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Selecting the Proper Size for a Variable Support

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FIGURE 5

The coil size (force required for compression) should be chosen to balance the expected force on the support when the system is operating. To do this choose the size (00 to 220) which has its center compressive force (blue line) in Figure 6 as close as possible to the OPERATING LOAD ("hot" load). Choose the shortest (least expensive) coil and operating range which will satisfy the expected movement the support will experience during the temperature changes expected going from COLD LOAD (non-operating force applied) to OPERATING LOAD.

Figure 5 illustrates the relation between the three quantities, OPERATING LOAD, COLD LOAD and MOVEMENT. The direction of MOVEMENT is defined by the change of load (force on the support) when the system changes from cold to operating. If the OPERATING LOAD is greater, the increase in the force will further compress the coil so the direction of the movement is DOWN. If the OPERATING LOAD is smaller, the decrease in the force on the support will allow the coil to expand so the movement is UP.

(click here for a larger image)
FIGURE 6

Variability Constraint

The change in the force on the support when changing from cold to operating is equal to the spring rate multiplied by the TRAVEL (magnitude of the movement). Good practice as specified in MSS code requires that this change experienced by the support should be no more than 25% of the OPERATING LOAD thus the inequality in Equation (a) must always be satisfied. Note that the SPRING RATE is a function of both the coil size (Z) and the coil length (X). Coil size Z is chosen to match the operating load, so X is chosen as small as possible to satisfy Equation (a). The possible values of X are 1, 2, 4, 6 and 8 which correspond to PTP-X and define the OPERATING RANGE of the support.

(a) Spring Rate (Z,X) * TRAVEL < 0.25 * OPERATING LOAD.

If you are interested in a computer program which will size variable spring supports shown in the PT&P catalog, please contact us.

Coil with Neoprene Coating

Latest Design for Guided Loaded Column

Coil Prior to Coating

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