How does jacking oil pump work in turbine?

A jacking oil pump also called a lift pump is commonly used on rotor shafts of steam driven Turbine Generators prior to startup or after shutdown to provide even cooling of the shaft and eliminate rotor distortion caused by sags due to weight and bows due to uneven cooling. The jacking oil pump uses high pressure oil supplied at the bearing journals to initiate an oil film and lift the shaft off its bearings. The rotor can then be put on a turning gear and rotated slowly to create even cooling and or roll out any distortions caused by the weight of the shaft while at rest. It also helps to maintain the oil film between shaft and the bearing till the rotor speed is adequate enough to maintain the film thickness and protects the shaft & bearing.