Frequently Answered Questions?
Following are the answers to some of the most frequently asked questions in the field of bearings and its allied industry.
What is the difference between a seal and a shield on ball bearing? Why can't we just use all seals instead of stocking both shields and seals?
Ans: The shield is attached to the outer race a clearance between it and the inner race. Now, the seal is attached to the outer with the inner race. The main reasons you can't use seals in all applications instead of shields are:
1. Speed 2. Heat and 3. Starting Torque
The seal could cause problem in these areas due to its rubbing against the inner race where the shield has a clearance at the inner race.
On single row deep groove type bearings' after they are mounted a perceptible end shake is grabbed and moved. Are these bearings too loose internally?
Ans: No, everything is fine. Look at the picture below. the bearing must have a slight amount of movement. This is necessary running clearance so it does not bind. And say, before you do this, remember to take a second to wipe off your fingers.
Are burrs on the shaft much of a problem?
Ans: They sure are. If a shaft due to any reason gets bumbed hard enough to produce a burr, you can't see, you can be sure that the bearing which it was meant for, no longer would slip aver a bumbed end. Any time spent checking for and removing burrs is certainly time well spent.
As we listen to all the things that are told about correct storage, installation, handling, lubrication, maintenance, etc., we can't help but wonder. "Isn't help this lot of nice theory, most of which isn't done?" How many plants can do all this as perfectly as desired?
Ans: Honestly, very few, if any at all. It is a cruel dirty world out there and where bearings have to be installed and handled is a far from perfect environment. But, we never predict immediate and catastrophic failure did we? No, both you and i know that bearings operate reasonably well in spite of all those horrible things that happen to them. The point we always try to make is that if you do the best you can in all these areas, talking into consideration the bearing life and improve performance.
Is selecting the right lubricant vital? It seems that bearings fail for other reasons- not from picking the wrong oil or grease?
Ans: Bearings do fail for many reasons, but improper lubrication is right at the top of the list. The breakdown why bearings fail would go something like this: 43 per cent due to improper lubrication;
27 percent due to improper mounting;
21percent due to other causes
and 9 percent to reaching the fatigue limit of the metal.
Reasons for lubrication failure include:
1) Incorrect selection of the grease base for a particular application.
2)Use of grease when condition dictate use of static or circulating oil.
3)Insufficent lubrication either in quantity or required viscosity.
4)Deterioratin of prolonged service without proper relube intervals.
5)Contamination of the lubricant and the bearing with foreign materials.
Is there a right or wrong to store bearings ? We usually put them on a shelf until we need them.
Ans: Yes, there is a right and wrong way. You should bearings in a clean, dry place. they should also type of order so there is no need to toss them about when looking for the correct bearing. Always use the oldest bearing first, by the way. Bearings are precision components and should be treated gently. Never drop them or handle them roughly. In fact, the less you handle them the better since finger printers can lead to rusting. Also watch temperature changes in your bearing storage area- drastic changes can harm the as condensed moisture forms.
Our bearings operating at high speeds seems to get unusually hot. What is causing? We are sure they are well lubricated so that can't be the problem.
Ans: we are afraid that just might be your problem- over lubrication. As unlikely as it may seem, aver lubrication of bearings used in high application can be as harmful as insufficient lubrication. Over lubrication creates action of oil or grease which increases the friction and torque characteristics of the bearings. This increase in friction cause the bearing and lubricant) to operate at a high temperature. The fact that your bearings are "well lubricated" is probably the source of your trouble!
So we take good care of the bearing during storage and then we install it in a dirty, nasty environment. Does that make sense?
Ans: No, it doesn't make such sense, but there are some thing you can do to clean to that environment. Start by cleaning the work bench area the tools you will use on the new bearing. You should also get rid of all the linty clothes. Put the bearing on its original wrapper or on some other type of clean paper when working on it. Never let a bearing rest on a dirty surface even for a short time; cover it with a clean, lint-free cloth. Bearings should not be washed unless it is for an oil mist or circulating oil system. these comments are pretty basic, but they be repeated enough times.
What is the main function of a bearing lubricant?
Ans: Bearing lubricant should establish and maintain a micro thickness separating film of oil between rotating and static members. The forming of the elasto hydrodynamic film is the result of a build-up wedging action of the lubricant between rolling elements and raceways.
Why does some new equipment come in with bad bearings? Often they are noisy or rough on first startup and often they fail in the first few hundred hours of operation.
Ans: This is not too surprising, really, when one thinks of all that might have happened in assembly, in transits, in setting up, etc. First, the subassembly might have been dropped or bumped hard or pressure might have been applied to the wrong ring during assembly which would have resulted in brine ling or a ball denting the raceway. Second, perhaps the jolting, jostling in shipment or the unit setting idle where there was extreme external vibration, resulted in metal to false brine ling. These are just samples of some of the more obvious and common problems. On the above, simply replacing the bearings should solve the shaft and housing dimensions must be checked.
What are some of the most common improper methods of bearings mounting or installation?
Ans: We would say contamination of the bearing during installation and applying force to the wrong ring of the bearing. ALWAYS apply force to the press fitted ring.
What are some of the best indicators that a bearing is failing?
Ans: Excessive heat, noise and vibration are three sure indicators that a bearings is failing.
When heat is used to expand the inner rings in mounting bearings and temperature is carefully controlled, is there anything else to worry about?
Ans: Yes-The tendency of the hot inner to pull away from the cold shoulder. Lock it against the shoulder with a lock-nut, if there is one, and after it cools, be sure to check and see that it is the correct location, snug against the shoulder where it belongs. In the cause of cones and cups, this could affect the bearing adjustment. Be sure to check the seating with a feeler gage.
Is it true that tapered roller bearings do not have to be handled as carefully as ball bearings? Are they a little "tougher" in this respect?
Ans: If you want to join the SPCB (Society For Prevention of Cruelty to Bearings), treat all bearings the same . Give them all the tender loving care you can. In one respect, the cone demands more care before it is mounted in the cup as careless can handling or mounting practices can damage the cage enough to allow the rollers to separate from the cone. Never use a bearing with a bent or damaged cage. we see many cases of this because of careless handling or poor mounting practices. often the punch and chisel are used instead of sleeve that contacts and applies pressure the entire cone.
There is allot of importance attached to keeping the bearings clean, but, is it not as important to keep the shafts and housing clean?
Ans: Look at the accompanying sketch what happens to that dirt if we don't wipe it off. the tight fit of the inner pushes it to the shoulder, where it builds up to the point where the bearing cannot to wipe off your fingers.
What is the single most important aspect of bearing care you can tell us about?
Ans: We would say that cleanliness in bearings maintenance is probably it. Amount the most important factors in bearing maintenance are cleanliness and working conditions in the assembly area. Many bearing difficulties can be traced to contaminants that have found their way into the bearing after the machine has been placed into operation. Contaminants are generally miscellaneous particles that, when trapped inside the bearing, will terminally indent the balls and raceways under the tremendous pressures generated. Due to the relatively small area of contact between the ball and raceway, contact area pressures are very high even for lightly-loaded bearings. when rolling elements roll over contaminants, the contact areas are greatly reduced and the pressure becomes extremely high. When abrasive material contaminates the lubricant, it is frequently crushed to finer particles which cause wear to the balls and raceways. therefore, it is important to maintain a clean environment when working on all servicing operations the assembly area should be isolated from all sources of contamination including moisture and corrosive elements. Work benches, tools, clothing and hands should all be free from dirt. lint dust and other contaminants that may harm the bearing. The surface of the work bench should be clean and rubber-coated, if possible. Plastic coating is also a good idea. This helps to avoid the nicking of spindle parts on too-hard surface and allows you to wipe the surface and allows you to wipe the surface clean frequently. In addition, plastic and rubber coating allow easy and economical replacement.
Is there any way we can "save" a new bearing that has been used but that is dirty?
Ans: Sure, if you handle it carefully. Don't spin it, even by hand while you are washing it. And remember, don't wash double shield bearings. Wipe them off to keep dirt from working inside and coat with lubricant before wrapping and storing.