Loosening of Mortise and Tenon in Office Chairs

The loosening of the mortise and tenon in office chairs can be repaired using the following methods, and an appropriate solution should be selected based on the degree of looseness:

I. Basic Repair Methods

1. Wood Glue Bonding
   Apply wood glue evenly on the surfaces of the tenon and mortise, insert them, and secure with clamps to ensure tight contact. Alternatively, you can reassemble after applying the glue directly and use it after the glue has fully cured (usually takes 24 hours). It is essential to choose a water-resistant glue, such as polyvinyl acetate, and strictly follow the instructions to control the curing time.

2. Mechanical Reinforcement
   If glue bonding is insufficient, nails can be used for fixation: drill small holes beside the mortise and drive nails diagonally into the tenon to form a concealed connection. Note that the nail length should not exceed two-thirds of the tenon thickness to avoid penetrating the surface.

3. Dowel/Wooden Peg Filling
   For severe looseness, remove the mortise and tenon and drill holes:

   • Dowel: The hole diameter should match the dowel diameter, and the surface should be sanded after insertion;
   • Wooden peg: The hole diameter should be slightly smaller than the peg diameter to create an interference fit when hammered in.
     The material should be consistent with the original wood to prevent secondary looseness due to differences in expansion coefficients.

II. In-Depth Repair Using the Wedge Expansion Principle

1. Basic Steps

   • Extract the wooden tenon: Use a screwdriver to gently pry it out. If adhesion is severe, heat the area around the hole (temperature ≤ 60°C);
   • Cut a groove: Saw a vertical groove at the top of the tenon with a depth of one-third of the tenon length;
   • Insert a wooden wedge: Choose hardwood to make a wedge-shaped block, insert it with the pointed end facing down into the groove;
   • Reinstall: Apply glue to the inner wall of the hole, insert the tenon, and gently tap until the wooden wedge fully expands.
     Key Point: The depth of the groove must be precisely controlled to avoid breaking the tenon if it is too deep.

2. Special Case Handling

   • Backrest disengagement: Cut double grooves on both sides of the tenon and insert bidirectional wooden wedges to secure them with bidirectional expansion force;
   • Hole wear: Repair the inner wall by pasting a 1-2mm thick wooden sheet, redrill the hole after the glue dries, ensuring the new hole has a clearance of ≤ 0.2mm from the tenon.

III. Post-Repair Maintenance

1. Curing Period Protection: Avoid moving or applying weight for 24 hours after repair to ensure the glue fully cures.
2. Environmental Control: Maintain indoor humidity between 40%-60% during long-term use to prevent wood deformation due to shrinkage and swelling.
3. Regular Inspection: Check the tightness of the mortise and tenon every 3-6 months and promptly apply glue or reinforce if slight looseness is detected.

Tip: If the mortise and tenon structure is complex (e.g., multi-directional connections), it is recommended to prioritize the wedge expansion method or consult a professional carpenter to avoid further damage from disassembly.