bearing mount, alignment and anything related

Gents,

thanks to D-man (cheers buddy!) my project is moving along rather smoothly, but it seems like I haven't thought it all the way through. As I have a rather limited understanding of bearing mounts and alignment problems I thought I'd ask here, maybe someone can give some tips / suggestions (fingers crossed).

I'm trying to mount a 20mm shaft in a case which holds two bearings, as shown on the following picture:

http://i1120.photobucket.com/albums/.../support-1.jpg

I'm using tapered roller bearings, which look like this:

http://i1120.photobucket.com/albums/...om/bearing.jpg

My initial though was mounting the bearings using the 'snug fit' approach ie. make a seat for the bearing (shown on the drawing above) and push it in a hole which is slightly smaller than the bearing diameter. The bearing is 12mm thick (on the outside, the middle part is 15.25mm) and 47mm dia, so I though of making a 12mm deep 'seat' and cutting it 46.9mm for a press fit. I'm using MDF so I'm hoping this will be possible, but I'm not so sure the bearing will be held properly in place. I know this depends heavily on the forces acting on the shaft itself and those will not be large as the whole mechanism will be hand driven. This means resistance easy to break by hand cranking the shaft without pulling muscles.

The shaft will need to align with a hole cutout in another part of the machine, which will be adjustable vertically, lets call it carriage. This carriage will move on rails using linear bearings, so it should be rather straight, but I need to make sure the shaft is parallel to the rails. The parts holding the bearings will be CNC machined, but I'm guessing I might end up with misalignment anyway. Both bearing holding plates are 90mm away, the MDF is 18mm thick, so all together the shaft is supported on a length of 116mm.

Third thing that I'm not so sure about is mounting the bearings on the shaft (which will be machined on a lathe with the help of another great forum member). I've bought a 300mm piece of 20mm dia aluminium rod simply to try how it fits in the bearings. Slides in quite nicely to my surprise, although that feeling quickly disappeared when I realised it will rotate freely in the bearing and I need a tight fit, so that the shaft rotates with the bearing rather than in it.

So to sum up this ridiculously long post, I have three questions:

1. is a press fit a good method of mounting the bearing in the MDF parts?

2. should I assume that the CNCed parts will be precise enough to hold both bearings and give me a straight shaft or are the chances of having a wonky shaft quite high?

3. what's the normal method of properly mounting bearings on a shaft without a press and heavy equipment? the shaft will be aluminium for the prototype.

Any suggestions and tips are more than appreciated.

Regards,
dsc.

1) It's either that or machine something which fastens to the MDF to hold them, but since presumably the MDF is a temporay measure I'd stick with just pressing them into the MDF. Press them up against a shoulder (see 3).
2) Yes
3) Machine the shaft such that the bearings are a sliding fit, so they just fit on with no play. You shouldn't press them on since the bearings need to be preloaded, which isn't possible if the inner and outer rings are all constrained. Put a thread on the shaft and a nut to tighten and pre-load the bearings against a shoulder on the shaft. A fine thread and two nuts to lock against each other would work well, or a locknut/washer and grooves in the shaft but that's a bit OTT.

Hi Jonathan,

massive thanks for your answers. Here's a quick 3d drawing I've made to show the concept:

http://i1120.photobucket.com/albums/...ndom/shaft.png

The orange bit is the bottom of the bearing which is pressed / pushed into the MDF. The light blue is the top of the bearing which sits against the shoulder (green, which will be higher up the shaft). The drawing is missing the locking nut above the bearings (the red sections).

I'll keep the points the same and fill them with my comments:

1) The seat I mentioned before will be 12mm deep so I'' be able to push the whole outside element of the bearing in it. The inside element (the rollers) will be pressed against a shoulder on the shaft. I reckon I can use a shaft collar to act as a shoulder? Then again the whole shaft will be machined, so adding a shoulder shouldn't be too hard

2) The reason I was asking about precision is because I've read about problems with alignment on diy CNC machines. I know it's probably harder to align things which are far apart as a small error grows as you move away from parts.

3) Am I correct to think that with this approach the shaft needs to have a thread on one side of the bearing? I've marked those in red on the sketch above. A 20mm shaft can have an M20 thread on it, but in order to get the nut where it should be there needs to be thread all the way up the shaft or the shaft needs to be thinner to let the nut through. Please correct me if this is utter rubbish:)

Cheers Jonathan!

Regards,
dsc.