It seems like everyone is jumping into the electric gearbox market. Even Schaeffler is in the electric drive module (EDM) market now with their own 800v system. With all these new entries, some companies are satisfied with following traditional bearing arrangements while some want new and unique solutions or some combination of both. There is heavy debate over which styles are the best.
In this article, we will talk about the different types of 2-bearing arrangements for each shaft with the pros and cons of each for a simple three shaft, single speed parallel axis gearbox (ala Tesla style).
Let’s start with the classic fixed-float ball bearing design. This is a great, simple, almost one-size fits all approach to fixing 2 bearings on a shaft. One bearing is going to be your designated fixed bearing, which means it is going to take 100 percent of the axial loads plus a proportional part of the radial loads, depending on where the loads are coming from. To force the fixed bearing to take all the axial loads, it must be constrained on all four corners by shoulders, snap rings, retaining plates, screws, whatever your flavor is. The other bearing obviously takes none of the axial load and this is done simply by leaving a loose fit on the housing with no constraints. In all of our textbook examples, we will have a tight fit on the shaft. There are a couple of drawbacks to this system. Usually, snap rings or a retaining plate is needed for the fixed bearing and there is no way to reduce endplay. The fixed bearing can have decent amount of axial play just due to the internal axial clearance of the bearing (a function of the installed radial clearance). If snap rings are used, those introduce additional axial endplay. A bearing plate eliminates snap ring tolerances though at a significant cost and added complexity. Some people like to try and press fit the outer ring of the fixed bearing as well. You will find that after you increase the bearing internal clearance to compensate for the housing press fit, you are not any farther down the road in terms of reducing overall runout. In either case, we never rely on press fits as a retaining feature when taking direct axial loads. A snap ring is still needed in the loose or tight fit configurations. You can usually get away without a snap ring on the press fit side of the float side – though it is always recommended to package protect for it.
Another popular design is called a full float or straddle mount type of design. In this configuration, each bearing takes the axial load in one direction – the bearings share the axial load. This is done by constraining the outer rings in only one direction. This can be handled from the inside corners or the outsides. In the outer shoulder arrangement below (Figure 3), the right bearing will take all axial loads going to the right and the axial loads going to the left will be taken by the left bearing. A big benefit to this system is that endplay can be reduced with a selective shim, mounting screw or nut. As you shim one side, the reaction is taken by the opposite bearing through the shared loose fit housing and the outer rings are slightly pushed together. Unlike the fixed-float system, in a paired shim system, internal axial clearance be reduced or even eliminated.
If you are looking at options outside of ball bearings, a ball-cylindrical combination is popular. This design is a different version of the fixed float system. In this case, the float bearing is the cylindrical since it does not take axial load anyway. You can order these with shoulder to take incidental axial loads, but they are not meant to take gear loads. In racing applications, for example, straight spur might be used with do not generate any axial loads. Those boxes will often run all cylindrical bearings with shoulders which just serve to hold the shaft in place. The drawbacks for cylindricals are the outer ring still needs to be retained even though it is the float bearing because it is not attached to the inner ring. It can walk and keep on walking. The sleeve side of the cylindrical (here shown on the inner ring in Figure 4) tends to be fairly robust against walking. While rolling losses in cylindricals are very low, oil churning losses can be high if too much oil is in the area due to the wide flat roller. Care must be taken in oil management if efficiency is a driver for using this arrangement.
The final example we will cover is a higher performance arrangement using preloaded angular contact ball bearings. This arrangement is typically reserved for applications that require high precision, speed and efficiency. This is a great combination that is much more precise than the ball or cylindrical combinations but comes at a price. The angular contact bearings need to be preloaded just like tapered roller bearings. Even if the piece price is reasonable, the preloading operating can chase people away.
And the last example is, of course, the king of bearings- the tapered roller bearing combination. There is nothing else that can touch tapers in terms of stiffness, load carry capability and dynamic runout. Unfortunately, there is no free lunch. The high accuracy comes with the preload operation, light load efficiency loss and high-speed limitations. With the rollers having substantial sliding in both the rolling direction and the sliding rib, the losses can be measurably higher than ball bearings at low loads. However, when loads increase, hysteretic losses of a ball bearing increase significantly and eventually surpass the sliding losses of the taper roller bearing. Don’t get into the habit of assuming that ball bearings are always more efficient, there are many factors to consider.
The truth is, there is no perfect answer. Every system has benefits and compromises. The type of vehicle you are designing for will drive your gear design which will drive the bearing system. If you need a high performance quiet gearset, you may be looking at preloaded shafts. If cost and efficiency are your primary drivers and NVH is secondary, you can use bearings that have less running accuracy and stiffness.