When designing the crane it was necessary for the boom to be able to swing, this adds to its utility allowing it to be better able to lift and move objects over a greater range.
The first and simplest mechanism would be a pin hinge and a bushing, this would be the simplest. The bushing would sit within the mast and the pin would sit within the bushing and attach to the base of the boom. The bushing uses a thin film of oil to allow for the lubrication and free rotation. This arrangement is very simple and cheap and can deliver good results over a long period of time with little to basic maintenance. However bushings are not suited for high radial or thrust loads. Essentially the bushing is typically used for rotating shafts, these need lubrication and a freely moving surface however they aren’t exposed to high loads.
In this instance the choice of using a bearing is made, as this use case requires the mechanism to bear a thrust load. The bearing does the same thing as the bushing however the bearings are enclosed within a raceway, this rolling component reduces the friction allowing for the free rotation of the shaft, thus the movement of the boom.
For this application, given the vertical thrust load, that is applied in only one direction. In this case the best would be a thrust bearing, be it cylindrical or ball. These were chosen specifically for the fact that it can accommodate a high thrust load.
The entire swivel mechanism will comprise of 3 parts and upper, mid and lower, the upper section will sit on top of the bearing and attach to the base of the boom. The mid section will be the bearing it will bridge the two sections. The lower will attach to the mast. To size the bearing we use the knowledge of forces acting on the mast which is F1 calculated to be 14.33 kN, and consult a bearing catalogue, in this case the SKF catalogue.
We know that the internal diameter of the selected mast is 140.4 mm, with a maximum diameter of 152.4 mm. Thus we now know that the bearing should have an internal diameter of 140.4 mm thus allowing for an un-obstructed mast centre. The outer diameter can be anywhere between 152.4 and 250 mm. The reason for this is that the bearing will fit in the space between the mast and the bottom of the boom, and still ensure proper contact between the upper and lower sections and allow for proper load distribution.
