Tales from the Bearings Blog
The following short articles first appeared on www.powertransmission.com. They are part of the ongoing series of hints, technical tidbits and inside knowledge presented by our resident blogger, Norm Parker. If you like what you see here and are interested in learning more, visit www.powertransmission.com/blog.
Why Are Cast Iron Housings More Problematic When Fitting Tapered Bearing Cups into Bearing Caps of Salisbury- Style Axles?
I was recently approached for the 9,000th time (at least) about fitting tapered bearing cups into bearing caps.
Bearing caps can be a fairly generic term, so let me clarify that I am specifically talking about bearing caps in a Salisbury-style axle — which are the majority style of light-duty truck axles used today.
Generally speaking, you don’t want an interference fit on these bearing caps because they will distort when you tighten them down.
If only it were that easy!
For as long as bearing caps have been around, keeping them round has been the issue. Typically, the caps will be mated to carrier and machined as one assembled pieces. The prevailing theory was that if the cap bolts were tightened down enough so that the cap wouldn’t move during machining, the cap should be able to be removed and reassembled with the same tolerances it was machined to — right? Wrong.
It has been found that after the cap/ carrier interface is machined, the measurements may be perfect — until you loosen the bolts for the first time. The cap will often contract and give you an out-of-round cap. There are a couple of theories; I’m of the opinion that the machining operation imparts some compressive stress into the machined surface of the cap which aids in the contraction when the cap is removed.
How much does the cap contract?
It can be substantial. Substantial enough to fail a bearing at the split line location. Figure 1 is a cap that was machined to perfection prior to removing the bolts and, upon removal, measured 120 μm out-of-round at the split line location — far beyond acceptable typitolerances. To make matters worse, the out-of-round is very localized — creating almost an edge at the split line. In testing, this cap wound up failing a bearing cup as the first failure in the axle, taking about one-half of the life out of the axle.
For reasons I don’t yet completely understand, this problem seems to be more prevalent in cast iron housings. Aluminum housings don’t seem to exhibit this problem nearly as often as iron housings. It could be because the AL doesn’t get as hot during machining, but that is speculation on my part.
We’ll really get into this deeper in a full article, but for now, keep your cap fits light. I like to start off somewhere around a line-to-line fit. There are occasions, if you are having really bad contraction with heavy loads, when you may have to back off farther than that.
Bolted or Welded Ring Gears? Which and Why?
There is often a lively debate early in a program when we are discussing the pros and cons of laser welded ring gears vs. bolted ring gears. Just about every company that makes gears has both styles to some extent.
There is no right answer for every application. Mass savings with welding often dominates the conversation, but let’s take a high level look at some of the other pros and cons to consider if you find yourself if this conversation.
There are clearly more check boxes in favor of the bolted assembly. However, if someone if offering up 1kg or more in mass savings for a welded assembly, depending on the vehicle, welding very well may be a worthwhile venture. Looking at the comparison, it becomes obvious why companies typitolerances cally employ both methods. For high performance, lightweight vehicles, laser welding becomes very attractive. For larger, heavy duty type applications where mass does not have as high of a premium, bolting wins the contest. The upside to welding is that after you make the investment, you will always have both technologies available to suit any application that comes your way.
Bearing Cup Fits in Aluminum Housings
The interference fit that cups should have in an aluminum housing is a subject that comes across my desk in regular intervals. Of course, there are numerous reasons why you would need certain fits in different areas. If there is a bearing cap involved, you may have a light or loose fit. If you need something that is going to be serviced at intervals, you are likely going to want a fit as light as possible. In gear housings, the highest priority is often the stiffness of the system, which drives a cup fit that you never want to lose contact because the cups will start to float in the bore, creating misalignment in the shaft. In a perfect world we could run a CAE stress analysis to determine how the cup interferes with the housing and then run a thermal study to make sure we hold our position at peak temperature.
But as a great person once said, “Ain’t nobody got time for that.”
The bore expansion can be simplified to a simple diametrical expansion. It’s not 100% perfect — but its close enough for our purposes. A perfect calculation would use the linear expansion around the circumference, but when you use the same simplification for both cup and cone, the end result is nearly identical as the more elaborate approach. Using this simplified equation, a 100 mm bore at 120°C will expand by 85 μm from room temperature, 25°C. If you were trying to maintain fit up to a 120°C, that is going to be your target interference fit. Depending on where your tolerances are, that might be your maximum or minimum target, with your bearing + housing tolerance defining the other end.
In my February post, I discussed how to use hoop stress calculations to determine how much the cup will grow and shrink with moving temperatures. Don’t make the rookie mistake of using the 85 μm as your potential cup expansion; your cup will only expand as far as you compressed it. In a 25 mm wall housing, most of the interference is going to expand the housing wall, with the cup only compressing by as little as one-fifth of the overall interference.
The article "Tales from the Bearings Blog" appeared in the June 2015 issue of Power Transmission Engineering.