Crankshaft Design - Journal Sizing

Required information:

Maximum load on the journals is either going to be as a result of gas (combustion) or inertia force. Usually it will be gas, but to be sure both need to be calculated to ensure the worst case loading is being used.

Maximum gas force is based on a combination of cylinder pressure [P] and engine speed [N] which give the greatest net force. The net force being the force of combustion pushing down LESS the inertia force of the reciprocating components at that engine speed. Maximum cylinder pressure may not necessarily be when the journals experience their highest loads due to the higher engine speed. A lower cylinder pressure may not have so much inertia relief, resulting in greater loading on the journals. So the right combination must be determined.

Maximum inertia force is simply the inertia force at the highest anticipated engine speed; maximum overspeed. This takes into account the safe operating maximum RPM (limiter) plus a bit more for over speed conditions, such as changing down a gear too early.

Ideally some test data or predictive analysis data or even just comparable engine data will be to hand, giving cylinder pressure vs engine speed. Using the following calculations, it can be quickly determined what the worst case loading is. Setting up these calculations in a spreadsheet will speed up the process.

Fgas = P * Abore - Finertia at that engine speed.

Finertia = mrω2 cosθ + (r/l) mrω2 cos2θ

where, m = mass of reciprocating components (pistons assembly, 1/3 con-rod) [kg], r = crank throw radius [M], l = connrod length (between centres) [M], ω = engine speed [rad/s] (rad/s = rpm * [2π / 60]), θ = crank angle

Once maximum load is determined (Newtons, [N]), the minimum journal sizes can be calculated by:

P = F/A

where, P = the pressure on the bearing material (specific load limit), F = load, A = projected area of journal. So for a known material and load:

A = F/P

Journal dimensions can then be derived using some width/diameter ratio applied to the calculated area. This gives us a starting point. Note that the reaction to these forces on the main bearings will be split over the two main journals. If it is a single piston per bay, i.e. the conrod / crank pin is directly in the middle of the two main journals, then the split is 50:50. If it is more than one piston per bay, i.e. two rods per crank pin as in a vee type configuration, then moments should be taken to determine the split of the reaction forces.

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