Learning topic
Design of Interference-Fit Bolts
Load transfer and strength checks for fitted bolts installed without clearance.
Interference-fit (fitted) bolts are installed in accurately machined holes with negligible clearance or a small interference. Unlike ordinary clearance bolts, they transfer transverse force directly by bearing against the hole and by shear of the shank.
Load transfer
For a symmetrical joint with \(z\) equally loaded bolts, the force per bolt is approximately \(F_i=F/z\). If a bolt crosses \(m\) shear planes,
$$\tau=\frac{F_i}{mA_b}\le[\tau],\qquad A_b=\frac{\pi d^2}{4}.$$
Bearing pressure in each connected plate is
$$p_b=\frac{F_i}{dt}\le[p_b].$$
All plates are checked separately because thickness and allowable bearing stress may differ.
Eccentric bolt groups
A load not passing through the group centroid produces direct force and moment. In the elastic distribution model,
$$F_{Mi}=\frac{Mr_i}{\sum r_j^2},$$
and the direct and moment components are added vectorially for every bolt.
Fit and construction
The smooth fitted portion must pass through every shear plane; threads should not lie in the shear interface. Hole accuracy, surface finish, lead-in chamfers, assembly force, and replaceability are specified together. Also check net-section tension, edge tear-out, local yielding, fatigue, and fretting.
Схема роботи посадкового болта з натягом
Поперечне навантаження в посадковому болті передається безпосереднім контактом гладкого стержня з каліброваним отвором; d визначає площу зрізу, t — площу зминання, а δ = d_b − d_h > 0 характеризує натяг посадки, а не осьове затягування гайки.
Distinction from a preloaded friction joint
The word interference describes the dimensional fit between the smooth bolt shank and the hole, not axial tightening force. A fitted bolt can also be tightened axially, but its defining function is direct transverse load transfer through contact. It therefore requires reamed or otherwise accurately finished aligned holes.
Shear-plane details
The fitted cylindrical portion must extend through the entire connected stack at every design shear plane. If a thread lies in a shear plane, the effective shear area is reduced and local bearing becomes poorly controlled. Transitions between the shank and thread should also be kept away from highly stressed interfaces.
Plate limit states
In addition to local bearing, each plate is checked for net-section tension, edge tear-out, block shear, and local yielding. Edge distance and pitch are chosen from both strength and manufacturing requirements. Thin outer plates may govern even when bolt shear capacity is high.
Fit selection and assembly
The fit must allow controlled assembly without damaging the hole while limiting initial movement. Hole alignment is commonly achieved by drilling and reaming the assembled parts. Lead-in chamfers and a defined insertion method reduce galling and edge damage. Replacement may require an oversize repair bolt and renewed machining.
Fatigue and fretting
Alternating transverse load can produce local pressure cycles, fretting, and fatigue at hole edges. Surface condition, residual stress from hole preparation, interference magnitude, and joint stiffness influence endurance. For important joints, elastic contact analysis or validated design rules replace the uniform bearing-pressure assumption.
Calculation sequence
- Determine the most highly loaded bolt from direct force and moment.
- Check bolt shear using the real number of shear planes.
- Check bearing in every connected part.
- Check net sections, edge distances, and group failure.
- Select fit, tolerances, hole process, and assembly method.
- Assess fatigue, fretting, corrosion, inspection, and repair.