Millwright

I have first hand experience with this problem. If there is clearance between the race and the shaft, the constant slight impact or scooting will wear the shaft. The shaft may be taking the brunt of the wear because it is not as hard or has lower toughness when compared to the race. The race does not have to spin on the shaft to cause wear inducing scoot or impact. Just a little clearance will do it.

The fit might have started tight but loosened over time because: The taperlock bolts backed out (if thats the type of joint), or the initial fit was not tight enough, or the set screws loosened, or fan vibration yielded the shaft material and caused clearance.

Depending on what is causing the problem you can see there are different fixes like: regular bolt tightening, or check all of your fans balance, or as they fail go to a tighter fit on the new joint, or as they fail go to taperlock type shaft/bearing joints.

Thanks Dave the fit seems fine new shafts and new bearings the inner race is fixed to the shaft by set screws that are sunk into the shaft a bit so there is no excerssive clearance present Thanks for the ideas. Bill

Bill,

Please give us the shaft diameter (measured) and the bearing number and a rough idea of the construction and we could give more specific answers.

Sounds very much like the clearnace is too big as stated by others.

set screws that are sunk into the shaft a bit

Do you mean a counterbore or a flat for the setscrew? or are you distorting the metal (shaft)?

What type of setscrews are you using? cup? pointed?

What is the diameter (range) of the shafts?

Are the shafts made of a harder material than the inner race?

If not, maybe they should be.

1. Shafts are almost never harder than the inner race (which is usually case hardened/ through hardened) to 60HRC or so. Where are shafts (under normal conditions) will be HT condition.

2. The case is clearly that of the roller drag being more than the bearing race pre-load.

What is the interference of the race on the shaft?

Type of bearing

The excess rollar drag may be caused by over greasing check.

There are some bearing adhesives available from loctite. Use it in case all other things are OK. But try to eliminate the cause rather than the effect.

You said the inner race has been drilled (and set screw fixed?) still slipping ?

GA for some very good important points......

Bythe way it sounds as though they can fit the bearings with little or no "encouragement" (does not need to be driven on, or have the shaft cooled prior to assy.)

It looks to me that there went something wrong with the calculation of the needed diameter of the shaft. The stiffness of the construction (shaftdiameter, impeller weight and bearing distance) defines at what speed the fan can run. You can calculate the critical speed of the fan and to make the construction save you must divide this crtical speed by 1,5 to get the save level.

Critical speed is: Nc = 1/2π √k/m = 1/2π√g/f
in which:

π = 3,14

g = 9,81
f = bending of the shaft by the weight (or forces)

I would check to see if they were all installed around the same time first. It may just be a useful life issue.

Was the bolt used to secure the inner race expand more than the casing in the heat and weaken the casing ?

Steel bolts could expand past what and alumin casing could stand in high heat. The casing would then change it's shape when cooled because the steel blot would hold heat longer .

I has this same problem once. The problem is that any clearance causes the shaft to move, sometimes eccentrically, which pushes the shaft back and forth which makes the wear worse. I fixed my problem initially by using Locktite Stud and Bearing Mount. Over time I changed out the shafts and used an SKF spherical roller bearing with concentric shaft mounting. I have not changed a fan bearing in 3 years (knock wood) and I was changing about half of them every 18 months.

Bill, The guest in post 8 did the same thing I have done by going to as he put it a concentric shaft mounting. One version of this I mentioned in post 1 was a taperlock.

I have had problems with the collar and set screw type of bearing fit. As you can imagine on one side of the shaft the only thing that is supporting the load are the set screws (after you tighten them) with their very small contact area on the shaft. A hardened shaft might work but I went for the change to a inner race with a tapered collar that squeezes the shaft all around instead of the point load of set screws.

Maybe one day you will get to use an AIR BEARING.

Check out this video NASA is testing for Capstone Corp.

http://www.capstoneturbine.com/prodsol/techtour/index.asp

Is there any build up on the fan blades? Could it be as simple as a fan that has gone out of balance? Good luck.

Where are these fans in BC?? Along the coast or further in? I can't quite understand the wearing out part. Is the shaft scored to the point that it can't be built up again and machined? Do you have shaft breakage at the bearing position? Have you cut the bearings in half and checked the races and balls? Questions questions and ????

I'll call this week and will discuss.

Cheers Robert

Thanks for all the info guys the shaft is a 2 7/16 size and is made of normal cold roll. It is like new I like the idea that the cause could be set screw type locking of the bearing to the shaft and could go to eccentric locking collars. I have sent the bearings out and cut them open to inspect after a failure and everything was fine. I am just having a hard time that the set screws are the culprit but it is easy to insure that the collars are of the eccentric type for the next round of bearings. Just call if you think of anything else and Thanks again Bill

How are the setscrews secured?

Are you using loctite? If so, make sure it is the right type. At one place we had an incompatible loctite and the stuff never dried. The setscrews loosened up and we had all kinds of warranty problems.

It would be a good idea to have a flat spot on the shaft for the setscrew to hold against.

Make sure the setscrews are torqued properly so they can't loosen up.

we are using the blue locktite and the shaft has been drilled to allow the set screw to be set into the shaft to alleviate any slipage. It seems as thought the locktite is holding as the screws are tight. Thanks

Is the loctite dry after 24 hours? Have you unscrewed them?

We unscrewed the ones we thought were good and the loctite was still wet.

The company had switched from zinc coated screws to black oxide and hadn't switched the types of loctite.

The locktite is all dry when we check on it Thanks