Technical: Understanding Urethane

Urethane - What is it?

Urethane raw materials are liquid. Component materials can be pumped, mixed, metered, and dispensed by machines very precisely while controlling material temperature and proportion. These blended liquids are poured into molds and cured. Very large urethane parts with thick cross-sections can be molded. It is also possible to hand mix many of these raw materials where required part quantities so justify.

Urethane Is Often Useful As A Vibration Isolator

Urethane's dynamic properties combine with its high load bearing capacity to make it an excellent choice for vibration isolation applications.

Urethane Natural Frequency (fn)
Expressed Hertz (Hz) or cycles per second: fn = 3.13 time the sq. rt of 1 / Static Deflection

Frequency Ratio
Vibration isolator effectiveness is expressed as: Frequency Ratio = ff / fn
Forcing Frequency units are Hertz or cycles per second.
Forcing Frequency = ff
Natural Frequency = fn

Damping
The gelatinous component of a urethane isolator is the hysteresis characteristic that converts mechanical energy into heat which is then dissipated. During vibration a percentage of the input energy is dissipated in the form of heat with each cycle, which ultimately stops the vibration. Diffusion Curves have been derived and are used when designing urethane isolating systems or components. Dampening Ratio (C = cycles), C/CDR is used to designate damping in the systems and can vary from .05 (highly resilient formulations) to .15 (lower resilient formulations). C/CDR is affected by both temperature and preload. UI engineering can help you select the proper material and develop an isolation system for most any application.

Abrasion Resistance
Urethane resistance to abrasion is better than metals, plastics, or rubbers and will last 8 or more times longer. The specific types of abrasion are: erosion, impingement, impact, scuffing, and sliding. Rate of wear is affected by lubricity, mass, pressure, temperature and velocity. Experiments has shown:

• Softer urethanes (50A-85A) are best for particle impingement applications.
• Mid-range urethanes (80A-95A) of medium resilience last longest in abrasive applications.
• Hard urethanes (90A-95A) work best in impact and sliding types of abrasion applications.
• Very hard urethanes (60D-75D) work best in bearing and bushing applications.

Coefficient of Friction

Generally, the harder the urethane the lower the coefficient of friction. 80A has twice the friction as 60D. Lubricants can be added to urethane to make them more slippery without compromising the urethane properties.

Load-Bearing Capability and Its Affects

Urethanes have unusually high load bearing capacity relative to other elastomers. Their deflection and recovery capabilities exceed that of plastic and metal.

Urethane that is not bonded to a surface and compressed will try to spread or slip laterally because compressed urethane does not lose volume but instead bulges around the edges. Conversely, if compression is applied to urethane bonded to a surface, the compression/deflection action will be different. The dynamics changes when one, two or no surfaces are bonded ,or when surfaces are clean and dry, or surfaces are lubricated. Significantly different compressive stress-strain curves result for the same urethane part when bonded as opposed to un-bonded.

We must understand compression/deflection in design. An awareness of the parts feature shape and the conditions under which the part is subjected is critical. Shape impacts tension and shear. For example: Two isolator pads (blocks of urethane), one square and the other round, of equal cross-sectional area and each having the same load applied will result in greater deflection for the rectangular block because the side walls [area free to bulge] are longer than that of the cylinder. The longer the side wall, the easier the bulging and consequently the greater vertical displacement. This shape influence can be expressed in a formula which is referred as Shape Factor (SF). Simply, the ratio of one loaded surface area to the total area free to bulge. Urethanes behave like incompressible hydraulic fluids where under load they do not change volume. UI will help you determine deflection based on design, material selection and load. An example follows:

Consider a 1" thick pad, 6" x 6" [36 in2]; 90A urethane;

For a load say 72,000 lbs;

72,000 / 36 = 2,000 psi

Load deflection for 2,000 psi for 90A material is ≈17% or .170 inches

The effect of impact cushioning or the best material for a project can be calculated by UI to ensure product success.

Fatigue

Fatigue is an important consideration in cyclic dynamic applications. Testing of fatigue resulting from cyclic stress-strain requires knowing strain energy experienced by the part in application. Strain energy is a function of modulus and the strain cycle seen. It is important to consider urethane stoichiometry (polymer to curative ratio). Significant improvement in flex fatigue resistance can be achieved through chemical alteration. Stoichiometry used is commonly between 90% to 115%.

Hardness

The hardness of urethanes is easy to measure with a durometer gauge. Hardness alone is not a good indicator of performance, because various formulae having the same hardness do not have the same physical and engineering properties. Each of the below materials will perform better in different applications but are not well suited as replacements for each others intended application.

Dynamic applications and the environment the parts will see are critical considerations. After a couple of dozen strain cycles, equilibrium is usually achieved and performance is predictable and reproducible. Some urethanes have consistent spring rates and are excellent for bumpers or spring applications. Low loaded gears / sprockets with relatively low loads and mating surface mismatch benefit from more flexible materials.

Shock and Impact Applications

Bumper design can be optimized. Kinetic Energy of a moving body is compared to the bumper cross-section under a force-deflection curve. Urethanes behave differently than spring steels. Dynamic spring rates range up to 2.5 times greater than their static spring rates. A significant percentage of input energy is converted to heat and is not readily given up due to urethane's natural resistance to heat transfer. Excessive heat build up [Hysteresis] can be avoided in bumpers through design by controlling the deflection per cycle to less than 20%.

K.E. = ½ mv2 = ½ (W/g) v2          K.E. = ½ (F) (deflection) = psi
          Where:
                    m = weight of moving object
                    g = 32.2 ft/sec2
                    v = speed of moving object in ft/sec
                    v2 = (ft/sec)2
          The answer is expressed in ft-lbs; converted to in-lbs by multiplying answer by 12

Suppose you have a urethane bumper with 4" diameter and 3" thick. The Shape Factor is .33 because the loaded area of the bumper is 12.6 in² and the bulge area is 37.7 in². Given the desired deflection is not to exceed 20% of the 3 inch thickness [.6 inches] and if the dynamic static spring ratio is 1.8, then proper material selection will deflect .60" x 1.8 = 1.08" maximum deflection. Final testing should be conducted to confirm the actual deflection in use. If different, adjust bumper physical shape or select another material.

Mold Tolerances

It can be assumed the tighter the tolerance, the higher the part cost will be. Urethane casting has limitations so it is important that tolerances be realistic for the application. Below are practical tolerances for helping to optimize part cost. If there is a requirement for very close tolerances on certain dimensions, they are achievable by proper sizing [tune-in] of the tooling or by secondary machining steps after the part is cast. Urethanes have a very high coefficient of thermal expansion, approximately 10 times that of steel and four times that of aluminum. (1.0 - 1.5 x 10-4/in./in./°). If a part is made to a +/- .005 inch tolerance, as measured at 75°, it will be out of tolerance at 0° or 150°. So, proper design tolerance should take into account final application environment and usage for best results. UI engineering can help you optimize your system.

Hysteresis

Shape factor & compression / deflection are critical in determining the right design for a specific application. Urethane bumpers do not return all energy after a compressive cycle. The amount not returned is converted to heat and is referred to as hysteresis loss. If frequency of deflection is low, usually a low resilience compound will absorb more impact.

Urethane elastomers consist of two physical response factors, elastic and viscous. The elastic part stores energy and returns it (rebounds).A viscous portion captures energy and converts it to heat.

Highly resilient compounds have a greater elastic/viscous ratio. Low ratio urethanes do not return much energy and are referred to as "dead." Low ratio compounds convert more input energy to heat than do high ratio compounds. Also, urethanes do not lose heat rapidly so where cyclic conditions exist, it is important to deal with ways to generate less heat. This can be achieved by: (1) reducing the deflection (strain) per cycle; (2) increasing the compression modulus of the urethane; (3) increasing the shape factor; or, (4) reducing the stress per unit area.

Shear

Shear is a combination of tensile and compressive forces acting at right angles to each other. Urethanes in shear are generally used in mounting and suspension applications. The weakness of such an application is dependent on having a good bond between the urethane and substrate to which it is attached. Deflection of a urethane in shear is a function of shear stress, modulus of the urethane, and the thickness of the urethane.

Stoichiometry

Stoichiometry is the chemical proportion of an reactive elastomer to a curing agent. Proper stoichiometry plays an important part in compression set resistance, cut and tear resistance, and flex fatigue resistance. Generally, higher stoichiometry leads to greater heat build-up and flex fatigue resistance. Therefore, application considerations need to ascertain whether heat build-up or flex fatigue resistance is of more importance.