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Physics of Spring Coils

Variable spring supports are attached to piping systems so the coil is in a vertical position as shown in Figure 1. Forces which compress the coil act in the same direction as the force of gravity. Movement due to temperature changes may be either up or down. MOVEMENT is a vector which has both direction (up or down) and magnitude. TRAVEL is a scalar which describes the length of movement or compression of the spring coil.

A spring can only be compressed so much before the individual coils come together and it "goes solid". The usual method used to provide for larger travel is to provide a longer coil or stack standard coils together as shown in Figure 2.

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FIGURE 2: VARIABLE SPRING MODELS

Newton's Third Law of Motion states that every force is accompanied by an equal and opposite force. A spring coil resists the force which compresses it. The more the coil is compressed the greater resistance it has to further compression. This is why pipe supports which apply forces directly to the spring coil are called variables.

The magnitude of the force required to compress a spring coil depends on the size of the wire used to form the coil and on the diameter of the coil. A "stronger" coil can be produced by using a larger wire or by winding the wire "tighter". The two industry standards are the PTP-1 (short spring) and the PTP-2 standard spring. Twenty-three different wire sizes from .23 inches to 2.5 inches are used to increase the size of the force required to compress the coil. At PT&P we use 00, 10,..., 200, 210 220 to designate the wire size (resistance to force) of the coil as shown in the top of Figure 6.

Figure 3 shows a graph of pounds-force required to compress a PTP-2, Size 90 coil as a function of the amount of compression. The graph is a straight line through the origin with a slope of 200 pounds-force per inch of compression. The slope of this line is the SPRING RATE of this variable support. We compress the coil three inches during assembly (using 600 pound-force). The variable produced can be further compressed an additional 3.5 inches ( range of deflection) by applying 1300 pounds-force. Each additional 200 pounds-force will compress the coil one inch.

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FIGURE 3

Manufacturers recommend that only 80% of the range of deflection (called the OPERATING RANGE) be used for variables. The PT&P Load and Travel Table uses red lines to enclose the loads in the operating range. Figure 3 marks both the minimum and maximum loads (700 and 1200 pounds-force) for the operating range with red lines. The load at the center of the operating range (950 pounds-force) is marked with a blue line. The total compression of the coil at 700 pounds-force is 3.5 inches and the compression at 1200 pounds-force is 6.0 inches so the OPERATING RANGE is 2.5 inches. Figure 6 shows how to obtain these values from the Load and Travel table of our catalog.

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FIGURE 4

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