Rubber Mount Creep: Specifying Loaded Height and Load–Deflection Acceptance

A rubber mount can fit correctly at installation and still need allowance for a gradual change in loaded height. For machinery with limited clearance, alignment requirements or closely spaced stops, that change deserves its own acceptance criteria. A useful mount specification therefore connects three things: the force carried by each support, the resulting deflection and the additional movement permitted over time.

Separate initial deflection, creep and compression set

Initial deflection is the movement measured when a defined load is applied under a defined procedure. Creep is the subsequent increase in deformation while that load continues. ISO 8013:2019 addresses creep in vulcanized rubber under continuous compression or shear; its scope expressly excludes intermittent deformation.

Compression set describes the deformation remaining after compression is removed and the rubber has recovered under specified conditions. The result depends on compression time, temperature and the recovery procedure, as explained in ISO 815-1:2019. A compression-set percentage therefore cannot be treated as a direct prediction of a machine’s settlement while its mounts remain loaded.

There is a third question when an assembly holds rubber at a fixed deformation: how much supporting force remains with time? That is stress relaxation, covered for constant-temperature compression testing by ISO 3384-1:2024. Select the test question that matches the restraint in the actual assembly.

Start with a loaded-height budget

Before choosing an allowable creep value, identify what movement the equipment can tolerate. Record the intended mounting height, nearby clearances, alignment limits and the position of travel stops. Specify whether dimensions refer to the unloaded part, a seating condition or the assembly carrying its service load.

Keep initial compression and later settlement as separate entries in this budget. Also reserve the space needed for operational movement and assembly tolerances. The acceptable additional settlement must come from the equipment’s requirements; a universal percentage copied from an unrelated mount can conceal a clearance problem.

Provide the expected force at each support and the loading direction. Where support loads have not been established, request that calculation before setting acceptance limits. Document any adjustment or re-leveling that the installation permits, together with the conditions under which it may be performed.

Make the load–deflection test reproducible

A useful test record shows how displacement changes across the intended load range. ASTM D575 provides compression-deflection methods for comparing rubber materials, excluding hard rubber and sponge rubber from its stated scope. For a complete mount, agree a component test procedure that captures the actual geometry and installation conditions.

For that component procedure, specify the following before samples are tested:

• The part revision, compound identification and sample count.
• Conditioning temperature, conditioning time and any prior loading.
• Loading direction, support surfaces, restraints and fixture arrangement.
• The displacement datum, loading rate, measurement points and dwell times.
• Which loading cycle and loading or unloading branch determines acceptance.

Report the measured curve and the permitted deflection window at agreed loads. Avoid reducing the report to a single stiffness number with no stated load interval. Where a mount uses bonded metal plates or inserts, include those interfaces in the test definition; IKSONIC’s rubber-to-metal bonding page provides examples of this construction category.

Give creep its own measurement timeline

For sustained-load qualification, agree the load, temperature, test duration and observation times in advance. Define the reference reading used for additional settlement and state when it is taken after loading. Without that timestamp, two reports can use different starting points while both label their result “creep.”

Keep the required load applied during the observations. Record temperature and any interruption, fixture movement or change of load alongside the displacement readings. Identify the measurement method so that movement of the fixture is not silently included as movement of the rubber.

If recovery after unloading matters, add a separate recovery stage with its own timing and acceptance requirement. Treat a short screening test as evidence for those stated conditions. Any prediction covering a longer service period needs an appropriate model and supporting validation, rather than an assumed straight-line extension of the early readings.

Approve a test plan, then approve the sample

A practical approval package combines the controlled drawing, component identification, load–deflection results, creep observations and agreed limits. Include visible-condition checks and an assembly fit check where relevant. Record deviations explicitly and identify who can approve them.

For repeat production, retain the approved procedure and baseline results. Ask for review when a change could affect the comparison, such as a different compound, geometry or bonded interface. This creates a clearer basis for evaluating subsequent samples.

When reviewing IKSONIC’s rubber buffers and mount-related products, prepare the loaded-height budget and test brief alongside the dimensions. That makes the discussion more useful: the objective is a mount whose initial position and later movement both remain within the machine’s agreed limits.