MIL-C-882 Fabreeka Pads

FAQS

What are the vibration isolation characteristics of Fabreeka pad?
Please refer to our Fabreeka Pad brochure.

Does all “cotton duck” reinforced pad material meet the MIL-C-882 specification?
No. The MIL-C-882 specification requires specific strength, deflection, density and permanent set properties.

How can I be sure the material I am buying meets the MIL-C-882 specification?
The supplier should supply a material certification stating that the material meets MIL-C-882. In addition, the deflection, creep and environmental test data should be provided or certified to.

What environmental tests are involved with the MIL-C-882 specification?
The pad material must pass several environmental tests per MIL-E-5272A.

What is creep?
Creep is the deformation of rubber is influenced by the length of time under stress. If the rubber is statically loaded to a given amount, as occurs for example by the support of a machine, then an elastic deformation takes place superseded after a longer period of time by creep. Creep behavior follows an exponential law and is concluded sometime afterward. If the isolator is released, then its shape returns elastically. High elastic grades show a small residual strain and minimal creep. With good elastic qualities, the creep lies between 5% and 10% of the total elastic strain.

What is permanent set?
Permanent set, or residual strain, takes place after prolonged loading and unloading of an isolator, and lies between 2% and 5% of its original thickness.

How is damping different than isolation/absorption?
The essential properties of an isolator are natural frequency (developed by the spring rate or stiffness) and an energy dissipating mechanism known as damping. In some types of isolators, the stiffness or natural frequency and damping properties are contained in a single element such as elastomers, cork, rubber mats, etc. Other types of isolators may have separate means of providing stiffness and damping as is the case with air springs (pneumatic isolators) and coil steel springs, which are relatively undamped until used in conjunction with auxiliary damping elements such as orifice flow restrictors and viscous dampers. The purpose of damping in an isolator is to reduce or dissipate energy as rapidly as possible. Damping is also beneficial in reducing vibration amplitudes at resonance. Resonance occurs when the natural frequency of the isolator coincides with the frequency of the source vibration.

The ideal isolator would have as little damping as possible in its isolation region as much as possible at the isolator’s natural frequency to reduce amplification at resonance. Damping, however, can also lead to a loss of isolation efficiency.

Can Fabreeka pad material act as an electrical insulator?
Fabreeka pad has a dielectric strength of 12,500 volts (210 volts/mil) and a resistivity of 8.5 x 10e9 ohm-cm. (Insulating material classification requires a resistivity greater than 10e5 ohm-cm.) As a comparison, natural rubber has a resistivity value of 10e15 ohm-cm. Fabreeka pad has a dielectric constant of 9.34, with a power factor of 0.201 and a loss index of 1.881. All of these values are for Fabreeka pad at a standard room conditions of 73°F (213°C) and 50% relative humidity.

How does static spring rate differ from dynamic spring rate?
The static deflection principle can only be used to determine the natural frequency of an isolator if the isolator under consideration is both linear and elastic. For example: rubber, felt, fiberglass and composite materials tend to be non-linear and exhibit a dynamic spring rate that differs from the static spring rate.

Similarly, the spring rate of a pneumatic isolator changes when undergoing a change from the static condition to a dynamic condition.

The natural frequency as calculated based on static load vs. deflection data will give inaccurate lower natural frequencies as compared to realistic experience during dynamic vibration.

Any isolator with a calculated natural frequency based on static deflection may not behave in the predicted way because the dynamic spring rate differs from the static spring rate. It is the dynamic natural frequency that has to be used in isolation calculations rather than the static natural frequency.

What are the thermal properties of Fabreeka material?
Thermal conductivity of Fabreeka material is expressed in power per unit of area divided by temperature gradient in degrees per unit of length. The Imperial units are 1.90 BTU INCH/HR/FT-SQ/Degrees F.

What are the manufacturing tolerances of the Fabreeka Pad?
Manufacturing tolerances vary by thickness and part geometry. Please contact our Engineering department at 800 580 4195 to discuss the tolerances for your application.

Please feel free to contact us today to discuss your project requirements and schedule a commitment free lunch and learn or webinar with one of our product specialists.