Polyurethane is a non-compressible elastomer, meaning its volume will not change. Polyurethane formulations are able to store up and return energy; for example, if you squeeze a urethane product in one place, it will bulge the same amount in another place, and then return to its original shape once the load is removed. This property makes polyurethane an excellent material for load-bearing applications. In fact, when comparing urethane and rubber of equivalent hardness, urethanes have higher load bearing capacity than rubbers. For this reason, many industries prefer standard urethane over rubber.
Key Properties: Deflection and Compression Set
In load-bearing applications such as isolation pads, bumpers, bushings, rollers and wheels, it is important to determine – and fully understand – two key properties: compression and deflection. An understanding of these properties allows designers to choose the optimal size and thickness for a part as well as the best polyurethane formulation – improving part performance and minimizing material costs.
- Deflection is a measure of a material’s resistance when a compressive force is applied. It is measured by the force required to compress a given material, and is expressed as a percentage. For most polyurethane applications a range between 5% to 15% deflection is acceptable. At PSI Urethanes, we recommend not exceeding 25% deflection regardless of the application as operations above this limit lead to much higher stresses within the part.
- Compression set indicates how well the polyurethane will retain its elastic characteristics under compression. Elastomers with low compression set values will spring back when the load is removed and recover most, or all, of their original thickness, whereas a high compression set indicates the part will not recover as well once the compressive force is removed and will likely have a permanent deformation. At PSI Urethanes, our extensive experience enables us to customize polyurethane formulations to achieve the desired compression/deflection to meet your specific needs.
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Compression Calculation Variables
Before diving into the calculations, let’s first define each variable used in the equation as well as other factors that impact polyurethane compression/deflection. It should be noted that although the formula for calculating compression/deflection was scientifically derived, the many variables involved can make it difficult to precisely predict the outcome. For this reason, it is invaluable to work with a proven partner, like PSI Urethanes, who has the requisite knowledge and experience to determine polyurethane compression/deflection.
- Load (L): the amount of force, or load, placed on the polyurethane part will, of course, directly impact the level of compression/deflection.
- Shape Factor (f): the shape factor is determined by dividing the area being pressed by the area that is able to bulge. Parts made from the same compound and having the same shape factor behave identically in compression, regardless of actual size or shape.
- Young’s Modulus (Y): this is a measure of a material’s elasticity. For polyurethanes, the value of Young’s Modulus varies by formulation and depend on shape factor and hardness. Hardness, or durometer, is an indicator of how a polyurethane product will perform – and the extent of deflection – under a given load. In general, the higher the durometer of the urethane, the higher its load bearing capacity and its resistance to deformation.
- Load Surface Conditions: the surface condition of a part can make quite a difference in deflection and the load bearing capability of a given polyurethane formulation. When a part is compressed, it wants to expand out laterally and that movement changes the shape factor. This is particularly noticeable if the surfaces are lubricated since little resistance is offered to lateral movement. However, if the surfaces are bonded to a substrate such as metal, the shape factor remains the same as no lateral movement is possible.
Below is an example of a compression calculation for a pad (this method can also be used for cylindrically shaped bumpers). Click here for an example of a compression calculation for a wheel.
You have a 92A durometer(hardness) isolation pad that is 6″ long by 3″ wide and 2″ thick, that is under a 3,600 lb load. You want to know how much deflection it will have.
The formula is: L=D x Y x (1+2f2)
D = % of deflection/inch of thickness
L = load or force in psi
Y = Young’s Modulus
f = shape factor
*The shape factor is determined by dividing the area being pressed by the area that is able to bulge.
L = 3600 lbs ÷ 18 square inches (6 x 3) = 200 psi
Y = 4500 psi for 92A durometer(hardness)
f = 18 (6 x 3) area being pressed ÷ by 36 (2×6+2×3+2×6+2×3) area able to bulge = .5
f2 = .52 = .25 therefore (1 + 2f2) = (1 + (2 x .25)) = 1.5
Using the formula above:
200 = D x 4500 x 1.5
200 = 6750D
200 ÷ 6750 = D
D = .0296 per inch of original thickness
2 x .0296 = .0592 or 5.9% deflection
About PSI Urethanes
With over 50 years of experience delivering the highest quality urethane products, PSI Urethanes stands unequaled in helping companies in hundreds of industries around the world apply urethane products to their critical business applications. Since 1966, through our own independent research and activity in the Polyurethane Manufacturers Association, PSI Urethanes has stayed on the leading edge of urethane technology – including developing advances in new materials, comprehensive designs, and processing technologies. Today we are among the most respected and trusted suppliers of custom cast urethane products.
We operate from our 45,000-square-foot manufacturing facility in Austin, Texas, which houses an extensive stock of polyurethane products. Our fully equipped manufacturing facility includes a versatile range of processing machinery, which allows us to work on unique, one-of-a-kind projects, as well as high volume components. Our in-house engineering, tooling, and custom mold shop, produces custom and stock production orders with exceptionally fast turn-around times. From conception to prototyping, to final production runs, our technical representatives and design engineers will work closely with your staff to meet your critical tolerances and specifications. You are guaranteed to receive experienced, professional, and personalized service from the first contact through your on-time delivery, with complete confidentiality and dedication to protecting your good name.
Contact us today to see how we can help with your next project, or give us a call at 1-800-888-5156.