Sleeve Bearings

[Tmoore]

Can anyone help me with the design of a sleeve bearing, I am currently trying to understand how much side load I can have on my sleeve bearings. Does anyone know if there is any rule of thumb with regards to how long you should make your sleeve bearing based on the side load. To help you answer my question, imagine a column that supports a tank head. The tank head weighs about 200 pounds with all its accessories, and the C of G of the tank head will be about 21 inches from the column axis. The tank head is about 36" above the pivot point of the column. Anyway, if anybody has any helpful advice, it would be most appreciated, I have no books that talk about this, and have been unsuccessful so far with my internet research.
Thanks
Tom

 

[EnglishMuffin]

Just to clarify - for me and others - Are you asking how much direct thrust and/or overturning moment a single sleeve bearing can handle ? Is it hydrodynamic lube (speed?) or marginal/dry lube ?

 

[Tmoore]

I am just wondering how much side load a sleeve bearing in this arrangement can handle safely, and how you would go about finding out the maximum load the shaft can handle when extending before it will stick and not move in and out of the bearing smoothly.
Thanks

 

[EnglishMuffin]

Maybe you are asking : what is the minimum L to D ratio of a bearing such that it will allow sliding with an offset axial load ? If that is the case, then this is a classic problem, and to avoid sticking the L/D ratio of the bearing must exceed:

mu*(1+2*R/D)

wher mu is the coefficient of friction
R is the radial distance of the axial load
from the axial centerline of the bearing
D is the diameter of the bearing
L is the length of the bearing

If this is not what you are asking - then I apologise for hopelessly confusing te issue !

 

[Tmoore]

I think that is something I need to know, I am new at designing sleeve bearings, and I did not study them at all in school, and all I have is my machinists handbook The load is most definitely a side load, imagine if you will, the game hang man, I basically have the gallow (where the man is hanging) and I have a lid that is about 200 pounds. I am trying to help you visual what I am dealing with so we can be sure that I need to use your formula to find out the sleeve bearing length I need. so, imaging a pipe coming out of a rectangular box, going up. The pipe is about 36 inches tall. Then, attached to the pipe is a tank (lid)head. It is about 7 inches tall and has a diameter of 42". The pipe is housed in a mounting flange that fastens to the rectangular box. The mounting flange has two sleeve bearings in it made of 660 Bronze. The pipe has a lever on it that we pull and it causes the pipe to move up, and this is how we lift the lid off of our tank. Ocassionally, the lid will be lifted, and when it is, the pipe will stay in the lifted position, and we have to pull the lever even further to lift the lid higher, and this "un-sticks" it, and the lid will come back down. I think you are right, adn the only problem is that our bearings are not long enough, but I just can't find any more information other than web sites that say the bearing lengths should be longer than what they are. I guess I just need to be able to prove to myself what is the minimun L/D ratio that I need for my specific application. I have found books that say that I need at least a ratio of between 2 to 4 for lifting situations. Anyway, I appreciate your help, do you know anywhere I can find a chart that will tell me what kind of L/D ratio I should have? thanks again and have a great night!
Cheers
Tom

 

[EnglishMuffin]

I don't quite follow it yet - I'm a little slow on the uptake sometimes, and I don't know the game "hang man" - but it appears as though you are lifting a 200 lb lid. However, it is not clear from your description what the offset distance is - if any - between the CG of the lid and the bearing axis (pipe centerline). As far as the formula I have provided is concerned - it is an important variable. As I understand your description, I wouldn't call this a "side loading" situation - its better described as an axial or offset axial load, or an overturning load. But maybe I haven't understood it correctly. If you don't know what the offset is, or there is none, I would assume one. Maybe you still would like it to work if someoone is sitting on it off to one side, for example?

 

[EnglishMuffin]

I was hoping for a quick response - but it sounds like you've turned in !

Re-reading everything you have said, I think my interpretation is correct, my formula should work (I hope), and it looks as though the following data apply :

R = 21"
L - unknown
D - unknown
mu - depends very much on lubrication conditions
machinery's handbook gives .16 for steel on
lubricated bronze, but .35 for steel on unlubricated
phosphor bronze. Unless you are sure about the
lubrication conditions, I would assume unlubricated.

You will note that the weight of the lid is not a factor, at least in whether the shaft self-locks. It would be a factor in sizing the bearings for adequate life, however.

If you can provide D, you can figure out what L has to be. L is either the length of the bearing, or, if there are two, the distance across the outside edges of the bearings.

Whatever the answer turns out to be - I would double it, and recommend that. Others may disagree on that point.
Clarify further if necessary, in light of the above.

 

[Tmoore]

Ok, I understand that you want me to find the L/D ratio, but I don't understand what value I am aiming for. For instance, I don't understand what is an acceptable value for L/D for this application. Lets say I get an L/D ratio of 1, I don't know if it is good enough for this application, or if I need to make it higher or lower. This is what I really need help with.
Thanks again, sorry I had to go to bed after my last post, it was 1 AM.
Tom

 

[EnglishMuffin]

Look - the equation I gave you gives you the L/D ratio ! The only unknown that you haven't provided is D ! (I presume you have a value in mind for this, or an existing value).
However, once you have found the L/D ratio to avoid lock- up from this equation, there are still a number of other things you need to check. One of these is the shaft deflection - if the shaft bends too much, you will get edge loading on the bearings. Another thing you need to check is the actual load on the bearings themselves.

 

[Tmoore]

I see that now, I was much too tired last night and now I have a better understanding of what you meant with your equation, I had skipped over the part that said the L/D ratio had to be bigger than the formula you gave me. I do have a value in mind for D, it is currently about 2.93". I do not think we will get too much shaft deflection, as we are only lift 200 lbs, and the load on the bearings themselves should not be too much.
Thanks
Tom

 

[EnglishMuffin]

OK - now one other thing - I don't want to sound preachy, but its always a bit risky just using someones formula on trust. Even some of the formulas in Roark are wrong sometimes, although the more people cross check them the better they get. So see if you can derive it yourself from first principles - after all, that's what I did. Maybe someone else will read this thread and confirm it. And don't forget what I said about doubling the result - if you have enough room and can afford to. Friction is a bit unpredictable. Good luck.