A pivoted drawbar joins a steam locomotive to its tender while allowing relative movement. Explore its plan-view articulation and calculate longitudinal projection, lateral offset and the change in projected length.
Rigid Drawbar Articulation Geometry
Enter pin-center length and a signed plan-view swing angle. The worksheet resolves the drawbar into longitudinal projection, lateral offset and geometric shortening; it does not rate a coupling or prescribe clearance.
Equation Used
- One rigid drawbar connects two ideal pin centers in plan view.
- Pin-center length remains constant and pins have no clearance.
- Vertical motion, buffers, springs, safety chains and hoses are omitted.
- Strength, fatigue, shock, allowable angle, required clearance and regulatory inspection are excluded.
The drawbar keeps the same pin-to-pin length as it swings. Its longitudinal and lateral components follow rigid-link geometry; they do not establish strength or allowable clearance.
Pivoted drawbar between two vehicle frames
Historical patents and locomotive rules identify drawbars, pins, buffers and safety appliances between engine and tender. The drawbar transmits draft while pivoting for relative alignment on curves.
The animation is a plan-view rigid-link construction. It does not simulate track curvature, suspension or draft forces.
Calculating drawbar projection
Use the calculator to resolve the entered pin-center length and angle. Inspection, strength and allowable movement must follow the actual locomotive's drawings and applicable regulation.
Rigid-link projection
Articulation example
A 1200 mm drawbar at 5 degrees has 1195.4 mm longitudinal projection, 104.6 mm lateral offset and 4.57 mm geometric shortening.
Geometry and coupling capacity
Coupling capacity depends on dynamic loads, buffers, materials, connections and the condition of the actual draw gear. Inspection criteria and allowable clearances are specific to the hardware and applicable requirements; the geometric offsets calculated here do not establish those limits.
Drawbar geometry questions
Is lateral offset required clearance?
No.
Does the length check rate strength?
No; it only verifies the trigonometric resolution.
Are buffers and safety chains included?
No.
Draw-gear references
- 49 CFR Part 230, Steam Locomotive Inspection and Maintenance Standards: draw gear, pins and safety appliances between steam locomotive and tender.
- US1954307A, Locomotive tender: drawbar and buffer arrangement between engine and tender frames.
- US742380A, Car coupling: pivoted drawbar arranged to swing with vehicle articulation.
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