KEVOS® MAKEOS · Springs
Relief Valve Spring Check (Cracking Pressure, Full-Lift Pressure, Overpressure, Spring Stress)
For a direct-acting relief or check valve held shut by a coil spring, what pressure does it crack at, how much higher is it at full lift, and is the spring stressed within limits?
Check of the spring of a direct-acting poppet or ball relief valve: the spring rate, installed load and shear stress at the closed length (as the compression spring tool), the cracking pressure from the installed load over the seat area, the pressure at full lift from the extra spring load, the overpressure (accumulation) between them, the stress at full lift and the clearance to solid, against the set pressure and tolerance wanted, the allowed overpressure and the allowable stress.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Seat diameter (sealing) | mm | Diameter of the seat where the poppet or ball seals (the area the pressure acts on). |
| Full lift of the poppet | mm | How far the poppet opens at full flow. |
| Set pressure wanted | MPa | Pressure the valve should start to open at. |
| Set pressure tolerance (±) | % | How far the cracking pressure may be from the wanted pressure. |
| Largest overpressure at full lift | % | Largest rise above cracking pressure allowed at full lift. |
| Spring | ||
| Wire diameter | mm | Wire diameter. |
| Mean coil diameter | mm | Mean coil diameter. |
| Active coils | coils | Active coils. |
| End type | choice | End type of the spring. |
| Shear modulus | GPa | Shear modulus of the spring wire. |
| Unloaded length | mm | Length of the spring with no load. |
| Installed length (valve shut) | mm | Length of the spring fitted in the valve with the valve shut. |
| Allowable shear stress | MPa | Allowable shear stress for the spring wire and duty. |
Method and formulas
- k = G d⁴ ÷ (8 D³ N_a); load F = k (L₀ − L); τ = K 8 F D ÷ (π d³) with the Wahl factor (as the compression spring tool).
- Seat area A = π d_s² ÷ 4; cracking pressure = F_installed ÷ A; full-lift pressure = k (L₀ − L_w + lift) ÷ A.
- Overpressure = full-lift ÷ cracking − 1; length at full lift against the solid length.
- Example: cracks at 0.92439 MPa, 1.0168 MPa at full lift (10% over); 481.58 MPa in the spring.
Limitations
- Static balance on the seat area; flow forces on an open poppet change the full-lift pressure.
- Spring at room temperature; hot oil lowers G slightly.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set the spring for a small relief or check valve
- Find why a relief valve chatters or overpressures
- Re-shim a valve to a new set pressure
Who it is for
- Hydraulic engineers
- Valve designers
- Maintenance engineers
What you get
- Spring rate, installed load and stress
- Cracking and full-lift pressure, overpressure
- Checks against the set pressure, stress and solid
Interpreting the result
- A softer spring (more coils) cuts the overpressure but needs more installed compression for the same set pressure.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 304–324. Topic reference; no source tables or recommended settings are reproduced.
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