1) Measure the torque required and see.
2) Try to get information from the bearing manufacturers- may or may not be easy.
3) Check a machine design or mechanical engineering reference and see if they have any info on bearing drag forces.
There will be some drag due to the bearings. There will be an additional torque required due to inertia, that may or may not be significant, depending on the speeds involved.
I would suppose that drag from the bearings would be highly variable depending on the amount, kind, and temperature of the grease; the construction of seals, if any; how worn the bearings were, lateral loads on the bearings, etc. The drag may depend on how tight the retaining nuts are fastened, also.
Maybe some additional information on your part would get more helpful answers here.
The friction of the bearings is a consideration and must be known as well as the dimensional specs of the rod, the speed you wish to spin it up to, the acceleration you wish to use to achieve this speed or the time you have to achieve it. Also the means at which you wish to impart motion to the shaft would be helpful to account for losses and give an idea of where the force will act on the rod.
Lets forget about the bearing for now. A steel shaft is currently in place and I am wondering if an Aluminum shaft would require less torque to spin. I am guessing they would be the same since polar moment of inertia has nothing to do with type of material... correct? But my instincts tell me the heavier shaft would be harder to spin.
A lighter shaft would cause less friction on bearings, thus less force to overcome bearing friction.
Area moment of inertia may be the same, but the mass moment of inertia is density-dependent. Less mass = smaller "J" = higher rotational acceleration for same applied torque.
Since Aluminum is generally lighter than steel and if the shafts share the same dimensions the aluminum shaft will take less power to initially spin it up to the same speed as the steel one.
If loading is significant, then an aluminum shaft could be seriously overstressed- especially with regards to fatique failure. Assuming you made the shafts the same size, that is.
The main difference in the shafts from a force standpoint would be the inertia- and that looks pretty low either way.
Loading is not an issue, this application is just an idler roller that film runs across, but if the roller does not turn in sequence with the film it will scratch it. So I figured I would try to get some lower friction bearings and a lighter shaft.
Not the answer to your question, but if you are doing a web handling application you might be interested in using lightweight aluminum rolls specifically made for web handling. Here is one source:
Does the film drive the roller via friction or does the roller maintain the speed of the film through a seperate drive mechanism? If the former then maybe a coatiing on the roller to increase friction is in order
The film drives the roller via friction. And there are diameter limitations. Like I said before there is a steel shaft in place now, so I am going to have maintenance clean and grease the bearings and see what effect that has, and if that gets it close I can replace steel with aluminum and that should help out some more.
I'm guessing scratching has become an issue as the machine ages or has this been from startup? Do the rollers slip during the time the machine is being brought up to speed or during normal operation. If its the latter then friction rather than inertia is probably the culprit and some bearing work is needed. Lowering the weight of the roller may have no effect if this is the case but the corresponding bearing replacement may solve the problem if spec'ed correctly
search on thomasregister dot com for "roller coating" and you'll get a few companies that provide friction coatings for rollers - it may help prevent the slipping you are experiencing without designing a new part. Else you can look at plastic rollers as well - lighter than aluminum and with the right material choice it may prevent scratching even if the rollers slip on the film. Rather than ball bearings if you redesign or rebuild - you also may want to check into composite bearings as for low loads they can provide less friction than ball or roller.
In an application like this, bearing friction will be significant, and a greased bearing will have MUCH more friction than an oiled bearing. You might consider using oil lubrication rather than grease. Also, contacting rubber seals will have more friction than felt or labyrinth seals.
If weight is your main consideration,
I think I would go with a hollow steel
shaft, I would be a little worried about
the expansion of the aluminum shaft
increasing the preload on the bearings.
If you simply want to compare torque forces between different bearing fits, lubrication, and shaft materials, I suggest a method I once used for a tool bearing analysis on a electro-mechanical device. I fabricated a pendulum that approximated a simple pendulum and with that and a stop- watch I determined the combination that resulted in the most free running set-up by simply comparing the time it took for the pendulum to come to a complete stop. It worked! The temperature of the tests must duplicate the actual running condition. moreover, with a little thought and a knowledge of Newton's laws of motion and general mechanics, real values can be calculated.
Area moment of inertia may be the same, but the mass moment of inertia is density-dependent. Less mass = smaller "J" = higher rotational acceleration for same applied torque
Lowering the moment of inertia of the roller will reduce, but probably not eliminate potential scratching. The significance of inertia depends on the acceleration profile of the film. A way to reduce the bearing friction (a small contributor, I'll bet) would be to switch to an air bearing. After thinking of that, it popped into my head that since this is an idler, maybe what you need is an air bearing for the film itself. Think of an air hockey table and construct the same thing over the arc of contact of the film over the roller. If this is in an industrial setting, you probably already have compressed air or N2 available and the flow rates would be quite small. You'd just substitute a plugged piece of tubing with a number of small holes in it. Depending on the film, you might need to worry about static electricity accumulation, but there are a number of off the shelf solutions for that.
someonesdad has the right idea I think. If possible install an air wand behind the roller so that air is aimed in the direction of travel of the film. Unless there is high load on the roller, you can get a bit of a layer of air against the roller that will cushion the film. Since you are interested in start-up forces, it would seem that the film starts and stops so use a light weight roller and even consider a porous roller that will pump air against the film as a cushion. Air transport chutes are used to move powder in this fashion, on flat surfaces and a similar material might work for a roller.
