Machine, Structural, and Energy Systems
Power flow
Expect power-flow questions: engine to PTO to drawbar to implement. Drawbar power (hp) = draft (lb) x speed (ft/min) / 33,000; with speed in mph, hp = draft x mph / 375. PTO power exceeds drawbar power because of rolling resistance and wheel slip (tractive efficiency typically 65-80% on firm soil, lower on loose or wet soil). Hydraulic power: hp = (psi x gpm) / 1714. Know torque-power relations (hp = T n / 5252 with T in lb-ft, n in rpm) and how to combine motor efficiency and power factor.
Structures
Structural items involve loads (dead, live, snow, wind, grain), load combinations, and simple member checks. Grain bin wall loads grow with depth but follow a Janssen-type distribution that flattens at depth rather than rising linearly. Post-frame and pole buildings depend on diaphragm action and bracing; check uplift and sliding under wind.
Energy systems
Energy audits (lighting, motors, ventilation, pumping), solar PV sizing (insolation x area x efficiency x derate), biogas from manure (methane yield per unit volatile solids), and biomass heating. Watch for efficiency chains: multiply efficiencies rather than adding them.
Traps
Reporting PTO power when drawbar power is asked; omitting the 1714 constant in hydraulic power; using nameplate rather than derated PV output; applying a single safety factor where a load combination is required; ignoring slip losses when sizing tractors.
Worked example
An implement needs 3,000 lb of draft at 5 mph. Drawbar power = 3,000 x 5 / 375 = 40 hp. At 75% tractive efficiency, required PTO power = 40 / 0.75 = 53 hp. Adding a 10% margin gives about 59 hp.