Crankshaft Deflection Measurement

Why we need to make the deflection measurement?

If the crankshaft deflection exceeds its limit, the stress to be applied to the crankshaft becomes excessive, and it results in the breakage of crankshaft if the stress is too large. Therefore, adjust the crankshaft deflection to the proper value when being installed, and periodically measure the deflection, so that it can be corrected whenever it is over the limit value.

OK. But why deflections are appearing?

For better understanding of this effect let's imagine the section of crankshaft together with the connecting rod and piston, held by the pair of main bearings.

The piston-connecting rod assembly weight creates the force attached to the crankshaft, especially during normal operation of the engine. These forces bend the crankshaft webs, and this deflection should be carefully monitored by engineers.

How to prepare the engine for the measurement?

a. Deflection of the crankshaft varies according to the temperature. Be sure to measure it when the engine is in cold state and with 24h stopped prelubrication pump.

b. Before measurement, push the contact piece of the deflection gauge with a finger, and confirm that both the contact piece and dial gauge return to the original positions correctly. Let the gauge to be a little bit heated to the crankshaft temperature.

c. The deflection varies according to the loaded cargo conditions, be minded to record the conditions of loaded cargo, draft, temperature, and so on, at the same time.

d. Switch the fuel lever to the "0" position. Close and drain the engine air supply, and block the engine.

e. Open the indicator cocks and engage the turning gear. Open the crankcase covers.

Finally! How to measure the deflection?

(1) Turn the crank pin at the point of 30° (position "B") past the bottom dead center.

Install the deflection gauge at the balance weight. Install it between the balance weights according to the engine manual requirements. No. 1 cylinder must be less distance from the joining surface between the crank web and the balance weight.

(2) Set the reading on the gauge scale to +20 at the position "B" in the figure (to make clear the positive and negative directions).

(3) Slowly turn the engine in the normal direction of rotation, and measure the reading on the scale when the crankshaft is at the angle of "B", "C", "D", "E", and "A" respectively, of which data shall be recorded.