When you consider the machinery that keeps the industrial operations running, the parts of the machinery that actually do the heavy lifting seldom come into consideration. One of those unsung heroes are turntable bearings. They are located at the center of rotating platforms and allow the movement of even the largest excavators and wind turbines to move easily even when under the influence of heavy loads.
The standard job is made out to be simple. These bearings favour rotation. However, the truth about what they deal with is much more compound than such a mere description.
What Causes Them to be different than standard bearings?
Lightweight ball bearings are ideal as they are used in high-speed applications. Consider a bicycle hub or a skateboard wheel. The bearings of turntables are in a whole new world. They are made to carry enormous loads that are capable of exceeding hundreds of tonnes and they are also capable of being able to take multiple directional forces simultaneously.
This challenging position can be traced in the construction. The majority of designs employ balls or rollers that are disposed of in raceways which spread the weight over a significantly greater surface area. In other types of arrangements, several rows of rolling elements are placed on top of each other in an effort to distribute the weight. The outer and inner rings are normally very thick unlike in conventional bearing designs which prevents the rings in the designs of bearings to deform under pressure.
Axial and Radial loads at the same time.
It is here that it becomes interesting. Rotating equipment is not that light. Cranes swing loads sideways. Excavators bore holes in uneven ground. The wind turbines deal with the gusts that push in all directions that can be imagined.
Combined loading is taken care of in turntable bearings. That is to deal with forces that are pushing down (axis) and forces that are pushing sideways (radial) simultaneously. A slewing bearing of a mobile crane may carry the full weight of the boom and whatever it carries. And then it must also have to withstand the side forces that result as that load swings about. Frequent cries would be crushed in such treatment.
This is possible due to the geometry of the bearing. Four point contact designs allow the balls to contact the raceway at more than one point thus distributing forces. The cross-roller designs alternate the direction of the adjacent rollers so that there are always some that are placed in a way that it would take the direction the load is in.
Special Lubrication? Do they really need it?
Short answer, absolutely. Lubrication of the bearing surfaces would be devastating to the surfaces without a protective film forming between metal components. Maintaining the rotating equipment in good condition in the right lubricated state does not come as easy as pouring some grease and declaring it ready.
The change of temperature is a real challenge. A solar tracker has a bearing mounted on a turntable that is positioned outside during freezing winters and scorching summers. This lubricant must be able to flow at low temperature and not disintegrate when hot. A good number of applications have automatic lubrication systems that dispense the correct quantity of grease at the predetermined frequency. This maintains the flow of fresh lubricant in the bearing, and forces out the contaminants before they can cause harm.
Environmental seals operate with lubrication to ensure everything operates in a smooth way. Construction and mining environment are always threatened by dust, water, and debris. Multi-lip seals provide protection that prevents the entry of harmful materials but prevents the lubricant to leak out.
Your Mountaining Process is More Than you Think.
The installation of these huge bearings is not a thing to be done lightly. It is not something that can be attached in an afternoon. The surfaces on which the mounts will be attached must be machined flat to very narrow tolerances. Even minute variations cause unequal distribution of stress that causes premature failure.
The vast majority of manufacturers are stipulating the order in which to tighten the bolts, which is a star sequence, which brings the bolts to the necessary tightness step by step. It will avoid deforming the bearing rings. There are installations where hydraulic tensioners are used in place of ordinary torque wrenches due to the size and the forces on the bolts being too large to be reliably applied by manual tools.
Another complication is to get the gear teeth meshed correctly. A great number of turntable bearings have gear rings built-in to rotate. The pinion gear that will be interacting with these teeth must be positioned accurately. Excessive backlash will result in sloppy motion. Excessively little and the gears get up.
Applications are Continued to Grow.
One of the earliest serious users of these bearings was likely to be mining equipment. Draglines and bucket wheel excavators cope with unbelievable volumes of material. The walking dragline rotating superstructure may be more than a thousand tonnes. In order to support that, bearings need to be designed to standards unmet by the masses.
Medical imaging applied turntable bearings into a whole new environment. The CT scanners and MRI machines require very smooth rotating without much vibration. The bearing also needs to be fine such that no images can be blurred but this is a completely different task compared to maintaining a crane on its balance legs.
Energy that is renewable generated tremendous demand. Every wind turbine contains numerous bearings that meet various rotation requirements. The bearing of the yaw allows the whole nacelle to rotate into the wind. Pitch bearings vary on the angles of blades. All these bearings are proceeding through decades under conditions that would have caused the operators of industrial equipment a shiver.
They are dependent on satellite dishes and radar installations. Tracking systems should be flowing and should be accurate in placement. This type of controlled and precise movement is made possible by the bearings even to equipment which may contain a large dish of several tonnes.
The engineering continues to change. New material, more effective sealing systems, higher accuracy of manufacture. With every generation, there is an increase in the load, duration, and the extreme conditions in which it has to work.