Most bearings are used in applications involving rotation: a shaft spinning inside a housing, a wheel turning on an axle, and so on. The thrust bearings, on the other hand, are used in applications involving something entirely different: a force pushing along the length of a shaft rather than around it. If you’ve ever worked with machinery in which parts are trying to slide or shift along the length of a shaft, you’ve probably encountered the problem a thrust bearing solves.
The applications are quite universal if one looks hard enough. A vertical pump shaft supporting the weight of the impeller assembly. A gearbox in which helical gears cause axial forces during operation. A ship propeller shaft in which a thrust load of massive proportion must be handled as it turns through the water. A marine crane turret. Heavy machinery compressors. Even the lazy Susan on a dining table uses a form of thrust bearing, though admittedly at a rather different scale.
What Makes Them Different
The problem with a thrust bearing is the distribution of force across surfaces which are intended to slide or shift in relation to one another in a very specific direction. While a typical bearing handles force at a right angle to the axis of rotation of a shaft, a thrust bearing handles compression in the very direction of the axis of rotation. All of which means a thrust bearing does not behave at all like a typical bearing.
Most thrust bearings are based on fairly simple principles: two surfaces with a rolling element or lubricant in between intended to handle forces pushing in one or both directions. However, arriving at a bearing which works in the real world from the theoretical concept of a thrust bearing requires dealing with a variety of factors involving heat, wear, lubrication, and the characteristics of the machine in which it will be used.
Ball thrust bearings use balls that are placed between two washers called races. They’re neat and efficient and will support various speeds and loads. Roller thrust bearings use rollers instead of balls. This gives them more capacity because of the increase in surface area. Of course, they’re bulkier and speed-specific.
And finally, fluid film thrust bearings use an oil film between the two parts. There’s no direct metal-to-metal contact, which means no wear and tear. Of course, they need an oil supply and won’t function if stationary. They’re generally used in high-speed applications such as turbines and electric motors. The speeds and pressure would quickly destroy any contact type of bearing.
Getting the Selection Right
Now it gets really interesting. If the wrong bearing is specified and fitted, it will be quickly apparent. Either it will be noisy and vibrate excessively, or it will simply wear out prematurely and have an effective lifespan that is far less than was anticipated.
The capacity of the bearing to support the load is the most obvious place to start. The bearing will be subject to various forces and their direction. There will be situations where the bearing will be subject to purely axial forces. There will be situations where the forces will be radial and axial. The bearing will either be able to support the combination of the two or the combination of two types of bearing.
The speed of the bearing is also critical. A bearing that will perform well at low speeds will catastrophically fail at higher speeds due to the generation of heat. The temperature will also play a role. The space in which the bearing is to be fitted will also be important.
Another set of factors that need consideration are the environmental ones. Will it be exposed to moisture, chemicals, and contamination from dust and/or debris? Will it be exposed to saltwater, as is the case with marine equipment? Will it be exposed to washdown procedures, as is the case with food processing equipment? Will it be exposed to harsh environments, as is the case with mining equipment?
The Boring Bits That Matter
Thrust bearings are the type of component that is never noticed unless it fails. They are not exotic technology, though it does take real knowledge to get them right. Get it right, and it is a maintenance item that you do not even think about. Get it wrong, and it will teach you a very expensive lesson in the importance of proper engineering principles.