Bearing basics

[thegary]

I have a customer that brought me a wood chipper to repair. It needs a new shaft for the chipper drum. He told me that the bearing keep loosening on the shaft and then they spin on the shaft and it wears down the shaft. This will be the 3rd shaft he will have put in it ( not by me ) . I pulled the machine apart and found the shaft worn down about 1/16 where the drive side bearing was located. He has been doing the repair work himself but can not figure out why the bearings keep coming loose on the shaft. The bearing used on the chipper have 2 set screw in each one 90 degrees apart.

The reason the bearings keep coming loose or more specifically why one bearing comes loose is that the shaft expands and contracts with temperature. He has been putting new bearing and new shafts in but has been using fixed bearings on both ends of the shaft. Now when you do this it is fighting the shaft expansion . That shaft is going to expand weather you like it or not and it will move at the weakest point and that is the set screws on this particular example. Now when it moves back and forth a few times the set scews either loosen up or in the case of locktited they will wear down or put grooves in the shaft.

He should have put a fixed or held bearing on one side and a floating bearing on the other end . In general the held is most commonly put on the drive side but not always. Another problem that is very common is not setting up the floating bearing correctly. Depending on size they are made with more or less float. Usually 1/8 - 1/2" available float. The floating bearing is generally set so that it is in the middle of its float so that as the shaft expands ot contracts it has room to float back and forth.


There are a lot of factors that must be looked at but generally the longer the distance between bearing the more critical the float becomes . The other factor that needs to be looked at is operational temperature. If a part is used in a heated building and is not sbjected to wide temperature swings you might get away with 2 held bearings or not having the floating bearing set up correctly. The wider temperature the equipment is used in the more critical it is to have the float set correctly.

I have a customer who has a wast water treatment plant. It has 2 agatators with 6" dia. shafts that are 20 ft long. He has been having the same issues as the guy with the chipper. Guess what his problem is?

On some bearings like taper lock style the weak point is not the connection to the shaft. The expansion will still be there but it will show itself in a different way. It might be a bracket or mount that keeps breaking or a bearing that keeps going bad or a frame that flexes and causes not aparent problem for some time but will eventually cause cracks or breaks.

[Bls repair]

Have they tried putting locktite on the inner race and shaft

[denrep]

Whatdawe got? Ball, tapered-roller? Shat size? Distance between bearings? Load? End-play/thrust?

I'm putting my dollar on a design issue or overloading.
Is it possibly bearing lock-up and "skidding" due to contamination?
Shaft deflection?

There are thousands and thousands of slip-fit bearing applications that live just fine even through extreme temperature swings; in practice, minor temperature changes are just not that critical. How much can the shaft "grow" with just typical temperature change 0.0005? I didn't read about cooling issues or grease melting off, so I'm guessing things aren't that hot?

IMHO this needs deeper digging. Something is going on that's making it easier for the bearing to skid on the shaft rather than to function as intended.

[Pete.S.]

Good point about the thermal expansion.

From your description it sound like the bearing are in bearing blocks?

That usually means that the shaft is slip fit into the bearing.
The only thing he should have needed to do is to only use the set screws on one side of the shaft. Then the shaft can expand with heat because one side is fixed and the other has a slip fit.

The question is, how much axial load is on the shaft? Set screws can't take much. Better to use the blocks with a locknut and a tapered sleeve.


There is another aspect with thermal expansion and that is the thermal expansion inside the bearing. In situations where for instance the shaft gets hot it will expand and heat up the inner race of the bearing which causes it to expand and the balls (ball bearing) doesn't have enough clearance anymore. That means that the design needs bearings with increased clearance, these are often called C3 bearings (there are more than classes than C3 though).

[thegary]

Whatdawe got? Ball, tapered-roller? Shat size? Distance between bearings? Load? End-play/thrust?

I'm putting my dollar on a design issue or overloading.
Is it possibly bearing lock-up and "skidding" due to contamination?
Shaft deflection?

There are thousands and thousands of slip-fit bearing applications that live just fine even through extreme temperature swings; in practice, minor temperature changes are just not that critical. How much can the shaft "grow" with just typical temperature change 0.0005? I didn't read about cooling issues or grease melting off, so I'm guessing things aren't that hot?

IMHO this needs deeper digging. Something is going on that's making it easier for the bearing to skid on the shaft rather than to function as intended.

I am starting to wonder why I started this thread. Steel has an expansion rate of .0000072 for ever degree in temperature for each inch of shaft length. The chipper I refered to in my opening post has 36" between bearings. It is used out side so the possible temperature swing it will be used in is aproxamatly 120 degrees that would be from -20 degrees to +100 degrees .0000072 x 120 =.0000864 per inch of shaft length. .000864 x36 = .031104 of expansion . That is aproxamatly 1/32 of an inch. If you do not think that that will make the set screws loosen I do not know what to tell you.

On the example of the agitator shaft that is 20 ft between bearings . the expansion of the shaft would be .20736 that is over 3/16 of an inch. That is only with 120 degree temp swing.

I did not take into account any thing for heat build up of the machine in operation. In all likelihood the 36" chipper shaft will see more like 180 degrees in temperature swing if the shaft reaches 160 degrees which is not out of the question at all.

[thegary]

Whatdawe got? Ball, tapered-roller? Shat size? Distance between bearings? Load? End-play/thrust?

I'm putting my dollar on a design issue or overloading.
Is it possibly bearing lock-up and "skidding" due to contamination?
Shaft deflection?

There are thousands and thousands of slip-fit bearing applications that live just fine even through extreme temperature swings; in practice, minor temperature changes are just not that critical. How much can the shaft "grow" with just typical temperature change 0.0005? I didn't read about cooling issues or grease melting off, so I'm guessing things aren't that hot?

IMHO this needs deeper digging. Something is going on that's making it easier for the bearing to skid on the shaft rather than to function as intended.

Diameter of the shaft makes no difference to speak of . The bearings used are spherical roller bearings double row . Thay can be purchased in many forms . The ones used are in pillow block configuration. I have a bearing chart for clearance on c3 and all other spherical roller bearings that are used with taper lock centers. When they are installed you must measure bearing clearance to make sure you only tighten it as much as spec calls for . That type of bearing has a seperate bearing housing that it is set into. The housings all have float machined into them. To make one a held bearing you install a C ring which takes up the float room to make it a held or locked bearing.

The bearings used are slip fit hense the set screws for the chipper shaft I am working on. They come in fixed and float configurations. You must buy one of each . Most people look at one and get both the same which is a big mistake. I have run into this problem many times . I have no hesitation that it will work for many years when I am done.

[thegary]

Good point about the thermal expansion.

From your description it sound like the bearing are in bearing blocks?

That usually means that the shaft is slip fit into the bearing.
The only thing he should have needed to do is to only use the set screws on one side of the shaft. Then the shaft can expand with heat because one side is fixed and the other has a slip fit.

The question is, how much axial load is on the shaft? Set screws can't take much. Better to use the blocks with a locknut and a tapered sleeve.


There is another aspect with thermal expansion and that is the thermal expansion inside the bearing. In situations where for instance the shaft gets hot it will expand and heat up the inner race of the bearing which causes it to expand and the balls (ball bearing) doesn't have enough clearance anymore. That means that the design needs bearings with increased clearance, these are often called C3 bearings (there are more than classes than C3 though).

You never ever leave a bearing loose on a shaft, ever. It must be locked to the shaft or it will spin on the shaft . The only exception that I can think of is a brass bushing or a babbit bearing where the shaft spins in the bearing by design.

[thegary]

Good point about the thermal expansion.

From your description it sound like the bearing are in bearing blocks?

That usually means that the shaft is slip fit into the bearing.
The only thing he should have needed to do is to only use the set screws on one side of the shaft. Then the shaft can expand with heat because one side is fixed and the other has a slip fit.

The question is, how much axial load is on the shaft? Set screws can't take much. Better to use the blocks with a locknut and a tapered sleeve.


There is another aspect with thermal expansion and that is the thermal expansion inside the bearing. In situations where for instance the shaft gets hot it will expand and heat up the inner race of the bearing which causes it to expand and the balls (ball bearing) doesn't have enough clearance anymore. That means that the design needs bearings with increased clearance, these are often called C3 bearings (there are more than classes than C3 though).

You are correct the bearings with the lock nut and tapered sleeve are much better but still need the float and come in fixed or held and float configurations.

[denrep]

.....

[denrep]

Without arguing all the speculative unknowns of assembly room temperature vs ambient temperature and operating temperature and expansion rates...

I'll guess that in all likelihood the framework of this machine is of low-carbon steel construction, therefore, whatever expansion rates are being calculated should also apply to the framework.

Thus, for example, if with ambient temperature swings the 3' shaft "grows" an iota between bearing centers, then so to the framework to which the blocks anchor will also grow an iota between bearing centers. I would think that the essentially equal growth of all components would in all practicality make insignificant any thermal influence to dimensions.

It's not as if this shaft were rolling hot steel and subject to temp swings much different than the rest of the machine.
And that's why I'm hesitant to simply pin the repeated costly failures on thermal expansion.

Even though I doubt it, maybe I'm wrong and it is an issue with thermal expansion?
Again, I'll readily admit that I don't know; heck, I've not so much as seen a picture of the application.
However, seeing as we're at strike two, my vote would be for a thorough, critical, open-minded failure analysis.

[flushcut]

What make and model chipper? Most chipper bearing shafts are not round they are slightly oval and cam in to the bearing.
I will guess Vermeer BC1800.

[thegary]

Without arguing all the speculative unknowns of assembly room temperature vs ambient temperature and operating temperature and expansion rates...

I'll guess that in all likelihood the framework of this machine is of low-carbon steel construction, therefore, whatever expansion rates are being calculated should also apply to the framework.

Thus, for example, if with ambient temperature swings the 3' shaft "grows" an iota between bearing centers, then so to the framework to which the blocks anchor will also grow an iota between bearing centers. I would think that the essentially equal growth of all components would in all practicality make insignificant any thermal influence to dimensions.

It's not as if this shaft were rolling hot steel and subject to temp swings much different than the rest of the machine.
And that's why I'm hesitant to simply pin the repeated costly failures on thermal expansion.

Even though I doubt it, maybe I'm wrong and it is an issue with thermal expansion?
Again, I'll readily admit that I don't know; heck, I've not so much as seen a picture of the application.
However, seeing as we're at strike two, my vote would be for a thorough, critical, open-minded failure analysis.

You are right about the framework to a degree. They will expand together some what. Not perfectly and will not take into account the heat of the machine. It does not take a lot of movement to make enough stress to create a problem. Set screwed bearings are probably the weakest form of locking a bearing to a shaft. The reaction to loosen a set screw can be as little as a few thousands of an inch and may take several heat cool cycles to loosen. The fact that the bearings can be purchased in the 2 configurations shows that the problem is not an isolated incident .

There is a lot of variables to bearings. This thread was to teach an aspect of bearings that is not very well known. It was not to dispute or dicuss all the variables that would take a book to cover. Could it be a different problem, sure. In all likelihood it is not but it could be an out of balance that is caused it. In fact that is how this all started . He installed a new bearing on the worn shaft and locked the set screws. I did not know this at the time but he had me come over to dynamic balance the drum . I could not get a good solid reading and told him there was something wrong with the drive side bearing. Now 6 months later that bearing is bad and he said the shaft looks about like it did when he put the bearing on. I can say that for as bad as the shaft was it ran fairly smooth at the time I had checked it. If there is vibration when I am done I will balance it but I will not do it if it runs smooth unless he is willing to spend the money but I do not expect he will do it if it runs with little vibration. The bearing is bad because the set screw collar broke the inner race not because the bearing failed from operation.

I do not post pictures one reason is I am not all that into it. But I also have personal reasons why I do not post pics that I will not discuss here.

[Pete.S.]

You never ever leave a bearing loose on a shaft, ever. It must be locked to the shaft or it will spin on the shaft . The only exception that I can think of is a brass bushing or a babbit bearing where the shaft spins in the bearing by design.

Yes, you're right. I was wrong there, my idea would have been a bad solution on a rotating shaft.

But there are other applications where the outer ring is an tight fit and the inner ring a loose fit.

[Pete.S.]

The fact that the bearings can be purchased in the 2 configurations shows that the problem is not an isolated incident .

Does bearings like the ones below also come in two different configurations? Or is it just certain types that comes in a fixed or floating configuration?

[thegary]

Pete those two bearings I do not believe come in a float configuration. Those are inexpensive bearings and used commonly on things like conveyors. I do not want to mislead you. There are many applications where the expansion will not cause a problem. Some are when the bearing centers are not very far apart. A big one is on equipment where the frame is not real ridged like on most conveyors. Almost all equipment used none commercially is not built ridgid enough to have thermal expansion be a problem. In an aplication like that even a set of bearings 30" apart will be fine because the framework gives little resistance to flexing enough to compensate for the expansion. You do not see many applications where a floating bearing is needed except in industry and heavy equipment with a very a ridged frame. One of the critical places a floating bearing is needed is in applications where the bearings are mounted on separate structures like concrete piers with a soal plate grouted in to hold the bearings. Another place that a floating bearing is needed is on large structures like machines of large size such as paper machines or steel rolling mills to name a few. These structures are so large that the induced loads and vibration can cause what look like a very masive structure to move. I once was working on a steel rolling mill install that the framework was 3 stories high . We hade to align a roll on the top floor to a roll ancored to the concrete floor on the ground floor. We hung plum lines made of piano wire from the upper roll with 10 lb weights on them to check the alignment with the bottom roll. We were getting too much movement on the plum lines so we figured there was too much air movement with door open and such so we figure we needed to stabilize the plum wires better. We got 5 galon pails and put a vew galons of oil in them then we put them under the plum lines so the weights were submerged in the oil. That did the trick to stabilize the plum lines but we found we still had a problem. The alinement would drift slowly back and forth over a period of about 5 minutes . It was the steel structure that was slowly moving back and forth as much as 1"

[thegary]

What make and model chipper? Most chipper bearing shafts are not round they are slightly oval and cam in to the bearing.
I will guess Vermeer BC1800.

Its a Rayco with a 225 hp diesel. I do not remember the model number.

[thegary]

As Denrep pointed out a steel shaft mounted to a steel frame at first thought you would say they will expand evenly together thus no problem. This does not work out to be true in real life. In fact a steel frame can cause the expansion problem to be even worse.

I will use the example of a piece of equipment like the chipper I am working on. It sits out in the weather all the time. Now it has an all steel frame. Most of the frame that supports the drum shaft is shaded from the sun because of the other frame work depending on the position of the chipper the sun hits different parts. The parts in the sun can reach 140-150 degrees very easy. The shaded parts of the machine will only be ambient temp. This makes for a lot of changing distortion and flex in the frame.

With an expansion or floating bearing setup this causes no real problem with expansion to the shaft of the drum. That same shaft with 2 held or ridged bearings would see much more stress than just the expansion rate of the shaft itself.

This spring I had 3 semi trailers to redeck. The ambient temps were 50-70 degrees the days I worked on them. The decks were so hot out in the sun that I had to put some insulation under my knees to keep them from burning from the heat of the deck. I was sweating like pig while working on them. The heat felt like a blast furnace on my face when I was not welding. I use this just to show how hot steel can get in sun light . That same job when I would take a break I would sit under the deck in the shade. Sometimes I would feel cold by the time I finished a break. There was easily 100 degrees difference in the temp of the steel from that part that was in the sun to the parts that were shaded.

When I was young and dumb. I took my family on vacation to a lake. We had to get fuel at the local marina on the lake in the middle of the day. I was bare foot. The docks were made of aluminum. I jumped out of the boat to go pay for the gas. When I steped on that deck I took about 3 steps and my feet started burning, I ran the rest of the way down the deck. I spent the next 3 days in the cabin with my feet up I had burn blisters on every part of my feet that touched the dock. Now that dock was only about 20ft long so I did not contact that deck more than a few seconds . It had to be very hot to burn me that fast. I would guess it had to be 300 degrees .

Its 3:50 am now , I could not sleep so I was bored. Could you tell?

[denrep]

It's good that your digesting it all.

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Interesting chart Pete.S posted at #13.
But we've all seen exceptions.
I have a hunch that ultimately cost and difficulty of assembly/service have as much say-so with design as anything
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500+ views, no action
Thegary, you're gonna hafta bend your rules and post some pics...
These WeldingWeb guys won't sit down at the table if they don't see some meat.

[thegary]

It's good that your digesting it all.

------------

Interesting chart Pete.S posted at #13.
But we've all seen exceptions.
I have a hunch that ultimately cost and difficulty of assembly/service have as much say-so with design as anything
------------
500+ views, no action
Thegary, you're gonna hafta bend your rules and post some pics...
These WeldingWeb guys won't sit down at the table if they don't see some meat.

Yes , I liked his chart also. In many of the applications the shaft or axel was stationary so locking was not needed. I know what you mean about pics, it is the american way. People do not want to think. I prefer to use word pics to explain things but there are times a pic is worth a thousand words. I might have to give in to bending my rules but not today.

[thegary]

I am following up on this thread for any who are interested. Its been a few months since I finished this chipper . The customer called me yesterday to say thanks and that the chipper is working great. The bearings are staying tight and it runs smoother than it ever has. I also wanted to follow up that I did not have to rebalance the rotating assembly when I got it done since it ran nice the way it was.

[thegary]

Just giving another update. The machine has been in operation for a 1 year and 9 months now. There is still no problems with bearings loosening on the shaft. The customer has been using it every day. It has given him no trouble at all. He said he even had a large piece of steel go through it by accident . He thought for sure it would destroy the chipper. All he did was replace the knives and go back to work.