BIZOMAT MAKEOS · Springs
Spring Clamp Hydraulic Release Check (Clamp Force, Release Pressure, Spring Stress)
For a clamp or brake held on by springs and released by a hydraulic piston, is the spring clamping force enough, what pressure releases it, and are the springs stressed within limits when released?
Check of a spring-applied, hydraulically released clamp, brake or tool holder: the rate, load and stress of each coil spring at the clamped length (as the compression spring tool), the total clamping force of the springs in parallel, the extra compression and load at the release stroke, the hydraulic pressure on the annular release piston needed with an allowance for seal friction, the spring stress at release and the clearance to solid, against the clamping force needed, the pressure available and the allowable stress.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Clamping force needed | N | Clamping or braking force the springs must give when clamped. |
| Number of springs | springs | Number of identical springs working in parallel. |
| Release stroke | mm | Extra compression of the springs to release the clamp. |
| Each spring | ||
| Wire diameter | mm | Wire diameter of each spring. |
| Mean coil diameter | mm | Mean coil diameter. |
| Active coils | coils | Active coils. |
| End type | choice | End type. |
| Shear modulus | GPa | Shear modulus. |
| Unloaded length | mm | Length of each spring unloaded. |
| Length when clamped | mm | Length of each spring when clamped. |
| Allowable shear stress | MPa | Allowable shear stress. |
| Release piston | ||
| Piston outside diameter | mm | Outside diameter of the release piston. |
| Piston inside diameter (0 = full piston) | mm | Inside diameter of an annular piston (0 for a solid piston). |
| Seal friction allowance | % | Extra pressure for seal friction as a share of the spring load. |
| Pressure available to release | bar | Hydraulic pressure available. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Clamping force needed
- What it is
- The total clamping or braking force the set of springs must apply when the clamp is closed and the hydraulic pressure is off.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 100000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line for clamping; the spring forces do not change.
- Get it right
- Enter the total for all springs together, not the load per spring; allow for pad or lining wear in the requirement.
What this means: Number of springs
- What it is
- How many identical springs share the clamping load side by side; the tool takes them as equally loaded and adds their forces.
- Units and format
- Enter a number in springs: springs. The form accepts 1 springs to 1000 springs. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 springs as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Clamping force and release pressure rise in proportion; stress in each spring does not change.
- Get it right
- Count only springs of the same size and length; mixed springs do not share the load equally.
What this means: Release stroke
- What it is
- The extra distance the release piston compresses the springs beyond the clamped length to lift the clamp free.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer stroke raises the load at release, the release pressure and the spring stress at release.
- Get it right
- Include the running clearance you need when released; the springs must not reach solid length at the end of it.
Each spring
What this means: Wire diameter
- What it is
- The diameter of the round wire each spring is wound from, measured across the wire itself.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Spring rate and spring loads rise with the fourth power of wire diameter; at the same fitted lengths the stress rises too.
- Get it right
- Measure the wire with a micrometer, not the coil; small errors matter because the rate goes with its fourth power.
What this means: Mean coil diameter
- What it is
- The mean diameter of each coil: the diameter measured to the centre of the wire, halfway between the outside and inside of the coil.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 22 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Spring rate falls with the cube of the mean diameter, so a larger coil gives a softer spring.
- Get it right
- Do not enter the outside diameter: the mean diameter is the outside diameter minus one wire diameter.
What this means: Active coils
- What it is
- The number of coils in each spring that are free to deflect, not counting the inactive end coils that sit on the seats.
- Units and format
- Enter a number in coils: a number of coils. The form accepts 0.5 coils to 10000 coils. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 coils as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More active coils make a softer spring, lowering clamping force and release load in proportion.
- Get it right
- Count active coils, not total coils; the end type adds the inactive coils for the solid length.
What this means: End type
- What it is
- How the spring ends are finished, which sets the total number of coils and the solid length the tool checks the release stroke against.
- Units and format
- Choose one of 4 options from the list: Closed and ground (squared and ground); Closed, not ground; Plain and ground; Plain (open).
- Example
- The form opens with “Closed and ground (squared and ground)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Changes only the total coils and solid length; the rate and loads do not change.
- Get it right
- Choose the end type shown on the spring drawing or catalogue; closed and ground ends are flat and square to the axis.
What this means: Shear modulus
- What it is
- The shear (torsional) modulus of the spring wire material, which sets how stiff each spring is.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 500 GPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 79.3 GPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Spring rate, clamping force and release pressure rise in proportion to it.
- Get it right
- Enter it in GPa, not MPa; take it from the spring wire supplier’s data sheet.
What this means: Unloaded length
- What it is
- The free length of each spring with no load on it, measured end to end before it is fitted.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 48 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer free length gives more compression when clamped, raising clamping force and release pressure.
- Get it right
- Measure several springs, as free lengths vary; a short spring carries less of the load.
What this means: Length when clamped
- What it is
- The fitted length of each spring with the clamp closed, set by the housing depth and the pad or lining thickness.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A shorter clamped length compresses the springs more, raising clamping force, release pressure and stress.
- Get it right
- It must not exceed the free length; measure it with new pads and check again with worn ones.
What this means: Allowable shear stress
- What it is
- The highest shear stress you accept in the spring wire, checked against the corrected stress with the clamp released, when the springs are most compressed.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 100000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 700 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line for spring stress; the calculated stress does not change.
- Get it right
- Take it from the spring wire supplier or spring design standard for your wire, duty and number of cycles.
Release piston
What this means: Piston outside diameter
- What it is
- The outside diameter of the hydraulic release piston, the bore of the chamber it slides in.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger piston has more area, so the release pressure falls.
- Get it right
- Use the piston or bore diameter where the seal runs, not the outside of the housing.
What this means: Piston inside diameter (0 = full piston)
- What it is
- The inside diameter of an annular release piston, where a shaft or spindle passes through its centre; a solid piston has no hole.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger inside diameter reduces the piston area, so the release pressure rises.
- Get it right
- Enter 0 for a full, solid piston; it must be smaller than the outside diameter.
What this means: Seal friction allowance
- What it is
- An allowance for the seal friction the release piston must overcome, as a percentage added to the spring load at release.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises the release pressure in proportion to one plus this share.
- Get it right
- Type the percentage itself; base it on the seal maker’s friction data or a test on the actual clamp.
What this means: Pressure available to release
- What it is
- The hydraulic pressure the circuit can deliver to the release piston, such as the regulated supply or relief setting for the clamp.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 1000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line for release; the required release pressure does not change.
- Get it right
- Use the lowest pressure that can reach the clamp in service, after valve and line losses.
Method and formulas
- k = G d⁴ ÷ (8 D³ N_a); load = k (L₀ − L); Wahl-corrected stress (as the compression spring tool).
- Clamping force = springs × clamped load; load at release = springs × k (L₀ − L_w + stroke).
- Release pressure = load at release × (1 + friction) ÷ piston area.
- Example: 3,177.6 N of clamping; 11.01 bar releases it with 422.29 MPa in the springs.
Limitations
- Springs share the load equally; uneven spring lengths load some springs more.
- Clamping force falls as linings or pads wear and the springs lengthen.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a fail-safe brake or clamp
- Find the release pressure of a spring clamp
- Choose the number of springs for a clamping force
Who it is for
- Hydraulic engineers
- Machine designers
- Toolmakers
What you get
- Spring rate, clamped load and stress
- Total clamping force
- Release pressure and stress at release
Interpreting the result
- A softer spring with more compression keeps the clamping force steadier as the pads wear, at the cost of a higher release pressure.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 304–324, 281–286. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Springs that set a force: detents, relief valves and spring-applied clamps
What it covers: How springs set forces and pressures in machines: ball detent release force, relief valve cracking pressure and overpressure, spring-applied hydraulic release clamps, and records.
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