Transformerwood bolt connections transfer transverse force at the hole and compression beneath the washer. Assess both contacts with their real footprints and seating to identify the governing material region. [1]
Calculation basis. Project-specific values and gates in the illustrative screens below are assumptions; cited catalogue values retain their source test conditions.
Keep them separate in the assessment. Their nominal areas, local stresses and directions relative to the veneers differ. Increasing the bolt’s tensile strength does not necessarily improve either contact.
Calculate the two nominal contacts
For a simplified hole-wall bearing check, use projected area dt, where d is the relevant pin or bolt diameter and t is the loaded material thickness. A fictional transverse load of 2 kN acting on a 10 mm diameter bolt through 20 mm thickness gives 2000/(10 × 20) = 10 MPa nominal bearing stress.
For a fictional annular washer contact with outside diameter 30 mm, inside diameter 10 mm and axial load 5 kN, the nominal area is π(30² − 10²)/4 = 628.319 mm². The mean pressure is approximately 7.958 MPa.
These are illustrative average stresses, not peaks or allowable values. Partial washer seating, bolt tilt and clearance can reduce the actual contact area and alter the distribution.
Inspect the failure path around each contact
- Loaded hole wall
- Look for local crushing, ovalisation and layer separation. Record the force direction relative to the veneer arrangement.
- Hole-to-edge ligament
- Check splitting or shear-out towards the free edge. Its duty is not described by the projected bearing area alone.
- Washer seat
- Inspect indentation, nonuniform contact and damage at the washer’s inner or outer edge.
- Material beneath the seat
- Use representative sections or load witnesses to identify damage below an apparently intact surface.
Choose contact tests that reproduce the restraint and geometry of the actual joint. A compression block loaded over its whole face does not reproduce an annular washer contact. A plain tensile specimen does not reproduce concentrated hole-wall bearing.
Datasheet compression directions must be interpreted correctly. Parallel or perpendicular to the lamination plane is a test convention, not a guarantee that all grain directions within that plane behave alike.
For multiple bolts, clearance and positioning affect which hole first takes transverse load. Equal force sharing is an assumption to verify. A joint can overload one local bearing region while the total force divided by the number of bolts appears modest.
Condition the joint specimens for their intended processing and oil exposure. Examine the contact after unloading as well as under load, because recovered appearance can hide a permanent indentation or internal damage.
Select the washer footprint from part evidence and verify its seating, edge proximity and electrical arrangement. Retain those conditions with the contact-pressure assessment.
Recalculate washer pressure when seating is partial
Keep the wood test direction and assembled load path identified [2][3]. The preceding assumed 5 kN axial load over the 30/10 mm annulus gives 7.958 MPa only for full contact. Assume tilt leaves half the 628.319 mm² annulus in contact; mean pressure on that area becomes 15.915 MPa.
Against an assumed 14 MPa mean-contact allocation, full seating passes but half seating fails. The separate 2 kN transverse force still gives a 10 MPa nominal hole-bearing screen; increasing bolt tensile strength changes neither calculation.
Resolve actual washer support, edge pressure, hole clearance and directional damage before accepting the joint. Neither mean pressure is a local peak or a material allowable from the data sheet.
References
- Röchling Industrial, Lignostone L I/2-E3-HQ (P2R), Technical Data Sheet, version 2.0, released 20 September 2023.
- IEC, IEC 61061-2:1992, Specification for non-impregnated, densified laminated wood for electrical purposes — Part 2: Methods of test.
- R. T. Barrett, Fastener Design Manual, NASA Reference Publication 1228, 1990.