BIZOMAT MAKEOS · Strength
Press Tie Rod Check (Cylinder Force, Rod Thread Stress, Stretch, Nut Engagement)
For a four-post hydraulic press, what load does the cylinder put into the tie rods, is the thread stress within limits, how much do the rods stretch, and are the nuts long enough?
Check of the tie rods of a hydraulic press frame: the press force from the cylinder bore and pressure, matching the bore a cylinder sizing gives for that force (as the hydraulic cylinder sizing tool), the load per rod, the thread tensile stress area and stress (as the metric thread tool), the elastic stretch of each rod over its length, and the nut length needed so the threads do not strip (as the thread engagement tool), against your allowable stress, stretch limit and nut height.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Cylinder bore | mm | Bore of the press cylinder. |
| System pressure | bar | Highest system pressure (relief valve setting). |
| Number of tie rods | rods | Number of tie rods sharing the load. |
| Rods | ||
| Rod thread nominal diameter | mm | Nominal diameter of the rod thread. |
| Thread pitch | mm | Thread pitch. |
| Rod length between nuts | mm | Length of rod between the nuts (the length that stretches). |
| Modulus of elasticity | GPa | Modulus of elasticity of the rods. |
| Allowable thread stress | MPa | Allowable tensile stress on the thread (lower for rods that cycle millions of times). |
| Largest rod stretch | mm | Largest stretch you accept (it shows as frame spring and uneven bolster loading). |
| Nuts | ||
| Nut height (thread engaged) | mm | Length of thread engaged in each nut. |
| Rod tensile strength | MPa | Tensile strength of the rod material. |
| Nut tensile strength | MPa | Tensile strength of the nut material. |
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: Cylinder bore
- What it is
- The bore of the press cylinder. With the system pressure it gives the press force the tie rods carry.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Press force grows with the square of the bore.
- Get it right
- Use the piston bore, not the rod diameter or barrel outside diameter.
What this means: System pressure
- What it is
- The highest hydraulic pressure that can act in the cylinder, normally the relief valve setting.
- 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 200 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Press force and rod stress rise in direct proportion.
- Get it right
- Use the relief setting, not the normal working pressure, so the rods are checked at the largest force.
What this means: Number of tie rods
- What it is
- How many tie rods share the press force equally.
- Units and format
- Enter a number in rods: rods. The form accepts 2 rods to 16 rods. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 rods as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More rods lower the force and stress in each.
- Get it right
- Off-centre loads do not share equally; the tool assumes equal sharing.
Rods
What this means: Rod thread nominal diameter
- What it is
- The nominal (major) diameter of the thread on each tie rod.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 42 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the thread size, for example 42 for M42; a turned-down rod body is not used here.
What this means: Thread pitch
- What it is
- The thread pitch of the tie rod thread: the distance between neighbouring crests.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- For inch threads, pitch = 25.4 ÷ threads per inch.
What this means: Rod length between nuts
- What it is
- The length of each rod between the nuts, which is the length that stretches under the press force.
- 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 1500 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rod stretch rises in direct proportion.
- Get it right
- Measure between the nut faces, not the overall rod length.
What this means: Modulus of elasticity
- What it is
- The elastic modulus of the tie rod material, used for rod stretch.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 1000 GPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 GPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter it in GPa, not MPa.
What this means: Allowable thread stress
- What it is
- The tensile stress you allow in the threaded part of each rod.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Set it from the rod material and fatigue requirements of the press duty.
What this means: Largest rod stretch
- What it is
- The largest stretch of the rods between the nuts you accept under full press force.
- 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 1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Set it from the press accuracy you need.
Nuts
What this means: Nut height (thread engaged)
- What it is
- The length of thread engaged in each nut, compared with the engagement needed so the rod breaks before the threads strip.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 42 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Count only fully formed threads in contact, not chamfers or washers.
What this means: Rod tensile strength
- What it is
- The ultimate tensile strength of the tie rod material, used to compare external and internal thread strength.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from the material certificate or bolt property class.
What this means: Nut tensile strength
- What it is
- The ultimate tensile strength of the nut material.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A weaker nut needs more thread engagement; take it from the nut grade or material certificate.
Method and formulas
- Press force F = p × π D² ÷ 4; the bore that gives F at this pressure (as the hydraulic cylinder sizing tool).
- Thread stress area A_s = π ÷ 4 × ((d₂ + d₃) ÷ 2)² (as the metric thread tool); stress = F ÷ (rods × A_s); stretch = (F ÷ rods) L ÷ (E × π d² ÷ 4).
- Engagement needed so the nut thread does not strip before the rod (as the thread engagement tool).
- Example: 628.32 kN of press force puts 157.08 kN on each rod: 140.14 MPa and 0.85034 mm stretch.
Limitations
- Load shared equally by the rods; off-centre work loads some rods more.
- Static stress; presses that cycle often need a fatigue check of the thread roots.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check the tie rods of a workshop or moulding press
- Size rods and nuts for a new press frame
- Check a press after raising the system pressure
Who it is for
- Design engineers
- Hydraulic engineers
- Toolmakers
What you get
- Press force and load per rod
- Thread stress and stretch
- Nut engagement needed
Interpreting the result
- Pre-tightening the nuts above the press load keeps the frame stiff and the threads out of fatigue.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 281–286, 1867–1890, 1586–1590. Topic reference; no source tables or recommended settings are reproduced.
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Open the Press Tie Rod Check (Cylinder Force, Rod Thread Stress, Stretch, Nut Engagement) calculator