BIZOMAT MAKEOS · Fasteners
Friction-Grip (Slip-Resistant) Bolted Joint
Will this preloaded bolted joint carry the shear load by friction without slipping, what tightening torque does that need, and what happens if it does slip?
Check of a friction-grip (slip-resistant) bolted joint in shear: bolt stress area and preload as a share of proof load with the tightening torque (as the bolt torque–preload tool), clamp force left after any applied tension per bolt, slip resistance from the friction coefficient and number of faying surfaces with your slip safety factor, and the bearing-type backup check of bolt shear and plate bearing if the joint slips (as the bolt and pin shear tool).
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of bolts | Number of bolts sharing the shear. | |
| Bolt diameter | mm | Nominal bolt diameter. |
| Thread pitch | mm | Thread pitch. |
| Bolt proof strength | MPa | Proof strength of the bolt grade (for example about 600 MPa for 8.8, 830 for 10.9). |
| Preload as share of proof load | % | Preload as a share of the proof load. |
| Nut factor | Nut factor for the thread condition (about 0.2 dry steel, 0.15 lubricated). | |
| Slip (friction) coefficient | Slip coefficient of the prepared faying surfaces (your value from the surface class). | |
| Faying surfaces (slip planes) | Number of contact surfaces the shear crosses (1 for a lap joint, 2 for a double cover plate). | |
| Slip safety factor | Factor you require against slip. | |
| Shear load on the joint | kN | Shear force on the whole joint. |
| Applied tension per bolt | kN | External tension per bolt, which reduces clamp force. |
| Thinnest plate (for bearing) | mm | Thinnest plate the bolts bear on if the joint slips. |
| Allowable bolt shear stress | MPa | Allowable shear stress of the bolt for the bearing-type backup. |
| Allowable bearing stress | MPa | Allowable bearing stress of the plate. |
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: Number of bolts
- What it is
- The number of bolts in the joint that share the shear load through friction between the clamped plates.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 200. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Slip resistance rises in direct proportion to the number of bolts.
- Get it right
- Count only the bolts that clamp the same faying surfaces and carry this shear.
What this means: Bolt diameter
- What it is
- The nominal (major) diameter of the bolts. With the pitch it gives the tensile stress area, and it is also used for the tightening torque and the backup bearing check.
- 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 16 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the nominal size, for example 16 for M16, not the stress area or shank measured over plating.
What this means: Thread pitch
- What it is
- The thread pitch of the bolts, the distance between neighbouring threads. It is used only to work out the tensile stress area from the nominal diameter.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the pitch in mm for metric threads; coarse and fine threads of one size have different pitches.
What this means: Bolt proof strength
- What it is
- The proof strength of the bolt grade: the stress the bolt can take without permanent stretch. Multiplied by the stress area it gives the proof load.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Preload and slip resistance rise in direct proportion to it.
- Get it right
- Take it from the bolt standard for the property class; do not use the tensile or yield strength.
What this means: Preload as share of proof load
- What it is
- The preload you intend to tighten each bolt to, as a share of its proof load.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 75% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Preload, tightening torque and slip resistance all rise in direct proportion.
- Get it right
- Type the percentage itself, not a fraction. Follow the share required by your structural code.
What this means: Nut factor
- What it is
- The nut factor that links tightening torque to bolt preload for the thread condition: plating, lubrication and washer surfaces.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The tightening torque rises in direct proportion to it.
- Get it right
- It changes only the torque you must apply, not the preload this tool assumes; scatter in it makes torque control uncertain.
What this means: Slip (friction) coefficient
- What it is
- The friction coefficient between the clamped plate surfaces, which depends on how they are prepared: blasted, painted, galvanised or as rolled.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.8. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Slip resistance rises in direct proportion to it.
- Get it right
- Use the value for the surface class your structural code assigns, and make sure the surfaces are prepared that way.
What this means: Faying surfaces (slip planes)
- What it is
- The number of contact surfaces between plates that the shear force must slip across. Each one adds its own friction.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 4. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Slip resistance rises in direct proportion to the number of planes.
- Get it right
- A lap joint has one slip plane; a joint with cover plates on both sides has two.
What this means: Slip safety factor
- What it is
- The factor of safety you require against slip, by which the friction capacity is divided.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 3. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.25 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger factor lowers the slip resistance you can count on.
- Get it right
- Use the factor your structural code or design specification requires.
What this means: Shear load on the joint
- What it is
- The total shear force trying to slide the plates past each other, for the whole joint, not per bolt.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 100000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the total joint load in kN; the tool shares it over all the bolts and slip planes.
What this means: Applied tension per bolt
- What it is
- Any external tension pulling each bolt along its axis, such as a prying or direct tension load. It is taken off the preload, leaving less clamp force for friction.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 10000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Each kN of applied tension removes a kN of clamp force from each bolt.
- Get it right
- This is per bolt, unlike the shear load, which is for the whole joint. Use 0 if there is no tension.
What this means: Thinnest plate (for bearing)
- What it is
- The thickness of the thinnest plate the bolts bear on, used for the backup bearing check if the joint ever slips into bearing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- In a double cover plate joint, compare the middle plate with the two cover plates added together and use the smaller.
What this means: Allowable bolt shear stress
- What it is
- The shear stress the bolt shank may carry if the joint slips and the bolts take the load in bearing, from your design code.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 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
- The backup check uses the nominal diameter area; use an allowable consistent with that basis.
What this means: Allowable bearing stress
- What it is
- The bearing stress the plate may carry against the bolt shank if the joint slips into bearing, from your design code.
- 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 300 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the allowable for the plate material, which often governs over the bolt.
Method and formulas
- Stress area A_s = π ÷ 4 (d − 0.9382P)²; preload F_p = share × S_p A_s; torque T = K F_p d (as the bolt torque–preload tool).
- Clamp left per bolt = F_p − F_t (applied tension taken fully off the clamp, conservative).
- Slip resistance = n × planes × μ × clamp ÷ slip factor.
- Backup: bolt shear τ = V ÷ (n × planes × π d² ÷ 4); bearing σ = V ÷ (n d t) (as the bolt and pin shear tool).
- Example: 4 M16 bolts at 75% proof clamp 70.501 kN each and resist 67.681 kN against 40 kN; torque 225.6 N·m.
Limitations
- Nut factor scatter makes torque-controlled preload uncertain by ±25% or more.
- Structural friction-grip joints must follow the steel code for bolt grade, surface class and inspection.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design a slip-resistant splice or bracket connection
- Check a joint that must not move under reversing loads
- Find the tightening torque for the clamp needed
Who it is for
- Structural engineers
- Design engineers
- Maintenance engineers
What you get
- Preload and tightening torque
- Slip resistance against the shear load
- Bearing-type backup if slip occurs
Interpreting the result
- Surface preparation sets the slip coefficient: blasted, unpainted surfaces grip far better than painted ones.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1535, 1720–1735. 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
Joints in shear: bearing type and friction grip
In Bolted Joints by the Numbers
What it covers: How bolted joints carry shear: by bolts bearing on their holes or by the friction their preload creates, how to check each, and what painted faying surfaces do to slip.
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