Ball bearings are an integral part of all kinds of machines around us. They are found in most anything that has rotating parts such as cars, machinery, appliances, electric motors etc. These bearings can range in size from smaller than a grain of rice up to 12 ft in diameter according to a website I found. (Some different types of bearings are even larger!) The size bearings I regularly deal with when restoring machinery are of a much more benign size in the 1″ to 3″ range.
When I go through a piece of machinery I like to check out the bearings. I say “check out” instead of inspect because bearing health is a technically complicated field done by professionals. I, on the other hand, am some random guy on the internet (though I think that makes one an expert in everything these days). I’m going to discuss my layman’s conservative approach to determining if a bearing is good. I’m going to define good as “like a brand new bearing” and bad as anything else. Sure, there’s a spectrum and bearings in between good and bad exist. I prefer to replace any bearing that I don’t think is good because bearings are relatively cheap, they’re not always easy to get to, and you might forget about the so so ones.
Ball bearings are composed of a few parts made to high precision. The outer ring of metal is called the outer race and the inner is called the inner race. Inside all the balls which are held in place by the cage. On either side may be the shields. Depending on the type of bearing there may be one or two rows of balls. Here’s a sampling of some bad ball bearings I’ve replaced.
Ball bearings fall into two categories: Open and “greased for life”. An open ball bearing has no shields on the sides and no lubrication applied from the factory. These bearings will have lubrication applied through another means such as an oil bath or greased by the installer. The “greased for life” ball bearings come with grease from the factory. They also have shields on the sides which try to keep the inside of the bearing clean. The shields are either metal shields or rubber seals. The metal shields are a thin piece of metal that blocks off the sides but does not perfectly seal the bearing. Rubber seals do form a sealed environment which offers better protection. Bearings typically have the same shield type on each side but don’t always. Below, in the picture, shows an open bearing (left), metal shielded bearing (center), and a sealed bearing (right).
To me, a good bearing is one that feels like it just came from a factory. A new open bearings should spin freely when spun by hand and be perfectly clean. If you spin it quickly with your finger it should continue spinning and slowly come to a stop. A shielded bearing should turn with light even resistance and almost immediately come to a stop when spun by hand. Both should feel absolutely smooth with no gritty or lumpy feeling. The races should also look good with no chunks missing or discoloration. The inner race should also not move in any other direction other than spinning in the same plane as the outer race. Finally, the shields should also look flawless and, in the case of rubber seals, the rubber should still be supple.
I know describing things by feeling is a little suspect. I’d encourage you to pick a few brand name ball bearings up and play with them. You’ll soon get a good idea of how they should feel when new.
Now I’m going to discuss the signs and symptoms of bad bearings I’ve come across. Open bearings are subject to contamination from the outside environment before installation and during use if the lubrication isn’t properly maintained. Dirt and debris can get into the bearings and mess up the precision surfaces inside of the bearing leading to vibration, increased heat, and higher rotating resistance. Low lubrication levels, the wrong type of lubrication, and water intrusion can also cause trouble for a bearing. Below is a picture of an old open bearing I took out. I cleaned it out and left it sitting around out in the open. As you can see it’s picked up a lot of junk from the environment.
You might guess that the shields on a shielded bearing are important and you’d be right. While an open bearing is used in a closed environment, shielded bearings are typically left exposed. Metal shields can become damaged which can reduce their effectiveness at blocking contaminants or cause the shield to rub against the balls inside.
Similarly, ball bearings with rubber seals can have issues with punctures and breakdown due to age. Older rubber seals become brittle over time and can crack easily. When I’m checking out seals I like to gently poke them with a flat bladed screwdriver. The blunt end of the screwdriver should push in on the seal some but the seal should quickly return to normal once the screwdriver is taken away. I did this test on the bearing below and it cracked and fell apart.
Ball bearings can also be subject to rust. At some point the bearing below seems to have come in contact with water for a bit leading to the rust. I’d assume that the amount of water that did this probably made its way inside too. When I opened this bearing up I saw pitting and discoloration on the inside of the outer race. The balls inside will roll over the rust breaking it up and wreaking havoc elsewhere inside over time. Not good.
Remember when I talked about “greased for life” bearings? Well, it should actually be “greased for the life of the bearing” bearings because the life of the bearing may be shorter than the life of your machine. Grease will break down over time and lose effectiveness. You can feel this when spinning the bearing by hand. In some cases you’ll feel lumpiness or grittiness when turning it. In other cases the bearing will actually spin more freely than a new bearing. In the picture below I’ve shown a new (cheap) bearing on the left and the bearing from above on the right. The new bearing has soft clean looking grease. The one on the right shows dirty looking hardened grease and a lack of grease overall. When spun by hand this bearing turned freely in between numerous lumpy spots.
I should note that when I say “turn by hand” I mean completely remove the ball bearing from whatever it is in and on. I’ve found that when a bearing is installed it will feel completely different than when it has been removed. When installed, problems cannot be as easily felt. I’ve spun motor rotors and pulleys before and the bearings felt fine. Yet, when removed felt dry and/or lumpy. Of course some bearings can be so bad you don’t have to pull them.
Ball bearings can also suffer physical damage from shock loading. Below is half of the inner race of a wheel bearing that shows deformation from shock aka going over a median. This damage could be felt and heard.
I’m reusing the picture from up above to talk about the middle bearing. This bearing is from the back end of the head stock on my wood lathe. Installed, it felt, sounded, and looked fine. When I removed it, it felt loose, spun more freely than expected, and the inside race could be moved around (out of plane). Being at the far end of the headstock I’m sure it had to deal with a lot of shock loading and the saw dust on it tells me some may have made it inside.
This brings me to symptoms of bearing issues that I can’t take a picture of. Bad bearings can also make noises, cause vibration, and runner hotter. Bearings can be listened to using a mechanics stethoscope or any kind of solid rod such as a dowel or screw driver. While in motion place the tip of the stethoscope/rod/screwdriver on the housing that the ball bearing is in near the bearing. Any rhythmic sounds, sequels, or other irregularities can indicate bearing problems. I’d recommend listening to multiple bearings to get a good idea what sounds good and what doesn’t. Bearing temperature can be checked with an IR thermometer gun, thermal imaging, or if it’s bad enough, discoloration.
Bad Bearings? Who cares?
So, you run across a bearing and it’s questionable. Maybe its a little gritty or has a small amount of damage. Should you replace it? In my opinion yes for several reasons. The first is that whatever the bearing is in or on is probably several orders of magnitude more expensive than the bearing itself. The second reason is that you’ve probably already got to where you can access the bearing or have it removed already. You’ve already done the work. Just replace it. The last reason is that most regular ball bearings are super cheap for what they are. If there’s any doubt in your mind, just replace the bearing.
Also, most of the time a shaft (such as a motor rotor) has bearings on each end. Replace them in sets. You don’t want to replace just one and have the other one develop problems in a year or two. Then you have to do all the disassembly and reassembly again.
Story of Woe
A long time ago when I started this blog, I ran into my first bad bearing issue on the Clausing drill press. I gave it a once over when I bought it but didn’t find this issue until I was as home. I grabbed the chuck and discovered I could move it side to side with a soft clunking sound. Once disassembled, I found that the bottom bearing on the spindle was extremely hard to turn and had worn away the spot on the spindle where it was supposed to sit. It wore the spindle away so much that the bearing (or a new one) would easily slide along the spot. Bearing races are extremely hard. If it’s a battle between a race and most any other steel, the race is going to win. The picture below shows the bad bearing. It shows damage to the metal shield from something and even damage to the edge of the race.
This picture shows the spot where the spindle was worn away. Note that the worn spot is even smaller in diameter than the rest of the spindle shaft. It must have been this way for a while but no one noticed or cared.
At this point I was left with a drill press that wouldn’t work properly because of a bad spindle. I looked into several possibilities for fixing the issue. I could have had the spindle spray welded back up and machined down which may have possibly warped the spindle. I also called Clausing to see how much a replacement spindle was. They wanted around $350 for it. It’s not really unreasonable considering they still supply a part for a 40-50 year old machine but it’s more than I wanted to pay. Eventually, a used spindle popped up on Ebay for around $120. I went with the used spindle. Not a terrible price to get my drill press running but you have to compare it to the $5-$10 ball bearing which caused it. Technically, whatever/whoever damaged it and neglect cause the issue but you get what I’m saying here.
Finally, ball bearings are cheap but don’t cheap out on bearings. These days you can have a good name brand bearing shipped to your door for less than $10 from eBay or pick one up from a bearing supply house in your area. You can also pick up no name super cheap bearings off of ebay. DO NOT DO THIS for any serious use. If a name brand bearing is $10 and the no name is $3 for 10 something is suspect. Don’t be a penny wise and a pound foolish!
Anyways, I hope this long winded post will help you gauge the health of your ball bearings and perhaps save you some heartache in the future.