Dynamic characteristics testing of vibration-damping leveling mounts (shock-absorbing mounting pads) is essential for verifying the vibration isolation performance, dynamic stiffness, damping ratio, and natural frequency of resilient machine installation elements. This article explains the test samples, testing items, standard test methods, and typical applications of dynamic characteristic evaluation, helping manufacturers and equipment users select reliable damping mounts and ensure compliance with international standards such as ISO 10846, ISO 18437, and GB/T 15168 in accredited third-party laboratories.
Technical Overview
Vibration-damping leveling mounts — often referred to as shock-absorbing mounting pads or damping wedges — are resilient machine installation elements that combine a height-adjustable leveling mechanism (threaded stud, wedge, or screw thread) with an elastomeric damping layer, typically made of NBR rubber, rubber-cork composites, or polymer materials. They are widely used to install machine tools, forging equipment, pumps, and precision instruments without the need for anchor bolting, while simultaneously attenuating vibration transmission between the machine and the foundation.
The static load-deflection curve alone cannot describe how these components behave under operating conditions. When subjected to dynamic excitation, a damping mount exhibits frequency-dependent and amplitude-dependent stiffness, energy dissipation, and resonance amplification. Dynamic characteristics testing therefore measures parameters such as dynamic stiffness, damping ratio, natural frequency, transmissibility, and the dynamic-to-static stiffness ratio under controlled excitation, preload, and environmental conditions.
The principal references for this type of evaluation include the ISO 10846 series (laboratory measurement of the vibro-acoustic transfer properties and dynamic stiffness of resilient elements), the ISO 18437 series (characterization of the dynamic properties of visco-elastic materials), GB/T 15168 (test methods for the performance of vibration and shock isolators), and ASTM D5992 (guide for dynamic testing of vulcanized rubber and rubber-like materials utilizing sinusoidal excitation). Testing is commonly performed in accredited third-party laboratories or manufacturers' certified dynamic test facilities.
Testing Samples
Adjustable Damping Leveling Mounts for Machine Tools
Wedge-type or screw-type leveling mounts incorporating a damped rubber pad beneath a steel leveling mechanism. These are the most common products, supplied in load classes from tens of kilograms to several tonnes per mount, and are tested in both loaded and unloaded states.
Threaded Stud-Type Damping Mounts
Cylindrical or frusto-conical rubber-metal bonded mounts with a central fastening bolt, used under engines, compressors, and power units. Testing covers vertical and lateral dynamic behavior under rated static preload.
Anti-Vibration Pads and Damping Plates
Flat resilient pads made of nitrile rubber, rubber-cork composite, or cellular polymers, supplied in sheet or strip form and cut to size. Specimens are tested as complete pad assemblies at the design compressive strain.
Material Coupons from Damping Elements
When full-scale mount testing is impractical, cylindrical or rectangular material coupons taken from the damping layer are characterized for storage modulus, loss factor, and temperature/frequency dependence in accordance with ISO 18437 or ASTM D5992.
Testing Items
Typical parameters measured in a dynamic characteristics test program include:
- Static stiffness and rated deflection under working load
- Dynamic stiffness (complex, storage, and loss components) in vertical and lateral directions
- Damping ratio and loss factor
- Natural frequency of the mounted system (vertical, roll, and lateral modes)
- Vibration transmissibility and resonance amplification factor
- Dynamic-to-static stiffness ratio (a key indicator of isolation efficiency)
- Frequency dependence of stiffness and damping (typically 5–500 Hz)
- Amplitude dependence and preload sensitivity
- Temperature dependence (e.g., −20 °C to +80 °C) and creep behavior
- Durability and fatigue performance under cyclic dynamic load
- Ultimate load capacity and safety factor
Testing Methods
Static Load-Deflection Test
A universal testing machine applies compression at a controlled rate to determine the static stiffness curve, rated deflection, and linear working range. The measured static stiffness serves as the baseline for calculating the dynamic-to-static stiffness ratio and provides input for isolator selection calculations.
Non-Resonant Direct Method (Force Method)
In accordance with ISO 10846, the mount is fixed between a rigid excitation block and a sufficiently stiff mass. An electrodynamic shaker or servo-hydraulic actuator applies broadband random or stepped-sine excitation while force transducers and accelerometers measure the dynamic force and blocked displacement. This method yields the complex dynamic stiffness across the frequency range of interest without requiring the test system to resonate.
Resonance Method (Mass-Spring Method)
Per GB/T 15168, a rigid mass is mounted on the test isolator to form a single-degree-of-freedom system. A frequency sweep identifies the resonance peak, from which the natural frequency is obtained directly. The damping ratio is calculated using the half-power bandwidth method, and dynamic stiffness is derived from the natural frequency and the known vibrating mass.
Transmissibility Measurement
The mount is installed between a vibrating table and a loaded inertia block. Base excitation is applied as a sinusoidal sweep or random signal, and the acceleration ratio between the receiving mass and the excitation table is recorded. The resulting transmissibility curve confirms the isolation region (typically required to be below 0.5 at operating frequencies) and the resonance amplification factor.
Impact (Hammer) Method
For rapid quality-control checks, an instrumented impact hammer excites the preloaded mount-mass system and the free decay response is analyzed to extract natural frequency and logarithmic decrement damping. This method is suitable for comparative acceptance testing.
Environmental and Endurance Testing
Conditioning per IEC 60068 (temperature, humidity, and dry heat/cold cycles) combined with dynamic excitation verifies that stiffness and damping remain within tolerance over the service environment. Endurance tests apply several million load cycles at rated amplitude to assess stiffness drift, creep, bond integrity, and set.
Applications
Dynamic characteristics testing of damping mounts supports design validation, product certification, and installation engineering across many industries:
- Machine tools: CNC machining centers, lathes, milling machines, and grinding machines where micro-vibration affects surface finish and geometric accuracy
- Metal forming equipment: mechanical and hydraulic presses, punch presses, shears, and forging hammers exposed to impact loads
- Rotating machinery: pumps, fans, compressors, diesel generator sets, and gearboxes requiring resonance avoidance at rotating frequencies
- Precision metrology: coordinate measuring machines, optical instruments, and laboratory balances needing low-transmissibility isolation
- General manufacturing: textile machinery, printing machines, packaging lines, and injection molding machines installed on raised floors without anchoring
- Quality control and supplier qualification for mount manufacturers and export compliance verification in accredited third-party laboratories
FAQ
What is the difference between static and dynamic stiffness of a damping mount?
Static stiffness is derived from a slow compression test, while dynamic stiffness is measured under oscillating excitation and includes both elastic (storage) and dissipative (loss) components. For elastomeric mounts, dynamic stiffness is typically 1.2 to 2.0 times the static value, and this ratio directly influences the actual natural frequency of the installed machine.
Which standards govern the test methods?
The ISO 10846 series defines direct measurement of dynamic transfer stiffness for resilient elements, ISO 18437 covers characterization of visco-elastic materials, GB/T 15168 specifies resonance-based and transmissibility methods for isolators, and ASTM D5992 guides sinusoidal dynamic testing of rubber-like materials. Environmental conditioning generally follows the IEC 60068 series.
What sample information should be provided before testing?
Applicants should provide the mount model, rated load per mount, overall dimensions, damping material specification, intended service temperature range, target natural frequency or isolation efficiency, and the installation orientation. For full-scale tests, a minimum of three to five identical specimens is recommended to account for material scatter.
How are the results interpreted?
The key acceptance indicators are the measured natural frequency (which should be well below the main excitation frequency, typically at a ratio of at least 1:√2 and preferably 1:3 or lower), a transmissibility below the specified limit in the isolation region, controlled resonance amplification, and adequate damping to limit response during machine start-up and shutdown through resonance.
What is included in a typical test report?
A professional report from an accredited third-party laboratory includes specimen identification, test setup and instrumentation, preload and excitation conditions, stiffness and damping values versus frequency, transmissibility curves, natural frequencies in each direction, temperature dependence if applicable, uncertainty statements, and full traceability to the applied standards.