press fitting

[jrgrim12]

If you can, tap a hole in to the middle of the saft and use a piece of all thread and a washer to draw the part into place. An arbor press is the best. I personally don't like hydrulic presses because it takes the "feel" out of it. Remember if you are pressing a bearing into place only press/hit on the bearing race that is in contact to the surface it is being moved on. Example, if you are pressing a bearing onto a shaft press/hit on the inner race, if you are pressing a bearing into a pocket press/hit on the outter race. This keeps you from damaging the bearing. If you can't afford a press a great tool is a punch. It helps direct your hammer blows to where you want them.

[Ken Patton]

If you are talking about retaining a bearing in a plate and you don't expect significant axial load (which you shouldn't with ball bearings), another approach if your hole is not small enough is to "stake" the bearing in place.

You stake it by using a pointy punch (gee Ken thats awful technical) and indenting the plate just outside the diameter of the hole. This pushes enough material into the machined bore to help retain the bearing. You might want to put indents at 3-6 locations around the hole. I would NOT use this method if the bearing is seeing any axial loads.

The advantage of this is that you don't need a press. The disadvantage is that you are hitting your nicely-machined and partially-assembled parts with a hammer.

Regarding press-fits: it is possible to do some relatively simple calculations to determine the theoretical holding power of a press-fit. Although, asking an experienced machinist is usually quicker.... )

Ken

[dlavery]

Also be aware that to successfully press fit a bearing will depend on the size of the hole into which you are pressing the bearing (or axle or shaft or whatever), the ratio between the hole diameter and the bearing/shaft diameter, and the materials being pressed together. All these elements are inter-related, and have to be considered.

Press fitting works by forcing a too-large peg into a too-small hole. To fit, either the peg, or the hole, or both, have to distort. That distortion generates huge friction forces between the parts, and they become "frozen" together (up to a point). The combined part can then withstand significant torques without moving relative to each other.

But how big should the "peg" be, or how small should the "hole" be, to make this happen? There is not a single answer to this. If one part is plastic and the other steel, then you would get a significantly different amount of distortion (higher) and resulting torque resistance (lower) than if both parts were steel. You have to calculate what size you want the "peg" to be after pressing, and how much the materials are going to distort, and from there solve backwards to figure our how large the original peg and hole should be to get what you want.

So it is not just a case of "drill the hole one size under and jam it in there." But, fortunately, a lot of the work has already been done for you. For most of the type of problems we are going to have with FIRST robots, you just need to identify the materials you are using and the size of the "peg," and then look up the reference tables in Machinery's Handbook or a similar publication. For simple cylindrical bearings as are mentioned above, this page gives a very good set of general answers.

There is a nice, fairly straightforward description of how this all works and the steps for properly press fitting bearings (including cartoons!) in the Installation Guide for Drill Jig Bearings. If you want to see a more detailed explanation of what is going on, check out this description in The Engineers Toolbox (which is a great resource you should know about anyway).


-dave

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