Design of joint 5-6 (tube-in-tube joint) — example

Design data

External forces: F_X = 1500 N, F_Y = 300 N, F_Z = 700 N act at distances L_c = 70 mm; X₁ = 80 mm; Z₁ = 180 mm; the moments at the centre of the joint are:

M_X6 = F_Y (L_c + Z₁) = 75000 Nmm,

M_Y6 = F_X (L_c + Z₁) + F_Z X₁ = 431000 Nmm,

M_Z6 = F_Y X₁ = 24000 Nmm.

Dimensional constraints: the square tube 5, of size h₅ = 50 mm and wall thickness t₅ = 5 mm, is assembled with the square tube 6, of size h₆ and wall thickness t₆.

Operating constraints: temperature T = (−30…40) °C; outdoor working environment with high humidity.

Design constraints: tube 5 is inserted into tube 6, with loose clearance fits on the sides of the outer and inner squares.

Materials and properties

Weldable S235 steel according to SR EN 10025-2 (STAS 500/2) was adopted for elements 5 and 6.

Material properties

Since all the materials in contact are steels, the allowable crushing stress is taken as σ_as = 70 MPa.

Sizing

From the sizing relations obtained from the crushing strength conditions of the tube-in-tube joint, with F_X = 1500 N, F_Y = 300 N, M_X = 75000 Nmm, M_Y = 431000 Nmm, M_Z = 24000 Nmm, H = h₅ = 40 mm and σ_as = 70 MPa, the following solutions result:

The length of the tube-in-tube joint 5-6 is therefore L = max (38.86; 16.29) = 38.86 mm; L₆ = 70 mm is adopted (a larger value was chosen to leave extra room for the pin joint).

Observing the constraints t₆ ≥ t₅, h₆ = h₅ + 2 t₆, t₆ = 5 mm and h₆ = 60 mm are adopted.

Constructive-technological shape

Constructive-technological drawings

Adopted dimensions and constructive-technological parameters:

– dimensions: h₅ = 50 mm, t₅ = 5 mm, h₆ = 60 mm, t₆ = 5 mm, L₆ = 70 mm; the dimensions L₅, l₁, l₂ are determined from the calculation of the pin joint, with diameter D_b;

– technological operations: milling the tube ends, drilling/reaming the pin holes;

– roughness of the machined surfaces: R_a1 = 12.5 µm, R_a2 = 6.3 µm;

– sharp edges are chamfered 1×45°;

– the position of the pin hole (and, if needed, of the adjustment pin hole) is set when drawing the assembly.