require a high degree of true alignment and whose driving and driven units are not mounted on a common foundation. In view of the fact that half the total value of the increase in spring deflection for an approx. 20-year time period is already reached after 24 h of stress, final alignment can be carried out after just one day of service.