What is the difference between a variable and a constant spring support?

In a variable support, the force acting on the spring and thus the reaction force varies during the displacement of the pipe, while the moment around the line of action is zero. In contrast, with a constant support, the fixed applied load remains the same during displacement, but the moment around a pivot point varies.

Constant spring support

A constant support is a device consisting of a spring or series of springs and an integral cam mechanism. The external load of a constant support is fixed, while the moment around the fixed pivot point varies during displacement (because the length of the moment arm changes). To maintain an equilibrium state, the external force moment is balanced by the internal moment created by the compression or decompression of the spring about the pivot point during the displacement of the pipe.

Constant spring support
Constant spring support
Image.. Carpenter & Paterson

With a proper choice of moment arms, as developed by the cam geometry, and spring properties (i.e. spring constant), a resistance force can be provided that is almost independent of position during displacement.

At each displacement location of the applied load, the moment caused by the external load is balanced by the counter moment produced by the (compressed/suppressed) spring force with the appropriate moment arm. Usually, the variation of the active and reactive forces is very small (with a maximum deviation of 6%) and can be considered as a constant force during upward or downward motion.

Variable spring support

A variable spring support is essentially a spring, or series of springs, in a holder. When the installed load is applied, the spring is compressed over such a distance that the reaction force exerted by the spring is also installed load in the equilibrium state.

Variable spring support
Variable spring support
Image.. www.lisega.de

As the tube moves as a result of thermal expansion, it causes a deflection that causes a differential load to act on the spring(s). Depending on the direction of movement, the change in load will increase or decrease the installed load to reach the final operating load.

To minimize stress variations, the differential load for a given variable spring support is limited to a maximum of 25% of the operating load.

Reference.. Piping Technology & Products

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