Deep Hole Drilling for the Aerospace Ground Support Equipment Industry: Towbar Shear Pin Bores, Jet Bridge Hydraulic Cylinders, and Baggage Conveyor Shafts

A manufacturer of aircraft towbars produced instrumented shear pins for Boeing 737 nose landing gear (4340 steel, 40 HRC, 32 mm diameter x 250 mm length, requiring a 6 mm x 200 mm axial bore for strain gauge wiring, 4 mm cross-bore at centre) using a carbide gun drill (6 mm, Vc = 60 m/min, f = 0.025 mm/rev, oil coolant at 50 bar). The intersecting bores were deburred by electrochemical deburring (10-15 V, NaNO3 electrolyte, 10-20 seconds) to prevent the sharp edges from cutting the strain gauge lead wires.

Aircraft Towbar Instrumented Shear Pin Drilling ​

Aircraft towbar shear pins are the critical mechanical fuse that protects the nose landing gear from excessive towing loads. The shear pin is designed to fail at a predetermined load (typically 30-50 kN for a narrow-body aircraft) if the towbar is subjected to a shock load. Modern instrumented shear pins contain strain gauges that measure the towing load in real time and transmit data to a display in the tow vehicle, alerting the driver if the load approaches the shear limit. The strain gauge is bonded to the pin's internal bore surface, and the lead wires pass through the axial bore and the cross-bore to a connector at the pin end. The axial bore (6-12 mm diameter, 150-300 mm depth) is gun-drilled through the full pin length, and the cross-bore (4-8 mm) is drilled at the pin centre, intersecting the axial bore. The intersection is deburred by electrochemical deburring to prevent sharp edges from cutting the lead wires.

Jet Bridge Hydraulic Cylinder BTA Drilling ​

Jet bridge hydraulic cylinders are long-stroke (3-6 m) double-acting cylinders that extend and retract the passenger boarding bridge to and from the aircraft door. The cylinder barrel is manufactured from seamless steel tube (E355 or ST52, 100-200 mm OD, 10-20 mm wall) and the bore is BTA-drilled to H8-H9 tolerance then honed to Ra < 0.4 microns for the piston seal. For tubes up to 6 m length, the BTA drilling is performed in a single pass; longer tubes require two passes (rough and finish). After BTA drilling, the tube is honed using a diamond-plated honing tool (400-600 grit, 0.01-0.03 mm removal) to achieve the final surface finish and tolerance. The cylinder is assembled with the piston, rod, and seals, then tested at 1.5x the working pressure (typically 200 bar for a jet bridge cylinder). Leakage past the piston seal must be less than 0.1 cc/min at the test pressure.

Baggage Conveyor Shaft and Aircraft Jack Screw Drilling ​

Baggage conveyor roller shafts are precision-machined steel shafts (20-50 mm diameter, 500-2000 mm length) that support conveyor rollers and transmit drive torque. The shaft has a gun-drilled centre bore (6-12 mm diameter) that distributes lubricating oil to the roller bearings through radial cross-holes at the bearing positions. Each shaft has 2-6 radial cross-holes (2-4 mm diameter) drilled at 90 degrees to the shaft axis at each bearing position, intersecting the centre bore. After drilling and cross-hole drilling, the shaft is ground to final diameter. Aircraft jacking point screw jack plungers require precision gun-drilled bores (10-25 mm diameter, 200-500 mm depth) in 4140 steel for hydraulic fluid passages that operate the jack extension mechanism. The bore must be straight and smooth (Ra < 0.8 microns) to provide a reliable hydraulic seal at pressures up to 350 bar for aircraft jacking operations.

FAQ ​

What is the shear load specification for an instrumented towbar shear pin? ​

The shear load specification for a Boeing 737 nose landing gear towbar shear pin is typically 40-50 kN, with the pin designed to fail at 45 kN +/-5%. The shear plane is located at the cross-bore position (the pin centre), where the cross-bore reduces the effective shear area by approximately 15-25%. The axial bore (6 mm diameter in a 32 mm pin) reduces the shear area by approximately 3.5%, which is accounted for in the pin design. The strain gauge is calibrated to measure the shear load with an accuracy of +/-2% of full scale, and the display in the tow vehicle shows the load in real time with audible and visual alerts at 80% and 100% of the shear limit. The strain gauge is bonded to the axial bore surface at the pin centre with a cyanoacrylate adhesive, and the lead wires pass through the cross-bore to a miniature connector at the pin end.

How are intersecting bores deburred without damaging the strain gauge bonding surface? ​

Electrochemical deburring (ECD) is the preferred method for deburring the axial-cross-bore intersection in instrumented shear pins. The process: a shaped copper or brass electrode is inserted into the axial bore and positioned at the intersection point. The pin is immersed in a NaNO3 electrolyte solution (10-15% concentration, 30-50 C), and a DC voltage of 10-15 V is applied between the electrode (cathode) and the pin (anode) for 10-20 seconds. The electrical current preferentially dissolves the sharp burr edges because the current density is highest at the sharp points. The deburring removes 0.05-0.15 mm of material from the burr without affecting the surrounding bore surface (which has a lower current density). After deburring, the bore is flushed with clean water to remove the electrolyte, dried, and inspected by borescope at 20x magnification to verify the burr has been removed and the intersection edge is smooth and rounded.

What straightness tolerance is required for a 6 m jet bridge cylinder bore? ​

The bore straightness tolerance for a 6 m jet bridge cylinder is typically 0.1 mm per metre of length, with a maximum cumulative deviation of 0.5 mm over the full 6 m length. This ensures that the piston seal maintains uniform contact with the bore surface over the full stroke, preventing premature seal wear and hydraulic leakage. The straightness is verified by laser straightness measurement: a laser transmitter is mounted at one end of the cylinder tube, and a position-sensitive detector is traversed through the bore at 500 mm intervals along the full length. The measured deviations are plotted against the tube length, and the maximum deviation must be less than 0.5 mm. If the straightness exceeds the tolerance, the tube can be straightened by press-bending (up to 0.2 mm correction) or must be rejected.

What causes hydraulic cylinder seal failure in jet bridge applications? ​

Three failure modes dominate: (1) Piston seal wear from bore surface roughness -- if the honed bore surface finish exceeds Ra 0.4 microns, the seal lip wears at an accelerated rate, with typical wear-through occurring at 50 000-100 000 cycles instead of the design life of 500 000+ cycles. (2) Contaminant ingress -- jet bridges operate in airport environments with dust, jet exhaust particles, and de-icing fluid residue. If the cylinder rod wiper seal is damaged, these contaminants enter the cylinder and abrade the bore surface, causing the piston seal to fail. (3) Seal extrusion at high pressure -- if the bore-piston clearance exceeds 0.2 mm (the recommended maximum for a polyurethane lip seal at 200 bar), the seal material extrudes into the clearance gap and is mechanically damaged on each stroke. Regular preventive maintenance (seal replacement every 5 years or 500 000 cycles, whichever comes first) prevents unexpected failures.

How are conveyor shaft cross-holes verified for centre bore intersection? ​

Each radial cross-hole must intersect the centre bore to allow lubricating oil to flow from the centre bore to the bearing. The intersection is verified by: (1) Visual inspection -- a borescope is inserted into the centre bore and positioned at each cross-hole location. The inspector verifies that the cross-hole is visible as a full circular opening in the centre bore wall, with no wall material blocking the intersection. (2) Flow test -- compressed air (2-3 bar) is applied to the centre bore, and the airflow from each cross-hole is measured using a flow meter. A blockage at the intersection (caused by a drill breakthrough burr or misaligned cross-hole) reduces the airflow to that cross-hole by 50% or more compared to the other cross-holes on the same shaft. (3) Pin gauge -- a thin wire (0.5 mm smaller than the cross-hole diameter) is inserted through each cross-hole; the wire must pass through into the centre bore without resistance.

Data are based on published research and industry experience as of 2026. Always consult your equipment manufacturer and applicable aerospace GSE standards (SAE ARP, IATA AHM, ISO 6966) for specific application requirements.